Which technique is primarily used for orbit determination of ocean altimeter satellites via ground transmitters and satellite receivers?

• A. DORIS
• B. GPS
• C. GLONASS
• D. Radar Altimetry

Answer: (A)

This relies on ground-based beacons transmitting to the satellite, with the satellite carrying a receiver that records the Doppler shift of those signals. The Doppler measurements, collected from a global network of ground stations with precisely known positions, let us infer the satellite’s velocity and, through orbit estimation, its position over time. This ground-to-satellite Doppler approach is what DORIS is built for, and it is especially effective for ocean altimeter satellites because the beacons form a wide, stable network that provides continuous, global tracking.

GNSS methods (using signals from the satellites themselves) can also be used for orbit determination, but the described setup—ground transmitters and a satellite receiver measuring Doppler—points to DORIS. Radar altimetry, meanwhile, is the instrument that measures sea surface height, not the method of determining the satellite’s orbit.