How many satellites does the PADS require for FTL and self-positioning, respectively?

• A. 4 for FTL and 5 for self pos
• B. 5 for FTL and 4 for self pos
• C. 6 for FTL and 6 for self pos
• D. 3 for FTL and 5 for self pos

Answer: (B)

The key idea is how many independent measurements you need to solve for position and time with a GNSS-like system. Each satellite provides one pseudorange measurement, which contributes one equation. To determine three spatial coordinates (x, y, z) plus the receiver’s clock bias (the time error), you need four independent equations, so four satellites are the minimum for a basic 3D fix with time.

For the PADS functions here, the FTL (firing time lock or time-critical function) needs extra redundancy to ensure accurate time alignment and to verify the measurements used to compute precise timing for firing. That redundancy comes from using an additional satellite, bringing the total to five. The extra satellite improves reliability and reduces the impact of any single satellite’s measurement error on the critical timing calculation.

Self-positioning, on the other hand, is the standard 3D position with receiver clock bias, which requires only four independent pseudorange measurements. No extra satellite is needed for this basic fix, so four satellites are sufficient.

So, five satellites for the FTL and four satellites for self-positioning.