In a four-satellite GPS solution, which additional parameter is solved for besides latitude, longitude, and altitude?

• A. Velocity
• B. Time
• C. Bearing
• D. Course

Answer: (B)

In a GPS position fix, the extra unknown you solve for comes from the receiver’s clock being imperfectly synchronized with GPS time. Each pseudorange observation ties the true distance to the satellite to the measured range, and this measurement includes the receiver’s clock error multiplied by the speed of light. With three satellites you can solve for latitude, longitude, and altitude, but you still have a missing unknown—the receiver’s time offset. Using a fourth satellite provides a fourth independent equation, which lets you solve for the receiver’s time bias as well. That time offset is effectively time in seconds (the difference between the receiver’s clock and GPS time). Velocity, bearing, and course are not the immediate unknowns in this basic four-satellite position solution; velocity could be derived later from Doppler or the position change, but the fourth unknown that completes the fix is the receiver time.