ATPL Inst 4.2 PDF
ATPL Inst 4.2 PDF
ATPL Inst 4.2 PDF
Chapter 4.2
Inertial Reference System
Introduction
Gyro-stabilised platforms are generally costly, heavy and require a lengthy alignment
procedure. With the introduction of relatively low cost, high performance digital computers,
these systems have been steadily replaced by mathematical computer software models, which
modify the output signals from accelerometers that are strapped directly to the airframe. This is
referred to as an Inertial Reference System (IRS), which works on the same fundamental
principles as the INS, and has the following functions:
it adds the above results to a start position, to obtain the present position.
The fundamental difference between the INS and the IRS is that the latter is a Strap-down
System. The IRS senses the aeroplanes displacement about 3 axes to provide:
primary attitude.
vertical speed.
ground speed.
Each IRS consists of three laser gyros, three accelerometers, power supplies, a
microprocessor, built in test equipment (BITE), and output circuitry. Three totally independent
IRSs are normally installed on an aeroplane, and each receives barometric altitude, altitude
rate, and TAS data from the Central Air Data Computer (CADC). Coupled with the gyro and
accelerometer data the aeroplanes vertical speed can be determined, and the wind parameters
calculated.
Description of the Strap-Down System
The strap-down system dispenses with the gimbal mounted stable element and instead uses
solid-state ring laser gyros (RLG). These gyros are not required to stabilise the accelerometers,
as in the case of an INS, but provide aeroplane orientation, by determining the rate of rotation
around each of the aeroplane axes. The orientation data is used to process (modify) the
accelerometer outputs to represent those, which, under the same conditions, would be the
expected outputs from the accelerometers, if they were positioned along the North, East and
vertical axes. The transform matrix (a Quaternion) is generated by digital computation, and
gives the analytical equivalent of a gimballed system.
4-2-1
In the IRS a triad of RLGs (orthogonal axes), with their sensitive axes positioned
mutually perpendicular is utilised. A block diagram of one of these is shown above. The
example shown has a triangular path of laser light, whose path length is normally 24, 32
or 45 cm. Other models alternatively use a square path, ie. one more mirror. The RLG
is produced from a block of a very stable glass ceramic compound, which has an
extremely low coefficient of expansion. The triangular cavity contains a mixture of
helium and neon gases at low pressure through which a current is passed. The gas (or
plasma) is ionised by the voltage, which causes helium atoms to collide with, and
transfer energy to, the neon atoms. This raises the neon to an inversion state, and the
spontaneous return of neon to a lower energy level produces photons, which then react
with other excited neon atoms. This action is repeated at speed and creates a cascade
4-2-2
High reliability.
Digital output.
High accuracy.
4-2-3
- 2nm/hr *
b. Pitch/roll
- 0.050
c.
- 0.40
Heading (T)
d. Groundspeed
- 8 kts
e. Vertical velocity
- 30'/second
f.
- 0.1/second
Angular rates
g. Acceleration
- O.0lg
4-2-4
Attitude.
Acceleration.
Vertical speed.
Present position.
Wind data.
A block schematic of the overall input/output functions for a typical IRS is shown below.
4-2-5
The IRS also integrates with navigation aids and other equipment on the aeroplane. A
multitude of data is additionally passed to the Horizontal Situation Indicator (HSI), which may be
either an electro-mechanical instrument on its own, or alternatively may form part of an
Electronic Flight Information System (EFIS) display.
The system normally consists of two independent IRSs, which can operate on either AC or DC
power. If AC is not normal the systems will automatically switch to backup DC power from the
battery busbar. Backup power to the right IRS is also automatically terminated if AC power is
not restored within 5 minutes.
4-2-6
4-2-7
4-2-8