How does GPS determine the distance from a satellite to the aircraft?

• A. By measuring the Doppler shift of the signal.
• B. By comparing signal strength with a reference beacon.
• C. By the travel time of a radio signal.
• D. By calculating a geometric distance from ground stations.

Answer: (C)

GPS determines the distance from a satellite to the aircraft by measuring the travel time of the radio signal. The satellite broadcasts a highly precise time-stamped signal, and the receiver notes the exact moment it receives that signal. By comparing the satellite’s transmit time with the receiver’s receive time, the system multiplies the time it took for the signal to travel by the speed of light to get the range to that satellite. Because the receiver’s clock isn’t perfectly synchronized with the satellite clocks, this measurement is a pseudorange that includes the receiver’s clock bias; solving with signals from multiple satellites lets the receiver determine its position and its own clock error. Doppler shift relates to velocity, not distance, and signal strength isn’t a reliable method for measuring range. Ground stations aren’t used to compute the instantaneous distance to a satellite.