What is the minimum number of satellites needed for GPS RAIM capability?

To have GPS RAIM (Receiver Autonomous Integrity Monitoring) capability, a minimum of five satellites is necessary. RAIM is a function that enhances the integrity of GPS signals by allowing the receiver to detect errors in the satellite data it receives. By using data from at least five satellites, the system can cross-check and validate the integrity of the signals.

Specifically, four satellites are needed for a basic three-dimensional position fix, but to effectively identify and solve for errors, an additional satellite is required. This fifth satellite provides the redundancy needed to determine both position and integrity, enabling the system to alert the user if an erroneous satellite signal is affecting the accuracy of the navigation solution. In essence, the fifth satellite is crucial for the integrity monitoring function, making it the minimum required for RAIM.

The other options of six or seven satellites would provide additional redundancy and higher accuracy, but they are not the minimum required for basic RAIM capability. Therefore, five satellites are the key number for establishing this important feature in GPS navigation systems.