A French geodetic technique in which transmitters on the ground communicate with receivers on satellites to provide precise orbit determination required by ocean altimeter satellites.

• A. DORIS
• B. GPS Tracking
• C. LORAN
• D. Radar Altimetry

Answer: (A)

Ground-to-space Doppler tracking with a French system called DORIS uses a network of ground transmitters (DORIS beacons) that continuously send signals to satellites equipped with DORIS receivers. By measuring the Doppler shifts of these signals, the satellite’s orbit can be determined with very high precision. This precise orbit knowledge is essential for ocean altimetry, because accurate sea-surface height measurements depend on knowing exactly where the satellite is in space. DORIS is specifically designed for this purpose, linking ground-based timing and positioning signals to satellite orbits. The other options don’t fit this description: GPS tracking relies on a different global system, LORAN is an old terrestrial navigation method, and radar altimetry is the satellite measurement technique itself, not the ground-to-satellite tracking method used for orbit determination.