411A, you are priceless.
RAIM - Receiver Autonomous Integrity Monitoring.
Basically, a system to monitor the integrity of GPS nav. Why do we need it? Well, a VOR monitors its own integrity at the transmitter. In terms of quality and speed for use as an approved aircraft navigation system, a GPS satellite doesn't.
So its done onboard the aircraft, in the receiver. For GPS nav you need to track 4 satellites. For RAIM you need to track 5, so that the system can sense a duff signal. However, the full monty is FDE (Fault Detection and Exclusion), for that you need to track 6 satellites in order to identify, and ignore, the duff one.
Some recievers will predict and display RAIM availability for a certain position and time. Some authorities also require you to prove RAIM availability for your flight before despatch. There are a number of websites that offer the facility, one is the AUGER tool on the ECACNAV site. If you want to get really smart, you can input a mask angle to the equation to take account of high terrain on GPS appoaches.
Although the GPS constellation is based on 24 satellites, there were 27 airborne last time I checked. Preflight RAIM checks are, I think, only required when the constellation drops below 22 satellites.
A somewhat general reply, but I can't find my paper with all the info on it. Despite what 411A says, there is little detail in most flight manuals that are available to line crews.