Refer to figure.
Receiver Autonomous Integrity Monitoring (RAIM)
RAIM is a technique which uses extra GNSS satellites to improve the integrity of GNSS location data. Instead of the 4 satellite ranges required for a 3D GNSS fix, using 5 satellites can mean that the onboard RAIM system can continuously measure different combinations of 4 signals, making sure that every fix is consistent with the others. Each fix excludes 1 satellite, so the receiver knows if they are all correct or not, as they will all be correct if all the positions are the same, thereby noticing if any one satellite becomes faulty, as all the separate fixes will then be different from each other. This can tell us within 6 seconds if one of the satellites that we are using becomes faulty, rather than waiting a potential 3 hours for it to be noticed by the master station and taken offline. This RAIM feature is called Fault Detection (FD).

If we use a second extra satellite, (a total of 6 for a 3D fix), then we can also reliably calculate which one is the faulty satellite and remove it from usage, called Fault Detection and Exclusion (FDE).

RAIM does not require anything more than a RAIM capable reciever and enough standard GNSS satellites to perform its integrity monitoring.

It is a type of Air Based Augmentation System (ABAS), the other of which is called Aircraft Autonomous Integrity Monitoring (AAIM), which uses onboard location data such as IRS and barometric altimeter information to verify the integrity of the GNSS position. AAIM can actually assist RAIM, if the two systems are suitably paired, and can allow for one less satellite to be required in some cases (2D fix).