What is the minimum number of satellites required for RAIM fault detection?

The correct answer identifies that a minimum of five satellites is required for RAIM (Receiver Autonomous Integrity Monitoring) fault detection. RAIM is a technique used in GNSS (Global Navigation Satellite System) to ensure the accuracy and reliability of the positioning data received from satellites.

To effectively perform fault detection, the system needs to compare the measurements from multiple satellites. With five satellites, the receiver can not only calculate a position fix but also have enough redundancy to identify whether any of the satellite signals may be erroneous. The additional satellite data aids the receiver in assessing the integrity of the position it computes, ensuring that any potential errors can be detected and compensated for.

In contrast, having fewer than five satellites would not provide sufficient data for RAIM to effectively perform its checks. For instance, with just three satellites, the receiver can only determine a 2D position fix, lacking the necessary redundancy to confidently detect faults. Similarly, while four satellites allow for a 3D fix, they may not have enough data points to reliably identify and isolate a fault, as they lack the extra signals needed for robust integrity monitoring. Therefore, five satellites are essential for the RAIM system to function properly, enabling reliable fault detection in GNSS applications.