GNSS and Performance-Based Navigation
Core concepts
GNSS position is derived from ranging measurements to satellites; position accuracy depends on satellite geometry, expressed as dilution of precision. GPS, GLONASS, Galileo and BeiDou are constellations; augmentation improves integrity. Performance-based navigation (PBN) specifies the navigation performance required for a procedure in terms of accuracy, integrity, continuity and availability. The two main PBN specifications are RNAV and RNP; RNP requires on-board performance monitoring and alerting. Key concepts are RAIM (receiver autonomous integrity monitoring), satellite geometry, the difference between horizontal and vertical guidance, and the approach classifications (LNAV, LNAV/VNAV, LP, LPV).
Traps in questions
Expect questions on: RAIM availability prediction before flight; the fact that GNSS accuracy is degraded by poor geometry; the difference between an LNAV approach (lateral only) and LPV (vertical guidance with localiser performance); and the requirement to check NOTAMs for satellite outages. Another trap is assuming GNSS is always available at all altitudes; terrain and interference can degrade it. Also remember that RNP AR approaches require special crew and aircraft approval.
Worked example
If the predicted HDOP is 4 and the ranging error is 5 m, the horizontal position error is about 20 m. If the procedure requires 0.3 NM (about 555 m), the margin is comfortable; if HDOP rises to 20, the error approaches 100 m and integrity monitoring may be unavailable.
Rule of thumb
Always check RAIM prediction and NOTAMs before a GNSS-dependent approach. Poor geometry multiplies ranging error, so a low HDOP is essential.