DORIS is based upon the accurate measurement of the Doppler shift of radiofrequency signals transmitted from ground beacons and received on board the spacecraft. Measurements are made at two frequencies: 2.03625 GHz for precise Doppler measurements and at 401.25 MHz for ionospheric correction of the propagation delay. The 401.25 MHz frequency is also used for measurement time-tagging and auxiliary data transmission. The selection of an uplink-only system allows fully automated operation of the beacons and easy communication links for the overall system, data being centralised through the satellite and its ground segment to the DORIS data processing center.
To perform its missions of spacecraft orbit restitution and of ground beacon locations, the DORIS system comprises an on-board package, a beacon network, and the DORIS Control and Data Processing Centre. For operational aspects, the DORIS control centre provides an interface to the spacecraft flight operation segment. The orbit determination beacons are deployed on a dense worldwide network. Installation and maintenance of the network is performed by the IGN (French national geographic institute). The time reference for the system is provided by the master beacons, located in Toulouse, France, and in Kourou, French Guyana, which are connected to atomic clocks.
On-board measurements of the Doppler shift are performed every 10 seconds. Resulting radial velocity values (accuracy of the order of 0.4mm/s) are used on ground in combination with a dynamic model of the satellite trajectory to perform a precise orbit determination with an accuracy of better than 5 cm in altitude. These orbit data are available with a time lag of 1.5 months, where the delay is mainly due to the availability of external data, such as solar flux. The Doppler measurements are also processed on board to obtain real time orbit data with less accuracy.
For the point positioning mission of DORIS, specific ground location beacons are available and are installed according to different users need. They are field packaged and equipped with autonomous power supply. The control and data processing center is located in Toulouse, France, and operated by the CLS company.