Inertial and Satellite Navigation Systems
Domain 6 covers inertial navigation (INS/IRS) and satellite navigation (GNSS), the two systems that give position without ground aids. The exam tests INS principles and error behaviour, GNSS segments and error sources, integrity monitoring, and the meaning of DOP.
Inertial systems
Three accelerometers and three gyros in a strapdown or stable-platform arrangement. Measured accelerations are integrated once for velocity and twice for position; the gyros maintain the reference frame. Alignment performs levelling and gyrocompassing, takes several minutes, and must be done stationary. Schuler tuning gives the platform an oscillation period of about 84.4 minutes so that horizontal accelerations do not produce growing position errors. Outputs are position, velocity, attitude, heading, track and drift.
INS errors: gyro drift, accelerometer bias, Schuler oscillation, transport wander and Coriolis. Position error grows with time, which is why INS is updated by radio or GNSS.
GNSS
Segments: space (constellation), control (ground stations) and user (receiver). The receiver measures signal travel time and computes a pseudo-range to each satellite. Four unknowns (x, y, z and receiver clock) require four satellites. Three satellites give a 2-D fix using a barometric altitude aid.
Error sources: ionospheric and tropospheric delay, satellite clock and ephemeris, multipath, receiver noise and geometry. DOP quantifies geometry: HDOP horizontal, VDOP vertical, PDOP overall; lower is better. GNSS altitude is geometric (WGS-84 ellipsoid), not pressure altitude.
Integrity
RAIM uses redundant ranges to detect a faulty satellite - five satellites for detection, six for exclusion. SBAS (EGNOS, WAAS) adds corrections and integrity; GBAS does the same locally for precision approaches.
Worked example
A receiver tracks six satellites with good geometry. One range becomes inconsistent: with six usable satellites RAIM can both detect and exclude the faulty one and continue. With only five it could detect but not exclude. Loss of RAIM raises an alert and may force a non-RNP procedure.
Traps that cost marks
- Saying GNSS gives barometric altitude.
- Thinking more satellites always improve accuracy - geometry (DOP) matters.
- Confusing SBAS/GBAS (augmentation) with RAIM (receiver autonomy).
- Forgetting that an IRS cannot be fully realigned in flight and continues to drift.