What additional requirement is needed to exclude a bad satellite in RAIM?

To exclude a bad satellite in Receiver Autonomous Integrity Monitoring (RAIM), the system requires input from multiple satellites to effectively determine the integrity of the signals received. Specifically, when five satellites are utilized, the system can achieve the necessary geometrical diversity to isolate and exclude any satellite that may be providing faulty data.

The reason a barometric altimeter is also included in this configuration is that it can provide an additional source of information for altitude, enhancing the overall integrity check. With this setup, the RAIM algorithm can accurately assess the health of the satellite signals and detect which one may be erroneous. This requirement for five satellites alongside a baro altimeter forms a more robust and reliable system for ensuring safety in navigation.

Using four satellites would not provide the necessary capabilities for detecting and excluding a faulty satellite, as the redundancy required for a complete integrity check would not be sufficient. Moreover, while six satellites could potentially work without additional equipment, the minimum requirement to effectively conduct an integrity check in the scenario described is five satellites, stressing the importance of having that additional data source to confirm accuracy and reliability.