Receiver Autonomous Integrity Monitoring
To receive a 3D fix your receiver would normally track 4 satellites, every 20 seconds a fifth satellite replaces the data of one of the 4. If your position "changes" then the 5th satellite or the one that it "disregarded" is in error, so by continuously replacing one of the 4 with a 5th a faulty satellite can be identified.
So your normal 3D fix would require 4 satellites, but if you are using RAIM monitoring then you need the 5th.
Let's say an error is found; we will need another satellite for RAIM to continue to function, so RAIM now uses a 6th satellite for redundancy.
3D fixing with RAIM requires 6 satellites
Basic explanation I know, but this is all I remember
I hope it helps
Dean