GNSS and Performance-Based Navigation
GNSS basics
GNSS receivers use pseudo-range measurements to at least four satellites to solve latitude, longitude, altitude and time. GPS satellites orbit at about 20,200 km in six planes inclined 55°, with a 12-hour period; the civilian L1 signal is at 1575.42 MHz, and L2/L5 add ionospheric correction and integrity. Position accuracy depends on user-equivalent range error and geometry, expressed as dilution of precision (HDOP, VDOP, PDOP).
Integrity and augmentation
RAIM uses redundant satellites to detect a faulty signal and, with enough satellites, to exclude it. ABAS is airborne (RAIM and fault detection and exclusion), SBAS uses geostationary satellites to broadcast corrections and integrity (WAAS, EGNOS), and GBAS gives local differential corrections at an airport. Coverage is not worldwide, so an LPV or SBAS-based approach must be confirmed before use. Typical errors are ionospheric and tropospheric delay, multipath, ephemeris and clock errors, and radio-frequency interference.
PBN
Performance-based navigation specifies the navigation performance required in terms of accuracy, integrity, continuity and availability. RNP requires on-board performance monitoring and alerting; RNAV does not. ICAO Doc 9613 lists the navigation specifications, and each State's AIP lists which are approved in its airspace.
Worked example
HDOP 1.2 and a user-equivalent range error of 4 m give a 1-sigma horizontal error of 4.8 m; doubling for a 95% figure gives about 9.6 m, inside RNP 0.3 but not by itself proof that integrity is available.
Traps
Thinking four satellites always gives RAIM; confusing accuracy with integrity; assuming SBAS coverage everywhere; and forgetting that interference or jamming simply removes the service.