Inertial Reference System (IRS)

Wg Cdr Rajagopal

Inertial Reference System

Index

Introduction to Inertial Reference System

  • Inertial reference system is a stand alone system providing accurate position
  • Ring laser gyro and computers with high computing power are used in IRS
  • Acceleration in pitch, roll and yaw axis is measured and integrated
  • To compare, INS measures accelerations in East West and North South
  • Inertial reference system is a strap down system
  • Gyroscopes and accelerometers are fixed to airframe
  • Reduction in moving parts increase accuracy and maintainability

Sagnac Effect

  • Ring laser gyro uses Sagnac effect of laser source to measure acceleration
  • Laser beams are made to travel from start to end points of a circular tube
  • The start and end points are diametrically opposite to each other
  • Path length of the beams will be same if the ring is not rotating
  • Rotation of ring will cause variation in path lengths at end point
  • Variation of path lengths will cause change in in frequency
  • Frequency shift between the received waves provides angular rotation
  • Laser ring gyro uses a triangular tube, but the principle remains same

Construction of Ring Laser Gyro

  • Laser is a coherent light of a single frequency
  • Cathode generates the laser source and travel towards two anodes
  • One ray travels in clockwise and other in anti-clockwise directions
  • Photo electric cell detects interference pattern when the waves collide
  • Interference pattern can be used to measure of movement of aircraft

Laser Lock

  • Laser lock in ring laser gyro to give zero output due to error
  • Synchronisation of two-part laser beams causes the error
  • Laser lock error occurs during change in attitude
  • Dithering technique by vibration of frequencies is used to avoid laser lock
  • Piezo-electric dither motor is used to dither the frequencies

Axis Transformation

  • Axis transformation is the process of converting accelerations in usable format
  • Accelerometers in IRS detects acceleration in pitch, roll & yaw axis
  • Unlike INS, where acceleration is detected in N-S and E-W directions
  • Matrix algebra is used to convert accelerations to movement of aircraft
  • Aircraft movement is calculated in N-S, E-W and Vertical axis
  • Matrix transformation computer also corrects the gyro outputs
  • Mathematical model of earth is used to correct for five computable errors
  • Earth rate, Transport wander, Coriolis, Central acceleration and Schuler

Initial Tetrahedron

  • Inertial Reference System is levelled with respect to aircraft
  • Unlike the INS, where levelling is horizontal to surface of earth
  • Computer detects the slope of parking space and lateral errors
  • Computed errors are used to provide heading corrections
  • The earth’s spin rate is used for heading corrections

Inputs for IRS

  • Inertial Reference System obtains its inputs from four sources
  • Three ring laser gyros
  • Two horizontal accelerometers in x & y axis
  • One vertical accelerometer in z axis
  • True air speed from air data computer

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