What are the two primary causes of RAIM outages?

• A. Insufficient number of satellites or unsuitable satellite geometry.
• B. Satellite ephemeris errors.
• C. Pilot navigation errors.
• D. Receiver firmware glitches.

Answer: (A)

RAIM relies on multiple GPS satellites to check the integrity of the navigation solution and to detect or exclude faulty measurements. If there aren’t enough satellites to provide redundant measurements, the receiver cannot reliably confirm that every measurement is trustworthy, so an integrity outage is declared. Likewise, if the available satellites are clustered together in the sky and provide poor geometry, the protection levels become too large and the fault-detection process can’t reliably identify a bad signal, leading to an outage. Ephemeris errors or other issues can affect accuracy, but they’re not the primary cause of an outage—the lack of redundancy or poor satellite geometry is what prevents RAIM from guaranteeing integrity.