What we commonly call ‘GPS’ is officially known as the Global Navigation Satellite System (GNSS).
Several GNSS systems have been developed across the world:
A satellite in Earth’s orbit transmits a signal that includes the time it was sent and the aircraft’s receiver compares the time difference between when the signal was sent and when it was received. This time difference is used to calculate the distance between the satellite and the aircraft.
When these signals are received from 3 satellites, a 2-dimensional position fix is made by the GNSS unit. When a signal from a 4th satellite is received, the aircraft’s altitude can be calculated as well.
Receiver Autonomous Integrity Monitoring (RAIM) is a ‘self-checking’ system that detects faults with a satellite’s signal and ignores that signal when a fault is detected.
At least 5 satellites are required for RAIM to work (the 4 needed for a 3-dimensional position fix plus the 1 that RAIM ignores). However, the CAA recommends at least 6 satellites so that RAIM still functions after 1 satellite has been ignored.