What effect does baro-aiding have on the number of satellites needed for RAIM?

• A. It increases the number of satellites needed
• B. It has no effect on the required count
• C. It eliminates the need for satellites
• D. It reduces the number of satellites needed

Answer: (D)

Baro-aiding adds an external measurement—barometric altitude—that constrains the vertical dimension of the navigation solution. RAIM relies on redundant observations to detect and isolate a faulty satellite. In a GPS-only setup, you need enough measurements to solve for the four unknowns (x, y, z, and receiver clock) while still having redundancy for integrity checks, which typically requires five satellites. When barometric altitude is available and trusted, the vertical position is effectively provided by the baro data, reducing the number of satellite measurements needed to maintain the same level of integrity testing. Therefore, you can achieve RAIM with fewer satellites, specifically one fewer than without baro-aiding. Remember, this depends on the baro data being reliable; if the baro input is suspect, the RAIM capability changes.