DME Concept

PROFICIENCY LINKED INTEGRATED COURSE
ON

Volume ‐ I
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AIRPORTS AUTHORITY OF INDIA
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Civil Aviation Training College, Allahabad, India
Edition: May, 2012

Volume ‐ I: ‐ Concepts

Table of Contents

Chapter Page
Chapter Name
No. No.
1. Introduction to DME 1
2. DME Concepts 7
3. ICAO Specifications 22
4. DME Antenna 54
5. Siting Criteria for DME 65
6. Doc – 8071 : Ground & Flight Testing of DME 68
7. Flight Inspection 77
8. DGCA Civil Aviation Requirements (CAR) 78
9. CNS Manual and Circulars 83

CNS Circular No: 01 of 2008 - Maintenance of Navigational 84
Aid Site
CNS Circular No. 01/2010: Updation of Station CNS Manual 89
CNS Circular No: 01 of 2011 - Guidelines for provisions of 91
power supply systems to CNS / ATM Automation Systems/
Facilities
CNS Circular No: 02 of 2011 – SOP for Opening the Watch – 112
CNS point of view
CNS Circulars 02/2012: Standard Operating Procedures 115
(SOP)
CNS Circulars 03/2012: Alternate means for provision of 125
information on the operational status of Radio Navigation
Aids.
Chapter - 01 Introduction to DME
Chapter- 01
Introduction to DME
1.1 NAVIGATION:
Navigation is the ‘ART’ of determining the position of an aircraft over earth’s surface
and guiding its progress from one place to another.
To accomplish this ‘ART’ some sort of aids are required by the pilots. In the early days,
voyages were accomplished by the navigators through the knowledge of terrain or
movements of sun, stars and winds. As the time progressed, some instruments such as
Compass, Chronometer and theodolite came on the scene.
In the twentieth century, electronics also entered in the aviation field, direction finders
and other navigational aids enabled the navigators to obtain fixes using electronics aids
only.
Radio Navigation:
This method is based on the use of Radio Transmitter, Radio Receiver and propagation
of electromagnetic waves to find navigational parameters such as direction, distance
etc., required to find the position of the aircraft. The Radio Navigational aids provide
information to the pilot regarding the position of his/her aircraft in azimuth and/or
elevation at any instant of time. Radio communication and navigational aids also
provide useful information to Air Traffic Control Officers for effective control of air
traffic.
CATEGORIZATION OF RADIO NAVIGATIONAL AIDS:
Radio navigational aids can be classified in different ways. The classification helps in
identifying the usefulness of a given facility. All navigational aids, which provide
guidance by using Radio waves, are called Non-visual aids.
According to service range, the radio navigational aids are broadly classified into three
categories:
a. Long range
b. Medium range
c. Short range
Civil Aviation Training College, India Page 1

a. Long Range navigational aids:

Some of the Aids operating worldwide in this category are OMEGA and Long Range
Aid to Navigation (LORAN). They operate in Very Low Frequency (VLF) and Low
Frequency (LF) bands of frequency spectrum, i.e. 10 KHz, 50 – 100 KHz and 100 – 200
KHz respectively to give very long ranges of the order of 7000 Kms and 700 Kms
respectively. They are based on hyperbolic system of navigation. Airports Authority of
India (AAI) does not provide these aids, although aircraft equipped with corresponding
receiving equipment can use these facilities while flying over Indian air space.
b. Medium Range navigational aids:

NDB (Non Directional Beacon) falls in this category. It operates in the LF/MF band of
frequency spectrum with a nominal range of 150 – 250 nautical miles (NM), and even
up to 350 NM over high seas.
c. Short Range navigational aids:

Some of the important and widely used short-range aids are: VHF DF, VOR, DME, ILS
and RADARS. These aids operate in and above VHF bands and hence the coverage is
dependant upon line-of-sight phenomenon.
According to use and Frequency, the radio navigational aids are broadly classified into
short range and medium range as tabulated below.
Short Range Aids:
NAME OF THE SYSTEM FREQUENCY BAND POWER RANGE

EQUIPMENT (IN WATTS) (NM)
NDB Locator 200 – 450 KHz <50 45
VOR Terminal VOR 108 – 112 MHz 13 25
Localizer ILS 108 – 112 MHz 10 25
Glide Path ILS 328 – 336 MHz 10 10
DME ILS - DME 960 – 1215 MHz 100 25

Medium Range Aids:
NAME OF THE SYSTEM FREQUENCY BAND POWER RANGE

EQUIPMENT (IN (NM)
WATTS)
NDB Homing & En- 200 – 450 KHz 500 & 150 &
route >1KW >250
VHF D/F Homing 118 – 136 MHz -- 150
VOR Homing & En- 112 – 118 MHz 100 200

route
DME Homing & En- 960 – 1215 MHz 1KW 200
route
Inter relationship in terms of Frequency, Power, Range and System
1.2 Navigation Fix:
DME's use as a navigation aid is based on the principles of Rho-Theta Navigation

System.
The Rho-Theta Navigation System is based on the Polar coordinate system of azimuth
and distance as shown in figure 1.1.
Figure 1.1 Rho-Theta Navigation Systems

The Very High Frequency Omni Range (VOR) and DME constitute the basic
components of the Rho-Theta Navigation System. While the VOR provides azimuth
information (Theta) to the pilot, the DME provides the distance information (Rho) so
that the pilot receives a continuous navigational fix relative to a known ground location.
The DME equipment on the aircraft is very simple to use, the pilot having only to tune
the equipment to the appropriate frequency and read the display once the DME has
locked on to the ground transponder. The frequency control for the DME receiver is
normally derived from the VOR/Localizer receiver, the DME channels being paired
with the VHF frequencies used by the VOR/Localiser equipment.
Because the distance measurement taken by the aircraft DME receiver is from Air-to-
Ground, DME records Slant Ranges which are greater than the actual distance between
the ground facility and the ground position of the aircraft. The difference between the
slant and actual range is dependent on the relationship of the aircraft height and
distance from the ground equipment. Unless the aircraft is 5000 feet or more, close to
the ground transponder, then the discrepancy is small and can, for all practical
purposes, be ignored.
1.3 HISTORY OF DME
As early as 1946 many organisations in the West took an active part in the development
of DME system. The Combined Research Group (CRG) at the Naval Research
Laboratory (NRL) designed the first experimental L band DME in 1946.
The L band, between 960 MHz and 1215 MHz was chosen for DME operation mainly
because:
a. Nearly all other lower frequency bands were occupied.
b. Better frequency stability compared to the next higher frequencies in the
Microwave band.
c. Less reflection and attenuation than that experienced in the higher Frequencies in
the microwave band.
d. More uniform omni directional radiation pattern for a given antenna height than
that possible at higher frequencies in the microwave band.

1.4 PURPOSES AND USE OF DME
PURPOSE OF DME INSTALLATION

Distance Measuring Equipment is a vital navigational Aid, which provides a pilot with
visual information regarding his position (distance) relative to the ground based DME
station. The facility even though possible to locate independently, normally it is
collocated with either VOR or ILS. The DME can be used with terminal VOR and
holding VOR also. DME can be used with the ILS in an Airport; normally it is collocated
with the Glide path component of ILS.
Association of DME with VOR

Associated VOR and DME facilities shall be co-located in accordance with the
following:
a. Coaxial co-location: the VOR and DME antennas are located on the same
vertical axis; or
b. Offset co-location:
For those facilities used in terminal areas for approach purposes or other procedures
where the highest position fixing accuracy of system capability is required, the
separation of the VOR and DME antennas does not exceed 30 m (100 ft) except that, at
Doppler VOR facilities, where DME service is provided by a separate facility, the
antennas may be separated by more than 30 m (100 ft), but not in excess of 80 m (260 ft);
For purposes other than those indicated above, the separation of the VOR and DME
antennas does not exceed 600 m (2,000 ft).
Association of DME with ILS

Associated ILS and DME facilities shall be co-located in accordance with the following:
a. When DME is used as an alternative to ILS marker beacons, the DME should be
located on the airport so that the zero range indication will be a point near
the runway.
b. In order to reduce the triangulation error, the DME should be sited to ensure a
small angle (less than 20 degrees) between the approach path and the direction to
the DME at the points where the distance information is required.

c. The use of DME as an alternative to the middle marker beacon assumes a DME
system accuracy of 0.37 km (0.2 NM) or better and a resolution of the airborne
indication such as to allow this accuracy to be attained.
The main purposes of DME installations are summarised as follows:
• For operational reasons

• As a complement to a VOR to provide more precise navigation service in
localities where there is:
o High air traffic density
o Proximity of routes
• As an alternative to marker beacons with an ILS. When DME is used as an
alternative to ILS marker beacons, the DME should be located on the Airport so
that the zero range indication will be a point near the runway.
• As a component of the MLS
USE OF DME INSTALLATION
The important applications of DME are:

• Provide continuous navigation fix (in conjunction with VOR);
• Permit the use of multiple routes on common system of airways to resolve traffic;
• Permit distance separation instead of time separation between aircraft occupying
the same altitude facilitating reduced separation thereby increasing the aircraft
handling capacity;
• Expedite the radar identification of aircraft; and
• Provide DME distance in lieu of fan marker beacons and radio range intersections
in connection with instrument approaches and holding operations respectively.
1.5 MODELS OF DME IN USE:
Various models of DME are being used in AAI. The facility is collocated with either
VOR or ILS. AWA GCEL DME, ASI DME, THALES DME; these makes are currently in
use at various installations of AAI.

Chapter - 02 DME Concepts
Chapter - 02
DME CONCEPTS
2.1 Principles of operation of DME

The operating principle of DME systems is based on the Radar principle i.e., the time
required for a radio pulse signal to travel to a given point and return. In fact it is
Secondary Radar.
2.1.1 Principles of Secondary Radar

In Secondary Radar system the targets' active participation is necessary for its detection
as against Primary Radars where the targets role is passive.
Secondary Radar system basically consists of two principle components namely the
‘Interrogator’, which is ground, based and the ‘Transponder’, which is carried on
the targets. Each of these components consists of a set of one pulse transmitter and one
receiver. The Interrogator radiates pulses which when received by a corresponding
transponder on a target will initiate a reply from that transponder. These replies are
received by the interrogator to extract information about the targets.
2.1.2 Simplified Block Diagram Of DME System

DME is Secondary Radar with the location of the Transponder and Interrogator
reversed. Figure 3.1 shows the basic block diagram of DME system and figure 2.2 shows
the elements of a DME system.
Referring to figure 2.2, the airborne transmitter repeatedly initiates a process of sending
out very short, very widely spaced interrogation pulses. These are picked up by the
ground transponder receiver whose output triggers the associated transmitter into
sending out reply pulses on a different channel. The airborne receiver receives these
replies. Timing circuits automatically measure the round-trip travel time, or interval
between interrogation and reply pulses, and convert this time into electrical signals,
which operate the distance indicator.

IDENT CODE
Rx
DUPLEXER TIMING
1
CIRCUI
Tx DISTANCE
INDICATOR
(NM)
INTEROGATOR (AIRBORNE)
DUPLEXER
Tx Rx
DECODE
R DELAY,
ENCODE
R IDENT
TRANSPONDER
Figure 2.1 Basic Block Diagram of DME System

5
DISTANCE INDICATION
TIME MEASUREMENT
TIMING 4
CIRCUITS
AIRBORNE
INTROGATOR
TRANSMITTER RECEIVER
1
INTERROGATION
PULSES
AIR-GROUND GROUND-AIR
CHANNEL CHANNEL
REPLY PULSES
3
RECEIVER TRANSMITTER
2
GROUND
TRANSPONDER AUTOMATIC TRIGGERING
(BEACON)
Figure 2.2 Elements of DME System
2.1.3 Modes of Operation
There are two modes of aircraft interrogations. These are called Search and Track.
The Search mode is automatically established whenever the airborne equipment is tuned
to a new DME ground Transponder, or if for some reason, a major interruption in the
replies occurs.

