GPS satellites somewhat regularly go in and out of service for various reasons. There are more satellites in the constellation than strictly necessary in order

GPS satellites somewhat regularly go in and out of service for various reasons. There are more satellites in the constellation than strictly necessary in order to compensate for this. Information on this is distributed to pilots (in the US) via NOTAMs, and you can use this information to decide whether it's safe to use GPS navigation for your flight. Practically speaking though, aviation GPSs have a function called RAIM that continuously monitors the integrity of the position information and will inform you on a loss of precision. I believe most GPSs also allow predicting RAIM along your time and route of flight to make sure you'll have integrity from takeoff to landing.