What information does the Doppler measurement provide in GNSS/INS integration?

• A. Absolute position.
• B. Relative velocity along the line of sight to the satellite.
• C. Ionospheric delay.
• D. Clock bias.

Answer: (B)

The Doppler measurement tells you how fast the distance between the receiver and a satellite is changing, projected along the line of sight to that satellite. In other words, it provides the relative velocity component along the line of sight, often called the range rate. This reflects how quickly the LOS distance is shrinking or growing as the satellite and receiver move, and it’s this rate that shows up as a Doppler frequency shift.

This observable is not used to determine absolute position by itself, and it doesn’t directly give ionospheric delay. It also isn’t a direct measure of clock bias, though it does include effects related to the receiver’s clock drift rate as part of the range-rate term. By combining Doppler measurements from multiple satellites, you can infer the receiver’s velocity vector, which is especially valuable when fusing GNSS data with INS to improve overall state estimation.