What is Earth rotation compensation (Sagnac effect) in GNSS?

• A. The Sagnac correction adjusts for atmospheric delays.
• B. The Sagnac correction is irrelevant to GNSS.
• C. The Sagnac correction is used to calibrate the receiver clock.
• D. The Sagnac correction accounts for the rotation of the Earth when computing line-of-sight distance.

Answer: (D)

The main idea is that the Earth’s rotation changes the geometry of the signal path during travel. In GNSS, we estimate distance by measuring how long the signal takes to go from satellite to receiver. Because the Earth is spinning while that light pulse is in flight, the receiver and the point on Earth where it sits move a bit during the round trip, so the straight-line distance isn’t the same as if the Earth were stationary. The Sagnac correction compensates for this rotation when computing the line-of-sight distance, yielding a more accurate geometric range.

This correction is not about atmospheric delays, and it’s not simply for clock calibration. It specifically accounts for the rotation of the Earth in the distance calculation, which is important for precise positioning.