How many satellites does a typical GPS receiver need to determine integrity using RAIM, assuming baro aiding?

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

Answer: (B)

RAIM relies on having more measurements than unknowns so you can test for faults by comparing observed and computed ranges (residuals). Without any aiding, you solve for four GNSS unknowns—X, Y, Z, and receiver clock bias—so you need at least four measurements for a solution. But to detect a faulty satellite, you need an extra measurement beyond what’s just enough to solve the equations, which means five satellites.

If altitude is provided by barometric data, that constraint reduces the GNSS unknowns by one (you don’t have to solve for Z). Now you’re solving for X, Y, and the clock bias, three unknowns, with four GNSS measurements giving a solution. Yet detecting a fault still requires an extra measurement beyond the minimum, so you need five satellites in total to perform the RAIM integrity check with baro aiding.