MCRP 3-15.2B Mortar Gunnery
MCRP 3-15.2B Mortar Gunnery
MCRP 3-15.2B Mortar Gunnery
Mortars Gunnery
PREFACE
This manual provides guidance for MOS 11C soldiers and their trainers on the
employment of the 60-mm (M224 and M19) mortars, 81-mm (M252 and M29A1) mortars,
4.2-inch (M30) mortar, and 120-mm (M120) mortars. It discusses the practical applications
of ballistics and a system combining the principals, techniques, and procedures essential to
the delivery of timely and accurate mortar fire. (See FM 23-90 for information on
mechanical training, crew drills, and the characteristics, components, and technical data of
each mortar.)
This manual is divided into four parts: Part One discusses the fundamentals of mortar
gunnery; Part Two summarizes the operational procedures of a fire direction center; Part
Three describes the capabilities and use of the mortar ballistic computer; and Part Four
describes the capabilities and use of the M16/M19 plotting board.
The proponent of this publication is US Army Infantry School. Send comments and
recommendations on DA Form 2028 (Recommended Changes to Publications and Blank
Forms) directly to Commandant, US Army Infantry School, ATTN: ATSH-INB-D (MPO),
Fort Benning, GA 31905-5000.
Unless this publication states otherwise, masculine nouns and pronouns do not refer
exclusively to men.
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*FM 23-91
MORTAR GUNNERY
CONTENTS
Page
Preface................................................................................................................................ vi
Part One
INTRODUCTION AND FUNDAMENTALS
OF MORTAR GUNNERY
CHAPTER 1. INTRODUCTION
1-1. Organization .................................................................................1-1
1-2. General Doctrine ..........................................................................1-1
1-3. Indirect Fire Team ........................................................................1-2
1-4. Mortar Positions ...........................................................................1-3
__________________
DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited.
__________________
*This publication supersedes FM 23-91, 6 December 1991.
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Page
Section III. FIRE PLANNING...............................................................................2-10
2-14. Terminology ...............................................................................2-10
2-15. Target Considerations ................................................................2-13
2-16. Support of Offensive Operations................................................2-14
2-17. Support of Defensive Operations ...............................................2-14
2-18. Fire Support Coordination Measures .........................................2-15
2-19. Company Fire Support Plan .......................................................2-16
2-20. Battalion Fire Support Plan ........................................................2-18
Section IV. TARGET ANALYSIS AND ATTACK..............................................2-19
2-21. Target Description......................................................................2-19
2-22. Registration and Survey Control ................................................2-19
2-23. Size of Attack Area ....................................................................2-20
2-24. Maximum Rate of Fire ...............................................................2-20
2-25. Amount and Type of Ammunition .............................................2-21
2-26. Unit Selection.............................................................................2-23
2-27. Typical Targets and Methods of Attack .....................................2-24
Part Two
FIRE DIRECTION CENTER
CHAPTER 3. INTRODUCTION
3-1. Principles of Fire Direction ..........................................................3-1
3-2. Organization .................................................................................3-1
3-3. Personnel Duties...........................................................................3-2
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Page
5-7. Message to Observer ....................................................................5-5
5-8. Call-For-Fire Format ....................................................................5-5
5-9. Authentication ..............................................................................5-6
Part Three
MORTAR BALLISTIC COMPUTER
CHAPTER 6. INTRODUCTION
6-1. Description ...................................................................................6-1
6-2. Audio Alarm.................................................................................6-7
6-3. Capabilities...................................................................................6-8
6-4. Memory Storage ...........................................................................6-9
6-5. Error Messages .............................................................................6-9
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Page
Part Four
M16 AND M19 PLOTTING BOARDS
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1 MARCH 2000
ERIC K. SHINSEKI
General, United States Army
Chief Of Staff
Official:
JOEL B. HUDSON
Administrative Assistant to the
Secretary of the Army
0000506
DISTRIBUTION:
Active Army, Army National Guard, and U. S. Army Reserve: To be distributed in accordance with the
initial distribution number 110470, requirements for FM 23-91.
FM 23-91
PART ONE
INTRODUCTION AND FUNDAMENTALS OF
MORTAR GUNNERY
CHAPTER 1
INTRODUCTION
The mission of the mortar platoon is to provide close and
immediate indirect fire support for the maneuver battalion and
companies.
1-1. ORGANIZATION
Mortars are organized as part of a company and battalion. They are either sections
or platoons in airborne, ranger, air assault, light infantry companies, and cavalry
troops. They are organized as platoons in all tank and infantry mechanized
battalions. Regardless of the organization to which they belong, mortars have the
battlefield role of providing the maneuver commander with immediate indirect
fires. They can fulfill that mission when all of the elements responsible for
placing effective mortar fire on the enemy are properly trained.
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massed fires cannot be achieved, the time required to bring effective fires on the
target should be kept to a minimum.
d. The greatest demoralizing effect on the enemy can be achieved by the
delivery of a maximum number of rounds from all the mortars in a mortar section
or platoon in the shortest possible time.
e. Mortar units must be prepared to handle multiple fire missions. They can
provide an immediate, heavy volume of accurate fire for sustained periods.
Mortars are area fire weapons; however, units can employ them to neutralize or
destroy area or point targets, to screen large areas with smoke for sustained
periods, or to provide illumination.
f. In the armor and mechanized infantry battalions, units can normally fire
mortars from mortar carriers. However, mortars maintain their ground-mounted
capability. Firing from a carrier permits rapid displacement and quick reaction to
the tactical situation.
1-3. INDIRECT FIRE TEAM
Indirect fire procedures are a team effort (Figure 1-1). They include locating the
target, determining firing data, applying data to the mortar, and preparing the
ammunition. Since the mortar is normally fired from the defilade (where the crew
cannot see the target), the indirect fire team gathers and applies the required data.
The team consists of a forward observer (FO), a fire direction center (FDC), and a
mortar squad.
a. The team mission is to provide accurate, timely response to the unit it
supports. Effective communication is vital to the successful coordination of the
efforts of the indirect fire team.
b. The forward observer (FO), as part of the fire support team (FIST), is
normally provided by a direct support (DS) artillery battalion. One 4-man FO
team supports each mechanized infantry company. The light infantry company is
supported by a 10-man company-level FO team. The team is composed of a
lieutenant, a staff sergeant, a radio-telephone operator, a driver with a HMMWV
at company headquarters, and six FOs (one 2-man team for each infantry platoon
in the company). The FO’s job is to find and report the location of targets, and to
request and adjust fire.
c. The FDC has two computer personnel in each section (except the 60-mm
squad, which does not have assigned FDC personnel) who control the mortar
firing. They convert the data from the FO in a call for fire into firing data that can
be applied to the mortar and ammunition.
d. Mechanized infantry and armor mortar squads consist of one squad leader,
one gunner, one assistant gunner, and one ammunition bearer. Airborne, air
assault, and light infantry squads in the battalion mortar platoon consist of one
squad leader, one gunner, one assistant gunner, and two ammunition bearers. At
company level, these light units have two 3-man sections consisting of one section
sergeant, one squad leader, two gunners, and two assistant gunners. The squad
lays the mortar and prepares the ammunition, using the data from the FDC fire
command. When the data have been applied, the mortar squad fires the mortar.
The squad must also be able to fire without the FDC.
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CHAPTER 2
FUNDAMENTALS OF MORTAR GUNNERY
This chapter discusses the elements of firing data, ballistics, firing
tables, fire planning, target analysis, and methods of attack. This
information enables the FDC to engage the enemy even during adverse
conditions.
2-1. DIRECTION
In mortar gunnery, direction is a horizontal angle measured from a fixed reference. The
indirect fire team normally measures direction in mils clockwise from grid north, which is
the direction of the north-south grid lines on a tactical map. The team emplaces its
mortars on a mounting azimuth, then uses the direction to make angular shifts onto the
target. Direction to the target may be computed, determined graphically, or estimated
(Figure 2-1, page 2-2).
NOTE: The unit of angular measurement in mortar gunnery is the mil. A mil equals
about 0.056 of a degree. There are 17.8 mils in a degree and 6400 mils in a
360-degree circle.
2-2. RANGE
Range is the horizontal distance, expressed in meters, from the mortars to the target. It is
computed, measured graphically, or estimated. The range of a projectile depends on its
muzzle velocity (which depends on charge and other factors) and the elevation of the
mortar.
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TARGET
DIFFERENCE IN ELEVATION
BETWEEN MORTAR
AND TARGET
(VERTICAL INTERVAL)
AIMING POINT
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(5) An increase in bore resistance at any time has a dragging effect on the
projectile and decreases velocity. Temporary variations in bore resistance are caused by
carbon buildup in the barrel.
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to initial projectile movement and lessening pressure buildup. Wear can be reduced by
careful selection of the charge and by proper cleaning of the weapon and ammunition.
e. Rotating Disks. Rotating disks allow proper seating, keep gases from escaping
between the bore and the projectile, and create proper resistance to the projectile's initial
movement. Also, disks allow uniform pressure buildup but minimum drag on the moving
projectile, and they help give it a proper spin. Dirt or burrs on the rotating disk cause
improper seating, which increases barrel wear and reduces muzzle velocity. If the bore is
excessively worn, the rotating disk may not properly engage the lands and grooves to
impart proper spin to the 4.2-inch mortar projectile. Not enough spin reduces projectile
stability in flight, which can result in dangerously short, erratic rounds.
f. Temperature of the Propellent. Any combustible material burns rapidly when it
is heated before ignition. When a propellent burns more rapidly, the resultant pressure on
the projectile is greater, increasing muzzle velocity. Firing tables show the magnitude of
that change. Appropriate corrections to firing data can be computed, but such corrections
are valid only if they reflect the true propellent temperature. The temperature of
propellents in sealed packing cases remains fairly uniform, though not always standard
(70 degrees F).
(1) Once the propellent is unpacked, its temperature tends to approach the
prevailing air temperature. The time and type of exposure to weather result in propellent
temperature variations between mortars. It is not practical to measure propellent
temperature and to apply corrections for each round fired by each mortar. Propellent
temperatures must be kept uniform; if they are not, firing is erratic. A sudden change in
propellent temperature can invalidate even the most recent corrections.
(2) To let propellents reach air temperature uniformly, ready ammunition should
be kept off the ground. Ammunition should be protected from dirt, moisture, and direct
sunrays. An airspace should be between the ammunition and protective covering.
(3) Enough rounds should be unpacked so that they are not mixed with newly
unpacked ammunition. They should be fired in the order in which they are unpacked;
hence, opened rounds are fired first.
g. Moisture Content of Propellent. Handling and storage can cause changes in the
moisture content of the propellent, which affects the velocity. This moisture content
cannot be measured or corrected; also, ammunition must be protected from moisture.
h. Weights of Projectile. The weight of like projectiles varies within certain weight
zones. For the lighter 60-mm and 81-mm projectiles, the difference is minimal and has
little affect on muzzle velocity. For the 4.2-inch mortar projectile, however, the
difference must be considered. The appropriate weight zone is stenciled on the projectile
as squares ([]) of weight. A heavier-than-standard projectile is harder to push through the
barrel and has less muzzle velocity. A lighter projectile is easier to push through the
barrel and has a higher muzzle velocity. The weight of the projectile is also a factor in
exterior ballistics.
i. Barrel Temperature. The temperature of the barrel affects the muzzle velocity.
A cold barrel offers more resistance to projectile movement than a warm barrel.
j. Propellent Residues. Residues from the burned propellent and certain chemical
agents mixed with expanding gases are deposited on the bore surface in a manner similar
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to coppering. Unless the barrel is properly cleaned and cared for, such residues increase
subsequent barrel wear by pitting, thus increasing abrasion by the projectiles.
k. Oil or Moisture. Oil or moisture in the barrel or on the rotating disk tends to
increase the velocity of a round by causing a better initial gas seal and reducing projectile
friction on the bore surface. Conversely, too much oil or moisture in the barrel decreases
velocity, causing a short round.
2-10. TRAJECTORY
Trajectory (Figure 2-2) is the flight path followed by a projectile from the muzzle of the
mortar to its point of impact. The ascending branch is the portion of the trajectory traced
while the projectile is rising from its origin. The descending branch is that portion of the
trajectory traced while the projectile is falling. The summit is the highest point of the
trajectory. It is the end of the ascending branch and the beginning of the descending
branch. The maximum ordinate is the altitude (in meters) at the summit above the point
of origin.
SUMMIT
DESCENDING
BRANCH
ASCENDING
BRANCH
MAXIMUM
ORDINATE
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Its descending branch is shorter than its ascending branch, and its angle of fall is greater
than its angle of elevation.
(1) The spin initially imparted to the 4.2-inch mortar projectile causes drift
(Figure 2-3). This characteristic has an effect on trajectory that must be considered when
aiming.
(2) A trajectory in standard atmosphere is effected by the following factors:
• Horizontal velocity decreases with continued time of flight.
• Vertical velocity is affected not only by gravity but also by air resistance.
c. Standard Conditions and Corrections. Certain atmospheric and material
conditions are accepted as standard. Those conditions are outlined in the introduction to
the firing tables given below. When conditions vary from standard, the trajectory varies.
Variations in the following conditions can be measured and corrected:
• Difference in altitude between the mortar and the target.
• Propellent temperature.
• Drift.
• Ballistic wind.
• Air temperature.
• Air density.
• Weight of the projectile.
TARGET
CHART DEFLECTION
MORTAR
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variations in the weapon, weather, and ammunition at a given time and place. The
atmospheric standards in United States firing tables reflect the mean annual conditions in
the north temperate zone. The main elements measured in experimental firing are angle of
elevation, angle of departure, muzzle velocity, attained range, drift, and concurrent
atmospheric conditions.
2-11. PURPOSE
The main purpose of a firing table is to provide the data required to bring effective fire on
a target under any condition. Data for firing tables are obtained by firing the weapon at
various elevations and charges.
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percent of air density (1 percent of drag) increases with an increase of charge (muzzle
velocity).
(c) Two projectiles of identical shape but different size do not experience the
same drag. For example, a larger projectile offers a larger area for the air to act upon;
hence, its drag will be increased.
(3) The finish of the shell surface affects the muzzle velocity. A rough surface on
the projectile or fuze increases air resistance, thereby decreasing range.
(4) The ballistic coefficient of a projectile is its efficiency in overcoming air
resistance compared to an assumed standard projectile. Each projectile and projectile lot,
however, has its own efficiency level. Therefore, to establish firing tables, one specific
projectile lot must be selected and fired. Based on the performance of that lot, standard
ranges are determined. The ballistic coefficient of that lot becomes the firing table
standard. However, other projectile lots of the same type may not have the same ballistic
coefficient as the one reflected in the firing tables. If one of the other lots is more
efficient—that is, has a higher ballistic coefficient than the firing table standard—it will
achieve a greater range when fired. The reverse is true for a less efficient projectile lot.
NOTE: For ease in computations, all projectile types are classified into certain
standard groups.
(5) Range wind is that component of the wind blowing parallel to the direction of
fire and in the plane of fire. Range wind changes the relationship between the velocity of
the projectile and the velocity of the air near the projectile. If the air is moving with the
projectile (tail wind), it offers less resistance to the projectile and a longer range results; a
head wind has the opposite effect.
2-14. TERMINOLOGY
Some of the common terms used in fire planning are defined as follows:
a. A target may be troops, weapons, equipment, vehicles, buildings, or terrain that
warrant engagement by fire and that may be numbered for future reference. A solid cross
designates a target on overlays, with the center of the cross representing the center of the
target. The target number consists of two letters and four numbers allocated by higher
headquarters. That numbering system identifies the headquarters that planned the target,
distinguishes one target from another, and prevents duplication.
b. Targets of opportunity are targets for which fires have not been planned.
Planned targets are scheduled or on call.
(1) Scheduled targets are fired at a specific time before or after H-hour, or upon
completion of a predetermined movement or task.
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(2) On-call targets are fired only upon request. They include targets for which
firing data are kept current, and targets for which firing data are not prepared in
advance—for example, a prominent terrain feature, such as a road junction, that the FO
may use as a reference point.
c. A group of targets consists of two or more targets to be fired at the same time.
Targets are graphically portrayed by circling and identifying them with a group
designation (Figure 2-5). Mortars are normally assigned groups of targets. The group
designation consists of the letters assigned to the maneuver brigade by the division TOC
with a number inserted between them. For example, if the brigade is assigned the letters
A and B, the first group of targets planned by the DS battalion FDC is designated A1B,
the second group A2B, and so on. Similarly, if the division TOC has designated the
letters A and Y, the first group is A1Y and the second is A2Y. The designation of a group
of targets does not preclude firing at any individual target within the group.
A1B
AB0061
AB0062
AB0062
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DOG
AB1
AB0052
AB0051
AB0049
AB0048
AB0050
C CO
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and control installations, and later the attack of enemy forward elements. The detail and
extent of preparation plans depend on the availability of intelligence.
b. Battalion Mortar Platoon. The battalion fire plan table for a preparation may
include fires by the battalion mortar platoon. Once the preparation is fired, the mortar
platoon is available for fire support of the battalion maneuver elements. In some
situations, the battalion commander may exclude the mortars from the preparation and
retain them for targets of opportunity throughout the attack.
c. Company Mortar Section. The company mortar section may be required to fire
preparation fires. Those fires are limited to the engagement of enemy forward elements.
Before committing the mortars to preparation fires, the commander should consider
ammunition resupply and availability of mortars to quickly attack targets of opportunity.
d. Fires Supporting the Attack. Fires planned in support of the attack are shifted
to conform to the movements of the supported unit. They are planned in the form of
targets, groups of targets, and series of targets. They may be fired on a time schedule or
on-call and may include targets from the LD to the objective, on the objective, and
beyond the objective.
e. Objectives. Supporting fires have several specific objectives. They assist the
advance of the supported unit by neutralizing enemy forces, weapons, and observation
short of the objective. They assist the supported unit in gaining fire superiority on the
objective so that the assaulting force can close to assault distance, and they protect the
supported unit during reorganization. (On-call targets are planned on likely assembly
areas and routes for enemy counterattacks.) Supporting fires prevent the enemy from
reinforcing, supplying, or disengaging his forces. Also, they quickly provide mutual fire
support to lower, adjacent, and higher headquarters.
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(1) The artillery and mortar FPF are integrated with the FPL of machine guns.
Each artillery battery normally fires one FPF. The mortar platoon of the battalion may
fire one or two FPF; however, the platoon's fires are more effective in one FPF than in
two.
(2) The FPF of the DS artillery are available to the supported brigade and its
battalions. The FPF of any artillery reinforcing DS battalion is normally available. The
brigade commander designates the general areas for available FPF or allocates them to
the maneuver battalions. The maneuver battalion commander, in turn, designates general
locations or allocates them to maneuver companies.
d. Fires Within the Battle Area. The precise location of an FPF is the
responsibility of the company commander in whose sector it falls. The exact locations of
FPF within each forward company are included in the fire plan and reported to battalion.
Fires within the battle area are planned to limit penetrations and to support
counterattacks.
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Fire planning at battalion level is initiated the same as in the company. The battalion fire
planning team consists of the battalion commander, S3, battalion mortar platoon leader,
and FSO. The battalion mortar platoon must always be directly responsive to the desires
of the battalion commander. The platoon leader takes a position that best assists the S3 in
planning and obtaining fire support. The FSO is normally the battalion FSO; however,
the battalion mortar platoon leader serves in the absence of the FSO.
a. The battalion commander and S3 present the commander's concept of the
operation, which, as in the case of the company, includes the scheme of maneuver and the
plan for fire support. After the FSO has consolidated the target lists prepared by the
company fire planners, the battalion commander approves the consolidated target list as
part of the battalion fire support plan. The written plan becomes an annex to the
operation plan.
b. The FSO is usually the battalion FSCOORD and receives target lists from the
company's FIST chief and from the battalion mortar platoon leader. Once duplications
are deleted, all fire plans are updated by assigning target numbers or by consolidating
targets. Then, the FSO submits all fire plans and target lists to the battalion S3 as the
proposed battalion fire support plan.
c. The S3 ensures that the proposed fire support plan supports the scheme of
maneuver. After the battalion commander approves the fire plan, the plan becomes an
annex to the battalion operation plan. It is disseminated to all subordinate elements to
include rifle companies and the battalion mortar platoon.
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up. Smoke is used to blind or confuse, but the effect lasts only as long as fires are
continued.
(2) Neutralization knocks the target out of the battle temporarily. Ten percent or
more casualties usually neutralize most units. The unit becomes effective again when
casualties are replaced and equipment repaired.
(3) Destructive fires put the target out of action permanently. A unit with 30
percent or more casualties is usually rendered permanently ineffective, depending on the
type and discipline of the force. Direct hits are required on hard materiel targets.
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(1) Quick and superquick fuzes. Quick and superquick fuzes are used for impact
detonation. When the HE projectile with a quick or superquick fuze passes through trees,
detonation may occur in the foliage. Therefore, its effectiveness may be either improved
or lost, depending on the density of the foliage and the nature of the target.
(2) Proximity fuzes. Proximity fuze is used with HE ammunition to obtain
airbursts. A proximity or VT fuze detonates automatically upon approach to the object.
It is used to obtain airbursts without adjusting the HOB. If the proximity element fails to
function, a fuze quick-action occurs upon impact. The HOB varies according to the
caliber of projectile, the angle of fall, and the type of terrain in the target area. If the
terrain is wet or marshy, the HOB is increased. Light foliage has little effect on a
proximity fuze, but heavy foliage increases the HOB by about the height of the foliage.
The greater the angle of fall, the closer the burst is to the ground.
(3) Fuze delay. Fuze delay produces a mine action caused by the round’s
penetration before detonation. Fuze delay can be used to destroy earth and log
emplacements. It is also effective against some masonry and concrete structures. Fuze
delay is not used against armor. The depth of penetration depends on the type of soil and
terminal velocity of the round.
(4) Multioption. Multioption fuze gives the user the option to select and use all
types of fuzes previously mentioned. It has the setting of delay, impact, near surface
burst, and proximity. This type of fuze will be replacing the other fuzes in the future.
(5) Three-fuze family. The M734 multioption fuze, the M745 point-detonating
fuze, and the electronic time fuze make up the three-fuze family. The current M734
multioption fuze has received a materiel change, which is designated the M734A1 and
fielded on the M929, 120-mm smoke. The M745 point-detonating fuze is fielded on the
60-mm/120-mm smoke, and the M933, 120-mm HE (training) round. These three fuzes
are used on all 60-mm 81-mm, and 120-mm service and training rounds.
(6) M734A1. The M734A1 multioption fuze is an air-powered fuze with four
selectable functions: PRX 60/81; PRX 120; IMP and DLY. All functions are selectable
by the soldier before firing. In the HE proximity mode, the height of burst is constant over
all types of targets. The impact mode causes the round to function on contact with the
target. In the delay mode, the fuze functions about 30 to 200 milliseconds after target
contact. The impact mode is the first backup function for either proximity setting. The
delay mode is the backup for the impact and delay modes. The impact and delay modes
have not been changed from the current M34 multioption fuze. The M734A1 uses ram air
and setback to provide two independent environment sensors to comply with the safety
requirements of military-standard 1316C. Radio frequency jamming can be detected.
Radio frequency jamming initiates a graceful desensitizing of the fuze electronics to
prevent premature fuze function. Once the fuze is out of the jammer range, the fuze
electronics recovers and functions in the proximity mode if the designed height of burst
has not been passed. To limit the time of fuze radio frequency radiation, the proximity
turn-on is controlled by an apex sensor that does not allow initiation of the fuze proximity
electronics until after the apex of the ballistic trajectory has been passed.
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and compounds the problem of locating and removing the mines by hand and of moving
equipment across the mined area. Mortars also require extravagant amounts of
ammunition to breach barbed wire and should not be used.
REMARKS
TYPE OF (SEE
TYPE OF
ADJUSTMENT PROJECTILE FUZE TYPE OF FIRE FOOTNOTES)
TARGET
GROUP I
VEHICLES OBSERVED, HE, WP SQ, VT NEUTRALIZATION, (1), (2), (3)
(RENDEZVOUS) UNOBSERVED DESTRUCTION
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REMARKS
TYPE OF (SEE
TYPE OF
ADJUSTMENT PROJECTILE FUZE TYPE OF FIRE FOOTNOTES)
TARGET
PERSONNEL OBSERVED, HE SQ, DELAY DESTRUCTION, (4)
(UNDER LIGHT UNOBSERVED ASSAULT, DIRECT
COVER)
DELAY, VT, Q
HE DESTRUCTION, CRITICAL POINTS,
HARASSING, DEFILES, FILLS,
INTERDICTION CROSSINGS, CUL-
VERTS, BRIDGES,
AND NARROW POR-
TIONS MUST BE
ATTACKED. DIREC-
TION OF FIRE
SHOULD COINCIDE
WITH DIRECTION OF
ROAD.
(1) Area is neutralized with projectile HE (airbursts if practical); surprise is essential to produce casualties.
(2) Materiel remaining in the area should be attacked for destruction by using the appropriate projectile and fuze.
(3) Projectile WP should be combined with HE when the target contains flammable material and when the smoke will not obscure
adjustment.
(4) Projectile HE with fuze quick is fired at intervals to clear away camouflage, earth cover, and rubble.
(5) The first objective in firing on moving vehicles is to stop the movement. For this purpose, a deep bracket is established so that the
target will not move out of the initial bracket during adjustment. Speed of adjustment is essential. If possible, the column should be
stopped at a point where vehicles cannot change their route and where one stalled vehicle will cause others to stop. Vehicles moving on
a road can be attacked by adjusting on a point on the road and then timing the rounds fired so that they arrive at that point when a
vehicle is passing it. A firing unit or several units, if available, may fire at different points on the road at the same time.
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PART TWO
FIRE DIRECTION CENTER
CHAPTER 3
INTRODUCTION
This chapter contains information on the principals of fire direction
procedures, organization of FDCs, and duties and responsibilities of FDC
personnel.
3-2. ORGANIZATION
The FDC is the element of the indirect fire team that receives the call for fire from the
FO, FIST chief, or higher headquarters; determines firing data; and announces the fire
command to the firing section. The FDC also determines and applies corrections to chart
data and to standard firing table values to achieve accuracy in firing. Firing data
normally are produced in the FDC. However, they may be produced by a squad leader
when the section is firing without an FDC. Accuracy, flexibility, and speed in the
execution of fire missions depend on
a. Accurate and rapid computation of firing data from the MBC and plotting board.
b. Clear transmission of commands to the mortar section.
c. Accurate and rapid verification of firing data.
d. Efficient division of duties.
e. Adherence to standard techniques and procedures.
f. Efficient use of FDC plotting equipment, MBC, and other data-determining
devices.
g. Teamwork and operating in a specified sequence.
h. Efficient use of communications, including the FDC switchboard.
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CHAPTER 4
MAJOR CONCERNS OF THE FIRE DIRECTION CENTER
This chapter contains information on some of the "tools" the FDC uses
to accomplish its mission. It also discusses the methods and techniques
used in FDCs to convert calls for fire into proper fire commands.
c. Open Sheaf. The open sheaf (Figure 4-3) is normally used to engage targets that
are wider than a standard sheaf can cover. With the open sheaf, the distance between
impacts of rounds from two or more mortars is half again the distance between the bursts
of the rounds in a standard sheaf. Normally, 81-mm and 4.2-inch mortar rounds impact
40 meters apart, and 120-mm rounds impact 75 meters apart. Thus, in an open sheaf with
60-mm mortars, which impact 30 meters apart in a standard sheaf, rounds would impact
45 meters apart. All mortars fire different deflections for an open sheaf.
d. Special Sheaf. The special sheaf (Figure 4-4) is normally used in an attitude
mission and when needed for the FPF. With the special sheaf, each mortar has a certain
point to engage. The mortars may have different deflections and elevations.
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e. Standard Sheaf. With the standard sheaf (Figure 4-5), rounds impact within the
total effective width of the bursts, regardless of the mortar formation.
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be a 200-meter zone. Should the target require it, the 4.2-inch mortar can fire a larger
zone.
(10) TIME OF OPENING FIRE—The fire control for the mission.
W/R = When ready
AMC = At my command (either the FO or FDC)
The chief computer/section sergeant usually completes the FDC order. This area
describes how the FDC will engage the target.
m. INITIAL CHART DATA. This includes the following:
(1) DEFLECTION—Initial deflection from the mortar position to the target being
engaged (plotting board only).
NOTE: When using the M16 plotting board with the drift, the drift used will be
annotated in this column. "Drift" will be placed in the left column of the
initial chart data (4.2 only).
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(5) ON HAND—The number, by lot number, the firing element has on the firing
position.
(6) RECEIVED—Number and type of rounds received.
(7) TOTAL—The combination of rounds on hand and those received.
(8) ROUNDS EXPENDED—The number of rounds expended for missions.
(9) ROUNDS REMAINING—The number of rounds remaining.
NOTE: If the chart data and the command data are the same, do NOT repeat the data
in the range/chart block.
(4) FIRING DATA. This is the base gun command data for the targets. This
information contains all corrections (when used) plus chart data to get the firing data
(command data) to the center mass of the target.
(a) DEFL (deflection)—Command deflection to hit the center mass of the
target.
(b) RG/CHG (range/charge)—The command range and charge to hit the
target.
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4-4. ANGLE T
Angle T (Figure 4-8) is the mil difference between the OT line and GT line. Angle T is
not important to the FDC when computing. However, to the FO, it must be considered
when making corrections to engage a target when the angle T is between 1600 to 3200
mils.
0 MILS
ANGLE T BETWEEN
1600 AND 3200 MILS
MORE THAN 500 MILS
FO
1600 MILS
ANGLE T
3200 TO 2800 MILS
(LESS THAN 500
MILS)
FO
MORTAR 2800 MILS
3200 MILS
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NOTE: The FO must send the OT azimuth in the call for fire for a shift and polar.
EXAMPLE 1
Consider OT = 2950 mils and GT = 3190 mils; then, 3190 - 2950 = 240 mils (angle T).
EXAMPLE 2
Consider OT = 6210 mils and GT = 0132 mils. Because the azimuths are on either side
of 6400 (0), subtracting the smaller from the larger would not yield the angle T. The
computer must add 6400 to the smaller and then subtract from the larger:
NOTE: This procedure is used only when one azimuth is between 0 (6400) and 1600,
and one is between 4800 and 6400.
b. Because the angle T is over 499 mils in the example above, the FDC would then
send a message to the observer that the angle T exceeded 499 mils. Otherwise, there is no
need to tell the FO what the angle T is unless he requests it. The observer would use this
information before making any correction. When the angle T exceeds 499 mils (Figure 4-
9, page 4-12), the FO would continue to use the OT factor to make deviation corrections.
However, if it is observed that the correction is more than asked for, the deviation
corrections should be reduced proportionately during the mission. Information about the
angle T is automatically given to the FO only if it exceeds 499 mils. If the FO wants to
know what the angle T is, then the FDC would announce it to the nearest 100 mils.
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TARGET
MORTAR
NOTE: Refer to appropriate firing tables for specific rounds that are not listed in this
manual.
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(2) Part I includes the M720 HE round; Part II includes the M49A4 HE round;
Part III includes the M302A1 WP round; and Part IV includes the M83A3 illumination
round. The appendixes contain the trajectory charts for the M720 HE round.
(3) FT-6-Q-1 contains information for M49A4 HE, M50A3 training practice,
M302A1 WP, and M83A3 illumination rounds for the M31 subcaliber assembly.
