How many satellites are required to achieve RAIM?

To achieve RAIM (Receiver Autonomous Integrity Monitoring), a minimum of five satellites is typically required. RAIM is a technology that allows a GPS receiver to determine the integrity of the positioning information it is receiving. This is crucial for ensuring that the navigation data is accurate and reliable, as it can alert pilots to any potential errors or faults in satellite signals.

When five satellites are available, the GPS receiver can perform a self-check of the data it is processing. Specifically, with five satellites, the receiver can determine a three-dimensional position (latitude, longitude, and altitude), while also using the additional satellite information to check for consistency and reliability of the data being received. This redundancy is vital for the integrity checks that RAIM performs.

While four satellites can provide a basic three-dimensional position, they do not offer the necessary redundancy to validate the data, hence RAIM cannot be achieved with just four satellites. Having six or seven satellites would provide even greater redundancy and enhance the integrity monitoring capability, but the minimum requirement to enable RAIM is indeed five satellites.