If baro-aiding is available, how many satellites are required for fault detection and exclusion?

• A. 6
• B. 5
• C. 4
• D. 3

Answer: (B)

Fault detection and exclusion needs more observations than unknowns so you can test each measurement for consistency. When baro-aiding is available, the vertical position is constrained, leaving three unknowns to solve for: the two horizontal position coordinates and the receiver clock bias. You need four independent pseudorange observations to solve for those three unknowns, but to reliably detect and exclude a single faulty satellite you need one extra observation to provide redundancy for the integrity check. That makes five satellites the minimum required to perform fault detection and exclusion.