What is a pseudo-range measurement in GPS?

• A. A distance measured between a GPS satellite and receiver based on a time shift that depends on the correlation of the codes each generates to reveal the biased time delay between their respective clocks.
• B. A range calculated from carrier phase measurements.
• C. The geometric distance between the satellite and receiver.
• D. A range derived from the Doppler shift.

Answer: (A)

Pseudorange is the distance estimate from a GPS receiver to a satellite that comes from timing information, obtained by correlating the satellite’s code with the receiver’s locally generated code. This correlation reveals how far the receiver’s clock is offset from GPS time, exposing the bias between the two clocks. Because the receiver clock is not perfectly synchronized, the measured range includes this clock bias, so it’s not the true geometric distance but a biased one—hence the name pseudorange. In practice, multiple pseudorange measurements are used to solve for the true ranges to satellites and the receiver’s clock bias, yielding precise positioning.

Carrier-phase measurements can be more precise but include an unknown integer ambiguity; the geometric distance is simply the direct line-of-sight distance assuming perfect timing; Doppler relates to velocity, not range.