Explain what Doppler shift provides in GNSS navigation.

• A. Doppler shift gives relative satellite velocity along line of sight, contributing to receiver velocity estimation and clock bias estimation.
• B. Doppler shift measures clock bias directly.
• C. Doppler shift determines satellite altitude.
• D. Doppler shift is used to correct atmospheric delays.

Answer: (A)

Doppler shift in GNSS comes from the relative motion between the satellite and the receiver along the line of sight, so the received carrier frequency appears shifted depending on how fast you’re moving toward or away from the satellite. This shift provides a measurement of the range rate—the rate at which the distance to the satellite is changing—which directly ties to the line‑of‑sight component of the receiver’s velocity. In the navigation solution, this Doppler information helps estimate the receiver’s velocity and, because the observed Doppler also depends on the receiver’s clock error drifting over time, it helps constrain and estimate the receiver’s clock bias as part of solving for position and time. It does not directly measure clock bias, determine satellite altitude, or serve as the primary tool for correcting atmospheric delays.