GPS position integrity (RAIM)
GPS ranges to satellites: minimum four satellites in view = accurate three-dimensional fix.
RAIM (receiver autonomous integrity monitoring) checks own satellites for bad data. Needs more satellites than the fix (TC AIM COM 5.3.1):
| RAIM function | Satellites in view | With baro-aiding |
|---|---|---|
| Detect one corrupt satellite | 5 | 4 plus baro-aiding |
| Isolate and remove it from solution | 6 | 5 plus baro-aiding |
- Baro-aiding: aircraft altitude encoder = stand-in satellite.
- RAIM raises one of two messages: not enough satellites to monitor, or error beyond limit for current phase of flight.
No RAIM (PHAK ch.16; TC AIM COM 5.11(a), 5.3.1):
- Many VFR receivers and all hand-held units: no RAIM alerting.
- No flag for dropped satellite or growing error: display looks fine either way.
- Distrust GPS position when it disagrees with other radio nav, pilotage, or dead reckoning.
Trap: Selective Availability (deliberate GPS error) ended 1 May 2000, but many receivers still act as if it is on, and a receiver without RAIM gives no warning when wrong.
References
- FAA-H-8083-25 PHAK ch.16 (Global Positioning System, RAIM Capability, VFR Use of GPS)
- TC AIM COM 5.3.1 (Aircraft-Based Augmentation System: RAIM, FDE, baro-aiding)
- TC AIM COM 5.11(a) (Proper Use of GNSS: IFR-certified avionics)