Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) measures the radio signals from 55 global ground stations that compose the International DORIS Service (IDS). Each ground station acts as a beacon to broadcast two stable radio frequencies at 2036.25 MHz (S-band) and 401.25 MHz (VHF). Every 10 seconds, the DORIS instrument aboard Sentinel-6 Michael Freilich measures the Doppler shift of the radio beacons' frequencies to precisely determine its line-of-sight velocity. Over time, an accurate 3D position of the satellite can be determined.
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