RNAV Approach in a GNSS Interference Window
An aircraft is inbound at night to an island aerodrome whose only instrument approach is RNAV (GNSS) RWY 27, with LNAV minima 480 ft and LNAV/VNAV minima 390 ft. There is no SBAS coverage in the region, so any vertical guidance is baro-VNAV. The destination alternate, 180 nm away, has a VOR/DME approach and a 1,200 m runway. A NOTAM states: 'GNSS signal may be unreliable within 40 nm of the aerodrome between 1800 and 1900Z due to military activity.' The ETA at the holding fix is 18:25Z. The onboard RAIM prediction for the LNAV approach at 18:25Z shows no outage, and the FMS indicates DME/DME updating is unavailable. At 18:25Z at the holding fix, fuel on board is 2,700 kg; holding burn is 2,000 kg/h; fuel to the alternate including the approach is 900 kg; final reserve is 1,200 kg; an approach and landing at the destination costs 200 kg.
1. Does the RAIM prediction clear the aircraft to fly the RNAV approach inside the NOTAM window? Explain the difference between what RAIM detects and what the NOTAM describes.
Read the worked answer
No. RAIM and the NOTAM address different failure modes, and clearing one does not clear the other.
RAIM
Receiver Autonomous Integrity Monitoring uses redundant ranging signals to check that the position solution is consistent. It detects a satellite failure or an unfavourable geometry that leaves insufficient redundancy, and it is a prediction of availability based on the published constellation and the approach's integrity requirements. A 'no outage' result means the geometry and the satellite health data look adequate at that time and place.
The NOTAM
A military exercise implies deliberate or incidental radio-frequency interference on the GNSS bands. That is an external, localised signal environment that the receiver cannot model and RAIM cannot predict. In practice, interference produces one of two failure modes: loss of signal (the receiver drops satellites, losing the approach capability) or, worse, a corrupted signal that degrades accuracy without an immediate annunciation.
Conclusion
The RAIM prediction is necessary but not sufficient. Inside a published interference window the assumption on which the RNAV approach depends - a reliable GNSS position - is exactly what is in doubt. Treat the approach as unavailable between 18:00 and 19:00Z unless the approach can be flown with another means of navigation that satisfies the procedure, and here DME/DME updating is unavailable, so there is no such means.
2. Compute the fuel and time trade-off between holding until 19:00Z and diverting now, and recommend a decision.
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Holding option
Hold from 18:25 to 19:00 = 35 minutes at 2,000 kg/h = 1,167 kg.
Fuel at 19:00Z = 2,700 − 1,167 = 1,533 kg. Then approach and landing (200 kg) leaves 1,333 kg at touchdown - only 133 kg above a 1,200 kg final reserve. One go-around (typically 300–350 kg) plus a second approach would put the aircraft below final reserve. So holding until the end of the window consumes the entire safety margin and leaves no contingency for the most likely event - a missed approach in the weather that caused the diversion consideration in the first place.
Diversion option
Divert now: 2,700 − 900 = 1,800 kg at the alternate, i.e. 600 kg above final reserve, with a conventional VOR/DME approach available that does not depend on GNSS.
Recommendation
Divert. The comparison is not 'hold 35 minutes versus divert' on fuel alone; it is between a legal state with 600 kg of margin and a legal state with 133 kg and no go-around capability. If the commander chooses to hold, it must be with a hard time limit shorter than the full window - no more than about 15 minutes (500 kg), keeping at least 500 kg above final reserve at touchdown - and with the diversion still available at the end of it. What must not happen is holding to 19:00Z and only then discovering that the interference has been extended by a second NOTAM.
3. Can the LNAV/VNAV minima be used at this aerodrome, and what conditions apply?
Read the worked answer
Only if all of the following hold.
- Approval and equipment: the aircraft and crew must be approved for baro-VNAV, and the approach must be flown with the barometric vertical guidance computed from the altimeter setting - not with SBAS or an SBAS-derived LPV, which is not available here.
- Temperature limits: baro-VNAV procedures are published with a temperature limitation. In a hot tropical environment the concern is at the other end of the scale from the usual cold-weather problem, but the chart limit still applies and must be checked before selecting the LNAV/VNAV line.
- Altimeter setting source: the local QNH must be available and current; an incorrect or remote altimeter setting introduces a proportional height error along the whole path.
- GNSS integrity for lateral guidance: LNAV/VNAV still depends on the GNSS lateral position, so the NOTAM interference problem applies to the vertical-guidance line as well. Baro-VNAV fixes only the vertical dimension.
If any of these fail, the LNAV minima apply - and if GNSS integrity is in doubt, no RNAV approach at this aerodrome may be flown at all. That is precisely why the alternate's VOR/DME approach matters: with a single RNAV approach and no conventional backup, the diversion is the only mitigation, and it must be planned before the fuel becomes the constraint.