How many satellites are required for RAIM and fault detection?

For Receiver Autonomous Integrity Monitoring (RAIM) and fault detection, a minimum of five satellites is necessary. This requirement arises from the need to have enough satellite signals for both the accurate positioning of the aircraft and for the system to evaluate the integrity of these signals.

With five satellites, the receiver can compute a position solution and still assess one of the satellites for potential faults or errors. If the receiver detects that one satellite's data is inconsistent with the others, it can exclude that satellite from the calculation and still maintain a valid position solution using the remaining four satellites.

This capability is crucial for ensuring safe navigation in IFR (Instrument Flight Rules) conditions, where precise positioning is paramount. Therefore, the option of five satellites stands as the correct answer for RAIM and fault detection requirements.