When the aircraft's transmitter is in Search mode, it transmits interrogations at a higher

rate (about 150 interrogations per second). When the aircraft receives at least 65% replies
to its interrogations Lock-on will be established and the transmitter changes to the Track
mode of operation. This process may take up to 30 seconds. Only when this is achieved,
the cockpit readout of the DME range is turned on.
In the Track mode the aircraft's interrogation rate reduces considerably (about 30
interrogations per second). The reduced interrogation rate of transmission in the track
mode will allow more aircraft to use the DME station.
While in Track mode, if the signal is lost momentarily, the equipment enters Memory
State. There are two types of memory states. They are called Velocity Memory and Static
or Position Memory states. During the Velocity Memory state, the distance display
continues to get updated at the same rate as it was prior to the time of signal loss. In the
Static Memory state, the position display is held stationary at the same reading as it was
prior to the time of signal loss. If the signal is not restored within about 10 seconds, the
equipment goes into Search mode of operation again.
2.2 SYSTEM TIMING
2.2.1 TIMING DIAGRAM
The airborne equipment measures the elapsed time between the transmission of the
interrogation pulse and the receipt of the reply pulse and converts time into a distance
indication. The system-timing diagram shown in figure 2.3 can easily explain this
measurement.
Timing starts at the range circuits of the airborne equipment with the starting of the
interrogation pulse. After a time delay t, depending on the distance between the aircraft
and the ground equipment, the interrogation pulse is received at the antenna of the
ground Transponder. The interrogation pulse is detected and after a fixed time delay, the
reply pulse is generated. After time delay t, the reply pulse reaches the airborne
equipment. The distance between the aircraft and the ground Transponder is thus
determined by measuring the total time elapsed between initial transmission of the
interrogation pulse and the reception of the corresponding reply pulse minus the time
delay.

Figure 2.3 DME System Timing
2.2.2 RANGE CALCULATION

The range, in nautical miles, between the aircraft and the transponder is obtained by the
simple formula:
Total time (μsec) - time delay (μsec)
Range = ------------------------------------------------
12.36
The denominator 12.36 μsec is the time taken by the pulse to travel 1 nautical mile to and
fro. This time is also called Radar Mile.
2.3 CHARACTERISTIC FEATURE OF DME

Following are the characteristic features of the DME systems.
2.3.1 DME Channels for ‘X’ & ‘Y’

DME frequency range:
Allotted 960 MHz to 1215 MHz
Critically used 962 MHz to 1213 MHz

Although the frequency band allocated for DME is 960 MHz - 1215 MHz, the lowest DME
operating frequency is 962 MHz and the highest operating frequency is 1213 MHz leaving
2 MHz on either side of the band. This resultant band of 962 MHz -1213 MHz is divided
into 126 one-MHz channels for interrogation, and 126 one-MHz channels for transponder
replies with the interrogation frequency and reply frequency always differing by 63 MHz.
i.e. Channel frequency spacing: 1 MHz between Rx channels and

1 MHz between Tx channels
Rx – Tx Frequency separation: 63 MHz on all operating channels
The number of channels available is doubled by the use of X and Y channels which define
specific pulse separation for the interrogator and reply pulses. The pulse separation for X
channels is 12 microseconds for both interrogation and reply pulses. For Y channels, the
pulse separation is 36 microseconds for interrogation pulses and 30 microseconds for reply
pulses.
The airborne interrogator operates in the frequency band 1025 MHz - 1150 MHz. The
frequency of operation of the X channel ground transponder is 63 MHz lower than the
corresponding airborne interrogator frequency for the first 63 channels and for the next 63
channels; it is higher by 63 MHz. The frequency of operation of the Y channel ground
transponder is 63 MHz higher than the corresponding airborne interrogator frequency for
the first 63 channels and for the next 63 channels; it is lower by 63 MHz.
2.3.2 Twin Pulse (Pulse Pair) Technique

In the present international system specification for DME, the interrogation pulse and the
reply pulse is actually a twin pulse (pulse pair). There is a fixed, pre-arranged spacing
between the components of the twin. Most DME receivers, ground and airborne, are
followed by discriminators or twin pulse decoders, which are adjusted so as to pass only
pulse pairs of the prescribed spacing. Isolated single pulses, or pulse pairs with some other
spacing will not pass the decoder.
The purposes of the twin pulse technique are:

o To increase the average power radiated; and
o To make the system less susceptible to errors or interference caused by false
signals.
By using twin pulse technique, the DME operating channels can be increased. This is done
by establishing new operating channels by using differently spaced twins to define a
number of channels on each radio frequency.

2.3.3 Characteristics of Gaussian Pulse

A waveform similar to Gaussian in shape is used for the DME pulses. Because of its shape
there is less side-band energy radiated therefore, less interference with other DME’s
operating on adjacent channels. The true Gaussian waveform is a symmetrical bell-shaped
statistical curve. However it is possible to create a facsimile of the Gaussian curve by
squaring the amplitude at each point in time of a sine wave. The resultant will be a co-sine
squared waveform, which will be in close approximation of a Gaussian waveform as
shown in figure 2.4.
Pulse envelope
c e
A
o 9A
Voltage amplitude (A)
b f
a g
5
3A
h i
o.1A
0.05A
Time
Tr Tpr Td
Fig. 2.4 pulse shape
The other reason for selecting Gaussian pulse shape for the DME pulses is due to the fact
that the Noise also has a Gaussian wave shape. Noise has been an essential ingredient in
most of the DME’s to generate Squitters.

Voltage amplitude (A)
0.3A
h
0.05A
0 Partial Time
rise time
Virtual origin
Fig 2.5 DME Pulse
2.3.4 Specified Characteristics of pseudo Gaussian pulse pair:

i. Partial rise time
The time as measured between the 5 and 30 per cent amplitude points on the leading edge
of the pulse envelop, i.e. between points h and a (i.e. Tpr) on figure 2.4
ii. Pulse rise time

The time as measured between the 10 and 90 per cent amplitude points on the leading
edge of the pulse envelope,( i.e. Tr ) between points h and c.
iii. Pulse Width

The time interval between the 50 per cent amplitude point on leading and trailing edge of
the pulse envelop,( i.e. Td ) between points b and f.
iv. Pulse decay time

The time as measured between the 90 and 10 per cent amplitude points on the trailing
edge of the pulse envelop, i.e. between points e and i.

v. Pulse Separation
The time interval between pulses of a pair, as measured from the 50 per cent amplitude
point on leading of the first pulse to the same point on the succeeding pulse.
Amplitude
0.5A
Pulse spacing t
Fig 2.6 pulse pair
vi. Virtual Origin

The point at which the straight line through the 30 percent and 5 percent amplitude points
on the pulse leading edge intersects the 0 percent amplitude axis (fig 2.5).
2.3.5 PULSE JITTERING

Since all aircraft using the ground DME Transponder operate on the same pair of
receiving and transmitting frequencies, all replies of the Transponder to all aircraft
interrogations are received by all these aircraft. It is therefore necessary for each aircraft to
select those replies, which result from its own interrogations. This is done as follows:
Transmission of aircraft interrogation pulse pairs is continuous, and, in turn, aircraft

reception of the ground Transponder replies is also continuous. Transmission of the
aircraft interrogation pulse pairs is semi-random; that is, the number of pulse pairs per
second (PRF) transmitted by a particular aircraft remains fairly constant, but the intervals
between pulse pairs vary. The variation in time spacing of the pulse pairs is unique to
each aircraft, and permits the aircraft to select the replies to its particular interrogations.
The aircraft receiver continuously compares the time spacing pattern of the reply pulse
pairs with the time-spacing pattern of interrogation pulse pairs. Only those pulse pairs,
which lie in matching patterns, are processed to extract the range information.
The variation in time spacing of the pulse pairs of the aircraft interrogation is termed as
Pulse Jittering.

2.3.6 Powers & Duty Cycle
2.3.6.1 PEAK PULSE POWER

It is the maximum value of power reached during the pulse, excluding the spikes.
2.3.6.2 AVERAGE POWER

The value obtained when the peak power is averaged over the time interval of the
pulses.
2.3.6.3 Duty Cycle

The time anything is working per unit of time.
Duty ratio = Average power / Peak pulse Power
2.3.6.4 EIRP
Equivalent Isotropically Radiated Power; the product of the power supplied to the
Antenna and the Antenna gain in a given direction relative to an isotropic antenna.
Airborne DME receivers require a minimum number of random pulses to ensure its
correct operation by providing appropriate AGC signal. However, unless sufficient
interrogating aircraft are present, the airborne receiver may not receive the required
minimum pulse rate. To ensure this requirement, the DME transponder will generate
extra pulses in a random fashion at a minimum pulse rate of 700 Hz (for GCEL DME it is
945 Hz). These extra random pulse-pairs are called Squitter.
At the time when no aircraft is interrogating, only the squitter is being transmitted, at an
average rate equal to the minimum pulse rate. However, as the number of authentic
aircraft interrogations increase, the squitter rate is reduced, and becomes zero when the
aircraft interrogation rate reaches the minimum pulse rate or above.
(Older versions DME equipments operated with a constant duty cycle of 2700 pulse-pairs
per second (2800 in later versions), done mainly to regularly activate high power
transmitters using Klystrons for example to increase the mean time before failures
(MTBF)).
2.3.7 Minimum and Maximum Reply Rate

As the pulse rate of the interrogations increases, a limit is reached above which the
Transponder will not allow any further interrogations to be serviced. This limit is reached
at a reply rate of about 2800 Hz, above which the transponder would become overloaded.
To avoid overloading, the Transponder detects the high rate of replies and causes the
receiver automatic gain control to limit the gain of the receiver until the weaker, more

distant, aircraft are excluded from the transponder, thus lowering the Transponder
loading. Should the system reply rate still exceed the 2800 limit, video output pulses are
randomly suppressed to limit the maximum reply rate to 2800 Hz.
Because of this limitation, in heavy traffic the aircraft may not receive 100 percent replies
to their interrogations. In DME system operation it is assumed that on an average 95
percent of aircraft interrogating a ground transponder at any given time will be in the
track mode and 5 percent will be in search mode. So for 100 aircraft the total interrogation
rate of the ground transponder will be:
(30 pps X 95 aircraft) + (150 pps X 5 aircraft) = 3600 interrogations per second.
As the maximum reply rate of the transponder is limited to 2800 pulse pairs per second,
the ratio of replies to interrogations is 78 percent. However, the airborne DME receiver is
designed to operate safely with a reply ratio as low as 70 percent. Hence providing a safe
margin of operation.
In the extreme case, when 100 aircraft are using the DME and the ground equipment fails,
all aircraft will be in the search mode and will produce a total of 15000 (100 X 150)
interrogations per second. When the ground transponder returns to normal service, its
receiver gain will drop until only the 2800 strongest interrogations are serviced. This
means that replies will be provided to the seventeen or so aircraft, which are likely to be
nearest to the ground transponder. Once these aircraft enter the track mode, their
interrogations will drop from 150 to 30 pulse pairs per second, thus gradually relieving the
ground transponder of about 2100 interrogations and allowing it to increase its sensitivity
and reply to more distant aircraft.
2.3.8 Transponder Identification

The need for transmitting Identification Message on a DME is to associate it with its paired
facility. This is essential because the pilot should know that he has selected the right DME
associated with a particular VOR or ILS that he is using.
Each operational DME is identified by a 2- or 3- character long Morse code message,

which is transmitted at regular intervals. During this time, the squitter and reply pulses
are replaced by regularly placed pulses at 1350 pulse pairs per second. These pulses
activate a 1350 Hz tuned circuit in the aircraft receiver producing an audio signal. Each
identification code (ident) is unique and identifies a specific DME site.

Frequently, DME is co-located with VOR or ILS equipment and for this reason the DME
may derive the identification code from the co-located facility. In such cases, any failure of
the external ident generator should cause the DME to internally generate and transmit the
ident in place of the failed unit.
2.3.9 System Delay

In the ground Transponder, the time delay of a received aircraft interrogation pulse pair to
the corresponding reply pulse pair is adjusted to exactly 50 microseconds. This is done to
facilitate the aircraft receiver to deduct a constant time delay from the total time elapsed
between interrogation and reply while calculating the slant range distance.
2.3.10 Echo Suppression (LDES & SDES)

The normal slant range for a normal DME operating at optimum efficiency is about 200
NM. However, the terrain surrounding the installations may seriously degrade this
maximum range. A major contributing factor to distance accuracy degradation is the effect
of echoes on the interrogation pulses arriving at the Transponder. Figure 2.7 shows the
generation of echoes.
Because of siting problems, the interrogations from aircraft may be reflected by

obstructions and arrive at the DME station delayed in time from the direct interrogations.
Therefore it is possible for the aircraft to lock on to the replies from these echoes and
provide the aircraft with erroneous distance information.
There are two types of echoes that may cause interference. They are short distance and
long distance echoes.
a. Short Distance Echoes

This type of echo is defined as an echo that occurs within 12 μsec of the interrogations.
This is caused by an obstruction behind and within close proximity to the DME station
(see figure 2.7).
b. Long Distance Echoes

This echo is defined as any echo occurring 60 μsec after the valid interrogations. This is
caused by an obstruction normally appearing between the DME station and the aircraft
(see figure 2.7).
Echo suppression is employed to eliminate the effect of echoes. The Short Distance Echo
Suppression (SDES) is used to eliminate echoes with short delays and Long Distance
Echo Suppression (LDES) is used to eliminate echoes with long delays

Figure 2.7 Echo Generation Diagram
2.3.11 Priority of reply pulses

There are three modulating inputs to the transponder transmitter. The encoder section of
the transponder attaches priority to these signals. The order of priority is as given below:
a. Identification Pulses
b. Reply Pulses
c. Squitter
2.3.12 Transponder Receiver Dead Time

In order to avoid oscillations of the transponder, and also to some extent eliminate the
effects of echo pulses, the receiver is suppressed during the process of transmission and
immediately after a transmitted pulse. This period of receiver suppression is typically 60
μsec and is called the Dead Time of the receiver. It is to be noted that the transponder
does not reply to interrogation pulses received during this time. The total time lost is
typically 20 percent, and this means that aircraft may only receive replies to 80 percent of
their interrogation pulses, even when well within range of the ground transponder.

