RAIM unavailable on an RNAV 5 route
You are flying a Dash 8 Q300 from NZAA to NZCH on an RNAV 5 route. The aircraft has a TSO-C146 GNSS receiver and serviceable VOR/DME. Before departure, an AIP New Zealand NOTAM warns of GPS signal interference within 40 NM of NZAA up to FL200. Your route joins the airway after 60 NM. In cruise at FL190, the GNSS receiver shows RAIM prediction 'unavailable' for the next 30 minutes and one satellite is flagged. The VOR/DME is serviceable, but the airway is RNAV 5 only. You have 45 minutes to NZCH. ATC asks whether you can maintain RNAV 5. You must decide whether the GNSS is usable, what alternatives exist, and what to tell ATC.
1. Does 'RAIM unavailable' stop RNAV 5? Explain.
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Yes for GNSS-based RNAV 5. RAIM is required to assure integrity. If RAIM is unavailable, the GNSS receiver cannot provide the required integrity for RNAV 5, so you cannot use GNSS as the navigation source. The aircraft may still be RNAV 5 approved using VOR/DME or DME/DME, but only if the route and coverage support it and the aircraft's approval includes those sensors. Check the AFM or approved supplement and the route. The NOTAM about GPS interference near NZAA reinforces the prediction. Do not assume that one flagged satellite is harmless; it may be the reason RAIM is unavailable.
2. What are your options?
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Options include: request an alternate route or vectors clear of the interference and re-check RAIM; use VOR/DME RNAV 5 if approved and within coverage; change level to one where RAIM is available; or divert. You have 45 minutes to NZCH; if the route remains RNAV 5 only, ATC may need to provide vectors or a conventional route. Advise ATC early. If no approved navigation source remains, declare a navigation failure and request assistance. Do not continue on GNSS with RAIM unavailable just because the map looks correct.
3. What would you tell ATC and what would you monitor?
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Tell ATC: 'Unable RNAV 5 due RAIM unavailable; request vectors or VOR/DME route.' Monitor RAIM status, satellite flags, VOR/DME raw data, and position versus route. If using VOR/DME, cross-check radials and DME at each fix. Keep a written note of the last confirmed fix. If RAIM becomes available, you may resume RNAV 5 only after confirming the receiver is in the correct mode and the position is consistent. Report any navigation degradation.