Figure 4-10. Sample pages from firing tables for 60-mm Mortar.
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NOTE: To round off range, look for the range at the lowest charge,
then round it off to the closer range.
Figure 4-11. Sample pages from firing tables for 81-mm mortar.
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Figure 4-12. Sample pages from firing table for 4.2-inch mortars.
(1) For the 4.2-inch mortar, FT 4.2-H-2 applies to the M329A1 HE, M328A1 WP,
XM630 chemical, and M335A1 and M335A2 illumination rounds. FT 4.2-K-2 applies to
the M329A2 HE rounds.
(2) Parts I, II, III, and IV of FT 4.2-H-2 give details on the different elevations that
can be used with the 4.2-inch mortar, with and without extension, for various rounds and
charges. These parts also provide Tables A, B, C, D, and E, which provide the same
information as in all firing tables. Part I includes the M329A1 HE round and the
M328A1 WP round; Part II includes the XM630 round; Part III includes the M335A1
round; and Part IV includes the M335A2 illumination round. The appendixes contain the
trajectory charts.
(3) Parts 1-1, 1-2, and 1-3 of FT 4.2-K-2 provide details of the different elevations
that can be used with the 4.2-inch mortar for the M329A2 round. These parts also
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provide Tables A, B, C, D, and E that reflect the same information as in all firing tables.
The appendixes contain the trajectory charts.
d. Short-Range Training Round Firing Tables (Figure 4-13) (can be used with the
81-mm and 120-mm mortars with M303 insert). FT 81-AR-1, C7 (PROV) contains
different elevations that can be used with the M880 SRTR (Figure 4-12A). These parts
also provide Tables A, B, C, and E, which provide the same information as in all firing
tables.
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(b) Parts I and II provide general data, ground data, and correction factors for
each round. Part I includes the M57 HE and M68 WP rounds. Part II includes the M91
illumination round.
Figure 4-14. Sample pages from firing tables for the 120-mm mortar.
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b. Source of MET Message. The MET message is received from the corps FA
target acquisition battalion and is usually transmitted by FM radio to battalion. Battalion
headquarters then sends the message down to the FDC. Prior coordination with the target
acquisition battalion will ensure that the FDC receives the MET in ballistic format instead
of computerized format.
c. Receipt of MET Message. The MET message is broadcast in six-character
groups. These groups are shown in Figure 4-15 for ease of explanation. Examples of
completed DA Form 3675 and DA Form 3677 are given in Figures 4-16A and 4-16B,
using the same six-character groups to show how they are entered into the form. The
message has two parts: the introduction and the body.
INTRODUCTION
002618 009976
012618 009978
022720 008978
032924 004981
042927 002982
053129 004987
063228 004010
073227 004008
083228 002007
BODY
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4-19
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(1) Introduction. The first four groups of six characters in the MET message are
the introduction, identifying the type of message and the MET station transmitting the
message. This is what the character groups mean:
(a) GROUP 1: MET B 31. (METCM) for computer MET.
MET - indicates that the transmission is a MET message.
B - type of fire; indicates that the message is a ballistic MET message.
3 - indicates that the message is for surface-to-surface fire. For use with
mortars, the number 3 must appear.
1 - indicates the octant of the globe in which the MET message applies.
When code 9 is sent for the octant, the area is in code and not in
numbers—for example, MIF MIF.
0 - indicates the duration of the MET message. For US armed forces, the
MET data are presumed valid until a later message is received.
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LINE 00 SURFACE
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Figure 4-18. Example of completed first seven lines for DA Form 3675.
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proper spaces on the form. These are data available at the mortar platoon or section
(obtained from the data sheet or section sergeant) and are interpreted as follows:
(1) CHARGE—the command charge used to hit the RP. This charge is used to
determine the line number to be used for computing the message.
(2) CHART RANGE—the command range from the mortar platoon or section to
the RP.
NOTE: The reason for using the command charge and range is that this puts the round
at its highest ordinate for that range, which is where the round is affected
most.
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NOTE: If the altitude of the section is above the MDP, the sign is plus (+); if below,
the sign is minus (-).
(1) Once the distance above or below the MDP is known, the computer can enter
Table B (Figure 4-20), which shows the correction that must be applied on the MET data
correction sheet (Figure 4-19) to the ballistic AIR TEMP AIR DENSITY. This
compensates for the difference in altitude between the platoon or section and the MDP,
and determines the corrections for AIR TEMP (difference in T) and AIR DENSITY
(difference in D). Those corrections modify the values of AIR TEMP and AIR
DENSITY determined at the MDP to what they would be at the mortar platoon or section.
Corrections for difference in T and difference in D are arranged in four double rows in the
table.
(2) The numbers 0, +100-, +200-, and +300- in the left column of the table
represent difference in H expressed in hundreds of meters. The numbers 0 and +10-
through +90- across the top represent difference in H in tens of meters. The corrections
can be found where the proper hundreds row crosses the proper tens column. The
numerical sign of the corrections is opposite of the difference in H sign.
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EXAMPLE
Assume that the difference in H is -30, the corrected value for the difference in H is +0.1,
and the difference in D is +0.3 (enter a 0 in hundreds column, go across to +30-column).
Those corrections entered on DA Form 2601-1 and the corrected values can then be
determined and recorded in the proper spaces (Figure 4-19).
Figure 4-20. Sample page from firing table for air temperature
and density corrections.
EXAMPLE
DOF 4300, DIRECTION OF WIND (MET) 2900: 2900 + 6400 = 9300 - 4300 = 5000
mils (chart direction of wind).
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Figure 4-21. Sample page from firing table for wind components.
(1) Crosswind (deflection correction). Multiply the component of the wind speed
(Table A) by the wind velocity (MET). This yields the lateral wind. Once the lateral
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FM 23-91
Figure 4-22. Sample pages from firing table for basic data
and correction factors.
(2) Range wind. Multiply the component by the wind speed. Carry the sign of
the component (H or T from Table D), determine to the nearest 0.1 mil, and record it in
the proper space on DA Form 2601-1.
h. Range Corrections. All values should be recorded in the proper spaces except
DV, which is found as follows: The computer enters Table C (Figure 4-23), which shows
the corrections to muzzle velocity for various temperatures of the propellent charges. He
finds the temperature closest to that recorded for the propellent; DV appears in the center
column on the same line as the temperature. The computer records that value in the
proper space. Then he determines the amount by which all the known values vary from
the standard values upon which the firing tables are based.
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NOTE: Within the firing tables: D = decrease from standard, and I = increase from
standard.
(1) Once those variations are determined, enter the firing table at Table D
(Figure 4-22) (command charge and elevation, 4.2-inch mortars; command charge and
range, 60-mm/81-mm/ 120-mm mortar), go to columns 8 to 15 (60-mm, 81-mm, and
120-mm) or 10 to 17 (4.2-inch mortar) and record the unit corrections for each variation.
NOTE: The sign of the unit correction must be recorded; numbers without a sign are a
plus (+). If the column ends, the last listed numbers are considered to
continue.
(2) Once the variations have been recorded, multiply the variations from standard
by the unit corrections and place the result (rounded to the nearest whole meter) in the
column with the same sign as the unit correction. Once all corrections have been
multiplied, compare the minus (-) and plus (+), subtract the smaller form the larger, and
use the sign of the larger. Determine the result to the nearest meter for 60-mm/
81-mm/120-mm mortars, or to the nearest 10 meters for 4.2-inch mortars, and record in
the proper space.
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Figure 4-23. Sample page from firing table for propellant temperature.
4-31
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The target area is usually larger than the transfer limits of the RP, and yet time,
ammunition, and the tactical situation will permit firing only one registration.
a. By assuming negligible error in survey or maps, lay of the weapons, and
preparation of the plotting boards or MBC computer, the FDC can divide the registration
corrections for the RP into two parts. The first part is a correction that is only a function
of the range fired, and it is constant for a given range, regardless of direction. The second
part is a function of the direction fired.
b. If the amount of the concurrent MET computed for the RP is subtracted from the
total registration correction, the result is an absolute registration correction that does not
change with the direction fired or the weather. The FDC can then plot an imaginary RP at
the same range as the original RP, but in other directions (usually 800 mils apart),
compute a MET correction for each of those directions, and, by adding the different MET
corrections to the absolute registration correction, determine different firing corrections
for each of the imaginary RPs. The firing corrections determined for the imaginary RPs
can then be applied when engaging targets within their transfer limits.
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4-33
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a. Complete the top section of the sheet and compute the difference in H corrections
and the corrected values for AIR TEMP and AIR DENSITY in the usual way.
b. Determine the CHART DIRECTION OF WIND as on a normal MET. Copy the
result into the box marked I (RP) and as many others as there are imaginary RPs (II is
800 mils clockwise from the RP, and the numbers increase in a clockwise direction to
VIII, which is 800 mils counterclockwise from the RP).
c. Add the directional variations to the CHART DIRECTION OF WIND subtracting
6400 if necessary to keep the result less than 6400.
d. Copy the wind velocity into the first row of boxes under DEFLECTION
CORRECTIONS and RANGE CORRECTIONS. Do not use any column that does not
have the CHART DIRECTION OF WIND written on top.
e. From Table A (Figure 4-21), extract the appropriate crosswind component (record
it in the DEFLECTION CORRECTIONS section) and range wind component (record it
in the RANGE CORRECTIONS section) for each value of chart wind to checkpoints.
f. Multiply the velocity by the components to get values for crosswind and range
wind.
g. Find the crosswind correction factor in Table D, (column 7, 60-mm/81-mm/
120-mm mortars; column 9, 4.2-inch mortar) corresponding to the adjusted RP charge.
Multiply it by the crosswind to get the MET DEFLECTION CORRECTION.
h. Find the proper range wind unit correction in Table D, (columns 10 and 11,
60-mm/81-mm mortars; columns 12 and 13, 4.2-inch mortar). Multiply it by the range
wind to get the RANGE WIND CORRECTION.
i. Compute the MET RANGE CORRECTIONS for POWDER TEMP, AIR TEMP,
AIR DENSITY, and PROJECTILE WT in the usual manner. The net of the four is the
ballistic range correction.
j. Combine the ballistic range correction with the various range wind corrections to
obtain the total range corrections.
k. Obtain the total MET corrections by bringing together the MET RANGE
CORRECTION and the MET DEFLECTION CORRECTION for each of the points.
l. Determine the absolute registration correction. First, calculate the registration
correction. The registration range correction is the difference between the chart range to
the RP and the range corresponding to the initial range at the RP; it is plus if the chart
range is smaller. The DEFLECTION CORRECTION is the LARS (left, add; right,
subtract) correction, which must be applied to the initial deflection read at the RP to get
the firing deflection that hit it. The RP MET correction, which has been recorded under I
(RP), is then subtracted from the registration correction; the result is the absolute
registration correction.
m. Add the absolute registration correction to each point MET correction to obtain
the corrections to apply at the points.
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determine corrections due to changes in conditions that affect the flight of rounds during
the periods between registrations. Those conditions include changes in powder
temperature, air temperature, air density, and the speed and direction of the wind. The
FDC assumes that all other factors remain relatively constant until the section displaces.
a. Corrections computed from the MET message are not adequate firing corrections
alone. To be of value to the FDC, a valid MET message must be received along with (or
within four hours) the registration. The registration corrects for all nonstandard
conditions. A MET message received and computed along with the registration tells the
FDC how much of the total registration correction is due to weather. By comparing the
corrections from a later MET message, the FDC can modify the registration corrections to
account for changes in weather. Therefore, the use of MET corrections eliminates the
need for reregistration.
b. For MET corrections to be of use, the FDC must receive two MET messages. The
corrections from the two are compared to determine the current corrections to update the
firing corrections determined from the registration. Once the two messages are
computed, the correcting areas (deflection correction and range correction) are compared,
and the product is used to update the registration corrections.
EXAMPLE
(Figure 4-26)
Assume that—
MET 1: Deflection correction L20
Range correction -100
MET 2: Deflection correction R10
Range correction +25
Place the correction from the MET messages on a
MET cross.
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+50
RANGE CORRECTION
+40 FIRST SUBSEQUENT MET (+25)
1 2
-20
MOVED UP 125
CORRECTION TO APPLY +125
-30
CORRECTIONS TO APPLY
DEFLECTION CORRECTION R30
RANGE CORRECTION +125 -40
-50
RANGE CORRECTION
INITIAL MET (-100)
-100 1
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(2) Range correction. To get from a -100 to a +25, first go from -100 to 0, then
up the scale to +25; in doing so, you went +100 then +25 for a total correction of +125.
EXAMPLE
(Figure 4-27)
MET messages on the same side of the MET cross. Assume—
MET 1: Deflection correction L30
Range correction +50
MET 2: Deflection correction L40
Range correction +75
Deflection correction L30 + L40 = L10
Range correction +50 + +75 = +25
Use the same procedure — "Where am I?" "Where am I going?" "What is required to get
there?" each time to determine the corrections. Remember, MET 1 is compared to MET
2, MET 2, to MET 3. This procedure continues as long as MET messages are received
and as long as the unit remains in the same position.
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2
+70 RANGE CORRECTION
FIRST SUBSEQUENT MET (+75)
RANGE CORRECTION
+50 1 INITIAL MET (+50)
+40
+30
DEFLECTION CORRECTION
FIRST SUBSEQUENT MET +20
DEFLECTION CORRECTION
CORRECTION (L40)
INITIAL MET (L30)
+10
2 1
CORRECTION TO APPLY;
DEFLECTION CORRECTION L10
-30 RANGE CORRECTION +25
-40
-50
d. Once the MET corrections have been determined, the FDC can then determine the
corrections to use for updating. MET is based on the RP, and therefore the corrections
from the MET messages are applied to corrections determined from the registration.
(1) Range correction. Compare the range correction from the RP and the MET
range correction. For difference signs, subtract the smaller from the larger and use the
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sign of the larger for the new range correction for the RP. If signs are the same, add the
values.
EXAMPLE
Range correction from the registration +150
Range correction from the MET +50
+150 + 50 = +200 range correction
(2) Range correction factor. Once the range correction has been determined, to
determine the RCF, divide the initial chart range (rounded to the nearest hundred and
expressed in thousandths) into the range correction.
EXAMPLE
New range correction: +200
Initial charge range: 3,050
(100's = 3100; 1000's = 3.1)
+64.5 = +65 RCF
+3.1/ +200.0
Deflection correction from registration L12
Deflection correction from METs R10
L2 = DEF CORR
(a) Once the new corrections have been determined, the FDC can update the
data sheet (RP and previously fired targets). Because the chart is based on the RP, the
first target to update is the RP.
(b) Chart data remain the same because the known points have not moved.
The MET message only told the FDC what is needed because of the weather changes.
Apply the new corrections to the chart to obtain the new command data (Figure 4-28).
(c) For previously fired targets, chart data remain the same. Apply the new
corrections to obtain the new command data. To obtain the range correction, multiply the
new RCF by the range (rounded to the nearest hundred and expressed in thousandths)
(Figure 4-28). (For a blank reproducible copy of DA Form 2188-R, see the back of this
manual.)
(d) For new targets within the transfer limits of the RP, apply the new
corrections the same as the previous registration corrections.
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METEOROLOGICAL: NEW
QUADRANT: 0
LATITUDE: 322
LONGITUDE: 845
DATE: DAY: 02
TIME: 100
DURATION: 0
STATION HEIGHT: 014
ATMOSPHERIC PRESSURE: 003
4-31.1
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b. Using the procedures above, repeat steps (5) and (6) to line 8.
c. Press SEQ switch. After line 8; UPDATE MET * is displayed. Using the
multiple choice entry, select the flashing asterisk (*) to update the NEW MET
stored in the MBC, placing the NEW MET in the CURRENT MET file, while
retaining a copy in the NEW file. Sequence to ready.
NOTE: The MBC, M23 calculates the effect of each line of the MET on the
round when determining firing data. Changes to the MET files can
only be made to the new MET file, then updated to the current file.
(1) To check MET, enter MET switch and select current (CURR) and
review the MET message.
(2) If a change is needed, enter NEW and make the needed corrections,
then select UPDATE MET *.
4-31.2
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CHAPTER 5
CALL FOR FIRE
A call for fire is a concise message prepared by the observer. It contains
all information the FDC needs to determine the method of target attack.
5-1. INTRODUCTION
The call for fire is a request for fire. It must be sent quickly and be clear enough to be
understood, recorded, and read back without error by the FDC. The observer should tell the
RATELO that he has seen a target. This enables the RATELO to start the call for fire while
the target location is determined. The RATELO sends the information, as it is determined,
instead of waiting until a complete call for fire has been prepared.
a. Regardless of the target location method used, the normal call for fire is transmitted
in a maximum of three parts, consisting of six elements, with a break and a read back after
each part. The three parts are as follows:
• Observer identification and warning order.
• Target location.
• Description of target, method of engagement, and method of fire and control.
b. The six elements of the call for fire are listed below in the sequence in which they are
transmitted.
• Observer identification.
• Warning order.
• Target location.
• Target description.
• Method of engagement.
• Method of fire and control.
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without adjustment is warranted when the target has been fired upon previously or when it
is within transfer limits of a registration point (+/- 1,500 meters; right or left 400 mils) and
its location is either surveyed or accurately specified by the observer.
(3) Immediate suppression or immediate smoke (IS). When engaging a planned target
or target of opportunity that has taken friendly maneuver or aerial elements under fire, the
observer announces, "Immediate suppression (target location)." If a hasty screen for
obscuration is the desired effect, then the FO announces, "Immediate smoke."
b. Target Locations. This element enables the FDC to plot (M16/M19) or enter (MBC)
the location of the target to determine firing data.
(1) Grid. If the target is located by the grid method, the FO announces, "Grid." In
a grid mission, six-digit grids are normally sent. Eight-digit grids should be sent for
registration points or other points for which greater accuracy is required. Since the FDC does
not need the OT direction to locate the target, the direction is sent at the end of the call for
fire or just before the initial correction. Direction is expressed to the nearest 10 mils.
(2) Shift from a known point. If the target is located by this method, the FO
announces, "Shift (known point)." In a shift from a known point mission, the point from
which the shift will be made is sent in the warning order. The point must be known to both
the observer and FDC. The observer then sends the OT direction. Normally, direction to the
target will be sent to the nearest 10 mils; however, the FDC can use mils, degrees, or cardinal
directions, whichever is specified by the observer. The lateral shift (how far left or right the
target is from the known point, expressed to the nearest 10 meters), the range shift (how
much farther [add] or closer [drop] the target is in relation to the known point, to the nearest
100 meters), and the vertical shift (how much the target is above [up] or below [down] the
altitude of the known point, to the nearest 5 meters) are sent next. The vertical shift is
ignored unless it exceeds 30 meters.
(3) Polar plot. If the target is located by use of the polar plot method, the observer
announces, "Polar." In a polar plot mission, the word polar in the warning order alerts the
FDC that the target will be located with respect to the observer's position. The observer's
location must be known to the FDC. The observer sends the direction (to the nearest
10 mils) and distance (to the nearest 100 meters). A vertical shift (to the nearest 5 meters)
tells the FDC how far the target is located above (up) or below (down) the observer's
location. Vertical shift may also be described by a vertical angle (VA) in mils relative to the
observer's location. (This method is used when the FO is conducting a laser polar.)
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e. The target size and shape if significant. When the target is rectangular, the length and
width (in meters), and the attitude (azimuth of the long axis) to the nearest 50 mils should
be given—for example, 400 meters by 100 meters; attitude 2,650. When the target is
circular, the radius should be given. Linear targets may be described by length, width, and
attitude.
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FM 23-91
delivered without adjustment. In the absence of observer methods of control, the firing
section fires when ready (W/R) or under the FDC control when controlling the fire. The
observer announces the method of control by use of the terms below:
(1) At my command (AMC). This announcement indicates that the observer desires
to control the time of delivery of fire. The observer announces, "At my command,"
immediately preceding "Adjust fire or fire for effect." When the weapons are ready to fire,
the FDC personnel announces, "Section is ready," to the observer. The observer then
announces, "Fire," when he wants the mortar section to fire. At my command remains in
effect until the observer announces, "Cancel at my command" or "End of mission."
(2) Cannot observe. This announcement indicates that the observer cannot adjust
fire. However, the observer believes that a target exists at the given location, and the target
is important enough to justify firing on it without adjustment.
(3) Time on target (TOT). The observer may tell the FDC when he wants the rounds
to impact by requesting, "Time on target (amount of minutes desired) minutes from now,"
or "Time on target zero six four five (0645) hours." The observer must conduct a time check
to ensure that his timepiece is synchronized with the FDC’s.
(4) Continuous illumination. If no interval is given by the observer, the section
sergeant determines the interval by the burn time of the illuminating ammunition in use. If
another interval is required, it is indicated in seconds.
(5) Coordinated illumination. The observer may order the interval between
illuminating and HE rounds in seconds. This order achieves a time of impact of the HE
round that coincides with optimum illumination, or he may use normal at-my-command
procedures. The preferred method is to have the FDC compute the intervals between the HE
and illuminating rounds.
(6) Cease fire. This command is used during firing of two or more rounds to stop the
loading of rounds into the mortars. The gun sections may fire any rounds that have already
been loaded (hung).
(7) Check fire. This command is used to cause an immediate halt in firing.
(8) Continuous fire. In mortars, this command means loading and firing as rapidly
as possible, consistent with accuracy, within the prescribed rate of fire for the mortar being
used. Firing continues until suspended by the commands CEASE LOADING or CHECK
FIRE.
(9) Repeat. This command can mean one of two things.
(a) During adjustment, REPEAT means to fire another round(s) at the last data
and adjust for any change in ammunition.
(b) During FFE, REPEAT means to fire the same number of rounds using the
same method of FFE. Changes to the number of guns, gun data, interval, or ammunition may
be requested.
(10) Followed by. This is part of a term used to indicate a change in the rate of fire,
the type of ammunition, or another order for FFE.
5-4
FM 23-91
(1) Unit(s) to fire—the number of mortars available that will fire the mission.
EXAMPLE
In a six-gun 120-mm mortar platoon, two guns are already involved in a fire mission.
The other four are available, but the FDC only wants to use three mortars on the new
target. The FDC would announce to the observer, "Three guns."
(2) Changes to the call for fire—any change to the observer's request in the call for
fire.
EXAMPLE
The observer requested IMP in effect, and the FDC decides to fire PROX in effect.
5-5
FM 23-91
• Special sheaf.
• Traversing fire.
• Range spread, lateral spread, or range lateral spread (illumination only).
f. Method of Fire and Control.
(1) Method of fire.
(2) Method of control.
• At my command.
• Time on target.
• Continuous illumination.
• Coordinated illumination.
• When ready.
5-9. AUTHENTICATION
Authentication is considered a normal element of the initial requests for indirect fire.
a. The FDC inserts the challenge in the last read back of the call for fire. The FO
transmits the correct authentication reply to the FDC immediately following the challenge.
Authentication replies exceeding 20 seconds are automatically suspect and a basis for
rechallenge. Subsequent adjustments of fire or immediate engagement of additional targets
by the observer who originated the fire request normally would not require continued
challenge by the FDC.
b. The two methods of authentication authorized for use are as follows:
• Challenge and reply.
• Transmission.
The operational distinction between the two is that challenge and reply require two-way
communications, whereas transmission authentication does not. Challenge and reply
authentication is used when possible. Transmission authentication is used only if
authentication is required and it is not possible or desirable for the receiving station to
reply—for example, message instruction, imposed radio silence, final protective fire, and
immediate suppression.
c. The observer is given a transmission authentication table IAW unit SOP. The table
consists of 40 columns with authenticators in each column. After each authenticator is used,
a line may be drawn through it to avoid using the same one.
5-6
FM 23-91
PART THREE
MORTAR BALLISTIC COMPUTER
CHAPTER 6
INTRODUCTION
This chapter describes the characteristics, capabilities, and memory
storage of the mortar ballistic computer.
6-1. DESCRIPTION
The M23 MBC is handheld, lightweight, and battery powered. It is used for automated
computations, digital communications, and displaying mortar-related information
(Figure 6-1). The MBC weighs 7 pounds (including battery) or 8 pounds (including battery
and case assembly). It is highly portable, can be used in all-weather operations, and has built-
in self-test circuits. The MBC requires fire mission data input to compute fire commands
needed to effectively execute a mortar fire mission. When the MBC is connected to an
external communication device (digital message device), the FO fire mission inputs are
automatically entered and may be reviewed and edited by the MBC operator. When the
MBC is not connected to an external communication device, all fire mission data are entered
manually by the MBC operator. The fire commands are then relayed to the gun line IAW
unit SOP.
a. Initialization Switches (Figure 6-2). These switches include the following:
(1) SET UP1. Starts the menu for entry of setup data: timeout, target prefix and
block number range, audio alarm, minimum easting and northing coordinates, location grid
declination, latitude, listen only mode, message transmission rate, transmitter warm-up delay
time, single or double message block mode, and owner identification.
(2) WPN DATA2. Starts menus for entry of weapon data or review of weapon data
for each unit: selection of up to three firing sections; grid location of the basepieces
(normally the registering gun) for each of these sections; up to six individual gun locations
for each section; and weapon type, carrier- or ground-mounted, altitude, azimuth of fire, and
referred deflection being used.
NOTE: At this time, the MBC revision III/A does not allow entries with the same
identifier. (For example, when using B02, the number 02 cannot be used again.)
(3) FO LOC3. Starts menus for entry of data: FO number (12 maximum), grid
location, and altitude.
(4) REG DATA4. Starts menus for manually entering a registration data file for
registration points (RP) or review of RP data: RP number, location, altitude, weapon unit
and number; elevation for 107-mm or charge for the 60-/81-/120-mm; and type of MET data
used when the RP was fired to include the range and deflection correction factors.
(5) BRT5. Selects the level of brightness for the display area. Controls the
background lighting for the keyboard. The MBC can be operated in total darkness if the
brightness is set at LOW. When set at LOW, the background (keyboard) is lit.
6-1
FM 23-91
(6) ON/OFF6. Turns the MBC on or off. When turned on, the display temporarily
shows POWERUP TEST, then shows READY.
(7) FIRE ZONES7. Starts menus for entry of or review of fire zone/fire line
boundaries: location points for fire lines, zone numbers, number of points for fire zone (no-
fire area), and location points for fire zone boundaries.
(8) MET8. Starts menus for entry of nonstandard MET: MET station data and
location; and entry of nine lines of MET data including wind direction, speed, temperature,
and pressure for each line of MET data.
(9) KNPT/TGT9. Starts menus for data entry of known points or target reference
points: known point or target number, grid location, and altitude.
(10) AMMO DATA10. Starts menus for entry of ammunition data for each caliber
weapon in use: ammunition types, powder temperature change, and correction factors for
projectile weight.
(11) TEST11. Manually starts self-test of microprocessor (ROM, RAM, and
instruction set) for all switches and keys, display (character generation), modem
(communication device), software revision number, and communications (transmit test
message).
6-2
FM 23-91
1 2 3 4 5 6
11 10 9 8 7
6-3
FM 23-91
1 2
10 3
8 4
7 6 5
c. Keys (Figure 6-4). Eleven keys are used to enter alphabetical (alpha) or numerical
(numeric) characters and minus sign. Alpha or numeric selection for combination keys is
either automatic or menu-selectable.
d. Fire Missions (Figure 6-5). The operator starts a fire mission menu by pressing the
grid, shift, or polar keys.
(1) GRID1. For manual entry of grid fire mission data when target location is
identified by grid coordinates. Entries are: FO ID number, FO direction to target, target
location, and altitude when known.
6-4
FM 23-91
(2) ADJ2. For manual entry of fire mission adjustment data (corrections) from the
FO. By menu selection, use registration point data or MET data. Correction entries are: left
or right deviations, plus or minus range, and up or down height.
(3) REG3. For review of registration data, and computation and storage of
registration point correction factors. Displayed output from computation includes range
correction factor and deflection correction amount. (To review registration data, use REG
DATA key.)
(3) TFC4. For manual entry of technical firing data. Use to enter or change
information for sheaf, method of control, and weapons to fire. Use registration point data
or type of MET data.
(5) FPF5. For manual entry of FPF line data, safety fan, and minimum/maximum
charge. Entries are: FPF location, target altitude, target width, and attitude.
(6) WPN/AMMO6. For manual entry or to change the weapon or ammunition data
for a fire mission. Entries are: weapon unit and number (A section, No. 3 gun), shell and
fuze combination, elevation (107-mm mortar) or charge (60-mm/81-mm mortar).
NOTE: When the 120-mm mortar data becomes available, the computer must be updated.
(7) BURST7. For manual entry of burst location data (corrections) supplied by a
laser-equipped FO. Entries, from laser to burst are: direction, distance, and vertical angle.
(8) POLAR8. For manual entry of either a normal or laser-designated polar fire
mission using polar plot data. A normal polar mission target is identified by direction,
distance, and up/down height from an FO. A laser polar mission target is identified by laser
direction, laser distance, and laser vertical angle.
(9) SHIFT9. For manual entry of shift fire mission data when a target location is
identified by a shift from an existing known-point target. Entries are: FO ID, known/target
number FO direction to target, direction, and amount of shift.
1 2 3
9
5
7
6
6-5
FM 23-91
(1) FIRE DATA1. For reviewing existing fire commands of active fire missions.
Data are the same as the COMPUTE switch output.
(2) SFTY DATA2 Data menus for active fire missions to review safety factors.
Enter boundaries for a safe firing area or a minimum and maximum charge for the safety
area.
(3) REPLOT3. To review target replot data and to increase target location
accuracy. Enter new target altitude then press REPLOT switch to compute a new grid
location.
CHARACTER BLOCKS
DISPLAY SWITCHES
6-6
FM 23-91
(1) Standby Indicator1. Indicates (when flashing) that the display timeout period
has expired. Flashes once every 6 seconds while the display is "time out." To bring the last
display back on, press any key once.
NOTE: It is recommended not to use the FIRE MISSION keys to bring the display back
ON. Some of these keys are highly sensitive and a fire mission can be initiated
accidentally. The safest key to use is the sequence key.
(2) Sequence Indicator2. Indicates (when flashing) that more data are available
for the current menu or display.
(3) BATT LOW Indicator3. Indicates (when flashing) that the internal 12-volt
battery is low. This indicator starts flashing when the battery output reaches 11 volts. The
MBC shuts off at 10 volts. If the BATT LOW indicator starts flashing in the middle of a fire
mission, continue with the mission, and change the battery as soon as possible.
(4) Message Indicator4. Indicates (when flashing) that the MBC has received one
or more digital messages. The flash rate increases with the number of messages received.
4 2
6-7
FM 23-91
6-3. CAPABILITIES
The MBC performs the following functions:
a. Communicates with the digital message device (DMD). Incoming messages are of
two types: fire request messages and information only messages. When the message
indicator is lit or the audio alarm sounds and the MSG switch is pressed, the first line of the
first message received is displayed. When the message is a fire mission, the MBC
automatically assigns a mission and target number, unless three active fire missions are in
progress. Therefore, the MBC displays NO AVAIL MSN and discards the message.
b. Handles a full range of mortar ammunition. The ammunition file in the MBC
contains the following ammunition for each mortar system. The first round listed by type is
the MBC "default" ammunition.
(1) M224, 60-mm mortar.