2.4 DME Terminologies
There are three types of DME installations as defined in ICAO Annex 10.
DME/N:
DME, primarily serving operational needs of en-route or Terminal Control Area (TMA)
navigation, where the "N" stands for narrow spectrum characteristics.
DME/P:
The distance measuring element of the MLS, where the "P" stands for precise distance
measurement. The spectrum characteristics are those of DME/N.
DME/W:
DME, primarily serving operational needs of en-route or TMA navigation, where "W"
stands for wide spectrum characteristics.
The high power DME used in co-location with VOR and the DME used with the ILS belongs
to the DME/N type. Hence this handout discusses the specifications for DME/N type only.
DUPLEXER:
A device, which, by using the transmitted pulse, automatically switches the antenna
from, receives to transmit at the proper time.
EIRP:
Equivalent Isotropically radiated power, the product of the power supplied to the
antenna and the antenna gain in a given direction relative to an isotropic antenna.
HIGH LEVEL Interrogation:

Interrogations produced by the monitor at greatly increased amplitude, which are used
to inspect the DME station system delay parameters.
LOW LEVEL Interrogation:

Interrogations produced by the monitor at a greatly reduced amplitude which are used
to check the DME reply efficiency parameter. They simulate long distance aircraft.
Key Down Time:

The time during which a dot or dash of a Morse character is being transmitted.

LOCK ON:
The instant the aircraft begins to track the DME station continuously.
Mode W, X, Y, Z:
A method of coding the DME transmissions by time spacing pulses of a pulse pair, so
that each frequency can be used more than once.
PRF:
Pulse Repition Frequency; the number of pulse pairs per second.
Reply Efficiency:
The ratio of replies transmitted by the transponder to the total of received valid
interrogations.
When considering the transponder reply efficiency value, account is to be taken of the
DME dead time and of the loading introduced by the monitoring function.
System Efficiency:
The ratio of valid replies processed by the interrogator to the total of its own
interrogations. System efficiency is the combined effect of down link garble (down link
garble occurs when valid interrogations at the ground transponder are interfered with
by coincident interrogations from other A/c and results in loss of signals or error in
time of arrival measurement), ground transponder dead time, uplink garble (this is due
to ground to air loading), and interrogator signal processor efficiency. These statistically
independent components efficiency combined together yield the system efficiency.
Transmission Rate:
The average number of pulse pairs transmitted from the transponder per second.

Chapter - 03 ICAO Specifications
Chapter - 03
ICAO Specifications
To ensure safety, regularity and efficiency
of international civil aviation operations,
international standardisation is essential in
all matters affecting them such as the
operation of aircraft, aircraft airworthiness
and the numerous facilities and services
required in their support such as
aerodromes, telecommunications,
navigational aids, air traffic services etc. A
common understanding among the
countries of the world on these matters is
absolutely necessary.
To achieve the highest practicable degree of
uniformity whenever this will facilitate and
improve air safety, efficiency and regularity,
the ICAO Council adopts International
Standards and Recommended Practices
(SARPS) and approves procedures for the
safety, regularity and efficiency of air
navigation. The necessary standardisation
has been achieved by the ICAO primarily
through the creation, adoption and
amendment, by the ICAO Council, as
Annexes to the Convention on International
Civil Aviation known as SARPS. The
standard is a specification the uniform
application of which is necessary for the
safety or regularity of international civil
air navigation while the recommended
practices are agreed to be desirable but not
essential. Annex 10 specifies the SARPS in
respect of Aeronautical
Telecommunications. The specifications for
UHF Distance Measuring Equipment
(DME) as specified in Annex 10, Volume I,
Chapter 3, Section 5 are discussed in this
handout.















Attachement C





AMENDMENT No. 84
TO THE
INTERNATIONAL STANDARDS
AND RECOMMENDED PRACTICES
AERONAUTICAL
TELECOMMUNICATIONS
ANNEX 10
TO THE CONVENTION ON INTERNATIONAL CIVIL AVIATION
VOLUME I
(RADIO NAVIGATION AIDS)
The amendment to Annex 10, Volume I, contained in this document was

adopted by the Council of ICAO on 6 March 2009. Such parts of this
amendment as have not been disapproved by more than half of the total
number of Contracting States on or before 20 July 2009 will become effective
on that date and will become applicable on 19 November 2009 as specified in
the Resolution of Adoption. (State letter AN 7/1.1.44–09/26 refers.)
MARCH 2009
INTERNATIONAL CIVIL AVIATION ORGANIZATION

3.5 Specification for UHF

distance measuring equipment (DME)
Note 1.— In the following section, provision is made for two types of DME facility: DME/N for general
application as outlined in Chapter 2, 2.2.2, and DME/P as outlined in 3.11.3.
Note 2. In the following paragraphs, those denoted by ‡ are applicable to equipment first installed after
1 January 1989 (Chapter 2, 2.2.2.1).
3.5.1 Definitions
...
Equivalent isotropically radiated power (e.i.r.p.EIRP). The product of the power supplied to the antenna and the
antenna gain in a given direction relative to an isotropic antenna (absolute or isotropic gain).
...
3.5.2 General
...
3.5.2.5 When a DME function is combined associated with either an ILS, MLS or VOR for the purpose of
constituting a single facility, they shall be considered to be associated in a manner complying with Chapter 2,
2.2.2, only when:
a) they shall be operated on a standard frequency pairing in accordance with 3.5.3.3.5;
b) they shall be collocated within the limits prescribed for associated facilities in 3.5.2.6; and
c) they shall complying comply with the identification provisions of 3.5.3.6.4.
Note. A single DME facility may be paired with both an ILS and MLS.
3.5.2.6 Collocation limits for a DME facility associated with an ILS, MLS or VOR facility
3.5.2.6.1 Associated VOR and DME facilities shall be collocated in accordance with the following:
a) coaxial collocation: the VOR and DME antennas are located on the same vertical axis; or
b) offset collocation:
1) for those facilities used in terminal areas for approach purposes or other procedures where the highest
position fixing accuracy of system capability is required, the separation of the VOR and DME
antennas does not exceed 30 m (100 ft) except that, at Doppler VOR facilities, where DME service is
provided by a separate facility, the antennas may be separated by more than 30 m (100 ft), but not in
excess of 80 m (260 ft);

2b) for purposes other than those indicated in 1) a), the separation of the VOR and DME antennas does not
exceed 600 m (2 000 ft).
...
3.5.2.7 The Standards in 3.5.3, 3.5.4 and 3.5.5 denoted by ‡ shall apply only to DME equipment first
installed after 1 January 1989.
3.5.3 System characteristics
...
3.5.3.1.3 Accuracy
3.5.3.1.3.1 System accuracy. The accuracy standards specified in 3.5.3.1.3.4, 3.5.4.5 and 3.5.5.4 herein shall
be met on a 95 per cent probability basis.
Note. The total system limits include errors from all causes such as those from airborne equipment, ground
equipment, propagation and random pulse interference effects.
3.5.3.1.3.2 DME/N accuracy. Recommendation. At distances of from zero to 370 km (200 NM) from the
transponder, dependent upon the particular service application, the total system error, excluding reading error,
should be not greater than plus or minus 460 m (0.25 NM) plus 1.25 per cent of distance measured.
‡3.5.3.1.3.3 The total system error shall not exceed plus or minus 370 m (0.2 NM).
Note 1. This system accuracy is predicated upon the achievement of an airborne interrogator error
contribution of not more than plus or minus 315 m (0.17 NM).
Note 2. In mixed DME/N and DME/P operations it is intended that the achieved accuracy be at least that in
3.5.3.1.3.2.
...
Editorial Note.— The remaining paragraphs will be renumbered, as necessary, in the final edition.
...
3.5.3.3.4 Area channel assignment
3.5.3.3.4.1 In a particular area, the number of DME operating channels to be used shall be decided
regionally.
Note. Standards and Recommended Practices on the utilization of the DME frequency band 960 1 215 MHz
are found in Volume V, Chapter 4.

3.5.3.3.4.2 The specific DME operating channels to be assigned in such a particular area shall also be
decided regionally, taking into consideration the requirements for co channel and adjacent channel protection.
3.5.3.3.4.3 Recommendation. Coordination of regional DME channel assignments should be effected

through ICAO.
Note. The above paragraphs permit the use of DME airborne interrogators having less than the total
number of operating channels where so desired.
3.5.3.3.45 Channel pairing. When a DME transponder is intended to operate in association with a single
VHF navigation facility in the 108 MHz to 117.95 MHz frequency band and/or an MLS angle facility in the
5 031.0 MHz to 5 090.7 MHz frequency band, the DME operating channel shall be paired with the VHF channel
and/or MLS angle frequency as given in Table A.
Note.— There may be instances when a DME channel will be paired with both the ILS frequency and an MLS
channel (see Volume V, Chapter 4, 4.3).
...
3.5.4.1.3 Pulse shape and spectrum. The following shall apply to all radiated pulses:
...
e) For DME/N and DME/P: the spectrum of the pulse modulated signal shall be such that during the pulse
the effective radiated powerEIRP contained in a 0.5 MHz band centred on frequencies 0.8 MHz above
and 0.8 MHz below the nominal channel frequency in each case shall not exceed 200 mW, and the
effective radiated powerEIRP contained in a 0.5 MHz band centred on frequencies 2 MHz above and
2 MHz below the nominal channel frequency in each case shall not exceed 2 mW. The effective radiated
powerEIRP contained within any 0.5 MHz band shall decrease monotonically as the band centre
frequency moves away from the nominal channel frequency.
Note.— Guidance material relating to the pulse spectrum measurement is provided in Attachment C,
Section 7.1.11. Document EUROCAE ED-57 (including Amendment No. 1).
...
Note 2.— The power contained in the frequency bands specified in 3.5.4.1.3 e) and f) is the average power
during the pulse. Average power in a given frequency band is the energy contained in this frequency band divided
by the time of pulse transmission according to Note 1.
...
3.5.4.1.5 Peak power output
3.5.4.1.5.1 DME/N. Recommendation.— The peak effective radiated powerEIRP should not be less than
that required to ensure a peak pulse power density of approximately minus 83 dBW/m2 at the maximum specified

service range and level.
...
3.5.4.1.5.6 The transmitter shall operate at a transmission rate, including randomly distributed pulse pairs
and distance reply pulse pairs, of not less than 700 pulse pairs per second except during identity. The minimum
transmission rate shall be as close as practicable to 700 pulse pairs per second. For DME/P, in no case shall it
exceed 1 200 pulse pairs per second.
Note.— Operating DME transponders with quiescent transmission rates close to 700 pulse pairs per second
will minimize the effects of pulse interference, particularly to other aviation services such as GNSS.
...
3.5.4.2.3 Transponder sensitivity
3.5.4.2.3.1 In the absence of all interrogation pulse pairs, with the exception of those necessary to perform the
sensitivity measurement, interrogation pulse pairs with the correct spacing and nominal frequency shall trigger the
transponder if the peak power density at the transponder antenna is at least:
a) minus 103 dBW/m2 for DME/N with coverage range greater than 56 km (30 NM);
b) minus 93 dBW/m2 for DME/N with coverage range not greater than 56 km (30 NM);
bc) minus 86 dBW/m2 for DME/P IA mode;
cd) minus 75 dBW/m2 for DME/P FA mode.
...
3.5.4.5 Accuracy
3.5.4.5.1 DME/N. The transponder shall not contribute more than plus or minus 1 microsecond (150 m
(500 ft)) to the overall system error.
3.5.4.5.1.1 DME/N. Recommendation.— The contribution to the total system error due to the combination
of the transponder errors, transponder location co-ordinate errors, propagation effects and random pulse
interference effects should be not greater than plus or minus 340 m (0.183 NM) plus 1.25 per cent of distance
measure.
Note.— This error contribution limit includes errors from all causes except the airborne equipment, and
assumes that the airborne equipment measures time delay based on the first constituent pulse of a pulse pair.
‡3.5.4.5.1.2 DME/N. The combination of the transponder errors, transponder location coordinate errors,
propagation effects and random pulse interference effects shall not contribute more than plus or minus 185 m
(0.1 NM) to the overall system error.
Note.— This error contribution limit includes errors from all causes except the airborne equipment, and

