GNSS RNAV Arrival with a RAIM Prediction Problem
An aircraft equipped with a basic GNSS receiver approved for RNAV 5 is planned to arrive at an airfield at 52°10'N 021°00'E at 1430 UTC. The arrival procedure requires RNAV 1 capability. The pilot's database is current for the area. A RAIM prediction for the destination at 1430 UTC shows a 25-minute outage beginning at 1415 UTC. The aircraft has a single GNSS receiver with no baro-aiding and no DME/DME backup. The weather at destination is forecast 2,000 ft broken, 6 km visibility, wind 240°/18 KT, and the only instrument approach is an RNAV approach requiring RNP 0.3. The pilot's alternate is 65 NM away with a conventional VOR approach.
1. Assess whether the aircraft can legally and safely fly the planned RNAV 1 arrival and approach.
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There are two separate problems.
First, the aircraft is approved for RNAV 5 only. RNAV 1 requires a higher specification, including a system that supports the required accuracy and integrity. A basic receiver approved for RNAV 5 cannot simply be used on an RNAV 1 procedure, so the arrival is not authorised.
Second, the RNP 0.3 approach requires even tighter accuracy and integrity, and the RAIM outage from 1415 UTC removes the integrity monitoring that the approach depends on. With a single receiver and no baro-aiding or DME/DME backup, there is no alternative means of meeting the integrity requirement during the outage.
The planned arrival at 1430 UTC falls inside the outage. The pilot cannot rely on GNSS for the arrival or the approach at that time. The flight must be replanned, either by delaying to arrive after the outage, by using a different arrival and approach not dependent on GNSS, or by diverting.
2. Explain what RAIM does and why the outage matters more for the approach than for the en-route phase.
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RAIM, receiver autonomous integrity monitoring, uses redundant satellite measurements to detect a satellite that is transmitting faulty information. With enough satellites in view, the receiver can check each measurement against the others and raise an alarm if one is inconsistent.
In the en-route phase the required accuracy is looser, so a larger integrity limit is acceptable and fewer satellites are needed. On an RNP 0.3 approach the required accuracy and integrity are much tighter, so more satellites are needed and the geometry must be good. During the 25-minute outage the receiver cannot guarantee the integrity needed for the approach, even if position accuracy appears normal.
For the en-route phase the pilot could continue with other navigation means, such as VOR or dead reckoning, and use GNSS only for monitoring. For the approach there is no such fallback on this aircraft, so the outage is decisive.
3. Given the weather and the alternate, decide on a course of action and justify it.
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The weather is above the likely approach minima but not so good that a visual approach can be assumed. The wind of 240°/18 KT gives a crosswind component that depends on runway orientation, which is not given, so the pilot must check it against the aircraft limit before committing.
The alternate is 65 NM away with a conventional VOR approach, which does not depend on GNSS. That is a viable fallback, but the pilot must confirm that the alternate's weather is suitable and that fuel allows for the diversion plus holding.
The practical decision is to delay departure or adjust speed so that arrival is after the RAIM outage ends, provided the aircraft can legally fly the RNAV 1 arrival. If it cannot, the pilot must plan a different arrival and approach, or plan to divert to the VOR alternate. The pilot should not plan to arrive during the outage and hope the receiver copes, because the integrity requirement cannot be met.