Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) is a French satellite system used for the determination of satellite orbits (e.g. TOPEX/Poseidon) and for positioning.
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A so called beacon is installed on the ground and emits a radio signal, which is received by the satellite. A frequency shift of the signal occurs that is caused by the movement of the satellite (Doppler effect). From this observation satellite orbits, ground positions, as well as other parameters can be derived.
DORIS is a French system which was initiated and is maintained by the French Space Agency (CNES). It is operated from Toulouse, where a master beacon ensures the communication with the satellites.
There are about 50-60 stations equally distributed over the earth and ensure a good coverage for orbit determination. For the installation of a beacon only electricity is required because the station only emits a signal but does not receive any information. Therefore it is possible to install beacons in remote areas such as the Mount Everest base camp.
The best known satellites equipped with DORIS are the two altimetry satellites TOPEX/Poseidon and Jason. They are used to observe the ocean surface as well as currents or wave heights. DORIS contributes to their orbit accuracy of about 2 cm.
Other DORIS satellites are the ERS, Envisat and SPOT satellites.
Apart from orbit determination the DORIS oberservations are used for positioning of ground stations. The accuracy is a bit lower than with GPS, but it still contributes to the International Terrestrial Reference Frame (ITRF).
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