assumes that the airborne equipment measures time delay based on the first constituent pulse of a pulse pair.
‡3.5.4.5.2 DME/N. A transponder associated with a landing aid shall not contribute more than plus or minus
0.5 microsecond (75 m (250 ft)) to the overall system error.
...
3.5.4.7.3 DME/P monitoring action
3.5.4.7.3.1 The monitor system shall cause the transponder radiation to cease and provide a warning at a
control point if any of the following conditions persist for longer than the period specified:
a) there is a change in transponder PFE that exceeds the limits specified in either 3.5.4.5.3 or 3.5.4.5.4 for
more than one second. If the FA mode limit is exceeded, but the IA mode limit is maintained, the IA
mode may remain operative;
b) there is a reduction in the effective radiated powerEIRP to less than that necessary to satisfy the
requirements specified in 3.5.4.1.5.3 for a period of more than one second;
...
3.5.5.1.8 DME/P. The peak effective radiated power (ERP EIRP) shall not be less than that required to
ensure the power densities in 3.5.4.2.3.1 under all operational weather conditions.
...
3.5.5.4 Accuracy
‡3.5.5.4.1 DME/N. The interrogator shall not contribute more than plus or minus 315 m (plus or minus
0.17 NM) or 0.25 per cent of indicated range, whichever is greater, to the overall system error.
...

Amendment to Attachement - C
7. Material concerning DME
7.1 Guidance material concerning both DME/N and DME/P
...
7.1.6 Siting of DME associated with ILS or MLS
...
7.1.6.8 In considering DME sites, it is also necessary to take into account technical factors such as runway
length, profile, local terrain and transponder antenna height to assure adequate signal levels in the vicinity of
threshold and along the runway, and required coverage volume (circular or sector). Care is also to be taken that
where distance information is required in the runway region, the selected site is not likely to cause the interrogator
to lose track due to excessive rate of change of velocity (i.e. the lateral offset of the DME antenna must be chosen
with care).
7.1.7 Geographical separation criteria
7.1.7.1 In order to allow consideration of actual antenna designs, equipment characteristics, and service
volumes, the signal ratios needed to assure interference-free operation of the various facilities operating on DME
channels are provided in 7.1.8, 7.1.9 and 7.1.107.1.9. Given these ratios, the geographical separations of facilities
may be readilyevaluated by accounting for power losses over the propagation paths.
7.1.8 Desired to undesired (D/U) signal ratios at the airborne receiver
7.1.8.1 Table C-4 indicates the necessary D/U signal ratios needed to protect the desired transponder reply
signal at an airborne receiver from the various co-frequency/adjacent frequency, same code/different code,
undesired transponder reply signal combinations that may exist. The prerequisite for any calculation using the
provided ratios is that the required minimum power density of the desired DME is met throughout the
operationally published coverage volume. For initial assignments, the D/U ratios necessary to protect airborne

equipment with 6-microsecond decoder rejection should be used. In making an assignment, each facility must be
treated as the desired source with the other acting as the undesired. If both satisfy their unique D/U requirement,
then the channel assignment may be made.
Table C-4. Protection ratio D/U (dB)
...
Note 4.— The frequency protection requirement is dependent upon the

antenna patterns of the desired and undesired facility and the ERP EIRP of the
undesired facility.
...
...
7.1.9 Special considerations for DME Y and Z channel assignments
The channel plan for DME is such that the transponder reply frequency for each Y or Z channel is the same as
the interrogation frequency of another DME channel. Where the reply frequency of one DME matches the
interrogation frequency of a second DME, the two transponders should be separated by a distance greater than the
radio horizon distance between them. The radio horizon distance is calculated taking into account the elevations
of the two transponder antennas. Assignment of a Y or Z channel whose reply frequency is 63 MHz removed
from the reply frequency of another channel (either a W, X, Y or Z channel) or vice versa requires a distance
separation of at least 28 km (15 NM) between facilities.
7.1.10 Special considerations for DME W or Z channel assignments
Assignment of a W or Z channel whose reply frequency is 63 MHz removed from the reply frequency of a Y
channel or vice versa requires a distance separation of at least equal to the service volume range of the Y channel
facility plus 28 km (15 NM).
7.1.11 Special considerations for making pulse spectrum measurements
The effective radiated power contained in the 0.5 MHz measurement frequency band specified in 3.5.4.1.3 e)
can be calculated by integrating the power spectral density in the frequency domain or equivalently by integrating
the instantaneous power per unit time in the time domain using the appropriate analogue or digital signal
processing techniques. If the integration is performed in the frequency domain then the resolution bandwidth of
the spectrum analyser must be commensurate with the 5 per cent duration interval of the DME pulse. If the
integration is performed in the time domain at the output of a 0.5 MHz five pole (or more) filter then the time
sample rate must be commensurate with the pulse spectrum width.
Editorial Note.— The following paragraphs will be renumbered, as necessary, in the final edition.

...
7.2 Guidance material concerning DME/N only
7.2.1 Effective radiated power (ERP) of DME/N facilities Coverage of DME/N
7.2.1.1 Whether a particular installation can provide the required frequency protected coverage volume can
be determined by using Figure C-20. The propagation loss for paths without obstructions uses the IF-77
propagation model.
7.2.1.2 Whenever a DME that provides coverage using either directional or bi-directional DME antenna, the
antenna pattern in azimuth and elevation has to be taken into account to achieve full benefit of the reduced
separation requirements outside the antennas main lobe. The actual radiation patterns of the antennas depend on a
number of factors, including height of the antenna phase centre, height of the DME counterpoise above ground
level (AGL), terrain surface roughness, terrain form, site elevation above mean sea level (MSL), and conductivity
of ground and counterpoise. For coverage under difficult terrain and siting conditions, it may be necessary to
make appropriate increases in the equivalent isotropically radiated power (EIRP). Conversely, practical
experience has shown, that under favourable siting conditions, and under the less pessimistic conditions often
found in actual service, satisfactory system operation is achieved with a lower EIRP. However to account for
lowest EIRP in notches between the lobes of the real elevation antenna pattern the values in Figure C-20 are
recommended.
Note.— Further guidance may be found in Doc 9718 (Handbook on Radio Frequency Spectrum Requirements
for Civil Aviation including Statement of Approved ICAO Policies).
7.2.2 EIRP of DME/N facilities
7.2.1.1 The power density figure prescribed in 3.5.4.1.5.1 of Chapter 3 is on the following assumptions:
Airborne receiver sensitivity 112 dBW
Airborne transmission line loss +3 dB
Airborne polar pattern loss relative to an isotopic antenna +4 dB
Necessary power at antenna 105 dBW
Minus 105 dBW at the antenna corresponds to minus 83 dBW/m2 at the mid band frequency.
Note. The power density for the case of an isotropic antenna may be computed in the following manner:
O2
st Pd Pa 10 log
4S
where Pd = power density in dBW/m2;

Pa = power at receiving point in dBW;
= wavelength in metres.

7.2.2.1 The power density figure prescribed in Chapter 3, 3.5.4.1.5.2 is based on the following example:
Airborne receiver sensitivity -120 dBW
Transmission line loss, mismatch

loss, antenna polar pattern variation
with respect to an isotropic antenna +9 dB
Power required at antenna -111 dBW
Minus 111 dBW at the antenna corresponds to minus 89 dBW/ m2 at the mid-band frequency.
7.2.1.2.2.2 Nominal values of the necessary ERP EIRP to achieve a power density of minus 83 89 dBW/m2
are given in Figure C-20. For coverage under difficult terrain and siting conditions it may be necessary to make
appropriate increases in the ERPEIRP. Conversely, under favourable siting conditions, the stated power density
may be achieved with a lower ERPEIRP.
7.2.1.3 The use of Figure C 20 is illustrated by the following examples. In order to achieve the necessary
nominal power density at slant range/levels of 342 km (185 NM)/12 000 m (40 000 ft), 263 km
(142 NM)/12 000 m (40 000 ft) and 135 km (73 NM)/6 000 m (20 000 ft), ERPs of the order of plus 42 dBW,
plus 36 dBW and plus 30 dBW respectively would be required.
Editorial Note.— Replace existing Figure C-20 with the following figure:

Figure C-20. Necessary EIRP to achieve a power density of -83 dBW/m2 as a function of height above
and distance from the DME.
Note 1.— The curves are based on the IF-77 propagation model with a 4/3 Earth radius which has been
confirmed by measurements.
Note 2.— The radio horizon in C-20 is for a DME antenna located 5 m (17 ft) AGL over flat terrain. Terrain
shielding will reduce the achievable range.
Note 3.— If the antenna is located significantly higher than the assumed reference antenna, the radio horizon
and power density will increase.
End of Figure C-20.

Insert new text as follows:
7.2.3 DME-DME RNAV
7.2.3.1 There is an increasing use of DME to support area navigation (RNAV) operations. Although the use
of DME to support RNAV operations does not impose any additional technical requirements on the DME system
it does raise some additional issues compared with the traditional use of DME with VOR to support conventional
operations. These are discussed briefly below.
7.2.3.2 DME/DME positioning is based on the aircraft RNAV system triangulating position from multiple
DME ranges from DME facility locations in the aircraft database. The resulting accuracy of the position solution
depends on the range to the DMEs and their relative geometry. Some additional measures are therefore necessary
to ensure that the DME infrastructure is adequate to support the RNAV operation, i.e. that sufficient DMEs are
available and that their location provides adequate geometry to meet the accuracy requirements. For approach and
departure procedures it is also necessary to confirm that there is adequate signal strength and that there are no
false locks or unlocks due to multipath. When ensuring there are sufficient DMEs, it is also important to identify
any critical DMEs (i.e. those which must be operational for the necessary performance to be assured).
7.2.3.3 Errors in published DME facility locations will result in RNAV position errors. It is therefore
important that DME positions are correctly surveyed and that adequate procedures are in place to ensure that the
location data is correctly published. For DME facilities collocated with VOR the DME position should be
separately surveyed and published if the separation distance exceeds 30 m.
Note.— Standards for data quality and publication of DME location information are given in Annex 15,
Aeronautical Information Services.
7.2.3.4 When using DME for RNAV, scanning DME aircraft receivers usually do not check the DME
identification. As a consequence, removing the identification of a DME during tests and maintenance operations
does not guarantee that the signals will not be used operationally. Maintenance actions that may provide
Misleading Information should be minimized.
Note 1.— Further guidance on flight inspection of DME-DME RNAV procedures is given in Doc 8071.
Note 2.— Further guidance on navigation infrastructure assessment to support RNAV procedures is given in
EUROCONTROL document “EUROCONTROL-GUID-0114” (available at http://www.eurocontrol.int/eatm
/public/standard page/gr lib.html) and on the PBN page of the ICAO website at http://www.icao.int/pbn.
End of new text.
...

Chapter - 04 DME Antenna
Chapter – 04
DME Antenna
4.1 Biconical, Discone Antennas
4.1.1 Infinite and Finite Biconical Antennas
An infinite Biconical antenna acts as a guide for a travelling outgoing spherical

wave in the same way that a uniform transmission line acts as a guide for a
travelling plane wave. The two situations are compared in the figure below. They
both have constant characteristic impedance Zk and since they are infinite the
input impedance Zi == Zk. These values are purely resistive
Figure: Biconical antenna
so the input resistance
Ri, = Zi - Zk ……. (3.13)
For the infinite Biconical antenna
Ri = 120 ln cot (θ/4) …….. (3.14)
Where θ = cone angle

The solid line in the figure below shows the variation of the input resistance Ri,
as a function of cone angle θ. If the lower cone is replaced by a large ground
plane resistance is ½ the value given by (Eq. 3.14) as shown by the dashed line.
Note that a single cone of 90° angle has an input resistance of about 50 ohms.
The input impedance Zi, is given by
Figure: Characteristic resistance Rk of infinite Biconical and single

cone antennas. Since the antenna is infinitely long,
the input resistance Ri = Zi - Zk.
With the infinite Biconical antenna as an introduction, let us now consider the
practical case of a Biconical antenna of finite radius r (Fig. below). When the
outgoing spherical wave reaches a radius r part of the energy is reflected,
resulting in energy storage. The remaining energy is radiated, with more
radiated perpendicular to the axis than close to the cones as suggested in figure
below.

