All RAIM does, as much as I can remember, is maintain a list of received satellite signals and then rotate processes the list. If the signal from one particular satellite is incorrect the constant cross comparison highlights it and deselects that satellite from the list.
RAIM prediction just lets you know when there will not be enough strong signals to populate the redunancy list.
In the event of there not being enough signals to populate the list then the accuracy of the received satellite information cannot be accurately determined and a RAIM failure is generated.
The upshot of all this technical moolah is that the accuracy of the GPS cannot be positively determined to be good enough for the GPS approach and therefore the approach is not allowed to be flown. Reasons given is that the loss of one or more GPS signals could lead to gross inaccuracies within the lateral nav of the aircraft.
I seem to remember that some airports, Munich for example, have a GPS ground station providing additional info, can't quite rmember what they are called though.
Cheers, lookout and fly accurately!
