Area Navigation (RNAV) and Basic GNSS Principles
RNAV concepts
Area navigation (RNAV) lets an aircraft fly a desired path using signals or data from various sources, rather than tracking directly to or from a ground station. A basic RNAV route is defined by waypoints and legs; the system computes track, distance and guidance. Sources include GNSS, DME/DME, VOR/DME and inertial systems. Performance-based navigation (PBN) specifies the accuracy, integrity, continuity and availability needed; RNP adds on-board performance monitoring and alerting. RNAV 5 and RNAV 1 are common navigation specifications, not equipment types. Route spacing and containment depend on the specification.
GNSS principles
GNSS satellites broadcast time and orbital data. The receiver measures pseudorange to each satellite and solves for latitude, longitude, altitude and receiver clock offset. Four satellites are the minimum for a 3D fix plus time; additional satellites improve geometry and allow RAIM. RAIM detects a faulty satellite and warns if the position cannot be trusted. SBAS (e.g. WAAS/EGNOS) and GBAS provide augmentation; errors include ionospheric delay, tropospheric delay, multipath, ephemeris and clock errors. GNSS position is in a geodetic datum such as WGS 84.
Worked example
A receiver has pseudoranges to four satellites: it can solve x, y, z and time, giving a 3D fix. With five satellites it can run RAIM and detect one bad measurement. If only three are usable, the receiver must assume an altitude (2D fix) or cannot provide a reliable navigation solution; RAIM may be unavailable, so the flight plan needs an alternative.
Traps
Do not confuse RNAV with RNP: RNP includes performance monitoring and alerting. Do not assume a GPS fix is always 3D; satellite geometry and RAIM status matter. GNSS track is true, while ATS routes and charts may be published in magnetic; convert correctly. Datum differences can shift positions. Do not use a receiver's direct-to function without checking waypoint, leg and fly-by/fly-over coding.