Figure: Finite Biconical antenna enclosed in hypothetical sphere where energy

flowing near the cone is reflected but with energy escaping perpendicular
to the axis in the equatorial region.
where
r = cone length, m
β = 2π/λ.
Zk = 120 In cot (θ/4)
Zm = Rm + jXm
The Rm and Xm values are given by Schellkunoff for thin cones (0 < 5°)]
Measured values of the VSWR for large cone angles over a 2 to 1 bandwidth are
Cone angle VSWR

200 <5
400 <3
600 <2

Figure: Coaxial fed conical antennas for mounting on masts with

omni directional radiation in the horizontal plane.
Figure above shows a progression of conical antennas with coax feed.
(a) A full Biconical.

(b) Has the upper cone replaced by a thin stub
(c) Cone is replaced by a disk.
The coax feed makes these antennas convenient for mounting on masts;
Radiation is maxima in the horizontal plane.
4.2 Discone
A discone antenna is a combination of a disk and a cone in close proximity as

shown in figures below. Figure ‘Dimensions of a typical discone antenna’ shows,
where L = λ/4 at the lowest operating frequency. This antenna has very high
bandwidth, both for the input impedance and radiation pattern. It behaves as if
the disk is a reflector. Visualize an inverted cone image above the disk, and
consider a line perpendicular to the disk, drawn from the bottom cone to the top
image cone. If this line is shifted to and from the center of the disk, its length
varies from a minimum near the center to a maximum at the edge of the cone.

The frequency of operation corresponds to the range of frequencies over which

this imaginary line is a half wavelength.
A discone is a wide band antenna because it is a constant angle antenna. For an

antenna of, fig. (b) below, the SWR on the coaxial cable connected to the
discone antenna can remain below 1.5 for a frequency variation of 7:1. Overall
performance remains satisfactory for a 9:1frequency ratio. A discone is an omni-
directional antenna but has low gain. It is used mostly as a VHF and UHF
receiving and transmitting antenna at airports, where communication has to be
made with an aircraft from any direction.
A PRACTICAL DISCONE ANTENNA CROSSSECTION OF DISCONE ANTENNA
FIG. (a) FIG. (b)

Fig.: Dimensions of Discone Antenna

4.3 DME Antenna
4.3.1 Transponder Antenna
There are number of requirements for the transponder antenna of the DME.
Since the same antenna is used for receiving the interrogations from the aircraft
and for transmitting the replies, the antenna must operate over the range 960 -
1215 MHz. The antenna has a Characteristic impedance of 50 ohm. The VSWR
should not be more than 1.5 or so.
The antenna must be vertically polarized and must have a uniform gain for all
azimuth angles. Antenna gain at the maximum power point of vertical pattern
should not be less than 6 dB at all azimuths. In the vertical plane, the antenna
must have a sharp pattern. A gain of the order of 8 dB is desired. It is also
desired that the vertical pattern be pointed upward from the horizontal between
2° to 5 ° to minimize the reflections from nearby objects on the ground.
The Wilcox 496 DME antenna contains 9 bi-conical elements vertically stacked
inside a waterproof laminated fiberglass radome. Each element is constructed
from the nickel-plated Aluminum and held on a metallic support tube by
fiberglass spacers. The antenna is vertically polarized producing an omni-
directional pattern. The vertical pattern has a 6° wide lobe with many minor
lobes. The up tilting is accomplished by a difference phase between the dipole
exciting currents. It is done to minimize the reflections from the nearby objects
at the ground level and as well as to provide a concentration of the radiation
where the majority of the aircrafts using the facility will be located. Suppression
of minor lobes in the vertical pattern is accomplished by feeding unequal amount
of power to the dipoles of central array. The necessity of feeding unequal power
and in different phase requires special coaxial feed system for the proper power
distribution and impedance match between the feed line and the antenna.
The antenna provides approximately 8-dB gain and no radiation below 4°.
A matching device is used for matching purpose and power division to provide
appropriate power to the individual antenna.
Vertical Field Pattern: The DME antenna is omni directional in the horizontal
plane. The vertical pattern is obtained with an array of dipoles in a vertical stack.
The principal lobe has an upward tilt of 5 degrees.

Minor
lobes 60
0
6
40 40
Fig.: Radiation Pattern of DME Antenna
Fig.: A DME Transponder Antenna having 9 Biconical Elements

4.3.2 Airborne Antenna
It consists of a vertical element having an electrical length of a half wave to give

it broad band characteristics. The projecting end is loaded so that the physical
length of the antenna is appreciably less than a half wave. The antenna-
terminates in a quarter wave matching transformer used to transform the
impedance to 50 Ω which is suitable for matching a standard transmission line.
The antenna element is housed in a plastic casing. As the antenna length is very
small, it is installed on the belly of fuselages. The antenna is subject to
shadowing by the aircraft structure. If the aircraft position is such that, its wings
lies between the antenna and the transponder, the signals get lost completely.
To overcome this difficulty, the aircraft is equipped with a memory circuit, so
that it does not go to search mode unless the signal is lost for more than 10
seconds. Also sometimes two aerials are used with the switching facility to have
uninterrupted service. In such case the antennas are mounted above and below
the fuselages. Normally belly antenna is connected to the interrogator.
4.4 Thales DME Antenna
The suggested antenna for the DME 415-435 DME equipment is the
omnidirectional DME antenna FAN-96 (or FAN-86) with 9-dB gain.
Other antennas (e.g. sectorial, or higher gain) are available on request.
This antenna is a sturdy, corrosion-protected unit well suited to withstand

adverse environmental conditions like high wind speed, extreme temperatures,
salt fog, rain, hail, snow and ice.
The antenna can be easily mounted on top of a steel pole.
Its radome houses two monitor probes to be connected with the equipment
monitors.
Antenna cables must be chosen according to the total RF loss that can be
admitted and to the distance of the antenna from the equipment housing.
The DME/N beacon available recommended antennas are shown on figure,

¾ FAN-88: sectorial antenna with 15 dB gain, usually used in case of

associations to an ILS equipment. This antenna covers a sector of ±35°,

which can be centered on the runaway, thus reducing effects of obstacles

and increasing signal-to-noise ratio.
¾ FAN-96: this is omnidirectional antenna with 9 dB gain. Its main

characteristics are:
• frequency range: 960 MHz to 1215 MHz;
• RF: 5 kWp modulated and transmission cycle not greater than 5%.
• vertical polarization;
• input impedance: 50 Ω unbalanced; vswr: (voltage standing wave
ratio) stationary wave, measured on antenna input, less than 1.8
and 20 dB decoupling between the antenna and its probes (±3 dB);
• gain: not less than 9 dB, in the direction of maximum radiation of
main lobe and at average point of horizontal lobe, whereas the gain
in the horizontal plane (0°) is at least 6.2 dB with respect to an
isotropic source;
This antenna is provided with two obstruction lights which may be turned on and
off during the day by an automatic night switch. Mechanical antenna collocation
is simple and straightforward with any type of existing VOR, DVOR or ILS
antenna. Suitable adapters can be supplied on request.

Chapter - 05 Siting Criteria for DME
Chapter - 5
Siting Criteria for Distance Measuring Equipment (DME)
1. Introduction:-
1.1 The DME operates in the 960 MHz to 1215 Mhz band to enable a properly equipped
aircraft to determine its slant range to the DME site by measurement of the travel time of
pulse modulated radio waves. .
2 Siting Requirements
2.1 Site
2.1.1 The basic requirements in siting a DME beacon are to ensure adequate
coverage and to avoid the possibility of interference to the correct operation of the aid.
Sites selected in open country should have hills, mountains, large buildings, etc. at the
smallest angle of elevation as practicable.
2.1.2 In mountainous terrain, the site should be located on the highest hill or
mountain within the tolerance area. See also Section 4.2.3 Antenna Height for restrictions
governing antenna height above large expanses of level ground or water.
2.2 Obstructions
2.2.1 The distant obstacle horizon should preferably not extend above an elevation angle
of 0.5° when viewed from near ground level at the proposed location of the DME.
2.2.2 Outside a distance of 10 m from the DME, small buildings, trees, power and
telephone lines, and fences can be tolerated provided they do not project above a height
of approximately 1 m below the bottom of the DME antenna.
2.2.3 Large obstructions such as multi storey buildings, steel bridges, gasometers, etc. are
potential sources of interference to correct operation. For new installations it is preferable
to keep at least 1.5 km clear of these types of structures.
2.2.4 For existing DME facilities the Systems Engineer should be advised of proposals
for erection of new structures of this nature within 1.5 km of the site.
2.3 Antenna Height
2.3.1 On a clear open site, an antenna height of 6 m is recommended provided that this
clears local obstructions.

2.3.2 On obstructed sites e.g., hill tops, the antennas should be raised to provide clear
UHF coverage in all directions. In this case the height of the antenna relative to any large
expanse of level ground or water should be less than 20 m otherwise deep minima will be
produced in the field strength pattern with consequent degradation of service.
2.4 Collocation of DME with VOR
2.4.1 When collocating a DME with a VOR the requirements of ICAO document
“International Standards, Recommended Practices and Procedures for Air Navigation
Services” - Annex 10, shall be adopted. The determination as to whether a Navigation
Aid is Terminal or Enroute shall be carried out by the procedure designer.
2.4.2 Terminal Aids
a) Coaxial collocation: the VOR and DME antennas are located on the same vertical axis;
or
b) offset collocation: for those facilities used in the terminal areas for approach purposes
or other procedures where the highest position fixing accuracy of system capability is
required, the separation of the VOR and DME antennas shall not exceed 30 m except
that, at Doppler VOR facilities, the antennas may be separated by more than 30 m, but
not in excess of 80 m.
2.4.3 Enroute Aids
a) Coaxial collocation: the VOR and DME antennas are located on the same vertical axis;
or
b) Offset collocation: the separation of the VOR and DME antennas shall not exceed 600
m.
2.5 Vehicular Movements
No restrictions.
2.6 Services
Overhead construction is permissible to the DME site provided the obstruction

requirements of Section 4.2.2 Obstructions are met. Low voltage power only is to be
brought to the site; high voltage lines must be kept clear to the distance specified in
Section 4.2.7 Electrical Interference.

2.7 Electrical Interference
Overhead high voltage lines and substations may cause degradation in coverage due to
the physical structures themselves, and also due to electrical noise. For this reason it is
preferable that these structures should be kept clear of the site by at least the following
distances:
2 kV to 22 kV: 350m
above 22 kV: 900m
2.8 Restricted Area
No special requirements.
2.9 Maintenance of the Site
No special requirements exist for DME sites in open country. At mountain top sites, trees
should not be allowed to grow to a height exceeding that of the mast or tower supporting
the DME antenna.

