What type of distance does GPS measure?

• A. True geometric distance
• B. Geodesic distance
• C. Pseudo-range, a time-based distance
• D. Ellipsoidal distance

Answer: (C)

GPS measures a pseudo-range, which is a time-based distance. The receiver determines how long the satellite signal took to arrive and multiplies that time by the speed of light to get a distance. However, the receiver’s own clock isn’t perfectly synchronized with GPS time, so this measured distance includes the receiver’s clock bias as an unknown error. That combination makes the result a pseudo-range rather than the exact geometric line-of-sight distance to the satellite. For positioning, you need signals from at least four satellites to solve for x, y, z, and the receiver clock error, which is why this time-based distance is used in the system. The true geometric distance would require perfectly synchronized clocks, and distances along the ellipsoid or geodesic paths describe surface measurements, not the line-of-sight range being determined here.