GNSS Integrity and Baro-Aiding Decision on an RNP Approach
You are preparing an RNAV (GNSS) approach to runway 09 with LNAV minima. The aircraft's RNP value is 0.3 NM. The GNSS receiver is tracking 7 satellites, but one is at 4° elevation and another is flagged unhealthy; the elevation mask is 5°. The fault detection and exclusion (FDE) function requires at least 5 usable satellites, or 4 if baro-aiding is available and confirmed. The RAIM prediction for the ETA shows a 12-minute outage beginning 5 minutes before the planned approach. The FMS shows HDOP 2.8 and UERE 4.0 m; the barometric altimeter is coupled and the correct QNH is set. The weather is VMC, and a VOR/DME procedure is available at the same airport.
1. Determine the number of usable satellites and whether RAIM/FDE is available with and without baro-aiding.
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Usable satellites = 7 − 1 below the 5° mask − 1 unhealthy = 5. Without baro-aiding, FDE needs 5, so it is available, but with no spare satellite. With baro-aiding confirmed, only 4 are needed, so FDE is available with one satellite in reserve. However, the predicted RAIM outage is a time-based integrity warning, not a count of satellites. If the outage overlaps the approach and the prediction does not include the approved baro-aided case, RAIM is not available for that window. The current geometry does not override the prediction.
2. Compute the horizontal accuracy from HDOP and UERE and compare it with the RNP 0.3 value.
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1σ horizontal error = HDOP × UERE = 2.8 × 4.0 = 11.2 m. 95% error ≈ 2 × 11.2 = 22.4 m. RNP 0.3 NM = 0.3 × 1852 = 555.6 m. The accuracy is only about 4% of the RNP value, so accuracy is not the limiting factor. RNP also requires integrity and continuity; passing the accuracy check does not cure a RAIM outage. Do not confuse HDOP-based accuracy with fault detection.
3. State your decision and describe a safe alternative.
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Decision: do not commence the LNAV approach during the predicted RAIM outage unless the approved RAIM prediction tool, with baro-aiding correctly configured, shows availability for the actual ETA. In VMC, the safe alternatives are to delay until after the outage, request the VOR/DME approach, or divert. If the approach is attempted, cross-check the FMS position against VOR/DME and be prepared to go around; do not use baro-aiding as a substitute for integrity monitoring.