Chapter - 06 Doc – 8071 : Ground & Flight Testing of DME
Chapter - 06
Doc - 8071
(Manual on Testing of Radio Navigation Aids)
Ground & Flight Testing of DME









Chapter - 07 Flight Inspection of DME
Chapter - 07
Flight Inspection of DME
1. DME Flight Inspection

All DME Checks are carried out in conjunction with checks on their associated facilities
(VOR & ILS) .
1.1 Parameters Checks Results and Tolerances:
1.1.1 Identification: The identification code should be clear and correct through out
the area of coverage. The ID Code frequency should be 1350 Hz. The ID should be
properly synchronized with that of the associated facility.
1.1.2 Distance accuracy: The indicated Slant range distance must be within the limits
1.1.3 Coverage: The area of coverage of the DME will be at least that of its associated
facility (VOR & ILS)
1.1.4 Signal Strength (AGC): The signal strength must be at least –82 dBm
throughout the area of coverage.
1.1.5 Squitter Rate: The normal squitter rate should be 2700 ± 90 pps. On certain type
facilities, rates as low as 700 pps are normal.
1.1.6 False replies : No false replies should be present which could result in false
locks-ons. Within the area of coverage. This may occur at any location especially in the
presence of vertical nulls.
1.2 Ground Adjustments:

When the measured distance is out of tolerance, then the system delay of both
Transmitters is to be adjusted by the ground personnel. System delay is to be increased
to reduce the range error and vice versa.
Tolerances:
For Terminal (ILS) DME : ± 75 meters
Enroute (VOR) DME : ± 150 meters

Chapter - 08 DGCA CARs
Chapter – 08
DGCA CARs
8.1 Introduction
Directorate General of Civil Aviation is an attached office of the Ministry of Civil Aviation.
The Directorate General of Civil Aviation is the regulatory body in the field of Civil
Aviation primarily dealing with safety issues. It is responsible for regulation of air
transport services to/from/within India and for enforcement of civil air regulations, air
safety and airworthiness standards. It also co-ordinates all regulatory functions with
International Civil Aviation Organisation.
The regulations are in the forms of the Aircraft Act, 1934, the Aircraft Rules, the Civil
Aviation Requirements, the Aeronautical Information Circulars. The Advisory and
guidance material is in the form of circulars.
8.2 CIVIL AVIATION REQUIREMENTS (CAR)
For CNS facilities the regulations are stipulated for standards and practices popularly
known as CAR.
In Section 9 (Air Space & Air Traffic Management), Series D (Part i to vi) it has specified
the various standards and recommended practices to be adhered for different CNS
facilities.
8.3 DGCA CAR Section 9 – Air Space and Air Traffic Management
Series D: NAVIGATION, LANDING AND COMMUNICATION AIDS
Part I: Requirements for maintenance/ inspections of communication/ Navigation, landing and other
equipments installed at airports and enroute.
Part II: Aeronautical Telecommunications – Radio Navigation Aids
Part III: Aeronautical Telecommunications – Communication Procedures
Part IV: Aeronautical Telecommunications – Digital Data Communication and Voice Communication
System
Part V: Aeronautical Telecommunications – Secondary Surveillance Radar
Part VI: Aeronautical Telecommunications – Aeronautical Radio Frequency Spectrum Utilization

GOVERNMENT OF INDIA
OFFICE OF DIRECTOR GENERAL OF CIVIL AVIATION
TECHNICAL CENTRE, OPP SAFDARJANG AIRPORT, NEW DELHI
CIVIL AVIATION REQUIREMENTS

SECTION 9 – AIR SPACE AND
AIR TRAFFIC MANAGEMENT
SERIES 'D', PART I
ISSUE II, 8th January 2010 EFFECTIVE: FORTHWITH
F. No. 9/38/2009-IR
Subject: Requirements of Maintenance/ inspection of Communication,

Navigation, Landing and other equipment installed at Airports and
en-route.
1. APPLICABILITY:
This part of the Civil Aviation Requirements lays down the requirements of
maintenance, inspection or Communications, Navigation, Landing and
other equipment installed at airports and enroute and used for aircraft
operations. These equipment may be owned and operated by Airports
Authority of India, Meteorological Department or any other agency.
This CAR is issued under Section 5A of the Aircraft Act 1934 and Rule
133A of the Aircraft Rules 1937 for compliance by all concerned agencies.
This CAR is issued in supersession of CAR Section 4 Series X Part I,

Issue I dated 4th February 1994.
2. SCOPE:
The requirements stipulated in this Civil Aviation Requirement will apply to

all Communication, Navigation and landing facilities including the
following:
1. Visual Landing Aids-VASI/PAPI etc.,

2. Approach lighting
3. Non-Directional Beacon

CIVIL AVIATION REQUIREMENT SECTION 9
SERIES 'D' PART I 8TH JANUARY 2010
4. VHF Direction Finding System

5. Locator Beacon
6. Instrument landing System
7. Microwave landing System
8. VOR/ T-VOR Doppler VOR
9. Distance Measuring System
10. Communication Facilities like VHF and HF Radio Telephone, AFTN,
Satellite based Voice and Data Communication System, Direct Speech
Circuits, VHF Data Links etc.
11. Airport Surveillance Radar
12. Air Route Surveillance Radar
13. SSR and MSSR
14. Airport Surface Detection Equipment
15. Computer based ATC - ADS etc
16. Airport Recorder and Replay System
17. Differential GPS system and connected equipment
18. RVR Measuring equipment
19. Meteorological equipment
3. MAINTENANCE:
3.1 Maintenance Schedule
The operator shall prepare maintenance schedules for periodic preventive

maintenance, including testing, functional checks and serviceability of the
equipment. These maintenance schedules shall be prepared in
accordance with the guidelines provided by the manufacturers of the
equipment. The schedules should also indicate the level of officer who can
carry out the check/ inspection and the periodicity of the schedules A copy
each of these schedules should be submitted to the DGCA. DGCA may
introduce additional checks if required.
3.2 Test Equipment
The operator shall ensure that all tools/ test equipment are available for
carrying out the maintenance/checks of the facility and also adequate
spares to ensure continued serviceability of the facility.
3.3 Calibration of Test Equipment
The operator shall ensure that all the test equipment used for
maintenance and periodical checks of the facilities are kept properly
calibrated and certified by recognized standards institutions
3.4 Maintenance Records

All records of daily and periodical maintenance schedules (preventive as

well as corrective) shall be preserved for a period of not less than two
years. However, DGCA may direct preservation of such schedules for
longer periods, if required.
3.5 Defect and Rectification Register:
The operator shall maintain a Register giving details of all the defects and
rectification actions taken, duly signed by the officer in charge of the
facility.
3.6 Maintenance Personnel:
All personnel entrusted with the maintenance/checks of a facility should

have undergone necessary training. They should undergo periodical on
the-job checks at least once in a year and refresher course at least once
in three years.
3.7 Responsibility of Officer in Charge.
The officer in charge of the facility shall be responsible for continued,

maintenance and safe operation of facility.
4. STATUS OF EQUIPMENT AFTER AN ACCIDENT/INCIDENT:
In case an aircraft is involved In an accident while making use of a facility,

the concerned unit of the facility shall be withdrawn and flight inspected
immediately. The unit shall not be declared operational till checked and
tested thoroughly and its performance IS found satisfactory. The standby
unit of the facility shall be utilized during this period In case of an incident,
DGCA may require the concerned unit of the facility to be withdrawn for
checking.
4.1 In respect of a facility that is or might have been involved in an air

accident/ incident, operational status data shall be recorded for both main
and standby equipment of the facility.
4.2 In order to ensure that operational data of a facility is not misinterpreted

the operator shall ensure that the data entries are complete, clear,
concise, accurate and correctly timed.
5. SELF INSPECTION:
The operator shall draw a programme for periodically inspecting and

checking the functioning of the facility. The operator shall ensure that the
functional and calibration checks of the facility required as per the ICAO

norms are carried out and proper records of the same are maintained The
operator, should ensure that the facility IS used for operations only when It
is fit for operation
6. INSPECTION BY DGCA:
Any officer designated/nominated by the Director General of Civil Aviation

shall be empowered to inspect at any time any facility to check. The
serviceability and maintenance records and procedures
7. CERTIFICATION:
Any new equipment or system procured and installed, by the operator for
providing facility as listed above, shall be declared operational only after it
is found lit for operation on satisfactory completion of the necessary
inspection/checks and calibration from air and ground as required and
after obtaining concurrence of the DGCA for the same.
8. LOGISTIC SUPPORT:
In order to ensure that the maintenance of a facility is not delayed for lack
of spares. The stock of spare units, modules, PCBs and components etc.
shall be maintained at the site of facility or at a place from where the
required spares can be transported to the site without any avoidable
delay. The storage facility shall be subject to inspection at any time by an
officer designated by DGCA for this purpose,
9. MONITORING OF SERVICEABILITY STATUS:
The performance of the equipment should be monitored regularly. The

operator shall prepare a quarterly report on Mean Time Between Failures
(MTBF) of a facility and the same shall be made available to DGCA. If an
equipment becomes unserviceable for more than one week, the same
should be reported to DGCA along with the details of the defect and
proposed rectification action.
(Dr. Nasim Zaidi)

Director General of Civil Aviation

Chapter - 09 CNS Manual and Circulars
Chapter ‐ 09
CNS Manual and Circulars

9.1 Purpose of CNS Manuals
To Establish CNS procedure and to provide information and instructions pertaining to

CNS facilities, which are essential for the provision of safe and efficient Air traffic
service by AAI.
CNS In-charge of the station will ensure that the process, procedure and instructions
Pertaining to CNS facilities contained in these manuals are strictly complied by all
concerned.
9.2 There are 7 Volumes of CNS Manuals
Volume 1 – Deals with Maintenance of CNS Facilities.

Volume 2 – Deals with Communication Procedure.
Volume 3 – Deals with Siting Criteria for CNS Facilities.
Volume 4 – Deals with Flight Inspection of CNS Facilities.
Volume 5 – Deals with Lightening, Surge Protection and Earthing of CNS equipments.
Volume 6 – Deals with Technical Specification of CNS facilities.
Volume 7 – Deals with Maintenances Schedule of CNS facilities.
These manuals basically reiterates the subjects covered under DGCA’s CAR (Civil
Aviation requirements).
Also, the basic inputs for these manuals are from ICAO Annex-10

File No. AAI/NS/ILS/General/04 Date: 7/01/2008
CNS CIRCULAR No. 01of 2008
Sub: Maintenance of Navigational Aids Site
In order to prevent unacceptable interference to ILS and other navigational Aids signals,
areas around antenna shall be protected as per provisions of ICAO Annex 10 Vol I.
For the above purposes following guidelines shall be followed for maintenance of
Critical and Sensitive Area of Instrument Landing System, restricted area around VOR
(As defined in Annexure1) DME, NDB and Marker beacons and Area around DGPS/LT
point used for flight calibration by FIU at the airports.
1. Localizer and glide path:

1.1. The height of grass is not to exceed 150mm in the critical area of ILS.
1.2. During ILS operation access of personnel and vehicle and performance of
maintenance activities in the critical area of ILS shall be done as per following
procedure.
¾ Access control to critical and sensitive area of ILS operating as CAT-II and CAT-
III will be as per promulgated provisions of Low Visibility Procedure (LVP) of the
airport in the absence of such procedure access to the area shall be done with
prior coordination with ATC tower.
¾ Maintenance activities like grass mowing maintenance of airport lighting etc in
the critical area of ILS shall be carried out in coordination with CNS personnel.
During watch hours of ILS operation, CNS personnel shall have prior
coordination with ATC for execution of work.
¾ No metallic objects including vehicles shall be permitted to enter into critical
area of ILS. For operational reasons if entry of vehicle into critical area becomes
necessary, prior coordination will be done with CNS personnel. Before issuing
permission guideline contained above shall be kept in view.

2. VOR
2.1.1. Site maintenance:
2.1.2. Grass and shrubs within 305m radius of the site must be mown or cut regularly
so that their heights do not exceed 600mm.
2.1.3. Grass cutting equipment is not to be parked within 305m radius of the VOR
building.
2.1.4. The vehicles used by airport maintenance staff are to be parked underneath the
counterpoised or beyond the radius of 305m
3. NDB , Marker Beacons And DME Facilities:-

The height of grass and other vegetation over the protected area (within 30mtrs of
100 watt NDB and 100mtrs for 500 watt NDB ) covering the Antenna Mast (s), the
earth mat buildings is not to exceed 600 mm.
4. DGPS/LT point maintenance:-

4.1. The DGPS point marked by FIU for flight inspection of Radio Navigational aids
shall be maintained by suitable marking for future flight inspection of the facilities
of the airport.
4.2. Grass and other vegetation at the site must be mown or cut regularly. The height
of grass and other vegetation around the point is not to exceed 100 mm.
5. Signage: - proper signage shall be provided to delineate the boundaries of critical

and sensitive areas of navigational aids Sign boards shall be made of non metallic
material.
6. Water logging: - Actions shall be taken so to avoid water logging in critical and
sensitive area and around antenna systems of Nav-aids.
7. Foreign objects:-
Nav-aids sites viz ILS, VOR, DME and NDB shall be free from all foreign objects.
8. Construction of structures: - Construction of any structure in the vicinity of NAV

aids are to be controlled as per DARA circular 5/2005. Unauthorized constructions/
obstruction which is likely to affect the performance of NAVAIDS, is to be brought to
the notice of station in charge for immediate action.
9. No drainage/water pipe should be allowed to pass through the critical area of ILS, if
this is already existing necessary actions are to be taken so that water logging does
not take place in the area.