High explosive: *M720; M49A4; (X)M888
White phosphorus: *M302A1; #M302A2; M722
Illumination: *M83A3; M721
(2) M252 and M29, 81-mm mortars.
High explosive: *M374; M374A2; M374A3; M821; M889; #M889A1; #M821A1
White phosphorus: M375; *M375A2; M375A3
Illumination: *M301A3; M853A1
Training/Practice: *M1; M68; M879; #M880 (TP ammunition must be ground-
mounted mode only)
Red phosphorus: M819
(3) M30, 107-mm mortar.
High explosive: *M329A1; M329A2
White phosphorus: *M328A1
Illumination: M335A2
Tactical CS: (X)M630
(4) 120-mm mortar.
High explosive: M933; *M934; **M57
White phosphorus: **M68; *M929
Illumination: **M91; *M930
(5) M303 insert, 120-mm mortar.
High explosive: *M374; M37A42; M374A3
White phosphorus: M375; *M375A2; M375A3
Illumination: M301A3
Training/Practice: M880
* _____________________
*Default ammunition.
**NDI ammunition.
#Revision III/A.
6-8
FM 23-91
NOTE: At this time, the MBC revision III does not allow entries with the same identifier
(for example, when using B02, the 02 cannot be used again).
@ = Alpha character
# = Numeric character
$ = Alpha or numeric character
6-9
FM 23-91
^ AZ TOO BIG Difference between safety fan LLAZ and RLAZ entries is 3200
mils or more.
^ AZ TOO SMALL Difference between safety fan LLAZ and RLAZ entry is less
than 400 mils.
6-10
FM 23-91
^ RANGE TOO SMALL Difference between safety fan MIN RN and MAX RN entries
is less than 200 meters.
BAD AIR DENSITY Temperature and pressure entries will not yield ballistics
solution.
BAD CHARGE ZONE SFTY DATA, MIN CHG entry is greater than MAX CHG
entry.
BAD KNPT:## SHFT Upon receipt of FR SHIFT message, known point message is
not stored in KNPT buffer.
6-11
FM 23-91
BAD ^ WIND ##-## Direction and velocity entries in consecutive MET datum
planes yield easting and northing wind components that differ
by more than 29 knots. The ##-## indicates MET datum
planes in error.
BAT @ NOT FOUND Initialization data not yet entered for this battery.
CHG TOO BIG Minimum range for user-selected charge is greater than range
to target.
6-12
FM 23-91
CHG TOO LOW User-selected charge maximum range is less than the range to
target.
DEFL TOO BIG Required deflection exceeds maximum left or right traverse
limitations for carrier-mounted 107-mm mortars.
DISP $$$ MEM $$$ Follows REV NO. FAILURE error message. Indicates
revision numbers for display/processor and memory
respectively.
DUPLICATE WPNS Same weapon number entered two or more times into TFC,
GUNS selection for multiple weapon missions.
ENTRY NOT FND Required FO, KNPT, or TGT initialization data not yet entered
into the appropriate memory file.
6-13
FM 23-91
FORMAT ERROR All valid data not entered into blank menu fields.
GUN IS ADJUSTED Adjustments have already been completed for this weapon.
ID ASSIGNED This KNPT number or TGT number entry has already been
used.
6-14
FM 23-91
ILL ENTRY Illegal value entered into blank field of data entry menu.
ILLEGAL TGT NUM Target number is within target number block range assigned
in SET UP.
MAX NOT GREATER MAX fire line is closer than MIN fire line.
6-15
FM 23-91
NO ADJUST DATA All required ADJust data have not been entered.
6-16
FM 23-91
NO SHEAF DATA SPECIAL sheaf selected but without width or direction entry.
NO TGT NUM Target numbers not yet assigned for target block definition in
SET UP data.
6-17
FM 23-91
POWER FAILURE MBC Powered down by means other than ON/OFF switch,
such as by removing battery or external power.
RANGE TOO SMALL Range to target is zero, or when entering FIRE ZONES data,
distance between points is less than 10 meters.
REG TOO BIG Range corrections exceed 999 meters when computing a
REGistration.
REV NO. FAILURE Memory CCA and display/processor CCA have incompatible
revision numbers.
6-18
FM 23-91
SINGLE WPN ONLY More than one weapon is designated on TFC sequence GUNS:
@# ________ menu but selected TFC CONtrol allows only
one weapon.
SPC SHEAF ERROR Weapon registration is illegal while in TFC CONtrol (SPECial
SHEAF).
TEMP OUT OF RNGE Powder temperature entry outside range (-70 to 140).
TEMP TOO LOWMBC cannot compute gun orders for 107-mm mortars with extension
when powder temperature is below -30 degrees.
TEMP TOO LOWAir temperature in MET data is below 1536 (153.6 degrees Kelvin or
-183.2 degrees Fahrenheit)-
6-19
FM 23-91
TGT TOO HIGH Target altitude is greater than 90 percent of MAX ORD of
computed flight trajectory. Reliable results cannot be
obtained.
6-20
FM 23-91
CHAPTER 7
PREPARATION OF FIRE CONTROL EQUIPMENT
This chapter discusses the different types of data entry for the mortar ballistic
computer and how they are entered into the computer. The different levels of
initialization are also explained. Figure 7-1 is an overview of the groupings of
switches and indicators used in setting up the MBC for the tactical scene.
INITIALIZATION
SWITCHES STANDBY
INITIALIZE MSC INDICATOR
AND TACTICAL
SCENE
DISPLAY
DISPLAY SWITCHES
INTERACT WITH
MESSAGE DISPLAY
INDICATOR
SEQUENCE
INDICATOR
KEYS AND ACTION
SWITCHES
DATA ENTRY, BATTERY LOW
COMPUTATION, INDICATOR
REVIEW, AND END
FIRE MISSIONS
OUTPUT SWITCHES
FIRE MISSION REVIEW ACTIVE
SWITCHES FIRE COMMANDS,
START AND CONTROL SAFETY
CONTROL FIRE FACTORS, AND
MISSIONS REFINE TARGET
LOCATION
ACCURACY
7-1
FM 23-91
a. The operator presses the ON/OFF switch to activate the MBC. The display shows
POWERUP TEST, then shows: READY.
NOTE: The self-test should be conducted when the MBC is first turned on. However, the
operator must first know how to make menu selections to conduct the self-test.
(See example on page 7-7, paragraph 7-2.)
(1) Default entry. Press the SET UP switch. The MBC displays the menu for setup
data: timeout, target prefix, target number block, grid declination, message transmission rate,
transmitter warm-up delay time, transmission single or double block mode, and owner
identification.
(a) The display window of the MBC shows TIME OUT: 15. Timeout means
that the computer will automatically shut off the display if another switch is not selected
before the given time runs out. The computer is not off, just conserving energy. If the
computer should shut off during these examples, press any key (except the fire mission keys,
which are grid, shift, or polar) to reactivate the display screen.
(b) The flashing cursor on the display screen (on the 15) indicates that a selection
can be made to the timeout of the computer. The timeout of the computer can be set at 15,
30, 45, or 60 seconds. The timeout period of 15 seconds is computer-assigned (a default
entry) to the lowest setting, thereby maintaining the highest energy conservation. During the
time needed to "train up" on the MBC, the timeout period should be changed to 60 seconds.
(2) Correction entry. Select the blue display switch beneath the flashing cursor in
the display window. The display shows: 15 30 45 60. There are flashing cursors on each
number. The four blue display switches interact with the display directly above them—for
example, if the switches were numbered from left to right 1, 2, 3, and 4, and the timeout is
to be changed (corrected) to 60 seconds, select the number 4 display switch. The computer
now shows TIME OUT: 60.
(3) Alphabetical entry. The target number block assigned to the mortar platoon is
AH0001 - AH0099. Use the keyboard to enter the target prefix, which is entered in the
underlined blanks. The target prefix is AH.
(a) Press the sequence key: the display shows: TGT PRFX:_ _.
(b) Press the 1/ABC key: the display shows: A B C. Since a numerical entry is
not required at this time, the MBC automatically deleted the number 1 from the display
screen.
(c) Press the number 1 blue display switch to select A. The display shows: TGT
PRFX:A _.
(d) Press the 3/GHI key. The display shows: G H I. Since a numerical entry is
not required at this time the MBC automatically deleted the number 3 from the display
screen.
(e) Press the number 2 blue display switch to select H. The display shows: TGT
PRFX:AH.
Once the prefix has been entered, the sequence switch activates the memory storage of the
computer. The target prefix selected will be used to identify all the targets that are
programmed through the MBC. The prefix will be used until changed by the operator or the
computer is cleared.
7-2
FM 23-91
(4) Numerical entry. After the sequence switch is selected to store the target prefix,
the display screen asks for the numerical half of the target block number: 0001 - 0099. The
display shows: TN:_ _ _ _ - _ _ _ _. To make the numeric entry—
(a) Press the 0 key three times. The display shows: TN:0 0 0 _ - _ _ _ _.
(b) Press the 1/ABC key. The number 1 is automatically entered onto the display
because the MBC knows that a alphabetical entry is not called for in this situation. The
display shows: TN:0 0 0 1 - _ _ _ _.
(c) Press the 0 key twice and the 9/YZ key twice. Once again the MBC is
programmed to know when a alphabetical or numerical entry is to be made, therefore when
the 9/YZ key is selected and the number 9 is automatically entered on the display. The
display should show: TN:0 0 0 1 - 0 0 9 9.
Once the sequence key is pressed, the target block numerical entries are stored in the memory
of the MBC. If a mistake is made in entering the target block numbers, the operator only has
to make a correction entry.
(5) Correction entry. If the sequence key is pressed before making the correction
entry, simply press the BACK key to bring the last screen information "back" on.
(a) Clearing the rightmost character only:
• The last digit entered for the target block number is a 9, but it is supposed
to be a 5. Press the CLEAR ENTRY switch one time and the display
shows: TN:0 0 0 1 - 0 0 9 _.
• Now select the proper number. Press the 5/MNO key. The display
shows: TN:0 0 0 1 - 0 0 9 5.
(b) Clearing the entire field. During firing your section leader tells you that the
target block numbers have been changed from AH-0095 to AH-8000. The flashing cursors
above the display switches 1 and 3 indicate that both fields may be changed. To clear the
entire field, in this case the 0095, follow these instructions:
• Press the number 3 (blue) display key. The field is cleared and the
display shows: TN:0 0 0 1 - _ _ _ _ .
• Enter the new number by pressing the 8/VWX key once and the 0 key
three times. The display should show: TN:0 0 0 1 - 8 0 0 0.
For the computer to use the target numbers, the sequence switch must be pressed. Once the
sequence switch is pressed, the numbers are stored in the memory.
NOTE: The next display is for the ALARM OFF/ON function, which is discussed in
Chapter 9. For now, sequence past this display. The computer defaults the
selection to ALARM:OFF.
(6) Minimum easting and minimum northing entries. The next two displays,
MIN E:_ _ _ 0 0 0 and MIN N:_ _ _ 0 0 0, are entered with numerical selections. The
minimum easting (MIN E) and the minimum northing (MIN N) are the coordinates at the
lower left corner of a map sheet. Each of these coordinates are entered into the MBC
preceded by a 0—for example, the grid intersection of a map sheet (lower left corner) is
50/89. The MIN E is entered into the computer as 050, and the MIN N is entered as 089. The
three trailing zeros are computer-entered for each display.
7-3
FM 23-91
(7) Direction entry (display-selectable). Select the sequence switch and the display
shows: E W GD:_ _ _. This display is one example of a direction entry with an amount.
East (E) or west (W) must be selected from the display before filling in the underlined
blanks for grid declination.
(a) Locate the grid declination (GD) in the map sheet legend of the area of
operations. Before entering the GD, round it off to the nearest 10 and express it in tenths—
for example, the GD of 132 is 130; expressed in tenths is 13 (Figure 7-2).
(b) Since the grid declination is easterly, make the selection of the blue display
switch beneath the E on the display. The display should show: E W GD:E _ _. The
declination diagram shows the declination in both degrees and mils. Use the mils value
given. The difference between the grid north and magnetic north is 100 mils. The entry
made in the MBC is in tens of mils. Press the 1/ABC key once, and the zero (0) key once.
The display shows: E W GD:E 1 0.
(c) Additional direction indicators found in other menus are:
H = Horizontal S = Slant
L = Left R = Right
U = Up D = Down
+ = Add - = Drop
+ = North - = South
When these symbols appear in later chapters, their meaning will be discussed in depth.
Select the sequence switch once and store the grid declination in the computer.
7-4
FM 23-91
NOTE: Latitude ( LAT -/+) comes from a map sheet of an area of operation. Enter plus
(+) for the northern hemisphere or minus (-) for southern hemisphere. The
latitude is an optional entry.
(8) Multiple choice entry. The keytone is the length of time required for a
communications device (FM radio) to enable the transmitter before sending data. When a
radio is hot from frequent use, it takes a lower keytone to send a message. Similarly, if the
radio is cold from the outside temperature, it takes longer to send a message. The normal or
default value is 1.4 seconds. For this example, change the keytone to 3.5 seconds as follows:
NOTE: The next three screens are not required at this time. They are explained in later
chapters. Press the sequence switch four times, advancing the display to the
keytone menu.
(a) Press the number 3 display switch under the flashing cursor. This rejects the
default value and gives the first four available selections: 0.2 0.7 1.4 2.1. The selection
3.5 is not yet available. The sequence indicator bulb should also be flashing at this time,
indicating that there are more selections to be viewed.
(b) Press the sequence switch again, and the remaining selections appear in the
display: 2.8 3.5 4.2 4.8. Press the blue display switch under the flashing cursor and 3.5.
The display should now show: KEYTONE:3.5.
(c) Return to ready display. Press the sequence switch twice and advance to the
last fill-in-the-blank selection in the SET UP menu. The display shows: OWN ID: __.
The owner identification code must be entered here. This code is found in the SOI. Enter
the OWN ID, A through Z or 0 through 9. For this example enter 1. Press the 1/ABC key
once. Press the blue display key (4) under the 1 once. The display now shows:
OWN ID: 1.
NOTE: Coordination must be made between the FO and FDC to ensure that both know
the owner's identification when using DMD.
7-2. INITIALIZATION
This paragraph discusses the initialization switches and how they are affected by the different
modes of operation.
a. SELF-TEST. The MBC can perform its own internal tests. When the operator turns
on the MBC or suspects a malfunction, he should initiate the self-test.
(1) Press the ON/OFF switch; the MBC shows: POWERUP TEST while
performing internal circuit checks, and then it shows: READY. If any other display appears,
turn the MBC in to the GS maintenance team. If the BATT (battery) LOW indicator flashes
or the display does not appear, replace the battery or check the power connections.
(2) Perform the four self-test in any sequence. The SELF-TEST switch provides
testing of the microprocessor (MICR), all switches and keys (SW), the display and indicators
(DSP), and the modem (MOD).
7-5
FM 23-91
(3) Press the TEST switch. If after pressing the TEST switch, the correct software
revision number (Revision III/A) is not displayed, turn the MBC in to the GS maintenance
team.
(a) Microprocessor. Press the SEQ switch. Use the multiple choice entry to
select MICR. If after the microprocessor test (about 38 seconds) a display other than MICR:
PASS appears, turn in the MBC to the GS maintenance team.
(b) Switches and Keys. Use the multiple choice entry to select SW. Press the
switch or key indicated in the display. When a switch fails or is pressed out of sequence, the
display shows ERROR. The display returns to the name of the switch to be pressed. If the
specified switch is pressed and ERROR reappears in the display, the switch is inoperative.
Failure of the MBC to respond to a normal key pressed indicates a malfunctioning keyboard
assembly and should be turned in to the GS maintenance team. After all the switches and
keys have been tested, END OF TEST is displayed, and then READY is displayed.
(c) Display. Use the multiple choice entry to select DSP. Press the SEQ switch
three times to check for unlighted dot segments in each character space. During the first part
of the display test, make sure all dot segments are lit in the 16-character display. In the
second part of the test, check for character generation and indicators. Even though one or
more dot segments may be out, use the MBC if characters are readable. When characters are
not readable or an indicator is not flashing, turn the MBC in to the GS maintenance team.
CAUTION
Do not test modem while connected to a radio. This could
cause internal damage to the MBC.
(d) Modem. Use the multiple-choice entry to select MOD. After modem test
(about 20 seconds), MODEM PASS or MODEM FAIL is displayed. If MODEM FAIL
shows, message transmission and reception are inoperative. The MBC still accepts manual
input data and computes fire missions.
b. Basic Data Input. Before computing a fire mission, the operator must use certain
initialization switches to input basic data. Overall MBC initialization is directly related to
the tactical scene. Operators must always initialize SET UP, WPN DATA, and AMMO
DATA switches, initializing other switches as needed.
(1) Manual mode. When the MBC is not connected to an external communication
device, all data are manually entered.
(2) Digital mode. When the MBC is connected to an external device (DMD-
supported), data are digitally entered into the appropriate switch memory. Data entered
digitally may be reviewed or supplemented manually.
c. Minimum Initialization. Minimum initialization is the least required data to
compute for a standard mission. For minimum initialization, operators use the following
sequence:
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(1) TEST and BRT. These keys are used first to check the overall MBC operation and
to set the display brightness. The LOW setting in the BRT menu also lights up the keyboard
for night or limited visibility usage.
(2) SET UP and WPN DATA. These two switches must be initialized. They are
always manually entered in the MBC. Data will never change due to other switch action;
however, the operator may review and update data as needed. When the AMMO DATA
switch default values are suitable, this switch is not needed for initialization. The default
values are:
• 60-mm mortar: HE, M720; WP, M302A1; and ILLUM, M83A3.
• 81-mm mortar: HE, M374; WP, M375; ILLUM, M301A3; TNG, M1; and
RP, M819.
• 4.2-inch mortar: HE, M329A1; WP, M328A1; ILLUM, M335A2; and CS,
XM630.
• 120-mm mortar: HE, M933, M934; WP, M929; ILLUM, M930.
• 120-mm (insert) mortar: HE, M374; WP, M375A2; ILLUM, M301A3.
d. Minimal Initialization. Once the MBC is turned on and the self-test is conducted
the following minimal initialization information must be entered to compute for a standard
grid mission.
EXAMPLE
SET UP (menu)
Timeout: 60 seconds
Target prefix: AH
Target numbering block: 0001 - 0200
Easting (area of operation): 096000
Northing (area of operation): 029000
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NOTE: If firing a parallel sheaf with all mortars on line, the only weapon needed is the
base piece. When the situation allows, enter the rest of the section.
WARNING
Using the default firing data for all guns in the firing section may
cause rounds to be fired outside of the safety fan or firing zone.
Therefore, always use the TFC menu when a safety fan or firing zone
is used. This gives the mbc operator a "WARNING" message
indicating if any of the rounds for any particular weapon will land
outside the safety fan or firing zone. For revision III/A, the operator
must override the message in order to continue.
(1) Press the ON/OFF switch. The display shows: POWERUP TEST momentarily,
and then shows: READY. Use the test switch to manually start the MBC self-test. Perform
the self-test as the situation permits or as advised by the supervisor.
(2) Use the BRT switch to select the level of display character brightness (LOW,
MED, HI, and MAX). Use the LOW level to turn on the keyboard background lighting.
Character brightness is always set at HI when the MBC is turned on or when the BRT switch
is pressed.
(3) Press the SEQ switch. The display shows: READY. Press the SET UP switch.
Use the multiple choice entry to change the time-out to the desired length. Use time-out to
set the number of seconds (15, 30, 45, or 60); the display stays on between switch actions.
The default value of 15 seconds provides for minimal battery drain. A time-out of 60 is
recommended for the novice FDC computer.
(4) Press the SEQ switch. Using alpha entry, enter the target prefix AH.
(5) Press the SEQ switch. Using numeric entry, enter the target numbering block
0001 through 9999.
(6) Press the SEQ switch. Use the default shown: ALARM:OFF. Use message
alarm for DMD-supported missions, if needed.
(7) Press the SEQ switch. Using numeric entry, enter the minimum easting
coordinate 096.
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(8) Press the SEQ switch. Using numeric entry, enter the minimum northing
coordinate 029.
(9) Press the SEQ switch until OWN ID: _ is displayed. The E W GD:, + -
LAT:, LISTEN ONLY: OFF, BIT RATE: 1200, KEYTONE: 1.4, and BLK:SNG
information may be entered into the computer for expanded initialization.
(10) The final entry in the SET UP menu is the OWN ID. Enter the unit identification
code located in the SOI.
e. Weapon Data. Use the WPN DATA switch to enter the weapon data for section A,
B, and or C. Assign weapons to one, two, or all three sections. A total of 18 weapons may
be assigned (six for each section): A1 through A6, B1 through B6, and C1 through C6. The
first weapon entered in a section becomes the basepiece. The basepiece is the reference point
for the MBC to locate and add weapons to a section. It does not have to be the No. 2 gun or
adjusting piece.
(1) Press the WPN DATA switch. Use the multiple choice entry to select the desired
section (A). With the weapon types displayed, select the caliber (107 mm).
(2) After the caliber of weapon is selected, the choice of carrier or ground-mount is
next (except for the 60-mm mortar). The MBC defaults to CARRIER: NO. Ensure all
weapons in the section are mounted the same. Using the multiple choice entry, select the
CARRIER mode for the section. CARRIER: NO indicates the section is to be ground
mounted. CARRIER: YES indicates the section is to be carrier mounted. The muzzle
velocity is figured differently for ground mounted to carrier mounted. After entering the
selection of carrier-mounted, press the SEQ switch and the display shows:
CARRIER MV ENTERED. Carrier-mounted muzzle velocity corrections are entered into
the memory of the MBC for that section.
(3) Press the SEQ switch. Enter the basepiece (BP) number using multiple choice
entry (A2). The basepiece is just a reference for the MBC to locate the other mortars in that
section. Time and effort are usually saved if one of the flank mortars is used as the
basepiece.
(4) Press the SEQ switch. Enter the BP easting and northing grid coordinates. Most
mortar locations are known to within eight-digit grid coordinates. To enter the coordinates,
follow these instructions:
(a) Given the grid location for the basepiece as 04004700, enter the first four
easterly digits by pressing the alphanumeric key for that number followed with a zero (0).
Press the 0 key. The display should look like this: E:0 _ _ _ _ N:_ _ _ _ _. Enter the rest
of the coordinates. The numeric function of the key is the only entry that can be made. The
alpha characters are not part of the selection process for grid coordinates. The final display
should show: E:04000 N:47000. Do the same if only a six-digit coordinate is known—for
example 123456 is entered as 12300 45600.
NOTE: All easting and northing grid coordinates require five-digit entries.
(b) Press the SEQ switch. Use the multiple choice entry to enter the altitude (in
meters) of the BP (0750). The altitude is a mandatory entry. If the altitude is unknown to
the FDC, then an entry of 0000 is used. This entry tells the MBC to compute from sea-level.
Altitude entries may be made from 9999 meters to a minus (-) 999 meters.
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(c) Press the SEQ switch. Use a multiple choice entry to enter (in mils) the
direction of fire (azimuth 0800) and referred deflection (2800).
(d) Press the SEQ switch and the display shows: CONT END. Select CONT
(continue) if the rest of the section is to be entered at this time. If not, select END and the
computer shows: READY.
(e) To continue entering weapons, select CONT and the MBC shows:
WPN:A_ NXT CLR. Enter the weapon number (1) using the 1/ABC alphanumeric key.
(f) Press the SEQ switch. Use the multiple choice entry to enter weapon
direction (1600 mils) and distance (035 meters) from the basepiece.
(g) Press the SEQ switch. Repeat the steps in paragraph (a) and (b) above until
all guns in the section have been entered. Select END and the MBC display shows:
READY.
f. Ammunition Data Default Values. If the AMMO DATA default values are
suitable, the minimum initialization is complete. If suitable, the operator uses the AMMO
DATA to select shell types for each ammunition type for the caliber in use. Powder
temperature default is 70 degrees and is correctable. Three 107-mm ammunition types are
weight correctable: the M329A1, M328A1, and XM630. When corrections are entered, the
word NO on the right side of the display is changed to CR. Weight changes are entered in
pounds or squares. When pounds or squares are entered, a conversion is made to show both
unit entries.
(1) Press the AMMO DATA switch. The display shows: 60 81 107 TEMP. Select
the caliber of weapon being used (107 mm) by pressing the blue display switch (display
switch No. 3) beneath the number 107.
(2) The display now shows: HE: M329A1 :NO. Flashing cursors are on the 2 and
the N. These cursors indicate that changes may be made to the display. HE: M329A1 is the
default value for the 107-mm mortar, so no changes are needed for the round type. However,
the round also comes in different weights as explained earlier. Square weight is usually
given on the exterior of the boxes that the ammunition comes in. The FT 4.2-H-2 has charts
(pages X through XII) on the mean weights of all rounds and their fuze combinations, except
for the M329A2. The weight of the M329A2 round is standard and requires no changes to
weight or squares. Once the entry has been made either by weight in pounds or in squares,
press the SEQ switch and the display shows: HE: M329A1 :CR.
(3) Press the SEQ switch. Continue the above procedures until all the ammunition
requirements are entered.
NOTE: The ammunition menus for all the mortars are similar in format.
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(3) Manually initialize and update FO LOC when in the manual mode. When the
MBC is DMD-supported, input is automatically entered when a valid observer location
message is received. This is also a good time to update the SET UP menu. The
communication data are LISTEN ONLY: OFF, BIT RATE: 1200, KEYTONE: 1.4, and
BLK:SNG.
NOTE: The bit rate and transmit block mode are located in the SOI.
(4) Initialize and update KNPT/TGT at any time regardless of the mode of operation.
The KNPT/TGT switch may be updated automatically by the use of the EOM, REPLOT, and
SURV switches, or by receiving digital messages related to the KNPT/TGT.
(5) Manually initialize REG DATA to maintain a registration file when enough data
are known from conducting a fire mission. Normally registration data are generated
automatically by using the REG switch during fire mission processing. However, data
manually entered with the REG DATA switch are automatically updated when the REG
switch is used to compute registration.
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CHAPTER 8
TYPES OF MISSIONS
All fire missions, except final protective fires, begin with the GRID,
SHIFT, or POLAR switches. The needed elements of the fire request are
entered into the MBC. The WPN/AMMO switch is used to identify the section
and the adjusting piece. The firing data are displayed after pressing the
compute switch.
NOTE: Once the firing section is selected, the weapon type is displayed along with the
adjusting weapon. (107C WPN:A2)
NOTE: The MBC selects the lowest charge possible, or the operator can manually input
a charge of his choice.
c. The MBC operator pushes the COMPUTE switch (green) to receive firing date.
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FM 23-91
NOTE: When the COMPUTE switch is pressed before the WPN/AMMO switch, the
computer automatically enters the WPN/AMMO menu.
NOTE: The fuze setting applies only to the fuzes that require a time setting.
NOTE: At this point, it is not necessary to continue in the SFTY DATA menu.
(2) Press the SEQ switch 11 times to receive the angle T. ANG T:0300MILS is
displayed.
(3) An option of exiting out of this menu is to press the MSN switch (light green),
and then press the BACK switch. READY is now displayed.
f. The MBC operator must now wait for FO adjustments (if any). To make FO
adjustments, the MBC operator must do the following:
• Press ADJ switch (red). ADJ MPI is displayed.
• Press display switch 1 under ADJ switch. ENT REV is displayed.
—To enter adjustments, select ENT.
—To review the last adjustment, select REV.
• Press display switch 1, ENT. ADJUST FO:W/12\- is displayed.
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NOTE: During the initial input of the mission, the MBC operator bypasses the direction
entry. The direction shown is the gun-target (GT) direction, also known as the
initial azimuth. At this point, the MBC operator ensures the FO's direction is
shown. To clear the portion of the display showing the direction, press display
switch 3 under the flashing display cursor or press the CLEAR ENTRY switch
once to clear a digit at a time. Once the GT azimuth is cleared, the FO's direction
(0500) may be inserted.
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FM 23-91
Once all adjustments have been made or the FO requests FFE, the MBC operator decides
how to engage the target. Based on the information given by the FO in the call for fire, he
must use the TFC key.
h. The MBC operator presses the TFC switch (red).
(1) TFC FO:W/12\- (FO calling in the fire mission).
Press SEQ switch.
(2) MSN:1 TN:AA0001 (mission and target number assigned to mission).
Press SEQ switch.
(3) SHEAF:PRL (sheaf type prefer by FDC for this mission; this can be changed
when necessary).
Press SEQ switch.
(4) CON:AF (method of control; this can be changed when necessary). ENTER
FFE
Press SEQ switch.
(5) GUNS:A2 13 (section and weapons assigned to FFE; this can be changed when
necessary).
Press SEQ switch.
(6) REG/MET:NO (no current registration and or MET data apply to this mission;
this can be changed when necessary).
Press SEQ switch.
(7) MET:STD (MET to be applied to this mission will be standard MET; this can be
changed when necessary).
Press SEQ switch.
(8) PUSH COMPUTE is displayed.
i. The MBC operator presses the COMPUTE switch (green) to receive firing data.
Press SEQ switch to receive firing data for each gun.
NOTE: Once EOM is received, the MBC operator obtains the burst point (BP)
coordinates, they are 06691 48764. Do this by using the SFTY DATA switch.
j. The MBC operator presses the EOM switch (green) to end the mission.
• EOM (ends the mission without saving).
• EOMRAT (ends the mission and records as target/known point).
NOTE: The flashing red light over the SEQ switch indicates additional information is
available for the current menu or display.
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j. The MBC operator presses the EOM switch (green) to end the mission.
(1) EOM (ends the mission without saving).
(2) EOMRAT (ends the mission and records as target/known point).
NOTE: The flashing red light over the SEQ switch indicates additional information is
available for the current menu or display.
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(4) CON:AF (the method of control; it can be change when necessary). ENTER
FFE
Press SEQ switch.
(5) GUNS:A2 13 (section and weapons assigned to the FFE; it can be changed when
necessary).
Press SEQ switch.
(6) REG/MET: NO (no current registration and or MET data apply to this mission;
it can be changed when necessary).
Press SEQ switch.
(7) MET:STD (MET to be applied to this mission is standard MET; it can be
changed when necessary).
Press SEQ switch.
(8) PUSH COMPUTE is displayed.
i. The MBC operator presses compute to receive firing data.
Press SEQ switch. Firing data for each gun is displayed.
NOTE: Once EOM is received, the MBC operator obtains the burst point coordinates.
This is accomplished by using the SFTY DATA switch.
j. The MBC operator presses the EOM switch (green) to end the mission.
• EOM (ends the mission without saving).
• EOMRAT (ends the mission and records it as target/known point).
NOTE: The flashing red light over the SEQ switch indicates additional information is
available for the current menu or display.
NOTE: Always use the TFC switch whenever using a safety fan and or fire zones.
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(1) SHEAF:PRL—This is the type of sheaf needed to engage the target. Sheaves
selectable within the menu are PRL (parallel), CVG (converge), and SPECIAL.
(2) CON:AF—CON stands for control of fires. The multiple choice selections
include AF (adjusting fire), FFE (fire for effect), DST (destruction), and REG (registration).
NOTE: In the adjust fire mode, the only weapon shown is the same weapon selected
through the WPN AMMO switch. When the operator enters FFE, all assigned
available weapons in that section are included in the computation of fire data.
When control is FFE or DST (destruction), some weapons (not the adjusting
weapon) may be deleted by using correction entry.
