What is the minimum number of satellites that must be available for a differential solution of latitude, longitude, height, and time?

• A. 3
• B. 5
• C. 4
• D. 6

Answer: (C)

You must solve for four unknowns: latitude, longitude, height, and the receiver’s clock bias. Each satellite measurement provides one independent equation relating those unknowns to the observed pseudorange. To determine four unknowns you need at least four independent equations, so you need at least four satellites. With fewer than four satellites, you don’t have enough information to uniquely solve for all four parameters. In practice, more satellites improve the geometry and accuracy, but the bare minimum to solve for all four is four.