10. Normally no electrical power line to be permitted to pass through critical area of ILS
and in the protected area in case of VOR, DME, NDB and Markers.
11. Civil work like new construction, excavation, digging and leveling is not allowed in
Critical and Sensitive Areas and around antenna system of ILS and in the protected
area as mentioned above for VOR, DME, NDB and Markers.
(V K Chaudhary)
Executive Director (CNS-OM)

Annexure-A
1. Instrument landing system
The occurrence of interference to ILS signals is dependent on the total environment
around the ILS antennas and antenna characteristics. Any large reflecting objects
including the aircraft vehicle or fixed objects could potentially cause multi-path
interference to the ILS course and path structure affecting its performance. The
environment for the purpose of developing protective zoning criteria can be divided
into two types of area the critical area and sensitive area-
a) Critical area: - the critical area is an area of defined dimensions about the
localizer and glide path antennas where vehicles, including aircraft, are
excluded during all ILS operations. The critical area is protected because the
presence of vehicles and/or aircraft inside its boundaries will cause
unacceptable disturbance to the ILS signal in space.
b) Sensitive area: - The sensitive area is an area extending beyond the critical
area where the parking and/or movement of vehicles including aircraft is
controlled to prevent the possibility of unacceptable interference to the ILS
signal during ILS operations. The sensitive area is protected against
interference caused by large moving objects outside critical area but still
normally within the airfield boundary.
1.1. Critical and Sensitive Area Dimension:- Depending of the type of ILS antenna
system, category of ILS operation and aircraft operation the critical and sensitive
area should be established and properly designated at an airport to protect ILS
operation from multi path effects. The typical dimensions as per ICAO ANNEX
10/VOL I/ attachment C and DARA circular 5/2005 are as given below.
1.2. LLZ Critical Area:- The area bounded by

i. A line 300meter in the direction of approaches from localizer antenna and
perpendicular to the runway.
ii. A line 60m from the centerline of localizer antenna on either side and
parallel to the runway.
iii. A line containing the centerline of localizer antenna and perpendicular to
the runway.
iv. Area within a circle of 75 meter radius with center at middle of antenna
system.
1.3. LLZ Sensitive Area: - The typical LLZ sensitive area for 12 and 14 elements
directional dual frequency LLZ antenna system which are used in AAI are as
given below for a 3000m runway.
The area bounded by:-
Category I ILS: - An area of 600M X 60M from center of LLZ array towards
approach end of runway.

Category II ILS: - An area of 1220M X 90M from center of LLZ array towards
approach end of runway.
Category III ILS: - An area of 2750M X 90M from center of LLZ array towards
approach end runway.
1.4. GP Critical Area :- The area bounded by-

i. A line 300meter in the direction of approach from the glide path facility
and perpendicular to the runway.
ii. A line containing glide path antenna and perpendicular of runway
iii. Near edge of runway from glide path.
iv. A line 30 meter in the directions away from the antenna and parallel to it.
1.5. GP Sensitive Area :- the area bounded by-

a) For Category I ILS :-
i. A line 900 meter in the direction of approach from the glide path
facility and perpendicular to the runway.
ii. A line containing glide path antenna and perpendicular of runway
iii. Near edge of runway from glide path including runway towards
direction of approach.
iv. A line 300 meter in the directions away from the antenna and
parallel to it.
b) CATEGORY II/III ILS :-

i. A line976 meter in the direction of approach from the glide path
facility and perpendicular to the runway.
ii. A line containing glide path antenna and perpendicular of runway.
iii. Near edge of runway from glide path including runway towards
direction of approach.
iv. A line 300 meter in the directions away from the antenna and
parallel to it.
2. DVOR Restricted/Protected area :-
An area within a 305 meter radius from the centre of antenna of the facility.
3. NDB protected area :-

a) Self radiating mast -30 meter radius from the centre of the radiating mast
of the facility.
b) Cage/ T antennae – the area bounded by
1. 30 meter radius from each mast
2. lines joining the locus of each circle made by above 1.



























Civil Aviation Training College, Allahabad, India Page 115
Hkkjrh; foekuiRRku Ikzkf/kdj.k SOP for inspection/Safety Oversight Audit of
lapkj] fnDpkyu ,Oe fuxjkuh&iz- ,o v- funs’kky; CNS/ATM Automation facilities by DGCA
CNS Circular 02 of 2012

Sub: standard operating procedure (SOP) to be followed for Inspection/Safety Oversight Audit
of CNS/ATM Automation Facilities by DGCA.
1. Introduction
1.1 As per Air Craft act, 1934 and its subsequent amendments, Section 4A- Safety oversight
functions. The Director General of Civil Aviation (DGCA) or any other officer specially
empowered in this behalf by the central government shall perform the safety oversight functions in
respect of matters specified in this Act or the rules made there under.
1.2 DGCA, CIVIL AVIATION REQUIREMENT (CAR) SECTION 9 – AIR SPACE AND AIR
TRAFFIC MANAGEMENT SERIES ‘D’ PART I ISSUE II dated 08th January 2010. “Requirements of
Maintenance/Inspection of Communication, Navigation, Landing and other equipment installed
at airports and en-route” lays down the requirements of maintenance/inspection of
communications, Navigation, Landing and other equipments installed at airports/en-route
stations and used for aircraft operations.
1.3 As per above CAR, in Para 6 – INSPECTION BY DGCA and Para 7- Certification, following
requirement is laid down:-
Quote
“Para 6 – INSPECTION BY DGCA:-
Any officer designated/nominated by the Director General of Civil Aviation shall be empowered
to inspect at any time any facility to check the serviceability and maintenance records and
procedures.
Para 7 – CERTIFICATION:-
Any new equipment or system procured and installed, by the operator for providing facility as
listed above, shall be declared operational only after it is found fit for operation on satisfactory
completion of the necessary inspection/checks and calibration from air and ground as required
and after obtaining concurrence of the DGCA for the same.”
Unquote
2. Inspection/Safety oversight Audit of CNS/ATM Automation Facilities by DGCA
As per provisions mentioned above, inspection/Safety oversight of CNS/ATM Automation

Facilities is normally carried out by DGCA for:-
i. Inspection/safety oversight audit of newly installed/transinstalled CNS/ATM facility’s

before commissioning to meet the facility certification requirements.
ii. Inspection/Safety oversight audit of operational CNS/ATM Facilities at field stations to meet
safety oversight function requirement.
iii. Inspection/Safety oversight of Operational CNS/ATM Facilities along with such
inspection/safety oversight audit of other departments by DGCA like ATM and for
requirements like Aerodrome License etc.
February 2012 Page 2 of 9

iv. Inspection/Safety oversight for operationalisation of an Airport or authorizing CAT-II/CAT –

III operations etc at airport.
v. Special Inspection/Safety Oversight Audit for any incident/accident investigation purposes.
3. Objective of the circular
3.1 The objective of this CNS Circular is to lay down an internal standard operating procedure
(SOP) to be followed by all concerned within CNS Department so as to have uniform
guidelines/response for inspection/Safety oversight Audit of CNS/ATM Automation facilities by
DGCA at field stations with the purpose for:-
i. Action to be taken by all concerned in respect of preparing for inspection/Safety oversight

audit of CNS/ATM automation facilities;
ii. To avoid common routine observation during inspection/Safety oversight audit of
CNS/ATM automation facilities so as to have minimum adverse audit observation;
iii. Post inspection/Safety oversight Audit of facilities, procedure to be foll9owed for
submitting Action taken Report/Action plan on the observations of inspection/Safety
oversight Audit of CNS/ATM Automation facilities.
3.2 As mentioned above, this SOP is for internal use of CNS Department personnel to have
uniform guidelines/response to DGCA Inspection/Safety Oversight audits and does not replace
other orders/guidelines issued by DGCA or any other authority on the subject.
4. Definitions
Following definitions will be used in conjunction with this SOP:-
Corrective Action Taken Report: Action taken to eliminate the cause of a detected non-conformity
or noncompliance or other undesirable situation as mentioned in the DGCA Audit observation
(Note:- Corrective action does not mean the action taken to restore a non-conforming situation to a
conforming situation. This is know as remedial action. If the root cause of non-conformity is not
addressed then it is very likely that similar non-conformities will recur).
Corrective Action Taken Plan:- An action plan submitted to DGCA by an audited/inspected

station, detailing the proposed action by the station to resolve identified audit observations.
Implementation of the corrective action taken plan should be always necessarily submitted with a
probable date of Completion (PDC) of plan.
CNS Manual: - A manual containing procedures, instructions and guidance for use by the CNS
personnel in the execution of their duties for the operation and maintenance of CNS/ATM
automation facilities.
Checklists: Checklists are an integral part of SOPs. They depict "sets” of actions relevant to specific
phases of operations that must be performed or verified. Checklists also provide a framework for
verifying systems configuration for guarding against vulnerabilities in human performance

Inspection/Safety Oversight audit Report: A standardized means of reporting the inspection

findings/observations to the designated authorities.
Standard Operating Procedure [SOP]:- Standard operating procedures (SOPs) specify a sequence
of tasks and actions to ensure that intended task can be carried out in a safe efficient, logical and
predictable manner. SOPs, should unambiguously express:-
¾ What the task is
¾ When the task is to be conducted (time and sequence);
¾ By whom the task is to be conducted;
¾ How the task is to be done (actions);
¾ What the sequence of actions consists of; and
¾ Mitigation plan on the findings/deficiencies observed, if any, during the performance of
task.
5. Inspection/Safety Oversight Audit responsibility at CHQ
Following is the general division of responsibility at CHQ for Inspection/Safety Oversight Audit
of CNS/ATM facilities by DGCA:-
5.1 Matters related to Inspection/Safety Oversight Audit of operational CNS/ATM Automation

facilities at field stations are handled by CNS-OM Dte.
5.2 Matters related to Inspection/ Safety Oversight Audit of newly installed/transinstalled

facility(s) are handled by project executing Dte i.e. CNS-P Dte.
5.3 Inspection/Safety oversight of Operational CNS/ATM Facilities along with such

inspection/Safety Oversight Audit of other departments by DGCA like ATM and for requirements
like Aerodrome licensing etc. is handled by CNS-OM Dte
5.4 Matters related to Inspection/safety oversight for operationalisation of a new

Airport or for authorizing CAT-II/CAT-III operations etc. at airports are handled by CNS - P Dte
as part of project.
5.5 Hence all the matters i.e. submission of CAR compliance reports, Action Taken Report and
other documents etc. related to Inspection/Safety Oversight Audit of facility should be accordingly
suitably addressed to CHQ. In case of common issues/observations internal coordination is done
by both the Dtes.
6. Dissemination of inspection/safety oversight Audit plan
6.1 Inspection/ Safety Oversight, Audit of operational CNS/ATM Automation facilities:-
Advance information, normally in the brining of calendar year is received from DGCA at CHQ for
their proposed plan of inspection/Safety oversight audit of operational CNS/ATM automation
facilities at field stations during the calendar year. CNS-OM Dte further disseminates this
information to all concerned.

6.2 Inspection/Safety Oversight Audit of newly installed/transinstalled CNS/ATM Automation

facilities, operationalisation of an Airport and authorizing CAT-II/CAT-III operations etc. at
airports.
6.3 Information regarding Inspection/Safety oversight of operational CNS/ATM facilities for

requirements like Aerodrome License and inspection/Safety oversight Audit of other departments
like ATM by DGCA is intimated by concerned Directorate. If required at CHQ CNS-OM Dte also
coordinates for such type of inspection.
7. Common avoidable observation of the audit
It has been observed from previous DGCA inspection/Safety oversight audits of CNS/ATM
facilities at field stations that following common observations of DGCA inspection/Audit as
mentioned below are avoidable:-
i. Non submission of CAR compliance check list before start of audit to DGCA inspectors:
ii. CAR compliance check list not filled up properly i.e. some of the fields left blank or use of
Use of words like sat and Adequate etc. instead of measured parameters.
iii. Non-availability of relevant DGCA CAR, relevant ICAO Annex i.e. Annex 10 and DOCs
i.e. Doc 8071 etc. at stations;
iv. Non-availability of required test equipments to carry out approved maintenance schedule;
v. Test Equipments not being calibrated periodically;
vi. Maintenance schedules/log books and other records not maintained properly i.e. loosely
bound not numbered not signed etc;
vii. Maintenance schedules not completed in time;
viii. Non availability of approved maintenances schedule in respect of a facility to be
commissioned or already operational at a station;
ix. Earthing resistance not being measured/recorded;
x. Remote status of facility not available;
xi. Remote Control (RC) line Routing Diagram in respect of facilities not available;
xii. Single Line Power Supply distribution diagram not available at the station;
xiii. Availability of redundant power supply at facility site and adequate power supply back
up;
xiv. Site related issue such as proper approach road, security of site proper illumination,
marking and protecting critical and sensitive areas;
xv. Non Availability of Lightning and Surge Protection Systems;
xvi. Use of word like SAT and Adequate in flight inspection report/maintenance schedules
instead of measured parameters;
xvii. Facility overdue for flight inspection; and
xviii. Station CNS manual not updated etc.

