How many satellites are required for fault detection and exclusion?

• A. 4
• B. 5
• C. 6 (5 if baro-aiding is available)
• D. 7

Answer: (C)

Fault detection and exclusion relies on having more measurements than you strictly need to solve for the unknowns, so you can check for inconsistencies and then identify the one measurement that doesn’t fit. To determine a 3D position and the receiver’s clock bias, four independent pseudorange observations are required. With one extra satellite observation you gain enough redundancy to notice when one measurement doesn’t align with the others, enabling fault detection. However, to confidently pinpoint which satellite is faulty (exclusion) you generally need even more redundancy, which comes from having a larger number of satellites. If barometric aiding is available, the barometer provides an additional independent piece of information, increasing redundancy and allowing fault detection and exclusion with one fewer satellite. So, the standard requirement is six satellites for autonomous fault detection and exclusion, and five satellites if baro-aiding is available.