INERTIAL REFERENCE SYSTEM (IRS)

With the advancement of technological development of navigational aids, specifically for military operations, the systems incorporated, needed to be self reliant, i.e. not depending upon ground based stations.

The requirement for this type of long range navigation is to determine the position of the aircraft in relation to its point of departure, and points enroute to reach a destination without depending on ground stations.

Core: Ring Laser Gyroscope (Non-Rotating)

Principle: Law of Inertia

Technological Design: Strap Down Feature

Calculation: Dead Reckoning

(process of calculating one’s current position by using a previously determined position, using estimations of speed and course over elapsed time)

Inertial theory is based on Newton’s second law, i.e. velocity is the rate of change of distance with time, and acceleration is the rate of change of velocity with time.

An aircraft’s position at any time after starting to move can be determined solely from its acceleration history, and by double integration of measured accelerations the distance travelled during a given time may also be determined.

It comprises of

• 3 Quartz accelerometers (Motion sensors)

• 3 Ring Laser gyroscopes (Rotation Sensors)

They are aligned in the 3 axes, namely:

• Longitudinal direction (North-South)

• Lateral Direction (East-West)

• Vertical direction (Up-Down)

When the aircraft starts to move, due to the law of inertia, the accelerometers registers its movement and the computer associated is able to record the acceleration history of the aircraft to determine its position in relation to its starting point.

Angles that fall between the north-south or east-west direction are calculated by the computer solving algebric vector problems.

RING LASER GYRO

Principle: Sagnac Effect with Laser light

Calculation: Frequency Shift

COMPONENTS

• 2 Anodes (opposite polarity)

• 1 cathode (Photon emitter)

• 1 Piezoelectric Dither motor

• 2 Photo-Electric/Readout Detector/photodiode

• 1 Prism

• 2 reflecting Mirrors

• 1 semi transparent mirror

• Gas Discharge Region( Filled with helium-neon gas mixture)

WORKING

When an aircraft experiences a motion, the signals are captured and fed to the gyroscopes.

Generally,The cathode emits the laser light/photons which gets divided equally (50%-50%) in both the directions, i.e. toward the anodes, due to the huge potential difference between them, with the help of the mirrors at the corner to form a closed path for the lasers.

After reflection from both the corners, 2 independent lasers meet at a certain point, which creates an interference pattern.

Interference Pattern: When two waves meet, the crests and troughs of both the waves react with each other.

When 2 crests combine, it forms a wave of greater intensity of light and 2 troughs combine, there is absence of light.

Due to the movement of the gyro caused due to the movement of the aircraft, one part of the laser light has to travel more distance, and the other laser light has to travel less distance.

But,The Laser light is of a fixed colour (normally pinkish-orange), and got a fixed frequency.Therefore, to travel an increased distance with the same speed, due to the movement of the gyro, the frequency has to increase for one part of the light.

This frequency phase shift/difference in frequencies, is measured and converted into a digital output signal, which solely depends on the generation of interference pattern. With a semi-transparent mirror and a prism, both the laser lights are sent to the Photo-Electric Detector/photodiodes.

The laser lights are transmitted parallel toward a pair of photodiodes.

When the aircraft is stationary, both the independent laser lights travel the same distance and will be in phase.

DISADVANTAGE

FREQUENCY LOCK/LASER LOCK

At very low input rate of motion, the frequencies of both the independent laser lights are nearly the same therefore, they shift frequency and lock each other.

This results in a zero output, in place where the aircraft is actually in a motion (roll/pitch/yaw). That's where the Piezo Electric Dither Motor, a vibration device, comes in the picture. The amplitude of the rotation is very small, but the frequency changes the motor produces, keep the laser lights out of the lock-in range.

Since the rotation is first one way and then the other, the sum over time is zero, and the motor does not affect the mean output in any way and even if it does, the computer ignores the vibrations.