Receiver Autonomous Integrity Monitoring (RAIM) is the ability of the GPS receiver/nav system to confirm that the satellites that is using to calculate its position are serviceable (healthy). RAIM needs 5 satellites in view to make the calculation (Fault Detection) and 6 if the faulty satellite is to be identified as well (Fault Detection and Exclusion) There are 3 protection levels - en-route (2NM), terminal (1NM) and final approach (0.3NM0
If you are planning to fly a GPS approach, or even to fly GPS-based RNAV routes in the US or Europe, you are required to perform a RAIM prediction before take-off. The RAIM prediction should determine how many satellites will be in view at any point along your planned route. The RAIM prediction algorithm in the airborne receiver uses information about the current status of the satellite constellation, received via the satellites themselves, however it has no knowledge of planned outages. RAIM prediction services such as those provided by Augur (
http://augur.ecacnav.com), the FAA (
http://www.raimprediction.net) and flight planning systems like Jeppesen take account of the planned outages as well.