(3) GUNS—This shows which mortars are available for the designated control of
fires. For example, if AF appears on the previous screen, the only mortar shown on this
display is the piece designated by the MBC operator in the WPN/AMMO menu.
(4) REG/MET—If a MET has been entered and made current, this display would
show REG/MET: YES. This tells the operator that MET or registration corrections will be
applied to the target firing data. If the display shows REG/MET: NO, there are no
corrections applied.
(5) MET:STD—This tells the operator what type of MET corrections are used for
the fire mission. There are two possible types: STD (standard) and CURR (current).
8-5. SHEAVES
The term sheaf denotes the lateral distribution of the bursts of two or more weapons firing
at the same target at the same time. The distribution of bursts is the pattern of bursts in the
area of the target. Normally, all weapons of the platoon/section fire with the same deflection,
charge, and elevation. However, since targets may be of various shapes and sizes and the
weapons deployed irregularly, it is best to adjust the pattern of bursts to the shape and size
of the target.
a. Individual weapon corrections for deflection, charge, and elevation are computed and
applied to obtain a specific pattern of bursts. These corrections are called special
corrections. These corrections are computed and applied based on the target attitude, width,
length, and adjusting point.
b. When the mortar section or platoon engages a target, different sheaves can be used.
The types of sheaves include the parallel, converged, open, standard, and special (see
Chapter 4).
(1) When mortars fire a parallel sheaf, the distance between impacts of rounds is the
same as the distance between mortars. The mortars all fire using the same firing data. A
parallel sheaf is normally used on area targets.
(2) When mortars fire a converged sheaf, the rounds, fired from two or more mortars,
impact on the same point in the target area. This sheaf is normally used on a point target
such as a bunker or a machine gun position.
(3) When mortars fire an open sheaf, the distance between impacts of rounds is half
again the distance between mortars. Normally, 120-mm mortars are 60 to 75 meters apart,
81-mm and 4.2-inch mortars are 35 to 40 meters apart; thus, in an open sheaf, rounds should
land about 60 meters apart. For the 60-mm mortars, which are normally 25 to 30 meters
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FM 23-91
apart, rounds should land about 45 meters apart. All mortars fire using different deflections.
The open sheaf is used when the target is slightly wider than the area a standard sheaf would
cover.
(4) When mortars fire a standard sheaf, rounds impact within the total effective width
of the bursts, regardless of the mortar locations.
(5) When mortars fire a special sheaf, each mortar has a certain point to engage. The
mortars may have different deflections, charges, and elevations. This sheaf is normally used
in an attitude mission.
NOTE: When mortars fire an open sheaf or a standard sheaf, the operator must use the
special sheaf function and enter the appropriate data to obtain the desired results.
NOTES: 1. The target attitude should be within 100 mils of the attitude of the mortar
section (WPN DATA menu).
2. The attitude of the target should be perpendicular to the gun's direction of
fire. When firing at targets with other than perpendicular angles, a
combination of traverse and search will result.
a. Upon receiving the call for fire, the section leader/chief computer determines from
the size and description of the target that traversing fire will be used to cover the target. He
then issues the FDC order (Figure 8-1).
NOTE: Distribution of mortar fire to cover area targets (depth or width) is computed at
one round for each 30 meters and four rounds for each 100 meters.
b. When using the information in the call for fire, FDC order, and FO corrections, the
FDC computes the data to adjust the base mortar (usually the No. 2 mortar) onto the center
mass of the target area. He computes the firing data to center mass. The FDC selects the
SFTY DATA switch and records the range and burst point grid coordinate on DA Form 2399
(Figure 8-2).
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c. After the adjustment is complete, the FDC must perform the following procedures:
(1) Divide the target into equal segments by dividing the width of the target by the
number of mortars in the FFE.
EXAMPLE
target width = 300 meters
number of mortars in the FFE = 3
300/3 = 100 meters each mortar has to cover.
(2) Determine and apply the modification (either +/- RNG correction or left/right
DEV correction). Divide the the segment width (100) by 2 to determine the amount of the
modification—for example, 100/2 = 50. Use one of the following methods to apply the
modification.
(a) Use Table 8-1 to determine the direction (plus or minus) for the modification.
As an example, let the GT be 5300 mils, traverse right.
Since the GT azimuth falls in the azimuth block of 4901-1499, the modification will be a
plus if traversing left and a minus(-) if traversing right. Since the mortars will traverse right,
their modification will be -50.
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OR
(b) When the FDC finds itself without the GT AZ chart, an alternative method
of computing for the modification is needed. Therefore, draw the situation to help new FDC
personnel develop an understanding of how and why the MBC computes for the traverse
data.
Guns must be placed so they are using the direction of the target attitude (400 mils). The
FDC determines if it needs a plus or minus correction to get to the starting point.
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OR
(c) Determine the perpendicular to the attitude (add or subtract 1600 mils; use
whichever is closer to the final azimuth of fire) and apply the modification as a left or right
correction. When computing for firing data using the perpendicular, copy the range and burst
point grid coordinate, and the final azimuth of fire.
(d) Add or subtract 1600 mils to the target attitude. Use the answer that comes
closer to the final azimuth of fire for the direction correction in the ADJ menu.
(e) Select the ADJ switch and change the direction to the perpendicular azimuth.
(f) Instead of making a range correction as in the previous examples, make a
DEV (deviation) correction. This correction is one-half the distance each mortar must cover.
(g) If traversing left, enter a right DEV correction; if traversing right, enter a left
DEV correction.
(3) Once the modification (regardless of the method used) has been entered into the
ADJ menu of the MBC, press the TFC switch and change or enter the following data:
(a) Change: SHEAF:PRL to read SHEAF:SPECIAL.
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EXAMPLE
Assume that the width of the target is 350 meters. Divide the area into equal
segments: 350/3 = 116. Each mortar covers 116 meters of the target area.
Multiply the even hundred by 4: 1 x 4 = 4. The remainder of the target width
(16 meters) is covered by adding one round. Therefore, rounds for each segment
equal 5.
(5) Determine the mil width of one segment. If the mil width of one segment is
determined, the other segments are the same. Use one of two methods to determine the
number of mils for one segment:
(a) In the first method, the start point deflections for all the mortars are given. By
comparing the mil difference between either No. 1 mortar and No. 2 mortar or No. 2 mortar
and No. 3 mortar (or No. 3 mortar and No. 4 mortar, if available). For example, No. 1 mortar
has a deflection of 2719 mils and No. 2 mortar has a deflection of 2773 mils. The mil
difference is 54 mils (subtract the smaller from the larger: 2773 - 2719 = 54 mils).
(b) The second method uses the DCT (Figure 8-6) to determine the mil width of
one segment. Enter the DCT at the final range, rounded off to the nearest 100. Go across
the deflection-in-meters line to the closest meters to cover the segment. The point at which
the range line and the deflection line meet is the number of mils that will cover the segment.
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(6) To determine the number of turns it will take to cover one segment, divide the
number of mils for each turn on the traversing hand crank by the mil width of one segment—
for example, 10 (divide by 5 when using the 120-mm mortar) (number of mils for each
turn)/54 = 5.4 (rounded off to the nearest one-half turn) or 5 1/2 turns to cover 116 meters.
(7) To compute the number of turns between rounds, the number of rounds to be fired
must be known for each segment (FFE). This information is in the FDC order. To determine
the turns between rounds, divide the total turns by the intervals (always one less than the
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number of rounds) between rounds—for example, 5 rounds = 4 intervals; 5.5 (total turns)/4
(intervals):
5.5/4 = 1.3 (rounded to nearest 1/2 turn)
= 1/2 turns between rounds
NOTE: Whether searching up or searching down, always determine the firing data for the
far edge of the target area first. This saves time if the charge designated at the
near edge is different than the one designated at the far edge.
(2) When using searching fire, enter an add correction that is half the total target
length. This places the mortars on the far edge of the target.
(3) Compute the firing data for the far edge and record the information.
(4) Enter a correction to place the mortars on the opposite edge of the target. The
correction to enter will be a drop, and the distance will be the entire length of the target area.
(5) Compare the charge needed to hit the near edge with the charge needed to hit the
far edge of the target. The charge must be the same. If they are not, select the charge
designated for the far edge by using the WPN/AMMO menu and recompute the near edge
firing data.
(6) Determine the number of turns between rounds by determining the mil distance
to cover the target area and by dividing it by 10 (approximate number of mils in one turn of
the elevation hand crank). Round off the answer to the nearest one-half turn. Compute the
distribution of mortar fire to cover area targets (depth or width) at one round for each 30
meters and four rounds for each 100 meters.
(a) Compare the far edge elevation to the near edge elevation and subtract the
smaller from the larger.
(b) Divide the mil distance by 10 (divide by 5 for the 120-mm mortar) and round
off to the nearest half a turn.
(7) Determine the turns between rounds by dividing the intervals into the turns and
by rounding off to the nearest half turn. The intervals are always one less than the number
of rounds in the FFE.
b. The 4.2-inch mortar does not fire a search mission the same as the 60-mm, 81-mm,
or 120-mm mortars. It does not have the same elevating characteristics as these mortars;
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therefore, the 4.2-inch mortar uses zone fire. The 4.2-inch mortar platoon/section usually
fires two standard zones: a 100-meter zone (three rounds for each mortar) for a platoon-size
target, and a 200-meter zone (five rounds for each mortar) for a company-size target. Cover
larger zones by firing more rounds.
c. These are the procedures used when using an MBC.
(1) The FO adjusts to center mass of the target area. He calls for an FFE (his last
correction may also include the FFE command). The FDC determines the center mass of the
target based on the FO's last correction and determines the burst point.
(2) The FDC obtains the information for the zone using the MBC by:
(a) 100-meter zone.
• Entering the ADJ menu.
• Entering an adjustment of +50 (far edge).
• Writing down all the firing data.
• Entering the ADJ menu again.
• Entering an adjustment of -100 (near edge).
• Writing down all the firing data.
• Entering the ADJ menu again.
• Entering an adjustment of +50 (to return to center mass).
• Determining the BP and compares it to the initial BP, they should be to
within 10 meters of each other.
(b) 200-meter zone.
• Entering the ADJ menu.
• Entering an adjustment of +50 (halfway to the far edge).
• Writing down all the firing data.
• Entering the ADJ menu again.
• Entering an adjustment of +50 (far edge).
• Writing down all the firing data.
• Entering the ADJ menu again.
• Entering an adjustment of -150 (halfway to the near edge).
• Writing down all the firing data.
• Entering the ADJ menu again.
• Enter an adjustment of -50 (near edge).
• Writing down all the firing data.
• Entering an adjustment of +100 (to return to center mass).
• Determining the BP and compares it to the initial BP, they should be to
within 10 meters of each other.
(3) Once the FO gives the FFE, the computer proceeds as follows to establish the
100-meter zone:
(a) Firing without extension. Add and subtract 3/8 charge from the base
command charge. (The base command charge is the command charge in the FFE center mass
of target.) This gives each mortar three rounds with a different charge on each to cover the
100-meter zone (Figure 8-7).
8-20
FM 23-91
CHARGE 10 3/8
50 METERS
100 CHARGE 10
BASE COMMAND CHARGE
METERS
50 METERS
CHARGE 9 5/8
(b) Firing with extension. Add and subtract 4/8 charge from the base command
charge and use three rounds for each mortar.
NOTE: A 3/8 charge correction to any charge without extension moves the round about
50 meters at any elevation used. A 4/8 charge correction to any charge with
extension moves the round about 50 meters at any elevation used.
(c) Firing the 100-meter zone. Once the mortars are up (rounds set for proper
charges) and the fire command is given, the rounds are fixed in any sequence—for example,
No. 1 fires long, short, center mass; No. 2 fires center mass, short, long.
(4) Once the FFE has been given by the FO, the computer proceeds as follows to
establish the 200-meter zone:
8-21
FM 23-91
(a) Firing without extension. Add and subtract 3/8 charge from the base
command charge for the rounds on either side of the base round and 6/8 charge for the long
and short round (Figure 8-8).
CHARGE 31
50 METERS
CHARGE 30 4/8
50 METERS
50 METERS
CHARGE 29 4/8
50 METERS CHARGE 29
(b) Firing with extension. Add and subtract 4/8 charge from the base command
charge for the rounds on either side of the base round and a whole charge for the long and
short rounds.
(c) Firing the 200-meter zone. The mortars can fire the rounds in any sequence.
NOTES: 1. If a larger zone is needed, use the same procedures, only firing more rounds
for each mortar and cutting the charges.
2. A 2/8 of a charge should be used with the M329A2 ammunition.
8-8. ILLUMINATION
Illumination assists friendly forces with light for night operations.
a. The FDC routinely obtains firing data. However, the FDC uses one of the flank
mortars to adjust the illumination, leaving the base mortar ready to adjust HE rounds if a
target is detected.
NOTE: Normally, when a four-mortar section is firing, the No. 4 mortar is used to adjust
the illumination, leaving the No. 2 mortar as the base mortar. When the No. 1
mortar is used to adjust illumination, the No. 3 mortar becomes the base mortar.
8-22
FM 23-91
b. The FO makes range and deviation corrections for illumination rounds in not less
than 200-meter increments. He also makes corrections for height of burst (up or down) of
not less than 50-meter increments.
c. Multiple mortar illumination procedures are used when single mortar illumination
does not light up the area enough. Two mortars are used to fire illumination only when
visibility is poor. Two mortars, usually side by side ( No. 1 and 2, No. 2 and 3, and so on),
fire rounds at the same time at the same deflection, charge, and time setting to provide a large
amount of light in a small area. If the FO suspects a large target or if he is uncertain of target
location and desires a larger area be illuminated, he may call for illumination: range, lateral,
or range-lateral spread.
(1) Range spread. Two mortars fire one round each at the same deflection but with
different charges so that rounds burst at different ranges along the same line. The spread
between the rounds depends on the type of mortar firing the mission. The 120-mm rounds
have 1,500 meters between bursts, the 4.2-inch mortar rounds have 1,000 meters between
bursts, and the 81-mm mortar rounds have 500 meters between bursts. When four mortars
are present in the firing section, the No. 2 and No. 3 mortars normally fire the range spread.
When firing a three-mortar section, the range spread may be fired with just one mortar, which
fires both rounds.
(a) Enter the type of target location called in by the FO into the MBC to initiate
the mission. The weapon selected by the FDC in the WPN/AMMO menu (to activate the
section) should be one of the mortars that is going to fire the mission.
(b) The initial firing data determined for the mission are center-of-mass target
data. These data are not fired but are used as the starting point for the adjustment of the
spread.
(c) Enter the ADJ menu. Change the OT direction to GT direction and enter a
correction for the first round of the spread. Compute the firing data and record.
(d) Select the ADJ menu and enter a correction to get the required distance
between rounds, which depends on the mortar system being used.
(e) Compute for firing data, record it, and fire the two rounds for the range
spread.
NOTE: The two rounds should burst at the same time. The far round must be fired first,
with the near round being fired after, at the difference between the time settings.
EXAMPLE
Assume the mortar selected to fire is the No. 2 mortar. Enter the initial target
location and determine the center mass data. Next, enter the ADJ menu and give
the No. 2 mortar a correction of +500 (for 4.2-inch mortars), +250 (for 81-mm
mortars), or +750 (for 120-mm mortars). Compute these data and record them.
Enter the ADJ menu again and make a correction of -1000 (for 4.2-inch), -500 (for
81-mm mortar), or -1500 (for 120-mm mortar). Compute and record these data.
Using both sets of data to fire the rounds, rounds will burst the desired length (1,500 meters
for the 120-mm, 1,000 meters for the 4.2-inch, or 500 meters for the 81-mm) between rounds
on the GT line.
8-23
FM 23-91
NOTE: A range spread should be fired using one mortar firing both rounds—one long and
one short.
(2) Lateral spread. Two mortars fire one round each at different deflections but with
the same charge. Therefore, the rounds burst at the same range along the same attitude.
(a) Using the No. 2 mortar, process the call for fire and determine firing data for
center mass.
(b) Using the ADJ menu to enter left and right corrections, use the GT as the
direction and enter the first correction.
NOTE: The No. 2 mortar is used for the initial round. The first correction can be either
a right or left correction. For example, the first correction for the 4.2-inch mortar
is R 500; the first correction for the 81-mm mortar round is L 250; the first
correction for the 120-mm mortar round is L/R 750.
(c) Compute for the firing data and copy it down.
(d) Select the ADJ menu and enter the reverse of the first correction the entire
distance required between rounds: L/R 1,000 meters for the 4.2-inch, L/R 500 meters for the
81-mm mortar, or L/R 1,500 meters for the 120-mm mortar.
(3) Range-lateral spread. If the target area is extremely large or if visibility is
limited, the FO may call for range-lateral spread. This procedure combines the two methods
(Figure 8-9). This results in a large diamond-shaped pattern of bursts. If mortars use the
flank mortars for the lateral spread and the center mortar(s) for the range spread, the danger
of rounds crossing in flight is removed.
ILLUMINATION ROUNDS
TARGET
OBSERVER
8-24
FM 23-91
b. The mark method is the method used most. The FDC and the FO must know which
round the illumination mark will be given.
c. When the illumination round has been adjusted to provide the best light on the target,
the FO gives the command, MARK ILLUMINATION. The FDC times the flight of the
round from the time it is fired, until the command, MARK.
d. When determining the time to fire the HE round, drop all tenths before computations
are made. Subtract the time of flight for the HE round and the illumination round.
EXAMPLE
ILLUMINATION ROUND—53 SECONDS AND THE HE ROUND—
19 SECONDS = TIME TO FIRE THE HE ROUND WILL BE 34 SECONDS,
AFTER THE ILLUMINATION ROUND IS FIRED.
e. When firing coordination missions, the computer operator uses a new computer
record to record the illumination mission. The data that was used to fire the first illumination
round is taken from the computer record that was used to adjust the illumination mission.
f. The FO sends corrections and precedes each correction with the type of round the
correction is intended for—for example, ILLUMINATION, UP FIVE ZERO, HE, RIGHT
FIVE ZERO, ADD FIVE ZERO. He records each correction on separate lines. The FDC
keeps track of the 50-meter increments by using the computer record of the illumination
mission.
g. There are two methods normally used to adjust illumination and HE. Coordinated
illumination using the mark method with the FDC controlling the firing of both the HE and
illumination rounds and coordinated illumination using shell at the FO commands. The FO
controls the firing of each round. The FO sends corrections and computes the data that is
sent to the mortars from the FDC. The mortars then report when they are UP. The FDC
notifies the FO, and the FO gives the command to fire each round.
h. When the FO is certain that he can hit the target with the next round, he commands,
CONTINUOUS ILLUMINATION, FIRE FOR EFFECT or CONTINUOUS
ILLUMINATION, HE, DROP TWENTY-FIVE, FIRE FOR EFFECT.
i. By requesting the continuous illumination, the FO is telling the FDC that he wants
the target illuminated during the fire for effect and illuminated afterward to allow him to
make his target surveillance. Upon completion of the mission, he records the data on the
data sheet.
8-25
FM 23-91
CHAPTER 9
SPECIAL PROCEDURES
Procedures for basic fire missions are simple and require little
coordination by the indirect fire team. The one element that is lacking in
these procedures is accuracy, which the indirect fire team strives to improve.
In-depth planning and prior coordination between elements of the indirect
fire team help ensure the delivery of timely and accurate fires. This chapter
discusses the special procedures needed to conduct registration missions,
final protective fires, and quick or immediate smoke.
9-1
FM 23-91
NOTE: Changing CON:AF to CON:FFE and pressing COMPUTE are mandatory steps
before adjusting individual guns.
f. The FDC initiates the adjustment of the sheaf. He tells the FO, "Prepare to adjust the
sheaf." The FO responds with, "Section left/right." The section left/right is fired without the
basepiece unless the FO specifies to fire the basepiece. The operator prepares to receive
corrections for each mortar not firing within the sheaf. Then, he records the corrections and
computes them separately.
NOTE: The MBC can only compute one correction at a time; therefore, if the computer
records the corrections, he may compute for the corrections as he desires. The
smaller corrections should be entered first since the mortars will not likely be
fired again.
(1) Use the adjust menu (press ADJ) and sequence to ADJ:AUF (adjusting:adjusting
unit of fire). Change the AUF to SHEAF.
(2) Sequence to WPN: and enter the weapon number to adjust and the correction.
(3) Compute the correction. The weapon number identified with the correction is the
only weapon affected by the correction. The other weapons will still be on the last firing data.
(4) Use the adjust (ADJ) switch and sequence to WPN:NXT CONT. The WPN is
for "weapon." The abbreviation NXT is for the "next" mortar to adjust. The CONT means
"continue with the same mortar" identified in (2) above for more corrections.
NOTE: If a correction is over 50 meters (L/R) then that mortar will be refired. If the
correction is less than 50 meters (L/R), the mortar will not be refired, but the
correction will be made, and the gun will be considered adjusted.
(5) Sequence to WPN: and enter the weapon to adjust and enter the correction.
(6) Compute the correction.
(7) Use the firing data menu to sequence through the data and record the new fire
commands.
9-2
FM 23-91
g. After the sheaf has been adjusted, the section/platoon must refer the sight and realign
the aiming post on the last (hit) deflection of the basepiece used for the registration. The
mission is ended using the EOM menu.
h. The computer uses the REG DATA (initialization switch) menu to store information
concerning the RP and to update the RP. Then the MBC applies the correction factors to all
subsequent fire requests that are within the transfer limits of the RP.
i. To update or reregister on the RP, the computer (MBC operator) follows the same
procedures as a grid mission, until the FO determines the update or reregistration is
complete. The operator will then—
(1) Press REG DATA (light blue) switch.
(2)Press display switch (dark blue) under NXT. It should read RP00.
(3) Press display switch (dark blue) under CLR. (CLEAR RP 00 *) is displayed.
(4) Press display switch (dark blue) under (*). (RP: NXT CLR) is displayed.
• Press BACK (light green) switch until READY appears.
• Press REG (Red) switch.
• Sequence through until PUSH COMPUTE appears.
• Press COMPUTE (light green) switch for new deflection correction and
RCF.
• Press EOM (light green) switch instead of EOMRAT. Data is stored already
from the initial registration mission (RP).
(2) Use the WPN/AMMO menu to assign the mission to an adjusting piece.
(3) Press COMPUTE to determine the firing data and record the needed information
to include the burst point to the target.
NOTE: The MBC does not allow access to the MPI menu under the ADJ (adjust) switch
until a mission has been activated. This is done by using the GRID and
WPN/AMMO menus.
9-3
FM 23-91
b. After the locations of the FOs and target point are known, the FDC can compute and
report the orienting data to the FOs. The FOs must be given their orienting data before firing.
To determine the orienting data of the observer—
(1) Press the ADJ switch. Select MPI:, and FILE CONT INIT is displayed.
(2) Select INIT to initialize the MPI mission. INIT YES NO (for verification) is
displayed.
(3) Select YES. The MBC prompts the operator for one of the FO's ID.
(4) Enter either one of the FO IDs.
(5) Press the SEQ switch. The orienting direction is displayed for the FO entered.
(6) Press the SEQ switch. The vertical angle is displayed for the FO entered.
(7) Press the SEQ switch. The target number is entered and displayed.
(8) Press the SEQ switch. The orienting data are ready to be transmitted to the FO.
If the MBC is DMD-supported, select YES to transmit via digital. If the MBC is not DMD-
supported, select NO. The MBC prompts the operator for the other FO's ID.
(9) Follow steps (4) through (8) for the other FO.
(10) If the MBC is not DMD-supported, transmit the orienting data to the FOs in the
following format:
FDC: "PREPARE TO OBSERVE MPI REGISTRATION. HOTEL 42
DIRECTION 2580 VERTICAL ANGLE +40; HOTEL 41
DIRECTION 2850 VERTICAL ANGLE +10; REPORT WHEN
READY TO OBSERVE."
c. The FOs should announce "Ready to observe" after they have received the orienting
data from the FDC and have set up their instruments.
d. The section leader/chief computer directs the firing of the orienting round using the
computed firing data. The FOs use the round to check the orientation of their instruments.
The orienting round should be within 50 mils of the expected point of impact.
(1) If the round lands 50 mils or more from the expected point of impact, the FO
reorients his instrument and announces the new direction to the FDC. If one FO reorients his
instrument, the spotting of the other FO is disregarded. When either of the FOs must reorient,
the operator must enter the new direction by using the ADJ menu.
• Enter the ADJ menu. Press the ADJ switch.
• Select MPI.
• Select INIT.
• Reenter the FO's ID, when prompted.
(2) If the burst impacts less than 50 mils from the expected point of impact, the FO
sends the FDC a spotting. The spotting contains the number of mils left or right of the
expected point of impact.
(3) When both FOs report their instruments are ready, the adjusting mortar fires the
number of rounds needed to get six usable spottings. To enter the spottings into the MBC—
(a) Press the ADJ switch and select MPI. The computer displays FILE CONT
INIT.
(b) Select FILE to enter the spottings. The MBC requests the sighting number.
(c) Enter the sighting (round) number.
9-4
FM 23-91
(d) Press the SEQ switch. Determine the azimuth from the FO to the target using
the RALS (right add, left subtract) rule. Add or subtract this correction from the FO's referred
(orienting) direction. Enter the azimuth as the FO's direction.
(e) Press the SEQ switch. The MBC prompts for the vertical angle from the FO
to the round. Enter the vertical angle, if any.
(f) Press the SEQ switch. The second FO's ID is displayed. Enter the sighting
(round) number. Determine the azimuth from the FO to the target using the RALS rule. Add
or subtract the correction from the FO's referred (orienting) direction. Enter the azimuth as
the FO's direction.
NOTE: The MBC computes for only one vertical angle correction. This correction applies
only to the first FO entry. When the vertical angle entry must be computed, the
operator ensures the proper FO is entered.
(g) Press the SEQ switch. The MBC prompts the operator for the next sighting.
(h) Press the COMPUTE switch and enter the FO's sightings as described until
all sightings have been entered. After the last sighting has been entered, select END on this
display.
(i) Press the COMPUTE switch and sequence to view the RP corrections.
(j) Press the EOM switch to end the mission.
9-5
FM 23-91
NOTE: Use 10-digit grid coordinates; add a zero to the end of each easting or northing
coordinate until there are 10 digits for each coordinate—for example, the grid
123456 becomes 1230045600.
Easting Northing
RP Grid 0381(0) 7158(0)
1st Round Grid -0355(0) -7120(0)
26(0) 38(0)
(b) By using a piece of blank scrap paper, the FDC can draw a large square to
represent a 1000-meter grid square.
(c) The FDC labels the bottom left-hand corner of the square with the grid
intersection of the RP (03/71) (Figure 9-1).
(d) He divides the large square into four smaller squares by drawing a line
through the center of the box from top to bottom and from left to right.
(e) He estimates the location of both grid coordinates and plots them inside the
box.
(f) By looking at these plots, the FDC can tell whether the round is left or right
and whether it is over or short of the RP. This is the spotting of the round. For this example,
the spotting is left (260 meters) and short (380 meters).
72
st
1 ROUND
71
03 04
Figure 9-1. Determination of a spotting.
(g) The spotting is then converted to a correction by converting the left spotting
to a RIGHT (R)260 correction and the short spotting to an ADD (+)380 correction. Using
the ADJ menu, the operator enters the corrections to apply.
• Change the direction to 6400 (or 0000).
• Enter R 0260 for the deviation correction.
• Enter + 0380 for the range correction.
9-6
FM 23-91
• Sequence to READY.
The operator then computes for the firing data and sends it to the guns.
(5) The second round is fired, and the FO sends the grid 04007180. The same process
is repeated as for the first correction.
(a) The grids are compared and the spotting is determined (RIGHT 190 and
OVER 220).
(b) The corrections (LEFT [L] 190 and DROP [-] 220) are made in the ADJ
menu, and the firing data are sent to the mortars.
(6) The computer repeats this procedure until the spotting is within 25 meters of the
RP and until the FO has given "End of Mission, Registration Complete." The FDC—
(a) Enters the final correction through the ADJ menu and computes.
(b) Presses the REG switch and sequences through the REG menu. He ensures
that the data pertaining to the RP are correct.
(c) Presses COMPUTE when indicated at the end of the REG menu to determine
the RCF and deflection correction (DEFK).
(7) After the registration is completed, the FDC informs the FO (radar operator),
"Prepare to Adjust the Sheaf." The following procedures are continued until the sheaf is
adjusted. To adjust the sheaf—
(a) The FDC converges the sheaf on the RP. Using the TFC switch, the operator
changes the method of control (CON) from adjust fire (AF) to fire for effect (FFE).
(b) The operator sequences through the rest of the menu, ensuring all data match
with the FDC order. He presses the COMPUTE switch when the MBC reads PUSH
COMPUTE.
(c) All mortars are fired (except the BP) at 10- to 20-second intervals.
(d) The radar operator sends the FDC the grid to the impact of each round fired.
(e) The FDC compares the grids to the impact of each round with the grid of the
RP, and it determines the deviation corrections for each mortar. THE FDC DOES NOT USE
RANGE CORRECTIONS. When adjusting for a parallel sheaf during a registration mission,
the FDC disregards range corrections.
NOTES: 1. The operator compares the full grid for all rounds fired. Any extreme
deviation or range spotting means there is a problem in the setup of that
mortar position(s).
2. If the operator is using the MBC to apply these corrections, he must first enter
and compute all corrections under 50 meters should be entered and computed
for first.
(f) All corrections more than 50 meters are refired, the new grids are compared
to the RP grid, and new data are computed for those weapons.
(8) Once the sheaf is adjusted, the FDC must open the sheaf. Using the deflection
conversion table (DCT), the FDC opens the sheaf mathematically the distance required based
on the mortar system used.
(9) The FDC now has the mortars refer their sights to the HIT deflection of the
basepiece and realign the aiming posts.
9-7
FM 23-91
a. Precautions. The target location given in the call for fire is not the location of the
FPF. The FO must add a 200-meter to 400-meter safety factor to the location of the FPF.
The FDC never adds a safety factor. Since the FPF is adjusted to within 200 meters of
friendly forces—
• The adjustment is danger close.
• The creeping method of adjustment is used.
b. Procedures. FPF adjustments can be fired using one of two methods, which are
discussed in order of preference.
(1) Adjustment Mortar by Mortar. In the call for fire, the FO may give a section left
(SL) or section right (SR) to determine the danger-close mortar. The danger mortar is the one
impacting closest to friendly forces. The operator uses the FPF switch to enter, compute,
adjust, review, and delete data for FPFs. Three FPFs may be stored and identified as line 1,
2, or 3. The stored data include the line number and fire commands for each weapon assigned
(up to six) for that FPF line. An FPF line is located by a set of grid coordinates, marking the
left or right limit. Then the altitude, width, and attitude are entered. When the corrections
for each adjusting weapon have been entered and recomputed, they are stored. Further
corrections are not applied after advancing to the next weapon. The corrections made to each
mortar are automatically applied to the next weapon to be adjusted.
9-8
FM 23-91
NOTES: 1. The FO will tell the FDC the left limit grid, or he will tell him the right limit
grid (for example, L140 versus FPF grid).
2. All adjusting rounds should be set for fuze delay to further reduce the danger
to friendly forces. After entering the FPF line, a safety fan may be entered.
(a) Press the FPF switch and select INIT. Enter the line number (1, 2, or 3) and
the section/weapon number. The display shows LINE: 1 WPN:A1.
