If a GPS receiver can choose not to use a particular satellite, what is the minimum number of satellites required to run RAIM with baro aiding?

• A. Four satellites
• B. Five satellites
• C. Six satellites
• D. Seven satellites

Answer: (C)

RAIM needs redundancy to verify that a satellite’s signals aren’t faulty. Even with baro aiding providing altitude to help the vertical solution, the RAIM process still requires enough GPS observations to perform a fault-detection check. If you’re allowed to discard one satellite, you must have enough remaining satellites to both solve the navigation equations and run the integrity check. With five satellites in view, discarding one leaves four, which isn’t enough for RAIM. With six satellites in view, discarding one still leaves five, which is the minimum needed to perform RAIM while using baro aiding. So the minimum is six satellites in view.