GNSS Position Error During RNP Approach
You are flying an RNP AR approach with a 0.3 NM lateral containment requirement. Ten miles from the final approach fix the navigation display shows a position that is 0.4 NM left of the intended track, and the integrity annunciation changes from normal to a degraded status. The receiver is using GPS with SBAS augmentation, and the aircraft is in a mountainous area with a high terrain floor. Autopilot is coupled. Fuel is 45 minutes above final reserve, and the weather is 800 ft ceiling and 3 km visibility.
1. Interpret the position offset and the integrity change together.
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The integrity annunciation is the controlling information. When the receiver can no longer guarantee the integrity needed for the RNP AR 0.3 NM containment, the displayed position and the deviation scale are not certified for the approach. A 0.4 NM offset exceeds the 0.3 NM containment, so even if the position were valid the aircraft would be outside the allowed lateral bound.
SBAS in mountainous terrain can lose the geometry or correction signal needed to maintain integrity; the receiver then flags degraded status rather than silently degrading. The combination of a lateral offset beyond containment and a degraded integrity flag means the navigation solution cannot be used to continue the approach. The autopilot may be tracking a false deviation, so it should not be left coupled without intervention.
2. State the actions and the decision about continuing the approach.
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Disconnect or reduce autopilot coupling to the navigation source and fly on basic instruments. Advise ATC immediately of the loss of RNP capability and the position uncertainty. Climb to the minimum safe altitude for the area if there is any doubt about terrain clearance, since the terrain floor is high and the weather is only 800 ft.
The approach cannot be continued: RNP AR requires the specified containment and integrity throughout, and both are lost. Execute the published missed approach using the navigation means still available, or request radar vectors. With 45 minutes above final reserve there is ample fuel to divert. The decision is to abandon the approach, not to continue visually, because 3 km visibility in mountainous terrain does not reliably allow terrain avoidance.
3. What would you do if the integrity flag cleared and position returned to centre after 2 minutes?
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A transient degradation that clears does not restore the guarantee for this approach unless the aircraft can re-establish the required integrity and containment before the final approach fix and the crew can confirm the navigation solution is valid. Re-check the position against an independent source, confirm the integrity status is normal, and verify the deviation is within 0.3 NM and stable.
Even then, the conservative choice is to break off and reposition for another approach, because the cause of the degradation is unknown and could recur at a critical point. If the crew does continue, it must be from a re-established, stable position before the final approach fix, with ATC informed, and with a clear trigger to go around if the flag returns.