(b) Press the SEQ switch. (Shell/fuze combination [default entry by MBC is HE
PD] is normally not changed.)
(c) Press the SEQ switch and select the GT or enter the FO direction to target.
(d) Press the SEQ switch and enter the FPF right or left limit.
NOTE: If the right limit grid is entered for the FPF, adjust the right flank mortar first. If
the left limit grid is entered for the FPF, adjust the left flank mortar first.
(e) Press the SEQ switch and enter the FPF altitude (if known).
(f) Press the SEQ switch and enter the left or right limit and FPF line width in
meters. The display shows L R WID: L 350. The coordinate point becomes the left or right
limit.
NOTE: The direction of the FPF should be left if the right flank mortar (No. 1) is
adjusting, and right if the left flank mortar (No. 3 or No. 4) is adjusting.
(g) Press the SEQ switch and enter the attitude of the FPF. This is a
MANDATORY ENTRY.
(h) Press the SEQ switch and follow the instruction by the MBC. Press the
COMPUTE switch to receive firing data.
(i) Sequence through the firing data (record needed data) until the ADJ * is
displayed.
NOTE: If the ADJ* selection is passed, the MBC displays READY. To continue
adjusting the FPF, press the FPF mission switch and select ADJ. Proceed to
paragraph (k).
(j) Select the blue display key beneath the asterisk (*).
(k) Enter the weapon number to adjust. If another weapon is to be adjusted, select
NXT.
NOTE: The MBC considers the previous weapon adjusted, and it saves the firing
commands in the FPF data file. When the last weapon is adjusted, select NXT in
this display to end the mission. The MBC displays FPF ADJUSTED.
(l) Press the SEQ switch. The MBC displays the direction to the target.
(m) Press the SEQ switch and enter the deviation correction (if any) from the
FO.
9-9
FM 23-91
(n) Press the SEQ switch and enter the range correction, if any.
(o) Press the SEQ switch. (The operator may change the height corrections from
the default given in meters to feet.)
(p) Press the SEQ switch and enter the vertical correction (if any) from the FO.
(q) Press the SEQ switch. The MBC displays PRESS COMPUTE. Press the
COMPUTE switch to receive the firing data.
(r) Repeat procedures in paragraph (i) through (q) until each weapon in the
section has been adjusted. Repeat procedures in paragraphs (i) through (k) to end the
mission.
(2) Adjustment of Danger-Close Mortar Only. In the call for fire, the FDC is given
the attitude of the target area. From this attitude, the FDC can determine the danger-close
mortar.
(a) The operator uses the FPF menu to fire and adjust as with the mortar-by-
mortar method.
(b) Once the danger-close mortar is adjusted, the other mortars involved in the
FPF will have firing data already computed.
(c) The difference between this method and the mortar-by-mortar adjustment is
that each mortar will not actually fire on the FPF. Rather, the firing data for the nonfiring
mortars are calculated based on the firing data for the danger-close mortar and the attitude
of the target area.
c. Data Review. The FPF data for the section may be reviewed at any time using the
FPF menu switch.
(1) Press the FPF switch and select DATA.
(2) Press the SEQ switch and enter the line number of the FPF to be displayed.
(3) Sequence through the display to review each mortar's data.
d. Safety Data. After an FPF has been initiated, the operator may review the safety data
may be viewed at any time.
(1) Press the FPF mission switch. The sequence indicator should blink, indicating
that another choice is available (for multiple entry).
(2) Press the SEQ switch; the fifth choice, SFTY, is displayed. Select the blue
displaykey beneath the flashing cursor to select safety (SFTY).
(3) Press the SEQ switch, and enter the line number of the FPF safety data to be
viewed.
(4) Press the SEQ switch. The display prompts the operator to press the SEQ switch
to view the burst-point grid coordinate.
(5) Press the SEQ switch. The easting and northing are displayed.
(6) Press the SEQ switch. The maximum ordinate of the last round to its burst-point
is displayed.
(7) Press the SEQ switch. The time of flight is displayed.
(8) Press the SEQ switch. READY is displayed.
9-10
FM 23-91
NOTE: The TFC menu may be deleted from this procedure if the mortars to fire are in
a straight line with the rest of the section and if they are all the same distance
apart (a perfect parallel position).
WARNING
Using the default firing data for all guns in the firing section
may cause rounds to be fired outside of the safety fan or firing
zone. Therefore, always use the TFC menu when a safety fan or
firing zone is used. This gives the MBC operator a "WARNING"
message indicating if any of the rounds for any particular
weapon will land outside the safety fan or firing zone. For
revision III/A, the operator must override the message in order
to continue.
(3) If any adjustments are needed, the entire section conducts them, firing the same
number of rounds each time as in the previous command.
9-11
FM 23-91
9-12
FM 23-91
(d) The wind speed in knots determines which column to use. The box where the
proper row and column intersect contains the number of RPM needed to maintain a screen
500 meters wide for one minute with a flank wind.
EXAMPLE
For conditions of 60 percent humidity, a neutral temperature gradient, and a 4-knot
wind, it would take 6 RPM to maintain a 500-meter screen with a flank wind. If the
screen is to be only 400 meters wide, use the following procedure:
400 divided by 500 (or 400/500) = 4/5 = 0.8
0.8 x 10 (number of rounds x 2-minute duration for establishment phase) = 8.0
The result (8.0 in this case) is always rounded up (no less than 10 rounds will be fired in the
establishment phase). Each mortar fires (4.2-inch mortar platoon) 2 rounds each, (4.2-inch
mortar section) 4 rounds each, (81-mm mortar platoon) 3 rounds each, and (81-mm mortar
section) 6 rounds each.
LAPSE 13 13 11 11 13
30 NEUTRAL 9 9 7 7 9 9 11
INVERSION 6 6 4
LAPSE 9 9 7 9 9
60 NEUTRAL 6 6 4 4 6 7 9
INVERSION 3 3 3
LAPSE 7 7 6 6 7
90 NEUTRAL 4 4 3 3 4 6 6
INVERSION 3 3 3
— FOR QUARTERING WINDS—MULTIPLE TABLE VALUES BY 2.
— FOR TAILWINDS—MULTIPLY TABLE VALUES BY 2.
— FOR HEADWINDS—MULTIPLY TABLE VALUES BY 2.5.
— FOR SHELL IMPACT ON LAND—-USE TABLE QUANTITIES SHOWN.
— FOR WATER IMPACTS—MULTIPLY TABLE VALUES BY 1.4.
— FOR CURTAINS GREATER OR LESS THAN 500 METERS IN WIDTH—SCALE THE TABLE
VALUES UP OR DOWN PROPORTIONALLY.
— FOR ESTABLISHING A SMOKE CURTAIN—EMPLOY VOLLEY FIRE USING TWICE THE TABLE
TABLE VALUE (BUT NOT LESS THAN 10 ROUNDS).
A. SMOKE CURTAIN. NUMBER OF WP ROUNDS PER MINUTE TO MAINTAIN A SMOKE
CURTAIN ON A 500-METER FRONT IN FLANK WINDS (AS SHOWN IN THE CHART ABOVE).
B. OBSCURING SMOKE EFFECT. THE NUMBER OF ROUNDS PER MINUTE REQUIRED TO
MAINTAIN AN OBSCURING SMOKE EFFECT ON A 100-METER FRONT (OBTAINED BY
DOUBLING THE VALUES SHOWN IN THE CHART ABOVE).
Figure 9-2. Smoke chart.
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(e) The total number of smoke rounds needed for the mission is computed as
follows:
Adjustment phase = 1 round (all missions)
Establishment phase = 2 x number of rounds to maintain for 1 minute; at
least 10
Maintenance phase = Number of rounds to maintain for 1 minute time the
number of minutes
Total = (a) + (b) + (c)
NOTE: The time used during the establishment phase is not to be considered as any part
of the maintenance phase time of the mission.
(2) Mortars required. Under favorable conditions the 4.2-inch mortar platoon can
screen an area about 800 meters wide and the 81-mm mortar platoon about 500 meters. The
60-mm mortar section does not fire a screening mission. A limitation, however, is their
maximum and sustained rates of fire. For the entire platoon, these rates of fire are multiplied
by the number of mortars firing. If the required number of RPM exceeds the rate of fire, the
platoon must request supporting fire from flank units or artillery.
(3) Effects desired. If smoke is to be placed directly on the target for blinding or
casualty-producing effects, the FO adjusts the center of impact of the rounds onto the center
of the target. The number of RPM to produce this effect is twice that for a normal quick-
smoke mission.
(4) Ordering of ammunition. When ordering ammunition for a mission, the FDC
estimates what weather could exist, remembering that it is better to have too much
ammunition than too little.
(5) Briefing of the observer. Due to the many clearances required to fire the mission,
the FDC chief/section leader normally has ample time to brief the FO on the quick-smoke
screen. This briefing should include a map reconnaissance of the area to be screened so that
the FO will be able to identify it on the ground and to select an OP from which the screen can
be observed.
(6) Call for fire. At the appointed time, usually 10 to 20 minutes before the mission
is to be fired, the FO sends the call for fire. This allows the FDC to process the data in
advance and to prepare the needed ammunition. The FO should have checked the wind so
that the call for fire will specify the wind direction.
(7) Exact ammunition requirement. About the time the call for fire is received, the
chief computer/section leader makes a final check on the weather and directs the computation
of the exact ammunition requirements for the mission. The section/platoon has at least this
amount of ammunition broken down and ready to fire.
(8) Mission computation. The chief computer/section leader issues the FDC order
(Figure 9-3). The method of FFE is the number of rounds computed to establish the screen,
divided by the number of mortars to FFE. The time of opening fire is at the chief
computer/section leader's command. Once the first round of smoke is fired, all commands
should be such that they can be applied with a minimum of reaction time.
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(a) The MBC operator, upon receipt of the FDC order, processes the fire
commands as he would a normal grid mission until the final correction.
(b) HE is adjusted to within 100 meters of the adjusting point. The FO splits the
100-meter bracket and calls for one round of WP (in adjustment) to see if the WP will strike
the adjusting point.
• The MBC operator uses the WPN/AMMO switch/menu to change the
SHELL/FUZE combination.
• After the shell and fuze correction, the MBC operator computes the final
adjustment and relays this information to the adjusting mortar.
(c) The FO makes corrections for the WP. When the FO requests FFE, the FDC
tells the mortars how many rounds to fire (employing volley fire). The maintenance phase
begins almost immediately after the establishment phase. If the FO notices the screen
thinning in one place (usually the upwind end), the rate of fire may be doubled for one or
more mortars.
(9) Four phases to screening mission. When a standard sheaf will not cover the area,
a screening mission is conducted in four phases.
(a) PHASE 1. Using HE ammunition, the FO adjusts the upwind flank mortar
to the upwind edge of the area to be screened.
(b) PHASE 2. At the end of the adjustment phase, one round of smoke is fired
to see if it hits the adjustment point.
(c) PHASE 3. The FO calls for the sheaf to be opened (not to be confused with
a normal open sheaf).
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(d) PHASE 4. The FDC presses the TFC switch and changes or selects the
following information:
• Changes SHEAF:PRL to SPECIAL
• Selects ADJ PT:FLANK
• Enters the direction and size of the screen based on the adjusting
(upwind) mortar. If number 1 is adjusting, selects L (left) and size of the
area to be screened. If the number 3 (or 4) mortar is adjusting, selects R
(right) and enters the size of the area to be screened.
• Enters the attitude of the target area.
• Changes CON:AF to CON:FFE
• Pushes compute and observes the firing data.
(10) End of mission. Control in ending the screening mission rests with the
commander who ordered it established. Normally, screens are fired according to a time
schedule; however, the commander may order the screen to be maintained beyond the
scheduled termination time. In the absence of external control, the FDC controls the timing,
ordering the section/platoon to cease fire. The squad leaders give the FDC a count of rounds
expended (or remaining) at the end of the mission.
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b. REVIEW Switch. This switch returns the display to the first line of a message or to
the beginning of the last main menu selected.
c. SURV (survey) Switch. This switch is used to solve one of three survey problems:
• Resection (RES).
• Intersection (INT).
• Traverse (TRV).
(1) These functions are used to determine the coordinates and altitude of an unknown
point using measurements from known point(s).
(2) These known points must be entered in the MBC under the KNPT/TGT menu
before using any of the SURV functions.
(3) Computed coordinates may be stored as a basepiece, FO, known point, or target.
d. MSN (mission) Switch. This switch is used to review current active fire mission data
and to specify which mission is operational. The MBC can store data for three active fire
missions and compute fire commands for each of these missions one at a time.
(1) A mission and target number are computer-assigned to a mission each time the
GRID, SHIFT, or POLAR switch is pressed. Use these switches only when starting a fire
mission to avoid misuse of the target numbers from the target numbering block,
(2) Access fire mission data (active missions only) through the MSN switch.
NOTE: Only an operational mission allows entry or change of data for that mission. A
mission must be active before input can be applied from the WPN AMMO, REG,
TFC, SFTY DATA, EOM, and REPLOT switches.
e. XMIT (transmit) Switch. This switch, in either manual or digital mode, is used to
display or send message to observer and command messages. Some of the information in this
menu is as follows:
(1) NR VOL. The number of volleys for the FFE.
(2) NR UNITS. The number of units to be used in the FFE.
(3) PR ERR:NOTGVN. The probable error entered by the computer (MBC); this
example reads NOT GIVEN.
(4) ADJ SF. Adjusting shell/fuze entered by the computer.
(5) 1ST SF:NOPR. Shell/fuze for the first round for FFE entered by the computer.
NOPR means no preference
(6) SUBS SF. Shell/fuze combination for subsequent rounds for FFE entered by the
computer.
(7) MOE. Method of engagement. Use the default value.
(8) CON: WR AF. Method of control (WR = when ready, and AF = adjust fire).
Use default shown.
(9) TOF. Time of flight for the next (or last) round.
(10) ANG T. Angle T entered by the computer.
f. SFTY (safety) DATA Switch. This switch is used to review safety factors in effect
for a current fire mission. Some of the data and information found in the safety menu are as
follows
(1) RN: AZ. Range and azimuth from the guns to the target (GT).
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(2) BURST POINT SEQF. The coordinate of impact for the round fired can be
found by sequencing forward (SEQF).
(3) BP. Burst point easting and northing grid coordinates.
(4) MAX ORD. The maximum ordinate (top of the trajectory) of the round fired,
measured in meters from sea level.
(5) SAFETY DIAGRAM. Entries can be made to store up to three safety fans (one
for each section/platoon in WPN DATA menu) identified as A, B, or C.
(a) LLAZ: Left limit azimuth in mils.
(b) RLAZ: Right limit azimuth in mils.
(c) MAX RN: Maximum range in meters.
(d) MIN RN: Minimum range in meters.
(e) MIN:_ MAX:_ Minimum and maximum charges (except 4.2-inch mortar).
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CHAPTER 10
DIGITAL MESSAGE DEVICE SUPPORTED
The MBC transmits and receives digital communications by use of the
digital message device, which is a technological advancement for the FDC.
This ability reduces the mission processing time and provides a more secure
communication network.
10-1. APPLICATION
All DMD-supported missions occur in response to the receipt of an FO message. The input
data for the mission are supplied by digital transmission from the FO’s DMD and are
automatically entered into the MBC memory.
a. To make a digital communications check, the operator performs the following:
(1) Presses the SELF-TEST switch. The MBC displays: MICR SW DSP MOD.
The sequence indicator blinks, indicating another choice is available.
(2) Presses the SEQ switch. The MBC displays: XMIT TEST MSG. Selects
XMIT. The MBC displays: ROUTE: *XMIT. (Route is found in the SOI.)
(3) Enters the route. Selects XMIT. The MBC displays: XMITING.
b. The MBC transmits the test message to the DMD. When the DMD accepts the
message, the DMD transmits an acknowledgement (ACK). If the message is not accepted,
the MBC displays: NO RESP RETRY 1. The operator should try to retransmit the message
at least three times. If the message is still not accepted, the communication system should
be repaired.
10-2. COMMUNICATIONS
The MBC can store a maximum of three incoming digital messages. Incoming messages are
of two types: fire mission messages and information only messages. When the message
indicator is lit or the audio alarm sounds and the MSG switch is pressed, the MBC displays
the first line of the first message received. When a message is a fire mission, the MBC
automatically assigns a mission and target number, unless three active missions have already
been stored. In this case, the MBC displays: NO AVAIL MSN and discards the message.
a. Receiving Messages. The flashing MSG indicator tells the operator that a message
has been received. To view the message, he presses the—
(1) MSG switch. The MBC displays a heading to identify the type of message. If the
type of message is not a fire request, such as FO LOC, the applicable data are automatically
stored in the correct menu.
(2) SEQ switch. The MBC displays the FO and net identification.
(3) SEQ switch. The FO authentication code is displayed. The operator validates the
code in the authentication table.
(4) SEQ switch. The operator reviews each line of the message.
NOTE: After the FDC order has been completed, the operator clears the message from
the message buffer. If the message is a fire request, the mission is automatically
activated. The operator must assign the mission using the WPN/AMMO switch
and compute the firing data.
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NOTE: The operator selects MANUAL for the MBC to provide the operator with an
audio warning when to orally transmit the splash. If manual is selected, the MBC
displays: *SHOT. He presses the asterisk (*) when the round is fired. The
MBC provides the operator with an audio warning when to transmit the splash.
The MBC displays: READY when any key is pressed.
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(4) SEQ switch. The FO identification is displayed. The operator must enter the
route number.
(5) SEQ switch. The operator enters the authentication (COMSEC) code from the
SOI to transmit SHOT.
(6) SEQ switch. The operator enters the authentication (COMSEC) code from the
SOI to transmit SPLASH.
(7) SEQ switch. The MBC displays: *XMIT. When the command to fire is given,
the operator presses the asterisk (*), and the shot is automatically transmitted to the FO.
XMITTING is displayed until it is time to send the splash. The splash is momentarily
displayed, and then XMITTING. ACK is received when the DMD accepts the message.
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PART FOUR
M16 AND M19 PLOTTING BOARDS
CHAPTER 11
INTRODUCTION
The M16 and M19 plotting boards are secondary means of fire control
for all mortars. The computer can determine deflection, azimuth, and range.
When plotting on the plotting board, he should only use a soft lead pencil.
Computers NEVER use map pins, needles, ink pens, or grease pencils since
these can damage the board.
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c. Range Arm. The range arm, made of plastic, is used when the mortars are plotted
at the pivot point. The arm has a vertical centerline with a range scale and a vernier scale,
both of which are the same as on the base.
d. Range Scale Arm. The range scale arm, a transparent plastic device, has a knob with
pivot pin, two range scales (one on each edge), a protractor on the right bottom, and a vernier
scale across the top. The range scales are numbered every 100 meters and graduated every
50 meters. The protractor is graduated every 100 mils from 0 to 1600 mils.
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a. Base. The base is a white plastic sheet bonded to a magnesium alloy backing. A grid
is printed on the base in green at a scale of 1:25,000. The vertical centerline is graduated and
numbered up and down from the center (pivot point) (from 0 through 32) in hundreds of
meters with a maximum range of 3,200 meters. Each small grid square is 100 meters by 100
meters.
(1) The index mark points to the center of the vernier scale at the top edge of the
plotting board. It is the point at which deflections or azimuths may be read to the nearest
10 mils. When plotting at the pivot point, the pivot point represents the location of the No.
2 mortar.
(2) In addition to the grid pattern, a vernier scale is printed on the base. It is used to
obtain greater accuracy when reading the mil scale on the azimuth disk. The vernier scale
permits the operator to read azimuths and deflections accurately to the nearest mil.
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(3) On the bottom of the base, a double map scale in meters with representative
fractions of 1:50,000 and 1:12,500 is used to transfer to and from a map that has one of those
scales.
b. Azimuth Disk. The rotating azimuth disk is made of plastic. It is roughened on the
upper surface for marking and writing. A mil scale on the outer edge is used for plotting
azimuths and angles. It reads clockwise to conform to the azimuth scale of a compass. The
scale is divided into 10-mil increments (from 0 to 6400) and is numbered every 100 mils.
Also, the disk has two black lines called centerlines. These centerlines are printed across
the center of the disk from 0 to 3200 and from 1600 to 4800 mils.
c. Range Scale Arm. The range scale arm is used when the mortars are plotted at the
pivot point. It is made of plastic and can be plugged into the pivot point. Two range scales
are on the range scale arm. On the right edge is a range scale that corresponds to the range
scale found on the vertical centerline. An alternative range scale ranging from 0 to
6,000 meters is on the left edge of the range scale arm and is used when plotting away from
the pivot point. The vernier scale at the upper end of the range scale arm is used to read
azimuths or deflection when plotting at the pivot point without rotating the disk back to the
vertical centerline. The direction of the FO can be kept indexed at the index point. The
vernier scale on the range scale arm is read in reverse of the one on the grid base. The left
portion is read for azimuth, and the right portion is read for deflection. The protractor lines
below the range scale arm knob may be used to place a sector of fire on the disk.
(1) To read azimuth to 1 mil, read the left portion, starting at 0, and read to the 10 in
the center.
(2) To read deflections, start at the right edge of the range scale arm and read to 10.
11-3. CAPABILITIES
The straightedge of the plotting board should always be on the user's right. Each plot is
circled and numbered for identification. To avoid distortion, the computer should place his
eye directly over the location of a plot and hold the pencil perpendicular to the board. The
plot should be so small that it is difficult to see. The computer must be careful when placing
a plot on the disk since a small plotting error could cause the final data to be off by as much
as 25 meters in range and more than 10 mils in deflection.
a. To determine azimuths, read the first three numbers from the azimuth disk, left of the
index mark. Read the fourth number, or the last mil, by using the azimuth disk and the right
side of the vernier scale (Figure 11-3).
b. For example, consider the azimuth 3033 in Figure 11-3. The first and second
numbers are the first 100-mil indicator to the left of the index mark (30). To obtain the third
number, count the 10-mil graduations between the 100-mil indicator and the index mark (3).
The fourth number, or last mil, is read by counting the 1-mil graduations from 0 to the right
on the vernier scale until one of the 1-mil graduations align with one of the 10-mil
graduations on the azimuth disk (3).
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CHAPTER 12
PREPARATION OF FIRE CONTROL EQUIPMENT
Three types of firing charts can be constructed on the M16/M19 plotting
board: the observed firing chart, the modified-observed firing chart, and the
surveyed firing chart. This chapter discusses methods of constructing all
three charts.
EXAMPLES
(DOF 3150 is already at the nearest 50 mils; there is no need to round off.)
(3) Referred deflection. The aiming circle operator gives the referred deflection to
the FDC after the section is laid. The referred deflection can be any 100-mil deflection from
0 to 6300, as long as all of the mortars can place out their aiming posts on the same
deflection. Normally, the referred deflection used is 2800 to the front or 0700 to the rear.
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(5) Determination of firing data. After plotting the first round on the DOF at the
determined range and superimposing the deflection scale, the computer rotates the azimuth
disk until the first round is over the vertical centerline. He determines the deflection to fire
the first round by using the deflection scale and the left portion of the vernier scale
(Figure 12-3).
(a) Read the first two digits from the deflection scale. Since deflections increase
to the left, read the first number (100-mil indicator) to the right of the index mark. In this
example, it is 27.
(b) Read the third digit from the 10-mil graduations between deflection scale
numbers 27 and 28 (100-mil indicators). Count the 10-mil graduations on the azimuth disk
(from 27 to the index mark) to find that the index mark is between the eighth and ninth 10-
mil graduations, making the third digit 8.
(c) Read the fourth digit at the vernier scale. For deflections, use the left half of
the vernier scale. Count the 1-mil graduations, starting at the 0, to the left until one of the
1-mil graduations of the vernier scale and one of the 10-mil graduations on the azimuth disk
are aligned. In this example, the fourth 1-mil graduation is aligned, making the fourth
digit 4.
(d) Determine the range by rotating the plot over the vertical centerline and
reading the range to the nearest 25 meters. Enter the firing table (such as FT 81-AI-3) and
determine the charge as follows: Open the FT at TAB "PART ONE" and turn back one page
(page XXXIX). This page is the charge-vs-range chart (Figure 12-4). It can be used to
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determine the lowest charge to engage the target. To use the chart, find the range to the
target using the range bar at the bottom of the chart. The range bar is numbered every 500
meters from 0 to 5,000 meters. Since the range to the target was determined to be 2,600
meters, estimate the 2,600 meters on the range bar. After determining the 2,600-meter point
on the range bar, place a straightedge at the point so that it crosses the charge lines
(Figure 12-5). The first charge line the straightedge crosses is the lowest charge possible to
engage the target.
(e) Another method that can be used is to turn to page II in the FT. There is a
listing of charges for M374A2 (HE) and M375A2 (WP) from charge 0 through charge 9.
Below that listing is the charge listing for M301A3 illumination (illum) from charge 0
through charge 8. Write in after each charge the minimum and the maximum ranges that
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each charge zone covers (Figure 12-6). By looking at the maximum range, the correct charge
to use can easily be determined.
(6) Plotting of observer corrections. To plot the FO's corrections, the computer first
indexes the FO's direction to the target. That OT direction is given in the call-for-fire or with
the first correction. Going from the last round, he applies the FO's corrections.
(a) For example, assume that the OT direction is 3050, and the FO sends these
corrections: RIGHT 50, DROP 200. Ensuring that OT direction is indexed, make these
corrections from the first plot (Figure 12-7).
(b) To do this, move to the right one small square (50 meters), then straight down
the board four small squares (200 meters). Then, make a small plot, circle it, and label it "No.
2." To determine the firing data, rotate the disk until the No. 2 plot is over the vertical
centerline. Then, read the deflection and range (Figure 12-8). Using the FT, determine the
charge and elevation to fire the round, and compute the subsequent fire command.
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(c) Once the end of mission (EOM) has been given, update the M16/M19 plotting
board (Figure 12-9). To do this, erase all the plots except the final plot. Then enclose that
plot with a hollow cross and number it with the target number (Figure 12-10).
(7) Engagement of other targets. To fire other targets on this chart, the computer
must perform the following actions:
(a) Grid. Go back to the map, plot the target location, and determine the range
and direction.
(b) Shift. Index the FO's direction to the target and apply the correction from the
known point, which must be plotted on the chart.
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b. Below Pivot-Point Method. The observed firing chart (with mortars plotted below
pivot point) is used when the ranges to the targets being engaged are over 3,200 meters.
(When the initial range to the target is 2,900 meters or more, mortars are always plotted
below the pivot point.)
(1) Two items are needed to set up the board for operation: a gun-target azimuth and
a range from the mortar position to the target. To construct the chart—
(a) Index the gun-target azimuth.
(b) Drop below the pivot point 1,000 meters for 60-mm mortars, 2,000 meters for
the 81-mm mortars, and 3,000 meters for the 4.2-inch and 120-mm mortars.
NOTE: When firing 800-series ammunition with the 81-mm mortar, drop 3,000 meters
below the pivot point to accommodate the extended range.
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(c) Plot the mortar position 500 meters left or right of the vertical centerline
(Figure 12-11).
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(2) Once these actions have been taken, ensure that the azimuth disk is still indexed
on the gun target azimuth. Then, from the mortar position, plot the first round at the range
determined using the parallel-line method of plotting (Figure 12-12). Determine the
mounting azimuth and referred deflection the same way as with the pivot-point method.
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(3) To determine the firing data to send to the mortar, align the mortar position below
the target being engaged using the parallel-line method of plotting. Then read the deflection
using the azimuth disk and vernier scale and measure the range between the mortars and
target. To align the mortar position and target, since the mortar position is being plotted
away from the pivot point, use the parallel-line method of plotting. With the mortar position
and target plotted, rotate the disk until the mortar position and the target are an equal distance
from, or on, the same vertical line (Figure 12-13).
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(4) Now determine ranges differently than with the mortar position plotted at the
pivot point. Count each of the 50-meter squares from the mortar position to the target or
place the edge of the computer's record alongside the two plots on the plotting board (mortar
and target). Then make a tick mark on the edge of the computer's record at each plot. Using
the alternate range scale to the left of the pivot point, lay the computer's record along this
scale with the mortar tick mark at 0 and read the range (Figure 12-14).
(5) To update the board after the EOM is given or to engage other targets, use the
same method as with the pivot-point method.
NOTE: When operating the M16 plotting board as an observed firing chart (pivot-point
or below-pivot-point methods), no correction factors are applied to the data.
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c. Mortars Plotted at Pivot Point. With the pivot pin inserted in the pivot point of the
plotting board, the computer can use the range scale arm the same as with the range arm to
determine deflections and range to both the initial and subsequent rounds.
(1) Determine the range and direction to the center of the sector from a map or by
visual observation. Round off the azimuth to the initial round or direction of fire (DOF) to
the nearest 50 mils to determine a mounting azimuth, and superimpose a deflection scale on
the azimuth disk.
(2) Make the initial plot by indexing the DOF (or initial azimuth) to the initial round
at the index mark. This may be different from the mounting azimuth because of the round-
off rule. Use the scale on the vertical centerline to make the initial plot at the correct range.
(3) When the FO calls in a target direction (the OT azimuth), index the azimuth disk
on the M16/M19 plotting board at the OT azimuth. It remains indexed on that azimuth until
the mission is completed. Plot corrections from the FO IAW procedures. Once a correction
has been plotted, rotate the range arm until the right edge of the range arm is over the new
plot. Determine the range to the nearest 25 meters, and read the deflection to the nearest mil
using the vernier scale.
(4) Plot additional corrections, and use the range scale arm to determine range and
deflection. Once the azimuth disk is indexed on the OT azimuth, the disk does not have to
be rotated to determine ranges or deflections.
d. Mortars Plotted Below Pivot Point. With the pivot pin inserted in the pivot point,
the computer can use the left edge of the range scale arm to plot the initial round. The
mounting azimuth and azimuth to the initial round are determined as for mortars plotted at
pivot point. The computer indexes the azimuth disk on the DOF and aligns the right edge
of the range scale arm on the vertical centerline. Next, he makes a small plot at the zero
range on the left edge of the range scale arm. Then, still using the left edge, he makes a small
plot at the range for the initial round. The mortar position plot must be marked with a hollow
cross to further identify its position. Once the initial round is fired, the range scale arm is
removed, and the left edge is used as a range scale.
e. Care and Cleaning of Plotting Boards. Plotting boards must be handled with care
to prevent bending, scratching, or chipping. They must be kept away from excessive heat
or prolonged exposure to the sun, which may cause them to warp. When storing a board, it
is placed in its carrying case, base down, on a horizontal surface. It is not placed on edge or
have other equipment stored on it. The plotting board can normally be cleaned with a
nongritty (art gum) eraser. If the board is excessively dirty, a damp cloth is used. The
contact surface of the disk and base are cleaned often. The disk is removed by pushing a
blunt instrument through the pivot point hole from the back of the base.
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NOTE: When using the M16 plotting board with the 4.2-inch mortar, the section sergeant
selects the grid intersection to be 2,000 to 2,500 meters forward of the mortar
position. With the 120-mm mortar, the grid intersection should be 3,000 to 4,000
meters forward of the mortar position.
