How many satellites are needed for RAIM to detect a bad satellite?

The correct answer is five satellites. RAIM, which stands for Receiver Autonomous Integrity Monitoring, is a technology used to ensure the integrity of GPS signals. For RAIM to effectively detect a faulty satellite, it needs to compare the data received from the satellites.

When a receiver is connected to a minimum of five satellites, it can determine if any of the satellites are providing erroneous data. This is accomplished through a process of mathematical redundancy: with five satellite signals, the system can identify inconsistencies among the measurements. If one satellite's data appears to deviate from the expected fix based on the information from the other satellites, RAIM can flag that satellite as potentially faulty.

In contrast, with only four satellites, the receiver does not have enough information to perform this check reliably, as it requires a minimum of one additional satellite to discern differences and validate measurements. This inability to cross-reference the data limits its capability to assure the integrity of the position calculations. Thus, five satellites are essential for RAIM to function correctly and ensure that at least one bad satellite can be detected.