Inertial navigation: INS and IRS

Navigation · Chapter 47 · DGCA CPL and ATPL

Accelerometers integrated twice, a level platform, Schuler, alignment, errors, the INS display, and the laser-gyro IRS.

Remember

Accelerometers (N, E) → integrate → speed → integrate → distance; tilt lets gravity in.
Earth rate 15.04°/h × sin lat; transport rate = V ÷ R; Schuler period 84.4 min.
Align: enter position, keep still; warm-up, coarse, fine level, gyro-compass to true north.
Bounded errors rock (84.4 min); unbounded grow (gyro wander, heading misalignment).
IRS: strap-down ring laser gyros; dither breaks lock-in; waypoint 0 = present position.

Navigation from acceleration

Feel every push • An ins (inertial navigation system) feels every push the aircraft makes. Measure every push and you know every change of speed • It needs nothing from outside: no signals to jam or lose
Accelerometers • The sensor is an accelerometer: a small weight on a pivot. Speed up and the weight lags behind • A pick-off measures how far it moved, and a torquer pushes it back. The push needed is the acceleration
Integrate twice • Acceleration × time = speed: the first integration • Speed × time = distance: the second • Distances north and east from the start position give the present position
What it gives • It gives position, groundspeed, track, true heading, drift, wind, attitude, cross-track error, and distance and time to go

Keeping it level

Gravity gets in • Tilt an accelerometer, and part of gravity pushes the weight • It then reports an acceleration that is not there. Drag the tilt, and read the bottom line