(2) The grid intersection should be outside the area of responsibility. This ensures
that the pivot point does not interfere with plotting targets or corrections. The grid
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intersection is also as close as possible to the area of responsibility. This ensures that as
much of the area of responsibility as possible will be on the plotting board.
b. Superimposition of Grid System on Plotting Board. Once the grid intersection has
been determined, the computer indexes "0" on the azimuth disk. He then drops down 2,000
meters below the pivot point and writes in the east/west indicator on the vertical centerline
at the 2,000-meter mark. Next, he goes 2,000 meters to the left of the pivot point on the
heavy center horizontal line and writes the north/south indicator. To complete the grid
system, the computer writes in the other north/south, east/west grid numbers as though
looking at a map. By numbering every other heavy dark line (two large squares) on the
plotting board, he retains a scale of 1:12,500 on the board (Figure 12-16).
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c. Plotting of Mortar Position. Now that a grid system is on the board, the computer
can plot any grid coordinates. To do this, he must—
(1) Ensure that the azimuth disk is indexed at 0.
(2) Read like a map: RIGHT and UP.
(3) Remember that the scale is 1:12,500 (each small square is 50 meters by 50
meters) (Figure 12-17).
To superimpose the deflection scale, the computer writes the referred deflection on the board
the same way as with the observed chart. Firing data are determined by using the parallel-
line method of plotting.
d. Field-Expedient Method for Construction. If the grid coordinates of the mortar
position are known but a map is not available for determining the grid intersection to
represent the pivot point, the computer can construct the modified-observed firing chart by
using the following procedures:
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(5) Determine the 1,000-meter grid that contains the mortars (Figure 12-19). The
first, second, fifth, and sixth numbers of the mortar grid give the 1,000-meter grid square.
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EXAMPLE
Assume that the mortar section is at grid 939756 (six digits: observed chart)
and two targets have been fired on (Figure 12-20). The platoon leader
determines that the eight-digit grid to the mortar position is 93937563
(modified-observed chart) and designates the grid intersection to represent the
pivot point. The computer constructs the chart and transfers the targets from
the observed chart (Figure 12-21).
NOTE: No firing corrections are used with the observed chart. Once transferred to the
modified-observed chart, the altitude of the target is assumed to be the same as
that of the mortar position.
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a. Target Plotting. After transfer, through coordination with the FO, an RP or target
may be identified to valid eight-digit coordinates. The plotting board is then reconstructed
as a surveyed chart. When the situation permits, a registration mission should be conducted
on the point for which the valid eight-digit coordinates were determined. Then firing
corrections are computed.
(1) When transferring targets from one type of chart to another, remember that the
target plots on the observed chart are plotted at the data it takes to hit the target. This is not
always the locations of the targets.
(2) The same holds true for the modified-observed chart, except that with some
targets, altitude correction (VI) may have been used. When replotting the target at the end
of the mission, strip this altitude correction from the command range and plot the target using
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this range. Using this procedure gives a more accurate picture of the exact location of the
target than the observed chart; however, it is not always the actual location of the target.
b. Plotting of Previously Fired Targets. At the completion of the surveyed
registration mission and the computation of the firing corrections, previously fired targets
plotted on the plotting board must be forward plotted. Since the surveyed chart is the most
accurate chart to use, all information on it should be the most accurate possible.
EXAMPLE
When targets AL0010 and AL0011 (Table 12-1) were fired before the
surveyed registration, the data and the plots included all firing corrections,
even though they may have been unknown at the time of firing. To forward
plot these targets, the computer strips the firing correction from the range and
deflection to plot them at their actual location.
CHART DATA
COMMAND DATA FIRING CORRECTIONS COMPUTATIONS FOR REPLOT
TARGET AL0010
DEFLECTION 2786 DEFLECTION R12 2786 + L12 = 2798
RANGE 1825 RCF - 18 +18 X 1.8 = +32 DEFLECTION 2798
TARGET AL0011
DEFLECTION 3115 DEFLECTION R12 3115 + L12 = 3127
RANGE 2850 RCF - 18 +18 X 2.9 = +52 DEFLECTION 3127
NOTE: If the target has been mechanically surveyed, enter the DCT at the initial range
plot. If the target is nonsurveyed (even if it is an eight-digit grid), enter the DCT
at the final range plot.
a. To use the DCT, first round off the range at which the section is firing to the nearest
100 meters. This is required because the ranges on the table are divided into 100-meter
increments. Next, go down the range column to find the range. The deflection is in meters
across the top of the card.
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b. Using the number of meters the FO requested to move the strike of the round, find
that number of meters and go straight down that column until it intersects with the range.
That number is the number of mils that would have to be applied to the mortar sight to move
the strike of the round the required meters. If the range is greater than 4,000 meters, divide
the range and mil correction by two.
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EXAMPLE
The mortar section has completed a registration mission and is prepared to
adjust the sheaf. The final adjusted range for the RP is 2,750 meters. The
No. 1 and No. 3 mortars fire one round each. The FO sends the following
corrections: NUMBER 3, R30; NUMBER 1, L20; END OF MISSION,
SHEAF ADJUSTED. Any corrections of 50 meters or more must be refired.
For this example, the last deflection fired from No. 1 and No. 3 was 2931 mils. Using the
DCT, round off the range to the nearest 100 meters (2,800). Find 2,800 meters in the range
column and, using the FO's corrections, find 30 and 20 in the deflection-in-meters column.
Go across and down those columns to where they intersect. The table shows that the
requirements are 11 mils for 30 meters and 7 mils for 20 meters.
Using this information, use the previous deflection fired, which was 2931 mils. Since the
FO's correction for the No. 3 mortar was R30, which equals R11 mils (using the LARS rule),
subtract 11 mils from 2931 mils. This gives a new deflection of 2920 mils. The correction
for No. 1 mortar was L20, which equals L7 mils. Using the LARS rule for deflection, add
7 mils to 2931, which gives a new deflection of 2938 mils.
If there is no deflection conversion table available, use the mil-relation formula (W/R x M)
to convert the corrections from meters to mils. To use the formula for the same FO's
corrections of R30 and L20 used in the example cited, cover the item needed (in this case M
[mils]). The remainder of the formula states: divide W (width in meters) by R (range in
thousandths).
These are exactly the same figures determined by using the DCT.
NOTE: Corrections for VI can be used on the modified and surveyed charts.
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correction applied is sent in the call for fire. For modified and surveyed charts, the same
procedure is used as for the observed chart.
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b. Direction and Distance (Figure 12-24). The FO sends the computer the grid to the
FO position. The computer then plots the grid on the map, determines the direction and
distance from the mortar position to that grid, transfers the direction and distance to the
plotting board, and plots the FO's location.
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c. Range and Azimuth from a Known Point. The FO must send the range from the
known point and the azimuth on which that point is seen. Once that is known, the computer
can index the azimuth, drop below the known point the range given, and plot the FO's
location (Figure 12-25). For modified and surveyed charts, the FO's location can be plotted
if the grid of the FO is known, by indexing "0" and plotting the FO grid. If the grid is not
known, then the computer can use resection, direction and distance, or range and azimuth
from a known point.
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CHAPTER 13
TYPES OF MISSIONS
Certain missions require that special procedures be applied to effectively
engage targets; therefore, these missions should not be fired on the observed
chart. Area targets have width or depth or both, requiring the mortar section
to use either searching or traversing fire, or a combination of these.
GRID INTERSECTION…….………………………..04/64
DIRECTION OF FIRE…………………………2700 MILS
MOUNTING AZIMUTH………………………..2700 MILS
MORTAR POSITION………………….…...…..02006500
MORTAR POSITION ATTITUDE……………1080 MILS
MORTAR ALTITUDE…………………...…400 METERS
REFERRED DEFLECTION……………...…..0700 MILS
a. Upon receiving the call for fire, the section sergeant determines from the size and
description of the target that traversing fire must be used to cover the target. (To effectively
engage a target using traversing fire, the section sergeant ensures the attitude of the target is
within 100 mils of the attitude of the firing section.) The section sergeant then completes the
FDC order (Figure 13-1).
b. The three or four mortars are plotted separately on the M16/M19 plotting board, using
the attitude of the section. During the mission, the computer ensures that the correct plots
are used to determine data required—for example, during the adjustment, the impact point
is aligned with the No. 2 mortar plot. Using the information in the call for fire, the FDC
order, and the observer corrections, the computer computes the data to adjust the No. 2
mortar onto the center mass of the target. After the adjustment is complete (Figure 13-2),
the computer must complete the following procedure:
• Plot the 250-meter length of target on plotting board using the attitude of the
target.
• Divide the target into segments.
• Determine the number of rounds for each segment.
• Determine the mil width of one segment.
• Determine the number of turns required to cover one segment.
• Determine the number of turns between rounds.
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13-2
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c. To plot the target on the plotting board, the computer rotates the azimuth disc until
the target attitude (taken from the call for fire) is indexed. The computer erases all the plots
except the last plot. After ensuring that the attitude is indexed, the computer divides the total
target area into segments. These plots represent the starting points for each mortar. The area
between the plots is the area each mortar must cover with fire (Figure 13-3).
d. The target is now divided into three segments. Once the remaining data for one
segment have been determined, the data will apply to all three mortars. Since each segment
of the target is 75 meters, if the computer determines the mil width of one segment, the other
two will be the same. The computer can use one of two methods to determine the number
of mils for one segment.
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(1) In the first method, the computer knows the deflection that was used to hit the
No. 3 point. By aligning the No. 2 plot and No. 3 mortar, the computer can determine the
deflection to fire to hit the start point for the No. 2 mortar (Figure 13-4). Subtracting these
two numbers determines the mil width of the segment:
(2) The second method uses the DCT to determine the mil width of one segment.
The computer enters the DCT at the final chart range that is rounded off to the nearest
100 meters. He goes across the deflection-in-meters line to the closest meters (75) to cover
the segment. The point at which the range line and the deflection line meet is the number
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FM 23-91
of mils that will cover the segment. Each turn of the traversing handwheel is about 10 mils.
By dividing the mil width of each segment (29) by 10, the computer obtains the total number
of turns to cover the segment (round off to the nearest whole turn):
e. To compute the number of turns to take between rounds, the computer must know
how many rounds will be fired for each segment. This information is given in the FDC order
(3 rounds). To determine the turns between rounds, the computer divides the total turns by
the interval between rounds (there will always be one less interval than the number of
rounds: 3 rounds = 2 intervals).
Turns between rounds are rounded to the nearest half turn. The number of rounds to fire is
based on the rule: four rounds per 100 meters of target width, or one round per 30 meters.
f. At this point, the computer must determine the deflection and range for each mortar
by aligning each mortar with its start point, completing the subsequent command, and issuing
it to the mortar section. If there is a range change of 25 meters or more, the mortar will
receive its own elevation.
g. Upon completion of the adjustment phase of the mission, the section is given the
command PREPARE TO TRAVERSE RIGHT (LEFT). The gunners then traverse the
mortars all the way in the direction opposite to that given, back off two turns, and await
further instructions (Figure 13-5).
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EXAMPLE
Assume that the depth of the target is 350 meters. Multiply the even 100's by
4: 4 x 3 = 12. For the remainder of the target depth (50 meters), one round
covers 30 meters, which would add one more round: 12 + 1 = 13 rounds. At
this point, 20 meters of target is left. To cover the 20 meters, add one more
round: 13 + 1 = 14 rounds to cover 350 meters).
(3) When determining the number of turns needed to cover the target, the computer
can use one of two methods. If the computer is using the unabridged firing table (all escept
for FT 4.2-K-2), the number of turns in elevation required for a 100-meter change in range
is given in column 4 of Table D (basic data).
EXAMPLE
Assume that the target is 350 meters in depth, the range to the target center of mass
is 2,125 meters (always use chart range), and the firing charge is 4. To determine
the turns, determine the range to the center of mass of the target (2,125), enter the
firing table at charge 4, range 2,125, and go across to column 4. Four turns are
needed to cover 100 meters. Multiply 4 by 3.5 (range in hundreds): 4 x 3.5 = 14
turns to cover the target. The mortars are adjusted to center of mass. To obtain the
range to the far edge (search up), add half the target area to the range to the center
of mass.
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FM 23-91
EXAMPLE
The range to the center is 2,125 meters; target area is 100 meters by 350
meters; half of target depth is 350 divided by 2 = 175 meters; and the range
to the far end is 2,300 meters. To search down, start at the near edge and
subtract half the target depth from target center.
(4) Applying the second method, the computer must determine the mil length of the
target by using the firing tables. He uses the elevation for the far end of the target (adjusting
point) and the elevation to hit the near end of the target:
(6) To compute the number of turns to take between rounds, the computer must know
how many rounds each mortar will fire. The computer computes this information or finds
it in the FDC order (14 rounds). To determine the turns between rounds, he divides the total
turns by the intervals between rounds (there will always be one less interval than the number
of rounds: 14 rounds = 13 intervals).
1.15 = 1 turn between rounds
13/15.0
13
20
(7) The computer rounds turns to the nearest half turn. The number of rounds to fire
is based on the rule: four rounds per 100 meters of target depth, or one round per 30 meters.
At this point, the computer has all the information needed to complete the subsequent
command. The command can then be issued to the mortars (Figure 13-6, page 13-10).
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13-10
FM 23-91
(8) The only difference between a search UP mission and a search DOWN mission
is the starting point. Normally, a search mission is fired by searching UP. This allows the
FO to better observe the effect of the rounds on target as the rounds walk toward him
(Figure 13-7).
b. Zone Fire. The 4.2-inch mortar does not fire a search mission the same as the 120-
mm, 81-mm, 60-mm mortars. It does not have the same elevating characteristics as the other
mortars; therefore, the 4.2-inch mortar uses zone fire when targets have more depth than a
platoon/section can cover when firing a standard sheaf. The 4.2-inch mortar platoon/section
usually fires two standard zones: a 100-meter zone (three rounds for each mortar) for a
platoon-size target, and a 200-meter zone (five rounds for each mortar) for a company-size
target.
NOTE: A larger zone can be covered by firing one round for every 50-meter increase in
the target area.
(1) Establishing the 100-meter zone. Once FO gives the FFE, the computer proceeds
as follows:
(a) Firing without extension (M329A1). Add and subtract 3/8 charge from the
base command charge. (The base command charge is the command charge in the FFE center
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FM 23-91
mass of target.) This gives each mortar three rounds with a different charge on each to cover
the 100-meter zone (Figure 13-8).
CHARGE 10 3/8
50 METERS
CHARGE 10
100 BASE COMMAND CHARGE
METERS
50 METERS
CHARGE 9 5/8
(b) Firing with extension (M329A1). Add and subtract 4/8 charge from the base
command charge and use three rounds for each mortar.
NOTE: A 3/8 charge correction to any charge without extension moves the round about
50 meters at any elevation used. A 4/8 charge correction to any charge with
extension moves the round about 50 meters at any elevation used.
(c) Firing with M329A2. Add and subtract 2/8 charge from the base command
charge.
(d) Firing the 100-meter zone. Once the mortars are up (rounds set for proper
charges) and the fire command is given, fix the rounds in any sequence—for example, No. 1
fires long, short, center mass; No. 2 fires center mass, short, long.
(2) Establishing the 200-meter zone. Once the FFE has been given by the FO, the
computer proceeds as follows:
(a) Firing without extension. Add and subtract 3/8 charge from the base
command charge for the rounds on either side of the base round and 6/8 charge for the long
and short round (Figure 13-9).
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CHARGE 31
50 METERS
CHARGE 30 4/8
50 METERS
CHARGE 29 4/8
50 METERS
CHARGE 29
(b) Firing with extension. Add and subtract 4/8 charge from the base command
charge for the rounds on either side of the base round and a whole charge for the long and
short rounds.
(c) Firing with M329A2. Add and subtract 2/8 charge from the base command
charge.
(d) Firing the 200-meter zone. Fire the rounds in any sequence.
13-3. ILLUMINATION
Illumination assists friendly forces with light for night operations. The M16/M19 can be set
up for illumination as any one of the three types of firing charts. Determining firing data is
the same as with any type of mission, only now the FDC uses one of the flank mortars to
adjust the illumination, leaving the base mortar (No. 2) ready to adjust HE. The FO enters
corrections for the illumination rounds in range—deviation not less than 200-meter
corrections, and corrections for height (up/down) not less than 50-meter corrections.
a. Observers. Observers who are to adjust illumination should be informed when the
81-mm mortars are firing M301A3 illumination rounds. The M301A3 has an HOB of 600
meters, while the M301A1 and M310A2 rounds have 400-meter HOBs. There is a
difference in adjustment procedure. The M301A1 and M301A2 rounds are adjusted to a
ground-level burnout; the M301A3 round should have a burnout 150 to 200 meters above
ground. This procedure is based on the fact that all three of the rounds fall at a rate of 6 mps
(Table 13-2, page 13-14).
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FM 23-91
b. Corrections. The ranges in the firing tables are in 50-meter increments. (Rule:
Always round up, such as range 2,525 meters = 2,550 meters, to enter Part II of the firing
tables.) Corrections to the HOB are obtained in columns 4 and 5. These corrections are used
to move the round up or down in relation to the HOB line (Figure 13-10 and Figure 13-11,
page 13-16).
13-14
FM 23-91
350
300
250
150
100
50
HOB LINE
50 THIRD CORRECTION
(DOWN 100)
100
150
200
250
300
350
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HOB LINE
600 METERS
13-16
FM 23-91
EXAMPLE
Chart range to the first round fired: 2,525 meters = 2,550 meters to enter the
firing table (FT 81-A1-3).
Optimum charge to use: charge 8
Basic data, columns 1 (Range to Burst), 2 (Elevation) and 3 (Fuze Setting) to
give the basic HOB for 600 meters above the mortar position:
Range to Burst = 2,550 meters
Elevation = 1107 mils
Fuze setting = 31.0
c. Adjustments. The round is fired and the FO sends: ADD TWO ZERO ZERO (200),
UP ONE ZERO ZERO (100). The computed range is now 2,725 = 2,750 (Figure 13-12).
The basic data only give an HOB of 600 meters, but the FO requested an UP 100, meaning
that the round needs more height. To compute this change, the computer must determine
where this round will be in relation to the HOB line: HOB = 600 meters; UP 100 is two
increments above the HOB line. Once the number of increments has been determined, the
computer goes to column 4 (change in elevation for 50-meter increase in HOB) and column
5 (changes in fuze setting for 50-meter increase in HOB), and multiplies the increments times
the correction factors given in these columns.
EXAMPLE
Range to burst 2,750 meters, +2 increments
Column 4 = -14 x 2 increments
(100 mils above HOB) = -28 mils
Column 5 = -0.7 x 2 increments
(100 mils above HOB) = -1.4 seconds
(1) Once the corrections have been determined, apply those to the basic data
(columns 2 and 3) to obtain the firing data for the next round.
EXAMPLE
Basic data: column 2 = 1034 (600 meters HOB)
- 28 mils (correction)
1006 (elevation needed to fire)
column 3 = 29.5 (600 meters HOB)
- 1.4 (correction)
28.1 (time set needed to fire)
(2) Assume that the second round is fired and the FO sends: DOWN FIFTY (50).
Note that a range change was not sent, but an HOB correction was sent. Again, determine
the relation to the HOB line and apply the correction factors to the basic data to obtain the
firing data.
13-17
FM 23-91
EXAMPLE
Range to burst 2,750 meters, charge 8, down 50.
The computer is now working with one increment above the HOB line.
Increments (relationship to HOB, 600 meters)
1 x -14 (column 4) = -14
1 x -0.7 (column 5) = -0.7
New data:
1034 mils (basic data) -14 = 1020 mils elevation
29.5 (basic data) -0.7 = 28.8 fuze setting
(3) When the correction is below the HOB line, use the opposite sign of the sign
found in columns 4 and 5 to obtain the same HOB. To compute the correction, assume that
the chart range to burst is 1,550 meters and the optimum charge is 6. The first round is fired
at an elevation of 1260 mils with a fuze setting of 29.0.
(4) The FO sends: DROP TWO ZERO ZERO (200), DOWN ONE FIVE ZERO
(150). Assume that the new range is 1,325 meters (= 1,350), and the optimum charge is 5.
The procedure for determining the increments is the same as with the last example:
600-meter basic HOB, down 150 = 3 increments below the HOB line.
(5) Determining the correcting factors is the same as before, except that when
computing below the HOB line, reverse the signs since columns 4 and 5 are set up for
increases in HOB.
(6) Assume that the second round is fired and the FO sends: DROP TWO ZERO
ZERO (-200), and the new range is 1,150 meters. Note that a range change is given but not
an HOB correction. When only a range change is sent, only the increments below the HOB
line for the old range must be applied to the new range to keep the HOB correct. To
determine the data, apply the steps as before:
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13-19
FM 23-91
CHAPTER 14
SPECIAL CONSIDERATIONS
This chapter discusses the special procedures applied to some missions
to effectively engage targets.
NOTE: The procedure to obtain the firing data is the same as with all firing charts.
(4) Determine correction factors after the registration has been completed. Apply
these factors to all other targets within the transfer limits of the RP.
c. Obtaining Firing Data. Obtaining the firing data is the same as with any mission,
except that the FO continues to adjust until a 50-meter bracket is split and the last fired round
is within 25 meters of the target (Figure 14-1). Refinement corrections are sent to the FDC
and the mission is ended. Table 14-1 provides information to be used in setting up the
plotting boards to fire a surveyed registration. (See FM 6-30.)
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d. Adjusting the Sheaf. The purpose of adjusting the sheaf is to get all mortars firing
parallel. Mortars are positioned with gun No. 1 through 4 for the 81-mm mortars, No. 1
through 6 for the 4.2-inch and 120-mm mortar (when employed as a platoon), or No. 1
through 3 for the 4.2-inch and 120-mm mortar (when employed as a section) from right to
left as seen from behind the guns. There is normally a 10-second interval between rounds.
The FO needs that time to observe the impact of the rounds and to determine corrections. If
the corrections are 50 meters or more (deviation left/right only), the mortar must be refired.
The corrections can be plotted on the board, or the DCT (Figure 14-2) can be used to
determine the number of mils to add or subtract from the base mortar deflection.
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NOTE: If the target has been mechanically surveyed, enter the DCT at the initial range
plot. If the target is a nonsurveyed target (even if it is an eight-digit grid), enter
the DCT at the final range plot.
EXAMPLE
The sheaf of a 81-mm platoon is being adjusted. No. 2 mortar conducted the
registration. The FDC has requested to prepare to adjust the sheaf. The FO
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FM 23-91
requests section right. The entire platoon then fires, in order, starting at the
right (No. 1, 3, 4) with 10-second intervals between rounds. The mortar that
was used to register (No. 2) will not fire. The sheaf is adjusted perpendicular
to the gun-target line. The observer notes where each round lands and sends
back deviation corrections in meters; range corrections are ignored if less
than 50 meters. If a deviation correction is 50 meters or more, it must be
refired. Corrections to be refired should always be transmitted first by the
FO.
If angle T is greater than 499 mils, each piece is adjusted onto the registration
point, and the FDC computes the data for the sheaf. In adjusting the sheaf,
all rounds must be adjusted on line at about the same range (within 50
meters) and with the lateral spread between rounds equal to the bursting
diameter of the ammunition used.
The spottings from the FO are No. 4, right 20, No. 3, left 60, and No. 1, left
30 (Figure 14-3). The FO then sends these corrections to the FDC; No. 3,
right 60 (because it needs to be refired), No. 4, left 20 No. 4 is adjusted, and
finally No. 1, right 30 No. 1 is adjusted. The No. 3 mortar is now fired, and
the round impacts 10 meters right of the desired burst point. The FO would
then send: No. 3, left 10, No. 3 is adjusted, sheaf is adjusted, end of mission.
40 M 40 M 40 M
4 4 3 2 1
3 1
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1,500 METERS
REGISTRATION
POINT
1,500 METERS
Range correction is the difference in meters between the initial chart range and the final chart
range for the RP. As the registration mission is fired and completed, the rounds on the
plotting board may not be plotted at the point where the RP was plotted. Because of wind
and weather, the rounds may have to be fired at a greater or lesser range and to the right or
left of the target to hit it. As shown in Figure 14-5, the initial chart range to the RP was
3,050 meters; the final adjusted chart range (range used to hit the RP) was 3,200.
14-5
FM 23-91
RANGE
3,200 METERS FINAL
PLOT
RP NO. 1
RANGE
3,050 METERS
(1) Range correction factor. The RCF is the number of meters per thousand to be
applied to the initial chart range of a target within the transfer limits resulting in a range
correction for that mission. Continuing the preceding example, since the ranges to other
targets will not be 3,050 (range to RP), the RCF (+150) will not be correct. Therefore, other
range corrections must be determined and used for other targets. Once the range difference
has been determined, round the initial chart range to the nearest 100 and then express that
in thousandths.
EXAMPLE
Initial chart range: 3,050. 48.3 = +48 RCF
Round to nearest 100 = 3,100 3.1/+150.00
expressed in thousandths = 3.1. 124
Divide the range in thousandths into the range 260
difference: +150. 248
120
93
27
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Determine to the nearest whole meter and use the sign of the range correction.
(2) Deflection correction. The deflection correction is the number of mils needed to
correct the deflection to hit the target since nonstandard conditions again caused the plots on
the board to be either left or right of the initial chart deflection (Figure 14-6). Compare the
initial chart deflection and the final chart deflection and subtract the smaller from the larger.
RULE: Final chart deflection (hit) larger = LEFT deflection correction; final chart
deflection (hit) smaller = RIGHT deflection correction.
EXAMPLE
Hit Larger
Hit deflection: 2801
Initial chart deflection: 2790
(2801 - 2790 = L11)
EXAMPLE
Hit Smaller
Hit deflection: 2790
Initial chart deflection: 2801
(2790 - 2801 = R11)
Range and deflection corrections are applied to all other targets within the transfer limits of
the RP.
h. Firing of a Total Range Correction Mission. The procedure for a mission on the
surveyed chart is the same as with the modified-observed chart. However, now the firing
corrections are applied to chart data to obtain command data (firing data sent to the mortars).
For example, the computer assumes that the board is still set up on the information for the
registration mission, and the mission in Figure 14-6 has been received. It is within the
transfer limits.
14-7
FM 23-91
Figure 14-6. Example of completed DA Form 2399 for firing a total range
correction mission on the surveyed chart.
14-8
FM 23-91
i. Applying Firing Corrections. Once the chart data have been determined, the
computer applies the deflection correction by either adding or subtracting the deflection
correction to the chart data determined. When working with deflection corrections, the
computer uses the LARS rule. The deflection correction must be applied to each chart
deflection throughout the mission.
EXAMPLE
2715 + L11 = 2726
(1) Range correction. Determine the initial chart range, then round to the nearest
hundred and express it in thousandths; for example, 2975 = 3000 = 3.0. Multiply the range
in thousandths times the RCF and use the sign of the RCF: 3.0 x +48 = +144. This gives the
total range correction for this target.
(2) Total range correction. The total range correction (TRC) is the total correction
that must be applied to get the command range to fire the target. TRC is the range correction
(RCF x range in thousandths) plus or minus the altitude correction.
EXAMPLE
Range correction = +144 - 25 (altitude correction) = +119 TRC
The two factors (RCF and altitude correction) are compared. If one of these factors is a
negative, subtract the smaller from the larger. The sign of the larger is used for the TRC. If
both factors are negative or positive, then add the two factors to get the TRC. This must be
applied to every chart range to obtain command range. To enter the firing tables, the
command range is rounded to the nearest 25 meters.
j. Firing Reregistration. The FDC must consider weather changes to ensure the
accuracy of the firing data (firing corrections) from the surveyed chart. Two methods can
be used to do this: reregistration on the RP or MET message. Of those two methods,
reregistration is the better because all the unknown (nonstandard) factors are fired out.
However, due to countermortar-radar, determining and applying the MET messages may be
safer. The choice is dictated by the tactical situation and the availability of MET messages.
(1) Fire the reregistration at the established RP using only the mortar that originally
fired the registration (Figure 14-7). (The FDC assumes that the sheaf is still parallel;
therefore, the sheaf should not need adjusting again.) The chart data are the same as with the
initial registration. Apply the firing corrections to obtain the command data (Figure 14-8).
(A blank reproducible copy of DA Form 2188-R, Data Sheet, is located at the back of this
manual.)
14-9
FM 23-91
14-10
FM 23-91
14-11
FM 23-91
(2) The chart deflection plus or minus deflection correction equals command
deflection. The chart range plus or minus the range correction plus or minus the altitude
correction equals the command range.
(3) Carry out the mission the same as with the initial registration. Once the EOM,
"Registration complete," has been given, determine firing corrections again.
(4) In the initial registration, the FDC compared the initial chart range and the final
chart range difference. Determining the range difference after the reregistration is the same;
however, now determine the final adjusted range. During the reregistration, firing corrections
were applied for each round. Now apply those same corrections.
(5) Adjusted range is the final range with the correction for altitude correction
deleted.
EXAMPLE
Final command range: 3,100 meters; altitude correction: -25.
Final adjusted range: 3,100 + 25 = 3,125.
The altitude correction is added since it was initially subtracted. If the altitude correction had
been a plus (+), then it would have been subtracted to obtain the final adjusted range.
(6) Once the final adjusted range has been determined, compare the initial chart and
the final adjusted range. Subtract the smaller from the larger to determine the RCF. The sign
(+/-) would be determined as with the initial registration. Again, divide the range (initial
chart range rounded to the nearest 100 expressed in thousandths) into the new range
correction to determine the new RCF.
(7) To determine the deflection correction, compare the initial chart deflection and
the final command deflection. Subtract the smaller from the larger and determine the sign
(L or R) to apply.
(8) Apply the new firing corrections to all targets that have been and are fired within
the transfer limits. For those targets that are already plotted on the board, apply the new
firing corrections and update the target data. (The chart data do not change. The target has
not moved; only the weather has changed.)
Because the FOs will be sending azimuth readings for the impact points of the rounds, they
must see the area of the RP using the M2 aiming circle.
14-12
FM 23-91
(3) Record all data on DA Form 2188-R. To determine each FO's direction to the
RP, rotate the azimuth disk until the FO's position is aligned with the RP. Read the azimuth
scale to the nearest mil. To determine each FO's vertical angle, compare the altitudes of each
FO's location and the RP, and subtract the smaller from the larger. This remainder is the VI,
which is used to determine the vertical angle and carries the sign of the larger. Determine
the range from each FO's location. Round the range to the nearest 100 and express it in
thousandths. Divide the range expressed in thousandths into the VI and determine the
product to the nearest whole mil. The sign (+/-) of the vertical angle (VA) is the same as the
VI sign (+/-).
EXAMPLE
Assume that the VI is -80 for FO 1 and +50 for FO 2. The range for FO 1 is 2,525
meters; for FO 2 is 3,000 meters.
- 32
FO 1: 2525 = 2500 = 25/-800 VA: -32 mils
+ 16.6
FO 2: 3000 = 3000 = 30/+500.0 VA: +17 mils
Send the direction and vertical angle to the FOs so they can set up their M2 aiming
circles.
(4) To determine the firing data, align the mortar position with the RP. Determine
the chart data and apply the range correction for altitude between the mortar and target.
During the registration, only the range correction for altitude is used. Give the firing
command to the base mortar. Three to six rounds will be fired at 10-second to 20-second
intervals. The FO uses this interval to give himself time to determine the azimuth readings
to each round. If the azimuth for one or more rounds is determined to be 50 or more mils
different, then another round may be fired for each erratic round. Six rounds are needed for
the most accurate MPI registration, but as few as three rounds give correction data.
(5) As the rounds are fired, the FO reads the azimuth to each round and records it.
