Area Navigation, GNSS and Performance-Based Navigation
RNAV and RNP
Area navigation allows flight on any desired path using inputs from various sensors, rather than tracking station to station. RNP adds on-board performance monitoring and alerting, so the aircraft can alert the crew when the navigation performance is not met. Required navigation performance is expressed as a containment value in nautical miles.
GNSS
GNSS (GPS, GLONASS, Galileo, BeiDou) uses satellite ranging. Position accuracy depends on geometry (dilution of precision), satellite health and integrity monitoring. Augmentation systems (ABAS, SBAS, GBAS) improve accuracy and integrity. RAIM detects faults; without sufficient satellites RAIM may be unavailable, requiring an alternate means of navigation.
PBN
Performance-based navigation comprises RNAV and RNP specifications. It specifies accuracy, integrity, continuity and availability. GNSS is the primary sensor for most PBN operations; the navigation specification, not the equipment alone, defines what is approved.
Traps
Do not confuse RNAV with RNP (monitoring and alerting on board). Dilution of precision is geometry, not signal strength. RAIM prediction is required before GNSS-dependent approaches.
Rule of Thumb
RNP 0.3 means total system error must stay within 0.3 nm for 95% of the flight time; know that containment is not the same as the alert limit.