RAIM outage on an RNAV route and an LNAV approach
You are flying an RNAV route with RNP 2 toward an airport that offers an LNAV approach with an RNP 0.3 lateral limit. About 60 NM from destination, the GNSS receiver annunciates that RAIM is unavailable for the next 25 minutes, although baro-aiding is available. Your ETA at the initial approach fix is 14:35 UTC, and the RAIM outage is predicted from 14:20 to 14:45 UTC. The airport has a VOR/DME 35 NM from the threshold, but it is not a landing aid. You have two VOR/DME receivers and no certified RNP capability without GNSS. The destination weather is at the LNAV minima, so a missed approach is likely if the approach is unstable. You must decide whether to continue, hold, or divert.
1. Explain what RAIM is and why the outage matters differently for RNP 2 en-route navigation and for the LNAV approach.
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RAIM is receiver autonomous integrity monitoring. The GNSS receiver uses redundant satellite measurements to detect a position error and, with fault detection and exclusion, to warn the crew that the navigation solution is not trustworthy. RNP 2 requires on-board performance monitoring and alerting to a 2 NM total system error; the en-route phase is relatively tolerant, but the integrity function must still be available. The LNAV approach has an RNP 0.3 lateral limit, so the same undetected satellite fault could produce a lateral error many times larger than the approach tolerance. Baro-aiding can improve RAIM availability by adding barometric altitude as an independent measurement, but it does not replace the need for sufficient satellite geometry and it does not improve lateral accuracy to 0.3 NM. The approach is therefore the critical phase, not the en-route segment.
2. Can you continue the en-route segment under RNP 2? Can you fly the LNAV approach at 14:35 UTC?
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En route, the aircraft can continue only if the operator's RNP 2 approval permits operation with baro-aiding and the receiver confirms integrity. If RAIM is unavailable because of poor geometry but baro-aiding restores fault detection, the en-route segment may remain usable; if the receiver cannot provide integrity, the crew must revert to an approved alternate means or advise ATC. The approach is different. The ETA of 14:35 UTC falls inside the predicted outage, which ends at 14:45 UTC. The LNAV approach requires RNP 0.3 and integrity at the final approach fix; a predicted RAIM outage at that time means the approach should not be commenced. The correct decision is to hold, delay, or divert; continuing to the IAF in the hope that RAIM returns is not acceptable because the missed approach may occur at the least convenient moment.
3. If you revert to the VOR/DME 35 NM away, estimate its likely position accuracy and explain why it cannot simply be substituted for RNP 0.3.
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A VOR radial has a bearing error of about ±1.5°, so at 35 NM the lateral error from the radial alone is 35 × tan 1.5° ≈ 0.92 NM. DME error is typically ±0.2 NM plus about 1.25% of distance, giving 0.2 + 0.44 = 0.64 NM at 35 NM. Combining the radial and distance errors gives a position uncertainty of roughly 1 NM, which is larger than the 0.3 NM approach limit and leaves no margin for flight technical error or charting. VOR/DME also lacks the on-board performance monitoring and alerting required to claim RNP 0.3; raw accuracy is not the same as a certified RNP system. It is adequate for en-route monitoring and for a conventional non-precision approach where the published minima allow it, but it cannot be used to fly the LNAV approach to the RNP 0.3 limit.