When the last round has been fired, he sends the data recorded to the FDC. Once the rounds
have been fired and the readings recorded in the FDC, plot the MPI as follows:
(a) Determine the total by adding all the readings from each FO.
(b) Divide the total by the number of readings to determine the average of the
readings to the nearest mil.
EXAMPLE
FO 1 FO 2
1 6104 0400
2 6110 0402
3 6105 0404
4 6106 0405
5 6107 0401
6 6109 0400
TOTAL 36,641 mils 2412 mils
14-13
FM 23-91
(c) Once the average azimuth for each FO has been determined, index the average
azimuth and draw a line from each FO position toward the top of the board; where the lines
intersect is the MPI. Determine and record the eight-digit grid coordinates and altitude of
the MPI.
b. Determination of Range Correction Factors. With the MPI and RP on the board
and the altitude determined, correction factors to be applied to other targets within the
transfer limits of the RP must be determined. Again, because of the effects of interior and
exterior ballistics on the round, the MPI may not be plotted in the same location on the
plotting board as the surveyed point. Therefore, the corrections to hit that surveyed point
must be determined. These corrections are noted on DA Form 5472-R, Computer's Record
(MPI) (Figure 14-9). (A blank reproducible copy of DA Form 5472-R is located at the back
of this manual.)
(1) Range difference. Compare the command range to the MPI point (minus the
altitude correction) and the initial chart range to the RP.
EXAMPLE
Command range MPI = M Alt 500 mils, MPI Alt 450 mils, VI = -50, Alt Corr -25.
Adjusted chart range to the MPI = command range 2,650 M + 25 (to delete altitude
correction, reverse the sign) = 2,675 adjusted chart range to the MPI.
The sign of the range difference is determined by how the move from the MPI to
the RP must be made. If the RP range is larger, the difference is a plus (+); if
smaller, it is a minus (-).
EXAMPLE
Initial chart range to the RP is 2,600 meters; adjusted chart range to the MPI is
2,675 meters.
2,675 - 2,600 = -75 range difference
14-14
FM 23-91
(2) Range correction factor. Once the range difference has been determined, divide
it by the chart range to the MPI rounded to the nearest 100 expressed in thousandths and
round it to the nearest whole meter. The sign is the same as the range difference.
14-15
FM 23-91
EXAMPLE
Range difference - 75; chart range to MPI 2,675 meters. -27.8 = -28 meters = RCF
Rounded to the nearest 100 = 2,700 meters 2.7/-75.00
Expressed in thousandths = 2.7 54
210
189
210
(3) Deflection correction. Compare the chart deflection of the MPI and the chart
deflection of the RP (Figure 14-9) to determine the deflection correction. The sign of the
deflection correction will be determined by how the move from the MPI to the RP must be
made.
RULE: RP deflection is greater than the MPI deflection = LEFT deflection correction.
RP deflection is less than MPI deflection = RIGHT deflection correction.
EXAMPLE
MPI chart deflection = 2810;
RP chart deflection = 2790.
2810 - 2790 = L20 (correction to apply R20)
The application of the correction factors to other targets, within the transfer limit of the RP,
is the same as with the other registration corrections except that the sign of the corrections
must be reversed.
NOTE: The only time the corrections will be applied with the signs as determined is when
the corrections are being applied to move the strike of the round from the MPI to
the RP.
14-16
FM 23-91
TARGET HIGHER;
A PLUS (+) RANGE
CORRECTION NEEDED
TARGET LOWER;
A MINUS (-) RANGE
CORRECTION NEEDED
TARGET
MORTAR WHEN TARGETS ARE AT THE SAME RANGE
FROM THE MORTAR POSITION, BUT ARE HIGHER
OR LOWER, RANGE CORRECTIONS ARE USED
TARGET
b. Correction for Vertical Interval. Because of the VI, a range correction must be
applied to the chart range to obtain the range to be fired (command). The range correction
to apply is half of the VI; it is determined to the nearest whole meter.
EXAMPLE
VI = 75 meters
1/2 = 38 meters (altitude [range] correction)
The altitude (range) correction must be 25 meters or more to be applied. The range
correction is then added to or subtracted from the chart range. If the target is higher than the
mortar, the computer adds the range correction; if lower, the computer subtracts to get the
altitude to be fired (command). The altitude correction is applied to every chart range
throughout the mission.
NOTE: A VI of less than 50 meters is not used when working with the modified-observed
chart.
14-17
FM 23-91
from contour maps, by estimating, or by measuring the angle of sight, and by using the mil-
relation formula.
(1) Grid missions. The target is plotted on the map and the altitude determined. If
the altitude of the target cannot be determined, then the computer assumes that it is the same
as that of the mortars.
(2) Shift missions. The target is assumed to be the same altitude as the point being
shifted from unless, in the call for fire, the FO sends a vertical shift (up or down). Therefore,
that shift is applied to the point being shifted from, and that is the altitude of the new target.
(3) Polar missions. The altitude of the target is assumed to be the same as that of the
FO's position if no vertical shift is given. If one is given, then the computer applies the shift
to the FO's altitude, and that is the altitude of the new target. Once the computer has
determined the altitude of the target, then it is possible to determine the VI for the mission
and, finally, the altitude correction to apply. Remember, VI is the difference in altitude
between the mortars and the target.
NOTE: Registration of the AN/PPS-5 is explained here for the 60-mm and 81-mm
mortars.
b. The M16/M19 plotting board must be set up as a surveyed firing chart. That is, the
mortar position, RP, and radar site must be plotted to surveyed accuracy. The procedure for
obtaining firing data is the same as with a regular registration mission. The altitude
correction is the only firing correction used. Because this is a polar-type mission, the VI is
now obtained as with a polar mission. The firing corrections are obtained in the same
manner as with the regular registration mission.
c. After the board is set up and the direction and distance from the radar to the target
have been determined, the FDC informs the radar operator of this information. The radar
operator then orients the radar set using the information and calls the FDC when the set is
ready. Once the radar is ready, the FDC then gives the initial data to the mortar section. The
base mortar will adjust and then the sheaf will be established.
(1) When the first round impacts, the radar operator sends the FDC the direction and
distance to that round.
(2) The FDC then indexes that direction and plots the round at the distance sent (the
plot is made from the radar position plot, using the distance sent).
(3) The FDC indexes the mortar RP azimuth and determines the spotting by
comparing the round's impact plot with the RP plot. The FDC, acting as the FO, determines
all spottings (Figure 14-11).
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FM 23-91
RP1
+100
R50
(4) Once the spotting has been determined, the FDC converts the spotting into a
correction to fire the second round. He does this by reversing the signs of the spotting. He
then applies that to the registration point on the azimuth of the radar position (Figure 14-12).
1 SPOT
L50
RP NO. 1
+100
14-19
FM 23-91
(5) The firing data are then obtained by aligning the new plot with the mortar
position.
(6) The spottings for additional rounds are spotted from the initial RP, but the
corrections (spotting reversed) are applied to the last fired plot. This procedure is repeated
for all adjustment rounds until a range correction of 50 meters is split.
NOTE: If the FPF is within 200 meters of friendly troops, the FO should call for HE delay
in adjustment (preferred method) and use the creeping method of adjustment.
e. When adjusting each mortar, the FO may (in the call for fire) give a section left (SL)
or section right (SR) to determine the danger-close mortar. The danger-close mortar is the
one impacting closest to friendly troops.
14-20
FM 23-91
(1) Once the danger mortar is known (Figure 14-13), it is adjusted onto the FPF line.
(2) Once the danger mortar has been adjusted, the next mortar (No. 2) is given the
danger mortar data and fired. The firing of the same data should put the impact of the next
mortar 40 meters left or right of the adjusted mortar.
STARTING POINT
14-21
FM 23-91
(3) This procedure is used for the remaining mortars until each is on the FPF line. As
each mortar is adjusted to the FPF line, the data are then given to each mortar and placed on
the mortar after each mission. Also, the predetermined number (unit SOP) of rounds are set
aside ready to fire (Figure 14-14).
14-22
FM 23-91
f. When adjusting only the danger-close mortar, the computer is given the attitude of
the target in the call for fire.
(1) The FDC can determine the danger-close mortar by indexing the target attitude
and drawing a line from the initial FPF plot (given in the call for fire) 50 meters above and
below (Figure 14-15).
14-23
FM 23-91
(2) After drawing the FPF line, the computer rotates the azimuth disk and aligns the
mortar plot with the FPF plot to see which side of the line is closest to the friendly troops
(Figure 14-16).
(a) To use this method, the frontline trace of the supported unit must be plotted
on the board.
(b) Once the danger mortar has been determined, that danger mortar is fired and
adjusted to the FPF line.
(3) After the danger mortar is adjusted to the FPF line, the computer then indexes the
FPF attitude and erases all but the last plot.
(4) Using the last plot, the computer draws the FPF symbol by extending a line 90
meters long toward the top of the board and 10 meters long from the plot towards the bottom
of the board. This shows the full 100-meter width of the FPF.
14-24
FM 23-91
(5) The remaining plots for the No. 1, No. 2, and No. 3 mortars are then plotted 40
meters apart (Figure 14-17).
(6) Once the plots are on the plotting board, the computer determines the firing data
for each mortar by aligning each mortar plot with its intended impact plot (Figure 14-18).
14-25
FM 23-91
(7) Again, these data are placed on the mortar after each mission, and the rounds are
readied to fire.
g. To compute data for FPF without adjustment, the computer indexes the attitude of
the FPF line and makes a plot 40 meters above and below the FPF starting plot.
(1) The computer then indexes the attitude of the mortar section and plots the No. 1,
No. 3, and No. 4 mortars 40 meters above and below the No. 2 mortar plot.
(2) Once the FPF and mortars have been plotted, each mortar is aligned with its
impact plot, and the data determined.
(3) These data are given to the mortars and, again, are set on the mortars between
missions.
(4) This method is used when ammunition is low and time or the tactical situation
does not permit the adjustment of the FPF.
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FM 23-91
APPENDIX A
MORTAR TRAINING STRATEGY
This appendix provides a comprehensive unit training strategy for
training mortarmen. Leaders have the means to develop a program for
training their mortar units to full mission proficiency. This training strategy
applies to ALL mortars in ALL organizations of the US Army. Although not
prescriptive in nature, it must adapt to a unit’s mission, local training
resources, commander’s guidance, and unit training status.
A-1. GENERAL
The examination includes situations similar to combat. The gunner is required to be
proficient in mechanical drill and FDC when computing the fire mission from the forward
observer. This training strategy helps the mortar crew become proficient and effective on the
battlefield.
A-1
FM 23-91
ammunition is wisely expended and that training is conducted safely and effectively.
Mortarmen are certified when they receive a passing score of 90 percent and 70 percent on
the two-part examination. (See Appendix D.)
b. Collective Training.
(1) External evaluation. The commander formally determines the status of his
collective training through external evaluation. The external evaluation gives the
commander an objective appraisal of this status by using mortar expertise found outside the
normal chain of command. The external evaluation is not a test in which a unit passes or
fails; it is a diagnostic tool for identifying training strengths and weaknesses. It must be
emphasized that an external evaluation is not a specific training event but a means to
evaluate a training event. Mortar units undergo external evaluations during an LFX, FTX,
or a combination thereof. The unit may be evaluated alone, as part of its parent unit, or with
other mortar units. The MTP provides guidance on planning, preparing, and conducting an
external evaluation.
(2) Evaluation of forward observer. The mortars can be no more effective than the
FOs. It is critical that FIST FOs are present and evaluated during an externally evaluated
mortar live-fire exercise. If an FO fails to meet his performance standards, the mortars
should not be penalized. However, only as a last resort should the fire mission be deleted
from the evaluation. The mortars should be given the opportunity to successfully complete
the fire mission. This can be accomplished in the following:
(a) Start the fire mission over. However, ammunition constraints during live-fire
may not permit this. The task may need to be repeated using devices or, less preferably, dry
fire.
(b) Correct the call for fire or correction. The mortars should not have to use
wrong firing data if the FO has made an incorrect call for fire or correction. This also wastes
valuable training ammunition. The FO evaluator at the observation point can change the call
for fire or correction to reflect proper procedures.
A-2
FM 23-91
APPENDIX B
SAFETY PROCEDURES
Minimum and maximum elevations, deflection limits, and minimum fuze settings
must be computed to ensure all rounds impact or function within the designated
impact area. These data are then presented in graphical form on a range safety
diagram. They are also arranged in a simplified format (the safety T) for each
mortar squad leader. This chapter discusses the computation of safety data using
tabular and graphical data.
NOTE: If, the designated impact area does not already consider areas A and B, it must be
reduced by the appropriate amount to ensure no rounds impact within or outside
of either area.
b. Range and Deflection Probable Errors. The initial impact area must be reduced
again to account for the normal dispersion of rounds fired. The safety officer must determine
the maximum probable errors for both range and deflection.
(1) The safety officer checks columns 3 and 4 of Table E in the tabular firing tables
for the mortar and ammunition to be used. He checks all possible charge and elevation
combinations to ensure he has found the maximum probable errors at the distance to the far
edge of the impact area.
(2) The safety officer then reduces the maximum range by a factor of 8 times the
range probable error. He also adjusts the minimum range toward the center of impact by a
factor of 12 times the range probable error.
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FM 23-91
(3) Once the ranges have been adjusted, the safety officer adjusts the left and right
limits inward by a factor of 8 times the maximum deflection probable error.
NOTE: The safety officer must determine whether range control personnel have already
performed this computation before designating the impact area.
B-2
FM 23-91
c. Vertical Interval and Crest Clearance. The safety officer must compare the altitude
of the mortar position and that of the impact area. If there are significant differences in the
VI between these two areas, he must adjust the safety limits to preclude any rounds
impacting short or long of the impact area (Figure B-2).
(1) The rule for determining the correct VI for safety purposes is called the mini-max
rule. At the minimum range, the maximum altitude is selected. At the maximum range, the
minimum altitude is selected. If the contour interval is in feet, it is converted to meters.
(2) The safety officer determines VI by subtracting the mortar firing position altitude
from the altitude of the applicable range line. The resulting number is either positive or
negative.
(3) The safety officer adds half the value of the VI determined for each applicable
range line, to that line. This either increases or decreases the apparent size of the impact
area, depending on whether the VI is positive or negative.
DESIGNATED
IMPACT AREA
CREST
CLEARANCE
TRAJECTORY
DATUM
ROUNDS OUT OF
DESIGNATED IMPACT AREA
(4) The safety officer must then make a map inspection to determine the highest point
between the mortar position and the edge of the impact area. He then compares this highest
point with the lowest maximum ordinate value found in Table E in the tabular firing tables.
As long as the maximum ordinate exceeds the VI of the highest point, no correction need be
made. If not, all charge and elevation combinations that do not allow crest clearance must
be noted and applied to the safety diagram.
d. Drift (4.2-inch only). The safety officer must modify the left and right limits of the
safety diagram to compensate for the drift. The left limit must be moved to the left by the
amount of the minimum drift for the charge and elevation combinations to be fired. The
right limit must be moved to the left by the amount of the maximum drift for the charge and
elevation combinations to be fired.
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FM 23-91
NOTE: Drift is a function of both time-of-flight and range. The safety officer must be
careful to ensure he chooses the correct charge and elevation combination (the
one that gives the minimum drift). A common mistake is to simply use the drift
at minimum range, which is not always correct.
e. Section Width and Depth (manual plotting only). If a mortar near the center of
the section is used as the adjusting mortar, any mortar significantly left or right of this "base"
can put rounds out of impact, unless corrections are made. If the mortars are arranged in the
firing position with any significant depth, the rearward or forward mortar can put rounds
short or long of the impact area unless a correction is made.
(1) The safety officer must determine the width and depth of the mortar section as
it is arranged on the ground (at the firing position). He then reduces the left and right limits
by half the section width.
(2) The safety officer adds half the section depth to the minimum range and subtracts
half the section depth from the maximum range.
f. Registration and MET Corrections. After a registration (survey chart), a
reregistration, or a MET update has been conducted and corrections have been determined,
the safety officer must modify the original basic safety diagram by applying the registration
corrections. New elevations are determined that correspond to the minimum and maximum
ranges. Deflections are modified by applying the total deflection correction to each lateral
limit.
B-4
FM 23-91
(2) Draw lines representing the lateral limits in proper relation to the line on which
the section is laid. Label the lateral limits with the appropriate azimuths.
(3) Draw lines between the lateral safety limits to represent the minimum and
maximum ranges. Label each line with the appropriate range. If the minimum range for fuze
(FZ) time (TI) is different from the minimum range, draw a dashed line between the safety
limits to represent the minimum range for FZ TI. Label the line with the appropriate range.
(4) Compute the angular measurements from the azimuth of lay to the left and right
safety limits by comparing the azimuth of lay to the azimuth of each limit. On the diagram,
draw arrows indicating the angular measurements and label them.
(5) Apply the angular measurements to the deflection corresponding to the azimuth
of fire to determine the deflection limits (LARS).
L370
DIRECTION OF FIRE
AZ 5100
DF 2800
FP72
GRID AF603223872
ALT 390
c. Once the basic safety diagram is drawn, the FDC uses the tabular firing tables to
determine the proper charges, elevations, and time settings. He then applies them to
complete the diagram.
B-5
FM 23-91
d. The safety T is a method of passing safety data on to the mortar squad leaders in a
simplified form. The information needed by the squad leader is extracted from the
completed safety diagram and placed on a 3-inch by 5-inch card or similar form. Figure B-4
shows the safety T taken from the completed range safety diagram.
1330 = CH3
1366 = CH4 MAXIMUM ELEVATION (ILLUM)
14.0 SEC
MINIMUM TIME SETTING
B-6
FM 23-91
APPENDIX C
FIELD-EXPEDIENT SURVEY TECHNIQUES
Surveyed locations may be provided by the artillery survey personnel.
Normally, a map spot location to six-digit or eight-digit grid coordinates is
estimated by the platoon supervisor that is the most qualified. With the
"roving mortars" concept, new methods of position location are needed. Two
such methods are described in this appendix. The mortar position should be
constantly improved to include more accurate platoon center location.
C-1
FM 23-91
4520 MILS
POINT 2 AIMING CIRCLE LOCATION
e. With a protractor aligned with the correct azimuth on the line (Figure C-3), draw two
lines from the dot on the measured azimuths (Figure C-4).
POINT 2
POINT 3
4520 MILS
POINT 1
AZIMUTHS
C-2
FM 23-91
POINT 8
POINT 3
4520 MILS
AIMING CIRCLE LOCATION
POINT 1
f. Place the tracing paper over the map of the area and slide it around until it is
positioned so that the three lines pass through their respective distant points (Figure C-5).
The dot on the tracing paper represents the location of the aiming circle (mortar position) on
the map.
C-3
FM 23-91
g. If the angles are plotted with a standard protractor (accurate to about 10 mils) and
oriented over a 1:50,000 scale map, the resection should be accurate within 100 meters.
a. To begin the hasty survey, set the M2 aiming circle over the known point, level it,
index the declination constant using the azimuth micrometer knob, and, with the
nonrecording (lower) motion, orient the magnetic needle toward north. Now the grid
azimuth can be measured.
b. While the "circle" man is measuring the grids azimuth, an assistant (the "post" man)
moves toward the desired mortar position with the two aiming posts. (Before moving, the
"post" man will have joined the post together and placed reflective or black tape strips
exactly 2 meters apart on each post.) The post thus becomes a subtense bar (Figure C-7).
C-4
FM 23-91
TAPE
1
2 METERS
TAPE
c. At this point, the first leg of the hasty survey can be done. The "circle" man directs
the "post" man to move toward the desired mortar position until he is within 290 meters and
to place the post into the ground. This point on the ground becomes traverse station 1
(TS-1).
C-5
FM 23-91
d. The "circle" man then rotates the azimuth motion (upper motion) until the vertical
crossline in the telescope is on the center of the post. He records the azimuth to the post and
labels it traverse leg 1 (TL-1) (Figure C-8).
TS-1
18.5 MILS
5790 MILS
TL-1
KNOWN POINT
e. Next, the "post" man removes the post and holds it parallel to the ground, facing the
aiming circle.
f. The "circle" man measures the mil angle between the two strips of tape on the post
(subtense bar) and records the mil reading along with the azimuth to TS-1 (Figure C-8).
g. The post is then replaced into the ground and the "circle" man moves forward to this
point and sets up the aiming circle directly over this point. This completes the first traverse
leg.
C-6
FM 23-91
h. This procedure is repeated until the desired mortar position is reached. Either the
information obtained may be written down as an azimuth, a mil angle and a traverse station,
or a diagram may be constructed (Figure C-9). (To avoid confusing others working with a
hasty survey, any diagram should reflect the route of the various traverse legs and should be
close to scale.)
MILS OF
4786 MILS TS-1
TRAVERSE AZIMUTH SUBTENSE = METERS 10.1 MILS
LEG (MILS) (MILS) 18.5 MILS
TL-2
TS-2
TL-1 5790 18.5 110 TL-1 5790 MILS
TL-2 4786 10.1 200
TL-3 0008 10.0 200
TL-4 4560 8.5 240
TL-5 5769 12.8 160
(1) The information recorded by the "circle" man goes to the FDC either as the
traverse legs are made or after all the legs have been completed. The beginning known point
is represented by the pivot point of the M16 plotting board.
(2) Starting at the pivot point, the data are applied on the board for each leg of the
hasty survey—for example:
(a) The azimuth on the first traverse leg was 5790 mils.
(b) Index that information on the M16 plotting board.
(c) The distance between the two strips of tape on the aiming posts was 18.5 mils.
(d) Refer to the distance tables (Table C-1, page C-8) for the 2-meter subtense
bar width; a mil angle of 18.5 mils is equal to a distance of 110 meters. (For the hasty survey,
make one square on the plotting board equal to 25 meters.)
(e) From the pivot point on the direction of 5790 mils, move 110 meters (4 2/5
squares) along the index line, place a dot, and circle it. This point, marked as TS-1,
completes traverse leg 1.
(f) The azimuth for the second traverse leg was 4786 mils.
(g) Again, index this information on the plotting board.
(h) At TS-2, the mil angle measured for the 2-meter subtense bar width was
10.1 mils.
(i) Refer to the distance table for the 2-meter subtense bar width; 10.1 mils
equals a distance of 200 meters.
C-7
FM 23-91
(j) With 4786 mils indexed on the plotting board, move up 200 meters from TS-1
along or parallel to a vertical line (eight squares), place a dot, and circle it.
(k) This point, marked TS-2, completes traverse leg 2. Repeat the same
procedure for traverse legs 3, 4, and 5.
(l) Rotate the M16 plotting board until TS-5 (mortar position) is directly over the
vertical centerline.
(m)Read the azimuth from the top of the plotting board; this is the direction from
the known starting point to the base mortar squad’s position.
(n) Count the number of squares along the index line between the pivot point and
TS-5 (remember: each square equals 25 meters). This is the straight-line distance from the
known starting point to the base mortar squad’s position.
(o) If given data were properly applied in the example, a known starting point-
base mortar squad azimuth should have been obtained of 5961 mils, and a known starting
point-based mortar squad distance of 690 meters (+/-5 mils and 10 meters).
(p) Apply these data to the map. From the known starting point along the
direction of 5961 mils, move 690 meters. The new point is the eight-digit grid coordinate
for the base mortar squad's position.
(p) The FDC now establishes a modified-observed firing chart or, if the FO can
find an eight-digit location in the target area, establish a surveyed firing chart.
C-8
FM 23-91
APPENDIX D
FIRE DIRECTION CENTER CERTIFICATION
The FDC certification tests the proficiency of soldiers to perform their
duties as FDC computers and section sergeants. This appendix provides the
commander with a means to verify that their mortarmen are trained in FDC
procedures. STRAC specifies that FDC personnel pass an FDC examination
semiannually.
D-2. QUALIFICATION
The FDCCP is designed to be a battalion-sponsored program that the battalion commander
can use to certify FDC personnel. The goal is to certify all leaders under a standardized
evaluation program.
a. Soldiers must receive a minimum score of 90 percent on the written and 70 percent
on the hands-on component (to include a passing score on the mortar gunner's examination).
b. Soldiers may retest only once on any part of the test that they have failed. Soldiers
who fail the retest will not be certified and will be required to repeat the FDCCP during the
next evaluation. Those who fail a second time should be considered for administrative
action.
D-1
FM 23-91
D-2
FM 23-91
• Compute data for searching fire (60-mm, 81-mm, and 120-mm mortars).
• Compute data for battlefield illumination.
• Compute data for a coordinated illumination/HE mission.
• Determine angle T.
• Prepare an FDC order (section sergeant).
• Compute data for a zone mission (4.2-inch mortar only).
• Locate an unknown point using intersection.
• Locate an unknown point using resection.
SITUATION A
The following tasks place the MBC in operation.
TASK: Place the MBC into operation using internal or external power sources.
CONDITIONS: Given a BA 5588/U battery, power supply cable, MBC, and a variable
power supply.
STANDARDS: Place the MBC into operation.
D-3
FM 23-91
D-4
FM 23-91
D-5
FM 23-91
(a) (b)
(c) (d)
NOTE: The first round is fired, and the FO sends: RIGHT 100, DROP 100.
D-6
FM 23-91
NOTE: That round is fired, and the FO sends: DROP 50, FFE.
(a)
(b)
(c)
(d)
SITUATION B
A fire mission is conducted using the call for fire and FDC order in Figure D-2.
D-7
FM 23-91
D-8
FM 23-91
(a) (b)
(c) (d)
NOTE: The FO sends: EOM, EST 30 PERCENT CAS. The computer records:
EOMRAT AB 0401, KNPT 01.
D-9
FM 23-91
SITUATION C
The FO calls in a polar mission. His location must be determined before the polar mission
may be computed.
NOTE: The FO's call sign is T43. T43 sees KNPT 00 at a direction of 5850 and KNPT
01 at a direction of 5590.
D-10
FM 23-91
NOTE: The initial round is fired, and the FO sends LEFT 100.
NOTE: The round is fired and the FO sends: LEFT 50, ADD 50, FFE.
D-11
FM 23-91
5. What is the correct subsequent fire command for the fire for effect?
(a)
(b)
(c)
(d)
NOTE: The FO calls back: EOM, POL POINT BURNING. The computer records:
EOMRAT ABO402, KNPT 02.
SITUATION D
Your platoon has moved to a firing range.
D-12
FM 23-91
CONDITIONS: Given an MBC with setup, weapon, ammunition, and FO location data.
STANDARDS: Enter the setup, weapon, and ammunition data into the MBC without
error.
LLAZ: 1200
RLAZ: 2000
MAX RN: 4000
MIN RN: 0350
MIN CHG: 1
MAX CHG: 8
TASK: Store MET data (Figure D-4) and update to the current file in the MBC.
CONDITIONS: Given an initialized MBC and a completed DA Form 3677.
STANDARDS: Enter MET data in the MBC without error.
D-13
FM 23-91
D-14
FM 23-91
D-15
FM 23-91
(a) (b)
(c) (d)
D-16
FM 23-91
NOTE: The FDC sends an MTO, "Prepare to adjust sheaf," and the FO replies, "Section
right.
D-17
FM 23-91
(a)
(b)
(c)
(d)
NOTE: The FO calls back: NUMBER 1 GUN RIGHT 60; NUMBER 3 GUN LEFT 20;
NUMBER 4 ADJUSTED.
(a)
(b)
(c)
(d)
NOTE: The FO spots the last round and sends: EOM, SHEAF ADJUSTED. The
computer records as: EOMRAT AA0200, KNPT 00.
SITUATION E
While the section is referring and realigning their aiming posts, the section leader hands you
a call for fire.
D-18
FM 23-91
D-19
FM 23-91
(a) (b)
(c) (d)
D-20
FM 23-91
NOTE: The FO spots the first round and sends: ADD 100. That round is fired, and the
FO sends: RIGHT 50, ADD 50, FFE.
15. What is the correct subsequent fire command for the FFE?
(a)
(b)
(c)
(d)
D-21
FM 23-91
SITUATION F
The FO calls in a new mission.
TASK: Compute data for a grid mission using the call for fire and FDC order in
Figure D-7.
CONDITIONS: Given an initialized MBC, call for fire using grid coordinates as the
method of target location, computer's record, and data sheet.
STANDARDS: Compute data for the mission's initial fire command to within 1 mil for
deflection and elevation.
NOTE: The initial round is fired, and the FO sends: RIGHT 100, ADD 100
D-22
FM 23-91
(a)
(b)
(c)
(d)
NOTE: The FO spots the round and sends: ADD 50, FFE.
TASK: Compute data for a traversing mission using the call for fire and FDC
order in Figure D-7.
CONDITIONS: Given an MBC with a mission already in progress.
STANDARDS: Compute data for the corrections to within 1 mil for deflection and
elevation, and determine turns to the nearest one-half turn.
(a)
(b)
(c)
(d)
NOTE: The FO sends: EOM, BRIDGE DESTROYED, EOMRAT AA0202, KNPT 02.
D-23
FM 23-91
SITUATION G
W13 sends in the fire request in Figure D-8.
W13 immediately sends in another fire request. The section leader assigns No. 1 and No. 2
guns to the first mission (SHIFT), and No. 3 and No. 4 guns to the second mission (POLAR).
TASK: Compute data for a shift mission using the call for fire and FDC orders
in Figure D-8.
D-24
FM 23-91
CONDITIONS: Given an initialized MBC, call for fire using shift from a known point,
computer's record, and data sheet.
STANDARDS: Compute data for the mission to within 1 mil for deflection and elevation.
TASK: Compute firing data for a polar mission using the call for fire and FDC
orders in Figure D-9.
CONDITIONS: Given an initialized MBC, call for fire, computer's record, and data sheet.
STANDARDS: Compute the firing data for the mission to within 1 mil for deflection and
elevation.
TASK: Compute firing data for a polar mission using the call for fire and FDC
orders in Figure D-9.
CONDITIONS: Given an initialized MBC, call for fire, computer's record and data sheet.
STANDARDS: Compute the firing data for the mission to within 1 mil for deflection and
elevation.
18. What is the correct range for the first round in mission one?
D-25
FM 23-91
19. What is the correct initial fire command for mission two?
(a) (b)
(c) (d)
NOTE: The first mission's initial round is fired, and the FO sends: RIGHT 50, DROP
100.
D-26
FM 23-91
(a)
(b)
(c)
(d)
NOTE: The FO spots the round for mission two and sends: DROP 50, FFE.
21. What is the correct subsequent command for the second mission?
(a)
(b)
(c)
(d)
NOTES: 1. The FO spots the second round for the first mission and sends: ADD 50, FFE.
2. The FO calls back on the second mission: EOM, BMP DESTROYED,
EOMRAT AA204, KNPT 04.
D-27
FM 23-91
22. What is the correct subsequent command for the first FFE mission?
(a)
(b)
(c)
(d)
NOTE: The FO sends: EOM, EST 80 PERCENT CAS, EOMRAT AA0203, KNPT 03.
SITUATION H
The company commander orders the mortar platoon to displace. The platoon occupies the
new position. The initialization data below is entered into the MBC.
WPN DATA
81-MM (M252)
CARRIER MOUNTED: NO
BP: A2 GRID: AP: 13225 92885
ALT: 0420
AZ: 5340 DEF: 2800
A1: Dir 0540 Dis 035
A3: Dir 3740 Dis 035
A4: Dir 3740 Dis 070
FO LOCATION
F21 AP: 09850 93100
ALT: 0300
NO FIRE LOCATION
ZN1 04 PTS
PT1 09450 93300
PT2 10650 93300
PT3 10650 93500
PT4 09450 93500
TASK: Store safety data in the MBC.