8. Action to be taken by RHQ/Stations before Inspection/Safety oversight audit of CNS/ATM

Facilities
8.1 Inspection/safety oversight Audit of operational CNS/ATM Automation facilities:-
Following action will be initiated by concerned RHQ/Station after intimation regarding

Inspection/Safety Oversight Audit of operational CNS/ATM Automation facilities is received :-
(i) RHO/ GM (CNS) in charge of Metro station shall initiate action for internal performances
monitoring of the facilities at station as per the check list circulated on the subject by CHQ as
soon as information is received. For the internal performance monitoring following
composition of team is suggested:-
a) One or two officer from concerned RHQ/Metro depending upon the number of facility at the
station:
b) One officer from CHQ CNS-OM Dept; and
c) One officer from the station where the next inspection/Safety Oversight audit is planned will
also be associated.
(ii) When the information regarding inspection/safety oversight audit is received from the DGCA
well in time i.e. in the beginning of calendar year, the internal performance monitoring action
will be initiated two months before the planned inspection/Safety Oversight audit.
(iii) In other cases action, should be initiated as soon as information is received by RHQ/station.
(iv) Two hard copies of updated station CNS Manual and DGCA CAR Compliance check list are to
be forwarded to the CHQ two months before the planned inspection/Safety Oversight, for
onward submission to DGCA. A soft copy of the same is also required to be forwarded to
CHQ. Soft copy may be sent by email to smmisns@aai.aero.
(v) While preparing the CAR Compliance check list, it must be ensured that no column is left
blank. Use of word like SAT and Adequate etc. in preparing CAR Compliance check
list/performing maintenance schedules etc is to be avoided and instead measured parameters
are to be indicated.
(vi) As soon as intimations of inspection/Audit is received, a review should be undertaken so as

preemptive /necessary action is initiated by all concerned so as to avoid the common
observations of Inspection/Safety Oversight Audit as mentioned in Para 7 above. .
8.2 Inspection/Safety Oversight Audit of newly installed/transinstalled facility(s) for

certification before commissioning:-
Following action will be initiated by concerned RHO/Station after intimation regarding

Inspection/ Safety Oversight Audit of CNS/ATM Automation facilities for certification before
commissioning is received:-

(i) CAR compliance as per DGCA format for the concerned facility (ILS, DVOR/DME, NDB, VHF
TX/RX etc) is to be prepared and forwarded to CNS-P Dept;
(ii) While preparing the CAR Compliance check list it must be ensured that no column is left
blank:
(iii) FAT Document of the facility;
(iv) Approved maintenance schedule of the facility:
(v) Details of the trained manpower on the facility;
(vi) Copy of commissioning/recommissioning flight check in case of Nav-aids; and
(vii) Necessary action is required to be initiated by all concerned so as to avoid the common
observations of Inspection/Safety Oversight Audit as mentioned in Para 7 above
8.3 Inspection/Safety oversight audit of CNS/ATM Automation facilities for Aerodrome

Licensing, operationalisation of an Airport or authorizing CAT-II/CAT-III operations or any
other departments like ATM etc by DGCA.
Following action will be initiated by concern RHO/Station after intimation regarding

Inspection/Safety oversight audit for Aerodrome Licensing, operationalisation of an Airport or
authorizing CAT-II/CAT-III operations or any other departments by DGCA like ATM etc is
received :-
(i) DGCA CAR Compliance checklist concerning CNS/ATM Automation facilities for
Aerodrome Licensing/Operationalisation of Airport is to be prepared and forwarded
to Aerodrome Licensing Dte/Operation Dte/ATM Dte as the case may be.
(ii) A copy of CAR compliance may please be forwarded to CNS-OM dte also.
9. Submission of Compliance, Action Taken Report (ATR)/Action Taken Plan(ATP) on the

observations of inspection/safety oversight Audit:-
i. After the Inspection/Safety Oversight Audit at a station, DGCA forwards the

observations/findings of audit to the concerned Dept at CHQ. Following are the available
DGCA guidelines for submitting compliance, ATR/ATP on the findings observations of
the audit:-
a) ATR Should be submitted to DGCA within 30 days.
b) In case of proposed action taken plan against findings/observation, it should be
submitted with PDC.
c) Internal communication within other departments of AAI is not to be reflected as
ATR or part of ATR/ATP.
d) Required proof/Data i.e. measured parameters, de briefing reports, flight
inspection reports etc. is to be submitted for showing compliance to DGCA
audit/observation.
e) After submitting initial ATR/ATP updated status on pending audit
findings/observations is required to be submitted by 15th day of every month. Till
compliance is achieved against all the findings/observations of the audit.

ii. In view of above and to enable the CHQ to submit consolidated ATR to DGCA in time it is
required that:-
a) RHQ/station should submit compliance report, TAR/ATP on audit observation
findings within 15 days of receipt of same to CHQ so that it is submitted to DGCA
within 30 days stipulated time as stated above.
b) Before forwarding the compliance report to CHQ, it should be reviewed by RHQ.
c) RHQ is also required to review ATR/plan at their end for all such observation for
which action is required to be taken by station/RHQ and forward a consolidated
compliance report, ATR/Plan to CHQ.
d) For all such observation/audit findings for which action is required to be taken by
VHQ, same may please be forwarded with projections/references made earlier is
any by RHQ/Station
e) Where ever action is required to be taken by other departments within AAI same
may please be pursued at appropriate level and PDC obtained.
f) In some cases, station may not be able to do/complete some specific maintenance
schedule due to faulty or non availability required test equipments at station. In
such cases, if required test equipment is available within region at some other
station same may be made available to concern station carry out required
maintenance schedule till the test equipment is serviced/made available at the
station.
g) Updated status report on pending audit findings/observations is required to be
submitted by 05th of every month for onward submission by CHQ to DGCA by 15th
of every month as stated above.
h) This updated status is required to be forwarded regularly till compliance is
achieved against all the findings/observations of the audit.
10. Action Taken by CHQ for DGCA inspection/Safety oversight audit of CNS/ATM facilities
Following action has been taken by CHQ to comply with various requirements of DGCA
Inspection/Safety Oversight Audit of CNS/ATM automating facilities:-
i. Corporate CNS Manual has be ensured in Seven Volumes;
ii. Guidelines for preparing station CNS Manuals have been circulated and Stations CNS
Manual have been prepared in respect of all the stations and approved by CHQ;
iii. Approved Maintenance schedules are available in respect of all the operational CNS/ATM
Automation facilities;
iv. Guidelines on some specific subjects like Updation of station CNS manual, SOP for
opening watch at station have been issued in form of CNS Circulars.
v. Guidelines have been issued for the provision of ancillary systems like power supply Test
Equipment calibration and maintenances and lightning and Surge protection System etc.
vi. Available DGCA CAR compliance check lists for oversight audit of CNS facilities have
been circulated and uploaded on the Infosaarthee.
vii. An internal performance monitoring check list also has been prepared circulated and
uploaded on Infosaarthee so as to avoid common avoidable observations.

9. Availability of check lists on Infosaarthee:-
Soft copy following check list as motioned above has been made available at “Infosaarthee “link –
“Home > Board Member> Air Navigation Services> Communications. Navigation and
Surveillance (CNS) > Operation and Maintenance”
i. CAR compliance check list receiver from DGCA for safety oversight Audit of CNS
facilities;
ii. Attachment V-Schedule of inspection for renewal of Aerodrome license (CNS facility check
list is at S. No 7 & 9 of schedule); Extract from DGCA civil Aviation requirement, Section 4
– Aerodrome Standards & Air Traffic Services Series ’F’ Part I Dated 16th October 2006
SUBJECT REQUIREMENTS FOR ISSUE OF AN AERODROME LICENSE; and
iii. Internal performance monitoring checklist for CNS/ATM Facilities at field stations.
[Ravi Prakash]
Executive Director [CNS-OM]

€
€

-;ii;;i;''tlt'.,
l1fftqr{q',f'
$Tr$rs' F{fiTaq-da Provision of information on the
T|Err, E-{qr-fd (rq F?nrdt-q qti r. Bhne-q operational status of radio navigation aids
3. Alternate means for Provision of Operational status of radio navigation aids:
Whenever remote operational status of Radio Navigational Aids could not be

provided by means of RSU, following alternate action is required to be taken
by all
concerned :-
!i) Navigational aids shall be suita )ly manned by maintenance personnel during
flight operations.
(ii) The maintenance personnel nranning the Nav-aid shall be provided with
suitable means of communication so as intimate any changes in opbrational status
of facility to the ATC.
(iii) where it is not'possible to provide status indication to a control point,
permanently in case of VOR/DME, action is also required to be
taken as per CAR
Para 3.3.7 -r "voR fVl,onitoring" and para 3,5.4.7"1 - DME Monitoring,,
as
mentioned above.
5. As per the existing inltructions, the RHer shall continue to send the
information
on the non availability of Remote status of Navigational Aids and Action Taken
Report to restore the samg in the form NS122A to CHe.
IRavi prakash]
-. ., Executive Director ICNS-OMl
References:-
1.DGCA, CIVIL AVIATION REQUIREMENT (CAR) SECTION 9 AIR
SPACE - AND AIR
TRAFFIC MANAGEMENT sERtEs 'D', PART tssuE il Dated il 08th January 2010 -
"AeronauticaI Telecommunication -Radio Navigation Aids,,.

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PROFICIENCY LINKED INTEGRATED COURSE

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5. Siting Criteria for DME 65

6. Doc – 8071 : Ground & Flight Testing of DME 68

9. CNS Manual and Circulars 83

CATEGORIZATION OF RADIO NAVIGATIONAL AIDS:

Civil Aviation Training College, India Page 1

a. Long Range navigational aids:

b. Medium Range navigational aids:

c. Short Range navigational aids:

Short Range Aids:

NAME OF THE SYSTEM FREQUENCY BAND POWER RANGE

NDB Locator 200 – 450 KHz <50 45

VOR Terminal VOR 108 – 112 MHz 13 25

Localizer ILS 108 – 112 MHz 10 25

Glide Path ILS 328 – 336 MHz 10 10

DME ILS - DME 960 – 1215 MHz 100 25

Civil Aviation Training College, India Page 2

Medium Range Aids:

NAME OF THE SYSTEM FREQUENCY BAND POWER RANGE

VOR Homing & En- 112 – 118 MHz 100 200

Inter relationship in terms of Frequency, Power, Range and System

1.2 Navigation Fix:

DME's use as a navigation aid is based on the principles of Rho-Theta Navigation

Figure 1.1 Rho-Theta Navigation Systems

Civil Aviation Training College, India Page 3

1.3 HISTORY OF DME

Civil Aviation Training College, India Page 4

1.4 PURPOSES AND USE OF DME

PURPOSE OF DME INSTALLATION

Association of DME with VOR

Association of DME with ILS

Civil Aviation Training College, India Page 5

The main purposes of DME installations are summarised as follows:

• For operational reasons

USE OF DME INSTALLATION

The important applications of DME are:

1.5 MODELS OF DME IN USE:

Civil Aviation Training College, India Page 6

2.1 Principles of operation of DME

2.1.1 Principles of Secondary Radar

2.1.2 Simplified Block Diagram Of DME System

Civil Aviation Training College, India Page 7

Figure 2.1 Basic Block Diagram of DME System

Civil Aviation Training College, India Page 8

Figure 2.2 Elements of DME System

2.1.3 Modes of Operation

Civil Aviation Training College, India Page 9

When the aircraft's transmitter is in Search mode, it transmits interrogations at a higher

2.2 SYSTEM TIMING

2.2.1 TIMING DIAGRAM

Civil Aviation Training College, India Page 10

Figure 2.3 DME System Timing

2.2.2 RANGE CALCULATION

2.3 CHARACTERISTIC FEATURE OF DME

2.3.1 DME Channels for ‘X’ & ‘Y’

Civil Aviation Training College, India Page 11

i.e. Channel frequency spacing: 1 MHz between Rx channels and

2.3.2 Twin Pulse (Pulse Pair) Technique