In general how many satellites must be in view of the GPS receiver for RAIM?

• A. 3
• B. 4
• C. 5
• D. 6

Answer: (C)

RAIM relies on redundant satellite measurements to independently verify the GPS solution. Since four pseudorange measurements are needed to solve for the four unknowns (X, Y, Z, and receiver clock bias), having only four satellites provides no extra data to test for faults. When five satellites are in view, there is one extra measurement that lets the receiver check the consistency of the observed ranges; if a satellite’s measurement is inconsistent, it will show up as a large residual and can be flagged or excluded, enabling an integrity check. Therefore, the general minimum for RAIM is five satellites in view. More satellites improve the reliability of the integrity monitoring.