Receiver Autonomous Integrity Monitoring (RAIM)

A technique whereby a civil GNSS receiver/processor determines the integrity of the GNSS navigation signals without reference to sensors or non-DoD integrity systems other than the receiver itself. This determination is achieved by a consistency check among redundant pseudorange measurements. (PCG definition).

A system used to verify the usability of the received GPS signals and warns the pilot of any malfunction in the navigation system. This system is required for IFR-certified GPS units. (PHAK definition).

Source: Pilot Contoller Glossary (PCG), Pilot Handbook of Aeronatical Knowledge