D-28
FM 23-91
SAFETY DATA
LLAZ 4940
RLAZ 5740
MAX RN 3800
MIN RN 0450
MIN CHG 1
MAX CHG 7
The company commander has directed that an FPF be placed at grid 10850 93410. The
platoon leader informs the FO, and the FO sends the call for fire in Figure D-10.
D-29
FM 23-91
23. What is the burst point grid for the first round?
NOTE: The FO spots the round and sends: NO. 4 GUN, LEFT 25, ADD 25.
NOTE: The round is fired and the FO sends: NO. 4 GUN ADJUSTED, REPEAT NO.
3 GUN.
25. What is the correct deflection and elevation for No. 3 gun?
DEF (mils) ELEV (mils) DEF (mils) ELEV (mils)
(a) 3134 1059 (c) 3126 3127
(b) 3124 1050 (d) 3134 0975
D-30
FM 23-91
26. What is the correct deflection and elevation for the No. 2 gun?
DEF (mils) ELEV (mils) DEF (mils) ELEV (mils)
(a) 3126 0974 (c) 3127 0975
(b) 3141 0977 (d) 3141 0950
NOTES: 1. The round is fired, and the FO sends: NO. 2 ADJUSTED, REPEAT NO. 1
GUN.
2. The round is fired, and the FO sends: EOM, FPF ADJUSTED.
SITUATION I
A short time after adjusting the FPF, you receive the call for fire and FDC order in Figure D-
11.
TASK: Compute data for a grid mission using the call for fire and FDC order in
Figure D-11.
D-31
FM 23-91
CONDITIONS: Given an initialized MBC, call for fire using grid coordinates as the
method of target location, computer's record, and data sheet.
STANDARDS: Compute data for the missions initial fire command to within 1 mil for
deflection and elevation.
(a) (c)
(b) (c)
NOTE: The FO sends: EOM, AREA SCREENED, EOMRAT AA0205, KNPT 05.
D-32
FM 23-91
SITUATION J
The commander wants a screen at grid 11850 94150. The platoon leader informed the FSO
and the FO. A short time later you receive the call for fire in Figure D-12.
D-33
FM 23-91
NOTES: 1. The FO spots the round and sends: LEFT 50, ADD 100.
2. The round is fired and the FO sends: ADD 100.
3. The FO spots the round and sends: REPEAT WP.
4. The FO sees the WP and sends: FFE, CONTINUOUS FIRE FROM THE
LEFT.
30. What is the total number of WP rounds computed for the mission?
NOTE: The FO calls back: EOM, AREA SCREENED, EOMRAT AA0206, KNPT 06.
SITUATION K
The platoon leader has been ordered to displace No. 3 and No. 4 guns to a new firing point.
Enter the following weapon data:
WPN DATA
BP: B3
CARRIER MOUNTED: NO
GRID: 10750 91300
ALT: 0350
AZ: 6400 DEF: 2800
B4: Dir 4900 Dis 040
D-34
FM 23-91
Shortly after the section occupies its new position, another fire request is received. Use the
call for fire and FDC order in Figure D-13 to compute the mission.
TASK: Compute firing data for a polar mission using the call for fire and FDC
orders in Figure D-13.
CONDITIONS: Given an initialized MBC, call for fire, computer's record, and data sheet.
STANDARDS: Compute the firing data for the mission to within 1 mil for deflection and
elevation.
D-35
FM 23-91
(a) (b)
(c) (d)
D-36
FM 23-91
(a)
(b)
(c)
(d)
NOTE: The FO sends: EOM, TANKS BURNING, EOMRAT AA0207, KNPT 07.
SITUATION L
The No. 3 and No. 4 guns have now displaced back to their position with the rest of the
platoon. Another mission is received in the FDC. Use the call for fire and FDC order in
Figure D-14 to compute the mission.
TASK: Compute data for a searching mission using the call for fire and FDC
order in Figure D-14.
CONDITIONS: Given an MBC with a mission already in progress.
STANDARDS: Compute data for the corrections to within 1 mil for deflection and
elevation, and determine turns to the nearest one-half turn.
D-37
FM 23-91
NOTES: 1. The FO spots the initial round and sends a correction: RIGHT 200, DROP
200.
2. That round is fired, and the FO sends his next correction: LEFT 50, DROP
100.
3. That round is fired, and the observer calls back: ADD 50, FFE.
D-38
FM 23-91
33. What is the correct deflection, charge, and elevation for the near edge of the target?
DEF (mils) CHG ELEV (mils) DEF (mils) CHG ELEV (mils)
(a) 2652 6 1062 (c) 2645 7 1072
(b) 2642 7 1083 (d) 2642 7 1072
34. What is the correct deflection, charge, and elevation to the far edge of the target?
DEF (mils) CHG ELEV (mils) DEF (mils) CHG ELEV (mils)
(a) 2649 6 0982 (c) 2645 7 1051
(b) 2649 7 0997 (d) 2649 7 0982
NOTE: The FO observes the FFE and sends: EOM, TROOPS DISPENSING, EOMRAT
AA0208, KNPT 08.
SITUATION M
Just at dusk of the same day, the FDC receives another fire request. Use the call for fire and
FDC order in Figure D-15 to compute the mission.
TASK: Compute data for a traversing mission using the call for fire and FDC
order in Figure D-15.
CONDITIONS: Given an MBC with a mission already in progress.
STANDARDS: Compute data for the corrections to within 1 mil for deflection and
elevation, and determine turns to the nearest one-half turn.
D-39
FM 23-91
NOTES: 1. The FO spots the round and sends the correction: LEFT 200, DROP 200.
2. The round is fired, and the FO sends another correction: RIGHT 100, ADD
25.
3. The round is spotted by the FO, and he sends the correction: LEFT 50, FFE,
TRAVERSE RIGHT.
D-40
FM 23-91
(a)
(b)
(c)
(d)
D-41
FM 23-91
SITUATION N
It is now dark and the platoon is prepared for night firing. The FDC receives a fire request.
Use the call for fire and FDC order in Figure D-16 to compute the mission.
D-42
FM 23-91
NOTE: The round is fired and the FO sends the correction: RIGHT 200, DROP 400,
DOWN 100.
(a)
(b)
(c)
(d)
TASK: Compute data for a coordinated illumination mission using the call for fire
in Figure D-17.
CONDITIONS: Given an initialized MBC, call for fire, computer's record, and data sheet.
STANDARDS: Compute firing data for the deflection and elevation to within 1 mil for
all high-explosive and illumination rounds for the initial and subsequent
fire commands.
NOTE: The round is fired, and the FO sends a coordinated illumination and HE call for
fire.
D-43
FM 23-91
(a) (b)
(c) (d)
D-44
FM 23-91
NOTES: 1. No. 1 gun fires an illumination round and the FO sends: ILLUM MARK.
2. The MARK TIME is 50 seconds.
3. ILL and HE rounds are fired and the FO calls back: HE, DROP 100.
NOTE: ILL and HE rounds are fired, and the FO calls back: HE, RIGHT 50, DROP 50,
FFE
40. What is the correct deflection and elevation for the No. 2, No. 3, and No. 4 guns in the
FFE?
DEF (mils) ELEV (mils) DEF (mils) ELEV (mils)
(a) 2946 1047 (c) 2946 1063
(b) 2946 1055 (d) 2946 1070
NOTE: The FO observes the FFE and sends: EOM, VEHICLES BURNING, EOMRAT
AA0409, KNPT 09.
SITUATION O
The following are questions relating to various MBC situations:
41. When the MBC is connected to a radio, it is proper procedure to conduct a MODEM
test.
TRUE FALSE
42. While operating the MBC, the computer becomes unusually hot and a hissing sound is
detected. The first thing to do is turn the MBC off.
TRUE FALSE
43. When storing the MBC, the battery can be left in the computer for an unlimited length
of time.
TRUE FALSE
44. While operating the MBC using an external power source in the vehicle, the vehicle
should not be started.
TRUE FALSE
45. Never use a sharp object, such as a pencil, to press the switches when operating the
MBC.
TRUE FALSE
D-45
FM 23-91
46. The MBC is waterproof when one switch on the keyboard is punctured.
TRUE FALSE
47. The first step before operating the MBC is to place a battery into the battery
compartment.
TRUE FALSE
48. The last check before operating the MBC is to conduct a self-test.
TRUE FALSE
49. How many types of messages can the MBC receive from a DMD?
a. 4 c. 14
b. 9 d. 2
50. When receiving a completed fire request (FR) message from the DMD, why must you
review it before processing the mission?
a. To prevent errors.
b. To be able to send an MTO.
c. To receive an ACK.
d. To manually enter the GRID switch.
51. When entering SET-UP, data what two entries must be the same as the DMD to
communicate digitally?
52. After pushing the COMPUTE switch during a mission and the display window displays
*RANGE ERR*, what is the correct action to take?
D-46
FM 23-91
53. When receiving an FR from a DMD or over the radio, the display window displays
SAFETY VIOLATION. What corrective action should be taken?
a. Recompute.
b. Send an MTO.
c. Send a CMD message.
d. Clear out safety diagram.
54. Which FM or TM is used when performing a PMCS on the M23 mortar ballistic
computer?
a. FM 23-90.
b. TM 9-1350-261-10.
c. TM 9-1300-257-10.
d. TM 9-1220-246-12&P.
55. After entering safety data into the MBC, the need for safety T's is no longer warranted.
TRUE FALSE
SITUATION A
You are going to the firing range. The platoon leader goes to range control and obtains the
safety information. Using the information below, construct a safety diagram.
D-47
FM 23-91
57. What is the minimum elevation (mils that can be fired at the maximum range)?
SITUATION B
You move out to the field. The platoon leader determines an eight-digit grid and an altitude
to the mortar position. He instructs you to construct a modified-observed firing chart.
TASK: Prepare a plotting board for operation using the modified-observed firing
chart.
CONDITIONS: Given an M16 plotting board, a Fort Benning Installation Map 1:50,000,
Edition 1-DMA, Series:V745Z; a mil protractor; area of responsibility;
a direction of fire (DOF); an eight-digit coordinate to the mortar position;
target or registration point (RP); and a grid intersection to represent the
pivot point.
STANDARDS: Superimpose a grid system on the M16 plotting board using the grid
intersection given without error.
D-48
FM 23-91
The section leader receives a call for fire and checks the map. He then hands you the call for
fire in Figure D-18 and instructs you to compute the mission.
TASK: Compute data for a grid mission using the call for fire and FDC order in
Figure D-18.
CONDITIONS: Given an M16 plotting board, sector of fire, 1:50,000 map, protractor,
computer's record, tabular firing tables, call for fire for a grid mission, FO
corrections, paper, and pencil.
STANDARDS: Determine the deflection to within 1 mil with a 10-mil tolerance and the
range to within 25 meters with a 25-meter tolerance.
TASK: Determine the vertical interval (VI) between the mortar altitude and the
target altitude.
CONDITIONS: Given the mortar altitude and the target altitude.
STANDARDS: Determine the VI to the nearest whole meter and the range correction to
apply without error.
TASK: Determine VI to the nearest whole meter and the range correction to apply
without error.
CONDITIONS: Given an M16 plotting board, altitude of the mortar position, call for fire
with the target altitude, and a firing table.
STANDARDS: Apply the VI correction without error when computing a mission. Record
and update firing records. Determine deflections to the nearest 1 mil with
a 10-mil tolerance. Determine the range to within 25 meters with a
25-meter tolerance. Convert the range to the correct charge and elevation.
D-49
FM 23-91
NOTE: The FO spots the first round and sends these corrections: RIGHT 150, DROP 50,
FFE; OT direction 1800.
D-50
FM 23-91
(a)
(b)
(c)
(d)
NOTE: The rounds are fired and the FO sends EOM. Update and mark as target AC071.
You receive the call for fire in Figure D-19 and see that it is in your area of operations. You
are instructed to compute the mission.
TASK: Compute data for a grid mission using the call for fire and FDC order in
Figure D-19.
CONDITIONS: Given an M16 plotting board, sector of fire, 1:50,000 map, protractor,
computer's record, tabular firing tables, call for fire for a grid mission, FO
corrections, paper, and No. 2 pencil.
STANDARDS: Determine deflection to within 1 mil with a 10-mil tolerance and range
to within 25 meters with a 25-meter tolerance.
TASK: Determine the vertical interval (VI) between the mortar altitude and the
target altitude.
CONDITIONS: Given the mortar altitude and target altitude.
STANDARDS: Determine the VI to the nearest whole meter and the range correction to
apply without error.
TASK: Determine VI and the correction to apply when computing a mission using
the M16 plotting board.
CONDITIONS: Given an M16 plotting board, altitude of the mortar position, call for fire
with the target altitude, and firing table.
STANDARDS: Apply the VI correction without error when computing a mission. Record
and update firing records. Determine deflections to the nearest 1 mil with
a 10-mil tolerance. Determine the range to within 25 meters with a
25-meter tolerance. Convert range to the correct charge and elevation.
D-51
FM 23-91
D-52
FM 23-91
(a) (b)
(c) (d)
You are handed the call for fire and FDC order in Figure D-20 and are instructed to compute
the mission.
D-53
FM 23-91
63. The initial chart range is 2,375. What is the command range?
D-54
FM 23-91
NOTE: The FO spots the first round and sends this correction: ADD 50, FFE.
You receive the call for fire, check the map, and issue the FDC order to the computers.
Using the call for fire and FDC order in Figure D-21, compute the mission.
D-55
FM 23-91
D-56
FM 23-91
(a) (b)
(c) (d)
NOTE: The FO spots the first round and sends: DROP 50, FFE.
D-57
FM 23-91
(a)
(b)
(c)
(d)
SITUATION C
Your platoon is moving to a defensive position for a few days. Your platoon leader has the
site surveyed. He then instructs you to set up a surveyed firing chart and to conduct a
coordinated registration. Using the information below, construct a surveyed chart. Using
the information in Figure D-22, conduct the registration mission.
D-58
FM 23-91
69. What is the command deflection and command range for the first round?
DEF (mils) RANGE (mils)
(a) 3373 3,775
(b) 3820 3,750
(c) 3820 3,675
(d) 3773 3,625
NOTE: The FO spots the first round and sends these corrections: LEFT 50, ADD 50.
D-59
FM 23-91
70. What is the deflection and elevation for the second round?
DEF (mils) RANGE (mils)
(a) 3831 0880
(b) 3801 0839
(c) 3959 0896
(d) 3781 0862
NOTES: 1. The FO spots the second round and sends: ADD 25, EOM, REGISTRATION
COMPLETE.
2. The FDC sends a message to the FO: PREPARE TO ADJUST SHEAF.
3. The FO sends: SECTION LEFT.
TASK: Compute firing data for a sheaf adjustment using the plotting board.
CONDITIONS: Given an M16 plotting board, an active registration mission, FO
corrections for sheaf adjustments, computer's record, and firing tables.
STANDARDS: Determine total range correction (TRC) to apply within 25 meters range
with a 25-meter tolerance.
(a)
(b)
(c)
(d)
NOTES: 1. The FO makes a spotting and sends: NO. 3, RIGHT 10; NO. 1, RIGHT 20;
NO. 4 ADJUSTED, EOM S/A.
2. The command range to the target is 3,750 meters.
72. What are the deflections for the No. 3 and No. 1 guns?
No. 3 DEF (mils) No. 1 DEF (mils)
(a) 3777 3780
(b) 3843 3840
(c) 3793 3797
(d) 3827 3824
D-60
FM 23-91
73. If the initial chart deflection was 3820 and the final chart deflection was 3830, what is
the deflection correction for RP No. 1?
74. The initial chart range was 3,700 and the RP was hit at a command range of 3,750.
What is the range correction factor (RCF)?
After updating and computing all the corrections, you receive a call for fire. The section
leader hands you the call for fire and FDC order in Figure D-23 and instructs you to compute
the mission.
TASK: Compute firing data from a surveyed firing chart for a total range
correction mission using a plotting board.
CONDITIONS: Given an M16 plotting board, an RP with deflection correction and range
correction factors, call for fire, computer's record, and firing tables.
STANDARDS: Determine total range correction to apply within 25 meters for range with
a 25-meter tolerance.
D-61
FM 23-91
D-62
FM 23-91
APPENDIX E
TERRAIN MORTAR POSITIONING
To increase survivability on the battlefield, a mortar platoon section must
take advantage of the natural cover and concealment afforded by the terrain
and existing vegetation. Each mortar is positioned to fit the folds and
vegetation of terrain without regard to the bursting diameter of the mortar's
ammunition. When mortars are positioned without regard to standard
formations, firing corrections (M16/M19 plotting boards) are required to
obtain a standard sheaf in the target area. These corrections compensate for
the terrain positioning of the mortars (Figure E-1).
DOF
E-1
FM 23-91
AZIMUTH OF LAY
FORWARD
L R
BEHIND
BASE PIECE
PLATOON CENTER
c. The hasty traverse technique is the most accurate and rapid technique for determining
piece displacement. The deflection and distance from each mortar to the aiming circle must
be measured to plot their locations on the M16/M19 plotting board. These deflections are
recorded and reported to the FDC. The distance from each mortar to the aiming circle can
be determined by the following methods:
(1) In straight-line pacing, each squad has one man to pace the distance from the
mortar to the aiming circle. The gunner can guide the man on a straight line by sighting
through the mortar sight.
(2) When using a subtense bar for TMPC computations, a 2-meter rod is used. It is
held parallel to the ground at the aiming circle location. Each gunner traverses his sight from
one end to the other and records the number of mils traversed by the sight. This value is used
to enter a subtense table (Table C-1, page C-10) to determine the number of meters between
the mortar and the aiming circle. Distances up to 250 meters can be measured to within a
fraction of a meter. (For details on the use of the subtense bar and the subtense table, refer
to Appendix C.)
d. Once the deflection and distance values are known for each mortar, their locations
can be plotted on the M16/M19 plotting board. The pivot point represents the location of the
base piece. The location of the aiming circle is plotted in relation to the base piece. The
other mortars are plotted in relation to the aiming circle.
E-2
FM 23-91
meters and drawing the burst lines parallel to the center line. The distance between burst
lines is equal to the bursting diameter of the mortar systems' HE ammunition. For the M224
mortar, the distance is 30 meters; for the M29A1 mortar, the distance is 35 meters; for the
M252 and M30 mortars, the distance is 40 meters; and for the M120, the distance is 60
meters. A burst line is drawn for each mortar in the platoon or section (Figure E-3).
b. The clear rotating disk of the plotting board is used to plot the location of each
mortar. The disk has an azimuth scale around the outside edge; a temporary lay deflection
scale must be superimposed on the disk. The lay deflection scale increases from left to right
as does the azimuth scale. Deflection 3200 always corresponds to the azimuth of lay when
determining piece displacement (Figures E-4a to E-4d). Once superimposed, the lay
deflection scale is used to plot the location of the aiming circle and the mortars.
E-3
FM 23-91
EXAMPLE
Given: Azimuth of lay is 6400 mils.
The deflection and distances from the aiming circle to each mortar are:
AZ5600
DF2400
AZ6400
DF3200
AIMING
CIRCLE
95 M
#3 (BASE PIECE)
Step 1. Index the lay deflection from the aiming circle to No. 1 (1800 mils over the center
line arrow).
Step 2. Count off 200 meters parallel to the center line down from the aiming circle.
Place a circled dot there and label it No. 1.
E-4
FM 23-91
AZ500
DF1800
AIMING
CIRCLE
AZ5400
DF3200
#3
(BASE
PIECE)
200 M
#1
STEPS 1 AND 2
Step 3. Index the lay deflection from the aiming circle to No. 2 (1900 mils over the center
line arrow).
Step 4. Count off 135 meters parallel to the center line down from the aiming circle.
Place a circled dot there and label it No. 2.
Step 5. Index the lay deflection from the aiming circle to the No. 4 (2950 mils over the
center line arrow).
Step 6. Count off 120 meters parallel to the center line down from the aiming circle.
Place a circled dot there and label it No. 4.
E-5
FM 23-91
AZ6150
DF2950 AZ6400
DF32000
AIMING
CIRCLE
120 M
#3
#2
#1
#4
STEPS 3 THROUGH 8
NOTE: Once all mortar locations are plotted, erase the temporary lay deflection scale and
superimpose a referred deflection scale as performed when setting up the
M16/M19 plotting board. For example, if the referred deflection is 2800, the
referred deflection scale is superimposed on the disk beginning with 2800
corresponding with the azimuth of lay. The deflection increases to the left and
decreases to the right.
Step 8. Index the azimuth of lay (6400 mils over the center line arrow) and read the
displacement of each mortar right/left and forward/behind the base piece.
E-6
FM 23-91
AZ6400
ANSWERS:
#2 #1
#3 (BASE PIECE)
#6
#4
#5
STEPS 7 AND 8
ANSWERS
Mortar Displacement
No. 1 130R, 30F
No. 2 60R, 30F
No. 3 (Base Piece) — —
No. 4 40R, 45B
No. 5 95L, 70B
No. 6 145L, 15B
(R —right; L—left; F—forward; B—behind)
c. TMPCs are computed before occupation of a position by the mortars when possible,
but they can be computed after occupation. They are applied to each mortar's firing data to
achieve standard sheafs in the target area. The TMPCs are computed and applied whenever
the mortar platoon occupies a position that is wider than the width of the mortar system's
sheaf or deeper than the bursting diameter of its HE ammunition.
d. The TMPCs are most accurate at the range and direction for which they were
computed. They are considered valid 2,000 meters over and short of the center range and
200 mils left and right of the center azimuth of the sector (Figure E-5).
E-7
FM 23-91
2,000 M
CENTER
RANGE
2,000 M
CENTER
DEFLECTION
(1) The TMPCs provide acceptable sheafs on targets as long as the platoon position
is within the dimension parameters below:
• Six guns—400 meters wide by 200 meters deep.
• Four guns—250 meters wide by 200 meters deep.
• Three guns—200 meters wide by 100 meters deep.
• Two guns—100 meters wide by 100 meters deep.
(2) The box formed by the dimension parameters is centered over the platoon and
oriented perpendicular to the azimuth of lay. If the platoon is spread out more than indicated
dimensions, a degradation in the effectiveness of sheafs can be expected as fires are shifted
throughout the sector away from the center range and deflection (Figure E-6).
E-8
FM 23-91
400 METERS
2
6
3
200 METERS
4 BASE
PIECE
(3) Since a mortar unit's area of responsibility covers an area larger than the TMPC
limits, TMPCs should be computed for three sectors: primary, left, and right. Sectors can
also be computed for shorter or longer ranges to provide valid corrections throughout the
mortar systems available range.
(4) When using TMPCs, the platoon leader must establish an SOP directing that
primary TMPC sector data are used unless otherwise indicated. If other than the primary
sector is to be used, it is indicated in the corrections to apply in the FDC order or
immediately following the announcement of MORTAR TO FOLLOW in the initial fire
command:
EXAMPLE
SECTION, LEFT SECTOR, HIGH-EXPLOSIVE
PROXIMITY, DEFLECTION..................….........
NOTE: The absence of any instruction concerning TMPCs in the initial fire command
indicates that corrections for the primary sector will be fired. The command,
CANCEL TERRAIN CORRECTIONS indicates that no TMPCs are to be used
for that mission.
E-9
FM 23-91
NOTE: The TMPC worksheet can also be used to compute individual gun corrections for
special missions such as attitude missions.
E-10
FM 23-91
E-11
FM 23-91
d. Using the mil conversion table (deflection conversion table) (Table E-1), determine
the 100/R value at the center range for the sector and record it in block 4. The largest 100/R
value used is 40; therefore, if 100/R is actually larger than 40, enter in block 4. Now,
perform the computation shown in the heading of block 5. Label the corrections L (for left)
or R (for right). The sign used in block 3 always carries to block 5. Express and record the
value to the nearest mil.
e. In column 7, add the position range correction to the center range to obtain the
corrected range. This value is used to compute the position time correction in column 9.
f. Enter the tabular firing table at the corrected range and extract the fuze setting.
Record this value in column 8. Subtract the fuze setting corresponding to the center range
from the value in column 8 and record the difference in column 9.
g. The values in columns 5, 6, and 9 are either sent to the guns and applied by the squad
leader to the command data for each mission fired, or the FDC computes and applies the
data, and it sends the corrected command data to each mortar for each mission.
E-12
FM 23-91
EXAMPLE
A 4.2-inch mortar platoon is engaging a target at a range of 5,000 meters and a
deflection of 2950. (The target is within the transfer limits of the primary TMPC
sector.) The FDC issues the initial fire command: PLATOON, HE QUICK,
NUMBER TWO GUN, TWO ROUNDS FUZE TIME, DEFLECTION TWO NINE
FIVE ZERO (2950), CHARGE 35 3/8, TIME 34.7, ELEVATION ZERO EIGHT
ZERO ZERO (0800).
b. Applying TMPCs for the No. 2 mortar, the squad leader adds 4 mils to the command
deflection 2950 to determine his deflection to fire (2954). To determine his charge to fire,
he enters the TFT at elevation 0800 with extension and charge 35 3/8. He extracts the
corresponding command range (5000) for that charge and adds his position range correction
(-30) to determine his range to fire (4970). He then reenters the TFT at the range to fire and
extracts the corresponding charge to fire (35 1/8). To determine his time setting to fire, the
squad leader adds his position time correction (-0.1) to the command time setting (34.7) and
fires a time setting of 34.6.
c. Coupled with a registration, TMPCs eliminate the need to adjust the sheaf, thereby
saving ammunition and decreasing the chances of detection by enemy countermortar radar.
d. Determining TMPCs for left and right sectors is accomplished with the same
procedure using the center deflection to each of the sectors. The same applies to computing
TMPCs for ranges that are outside the original TMPC sectors.
NOTE: The procedures are the same for the 60/81/120-mm mortars with the exceptions
mentioned.
E-13
FM 23-91
provide cover and concealment for the platoon while placing acceptable sheaves on target
(Figure E-8).
2 100 M
DOF
6
50 M
20 M
3
50 M BASELINE
BASE
100 M PIECE
4
5
(NOT TO SCALE)
a. To use the modified technique, the platoon occupies the position, conforming to the
folds and tree lines of the terrain. It maintains a lateral dispersion between mortars equal to
the bursting diameter of an HE round.
b. An imaginary line (base line) is drawn through the base piece perpendicular to the
direction of fire (azimuth of lay). From this line, the squad leader determines the distance
to his mortar. Mortars, other than the base piece, will either be on line with, forward of, or
behind the basepiece. The distance from the base line can be measured by a squad member
while the mortar is being laid or estimated by the squad leader. This distance is referred to
as the position range correction and is recorded for future use by the squad leader. This
position range correction is also given to the FDC for future use in computing TMPCs for
the left and right sectors of fire. This position range correction is applied to the command
data and issued by the FDC for a fire mission in the same manner as described in applying
normal TMPCs.
c. The modified terrain mortar positioning technique establishes TMPCs for the primary
sector and allows the platoon to rapidly engage targets, upon occupation of the position, up
to 200 mils left or right of the azimuth of lay and achieve an acceptable sheaf on target. As
soon as time allows, the FDC must compute TMPCs for the left and right sectors using the
same procedures described in computing normal TMPCs to achieve acceptable sheaves on
targets in those sectors.
d. There are no position deflection corrections for the primary sector. There will be
position deflection corrections for the left and right sectors. Position time corrections should
be computed as quickly as possible for the primary sector if fuze M564 is to be used.
E-14
DATA SHEET
For use of this form, see FM 23-91. The proponent agency is TRADOC.
LOT NUMBER
WEIGHT
ON HAND
RECEIVED
TOTAL
ROUNDS
EXPENDED
ROUNDS
REMAINING
TARGET DATA
TARGET CHART FIRING FIRING INTELLIGENCE ROUNDS
ID DATA CORRECTIONS DATA
TGT GRID ALT DEFL RG DEFL RANGE ALT ALT DEFL RG FUZE TIME ELEV TIME TARGET METHOD OF SURVEILLANCE EXP REM
NO. CHG CORR CORR VI CORR CHG SETTING FIRED DESCRIPTION ENGAGEMENT
DA FORM 2188-R, MAR 1991 REPLACES DA FORM 2188-R, MAR 1977 WHICH IS OBSOLETE. USAPA V1.00
FM 23-91
GLOSSARY
AAR after-action report
AC Active Component
ACCP Army Correspondence Course Program
A/F adjust fire
AMC at my command
ANCOC Advanced Noncommissioned Officer Course
ARTEP Army Training and Evaluation Program
bn battalion
BNCOC Basic Noncommissioned Officer Course
BLTM battalion-level training model
D delta
DA Department of the Army
DCT deflection conversion table
DEPEX deployment exercise
DMD digital message device
DOF direction of fire
DS direct support
FA field artillery
FDC fire direction center
FDCCP Fire Direction Center Certification Program
FFE fire for effect
Glossary-1
FM 23-91
GD grid declination
GMT Greenwich mean time
GS general service
GT gun-target
GTA graphic training aid
HE high explosive
HEQ high-explosive quick
HOB height of burst
HTA Hohenfels training area
LD line of departure
LED light-emitting diode
LFX live fire exercise
LRTR long-range training round
M meter(s)
MAPEX map exercise
max maximum
Glossary-2
FM 23-91
ra range
RALS right add, left subtract
RATELO radiotelephone operator
RC Reserve Component
Glossary-3
FM 23-91
TC training circular
TEC training extension course
T&EO training and evaluation outline
TEWT tactical exercise without troops
TFC technical fire control
TFT tabular firing table
TG training guide
TM technical manual
TMPC terrain mortar positioning correction
TOC tactical operations center
TOE table of organization and equipment
TOF time of flight
TOT time on target
TRADOC Training and Doctrine Command
TRC total range correction
VA vertical angle
VI vertical interval
Glossary-4
FM 23-91
vs versus
VT variable time
WP white phosphorus
wpn weapon
W/R when ready
Glossary-5
FM 23-91
REFERENCES
DOCUMENTS NEEDED
These documents must be available to the intended users of this publication.
ARTEP 7-90-MTP Mission Training Plan for the Infantry Mortar Platoon, Section,
Squad. August 1989.
DA Form 2601-1 MET Data Correction Sheet for Mortars. October 1971.
DA Form 2601-2-R MET Data Correction Sheet 6400 Mils (Mortars). October 1971.
DA Form 4176 Target Plotting Grid Field Artillery Graduated in Mils and Meters,
Scale 1:25,000. October 1973.
*FT 81-AQ-1 Firing Table for Mortar, 81-mm: M29A1 and M29. August 1981.
References-1
FM 23-91
STP 7-11C14-SM-TG Soldier's Manual, Skill Levels 1/2/3/4 and Trainer's Guide, MOS
11C, Indirect Fire Infantryman. August 1994.
DOCUMENTS RECOMMENDED
These readings contain relevant supplemental information.
FM 6-30 Tactics, Techniques, and Procedures for Observed Fire. July 1991.
References-2
FM 23-91
INDEX
Index-1
FM 23-91
Index-2
FM 23-91
Index-3
FM 23-91
Index-4
FM 23-91
Index-5
FM 23-91
zone
surface danger, B-1
zone fire, 8-19, 13-8, 13-11
100 meters, 8-21 (illus)
200-meters, 8-22 (illus)
Index-6