Which term describes the GPS distance measurement that is based on time?

• A. True range
• B. Pseudo-range
• C. Geometric distance
• D. Ellipsoidal distance

Answer: (B)

Pseudo-range is the GPS distance measurement based on time. The receiver measures how long the satellite signal takes to reach it by comparing the received code phase with the receiver’s locally generated code. Multiplying that time by the speed of light converts it to a distance. However, the receiver’s clock isn’t perfectly synchronized with the satellites’ atomic clocks, so an unknown clock bias is added to this distance. That bias makes the measurement only an estimate of the true geometric distance, hence it’s called pseudo-range. To determine position, the system uses pseudorange measurements from multiple satellites and solves for the receiver’s three coordinates plus the receiver clock bias. True geometric distance would assume perfect clock synchronization, while ellipsoidal distance refers to distance on the reference ellipsoid, not to signal transit time.