RAIM stands for Receiver Autonomous Integrity Monitoring. It is a form of integrity monitoring performed within the GPS receivers themselves. It ensures available satellite signals meet the integrity requirements for a given phase of flight. By comparing the pseudorange measurements of a number of satellites, the RAIM function can identify a satellite failure and issue an alert to the pilot.

Many VFR GPS receivers and most hand-held receivers do not have RAIM capability. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position (and in a non-WAAS environment, may not file IFR). A minimum of five satellites is required to detect a bad satellite. (Note: At least six satellites are required to detect and exclude a bad satellite from the navigation solution if the receiver has a fault detection and exclusion (FDE) RAIM algorithm.)

The GPS receiver must report when its RAIM function is unavailable, at both present time/position and at any selected future time/position. Pilots can get information on satellite outages through the NOTAM system. The effect, however, of an outage on the intended operation cannot be determined unless the pilot has a RAIM availability prediction program which allows exclusion of a satellite which is predicted to be out of service.

If you are using GPS to fly an instrument approach and receive a RAIM annunciation prior to the final approach waypoint, you may not have sufficient accuracy to complete the approach.

RAIM function is required for instrument flight (except when flying under WAAS).