This is called RAIM prediction and is used to determine in advance if there will be sufficient satellites in view to permit the receiver to perform the RAIM check. This enables the receiver to predict at a specific location and time how many satellites will be in view. The GPS satellites transmit data necessary to determine the locations of all satellites. If they cross at 90 degrees and are close to the station, the position error is small, whereas if they cross at 20 degrees and are far from both stations, the position error is large. This is similar to determining a position with two intersecting VOR radials. Not all geometries of satellites provide the same accuracy of position. For enroute mode the RAIM value should be under 2 NM, terminal mode under 1 NM, and approach mode 0.3 NM. This calculated value can be compared to the required integrity for the phase of flight and if it is not acceptable, warn the pilot. The difference in the positions can be used to estimate the integrity of the position. This generates 5 possible positions or more. If more satellites are in view, then one of the four satellites being used for the position can be substituted one at a time with one of the extra unused satellites.

It works on the principal that a position takes 4 satellites in view. RAIM is a calculation that a GPS receiver makes to determine if the position information is suitable for the navigation mode the receiver is using.

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