GNSS and required navigation performance
This domain tests GNSS as the primary sensor for PBN, the integrity concept behind RNP, and the limits that decide whether a GNSS approach may be commenced.
Core rules
- GNSS positioning needs at least four satellites; RAIM generally needs five, and fault detection and exclusion needs six. Without RAIM, an approach that depends on it cannot be flown.
- Integrity, accuracy, continuity and availability are separate qualities. A receiver can be accurate and still unusable because integrity monitoring is unavailable.
- In New Zealand, GNSS approaches are published as RNAV (GNSS) with LNAV and, where surveyed, LNAV/VNAV minima using Baro-VNAV. SBAS (LPV) is not available, so RAIM prediction is part of pre-flight planning.
- Baro-VNAV is temperature-limited; below the charted limit, LNAV/VNAV minima must not be used unless the aircraft has approved temperature compensation.
Worked example
A flight plans an RNAV (GNSS) approach requiring RAIM at 10:00–10:15. The prediction shows a RAIM outage from 10:05 to 10:10. The crew cannot treat the approach as available for that window: they must delay, replan, or nominate an alternative approach that does not depend on RAIM. If only four satellites are usable, positioning may continue but the approach is not authorised.
Traps
- Using GPS altitude as a primary altimeter reference.
- Ignoring the charted Baro-VNAV temperature limit on a cold morning.
- Assuming SBAS or GBAS coverage exists in New Zealand.
- Believing an approach is available because the airport is in the database, without checking RAIM and NOTAMs.