What error does the fourth GPS satellite help minimize?

• A. Time error
• B. Signal strength deterioration
• C. Receiver bias
• D. Multipath error

Answer: (A)

In GPS positioning, you’re solving for four unknowns at once: the receiver’s position in three dimensions and the receiver’s clock bias relative to GPS time. Each satellite provides a pseudorange measurement that ties the satellite’s known position to the receiver’s unknown range, but this measurement includes the receiver’s clock error as part of the unknown. With three satellites you’d have three equations, which isn’t enough to determine all four unknowns unless you assume the receiver clock is perfect. Adding a fourth satellite gives a fourth independent equation, so you can solve for the receiver’s time offset as well. That time offset is the time error you’re minimizing.

Multipath error, signal strength deterioration, and receiver bias (distinct from the clock bias) aren’t solved or minimized simply by having an extra satellite; those issues are addressed through signal processing, system design, and careful measurement, while the fourth satellite specifically enables computation of the receiver’s clock error.