DORIS precise orbit determination and localization system description and USOs performances

The Centre National d'Etudes Spatiales (CNES), the French Space Agency, has designed and developed the Doppler Orbiting and Radiopositioning Integrated by Satellite (DORIS) system, able to determine within a few centimeters, the location of satellites and ground beacons. DORIS payload has been integrated on three French SPOT satellites, on TOPEX NASA satellite, and will be embarked on ENVISAT ESA satellite. Based on one way Doppler accurate measurements from a network of 50 ground beacons, localization performances depends strongly on the frequency stability of on-board and ground Ultra Stable Oscillators (USO). The paper describes elements of the DORIS system, and gives performances of orbit computations and precise positioning. It emphasizes performances analysis of on-board and beacons USOs, measured by CNES T/F lab, and via orbit measurements.