RAIM and RAIM outage?
A question from a controller to which I simply cannot wrap my head around, so help me a bit here.
RAIM, or Receiver Autonomous Integrety Monitoring, is a system built into a GNSS receiver in the aircraft, that is able to detect, FD, or detect and exclude, FDE, any faults in the signals received from GNSS satellites.
So far so good -> The aircraft GNSS reciever is itself able to monitor the integrety and alert pilots if any fault in the signal.
Now comes the real question:
Why, if the aircraft reciever is able to do so itself, do we issue "GPS RAIM prediction" -NOTAMS? And what does that mean?
Is it a safety layer on top of the RAIM onboard the aircraft, to advice that there in fact is faults on the signal within an area, and allow aircraft without built-in RAIM to operate? Or does it mean that the RAIM function in the aircraft simply does not work, which I don't really understand, since the function itself should always be working, it'll just show a warning (in the cockpit) if there is a fault?
And last but not least, if using SBAS (EGNOS/WAAS), can you then fly RNP approaches even if the RAIM is out of service? Considering that SBAS is a requirement for LPV minima (but RAIM not).