Area Navigation, GNSS and Onboard Systems
RNAV, RNP and PBN
Area navigation lets an aircraft fly a chosen path between waypoints rather than tracking directly to or from a ground station. RNAV specifies the navigation capability; RNP adds an on-board performance-monitoring and alerting requirement. The RNP value, for example RNP 0.5, is the lateral containment limit in nautical miles for 95 per cent of the flight time. Fly-by waypoints cut the corner; fly-over waypoints require overflying and are used where terrain or procedure design demands it.
GNSS
GPS, GLONASS, Galileo and BeiDou are satellite constellations; position is derived from timing signals, with the receiver solving for latitude, longitude, altitude and clock offset. Accuracy is degraded by ionospheric delay, multipath, satellite geometry and clock errors. RAIM checks whether enough satellites exist to detect, and with fault detection and exclusion to isolate, a faulty signal; the prediction must be checked before departure for the route and time. Augmentation systems are ABAS (on-board), SBAS (geostationary, wide area, for example EGNOS and WAAS) and GBAS (ground-based, for precision approach). WGS 84 is the standard coordinate frame.
Onboard systems
Inertial systems integrate accelerations and drift slowly, typically around 1 nm per hour without updating; they are blended with radio or GNSS updates in the flight management system. The FMS holds the navigation database, performance data and flight plan, and drives the electronic flight instruments and autopilot. Safety systems include TCAS/ACAS, EGPWS and ADS-B.
Worked example: with RNP 0.3, the aircraft must remain within 0.3 nm of the path for 95 per cent of the time and alert the crew if the limit is exceeded.
Traps: confusing RNAV with RNP, treating accuracy as integrity, and ignoring RAIM availability, which depends on time and position.