Analysis of DORIS range-rate residuals for TOPEX/Poseidon, Jason, Envisat and SPOT

Three currently operational radar altimeter satellites are equipped with the Doppler orbitography and radiopositioning integrated by satellite (DORIS) tracking system for precise orbit determination and two more are already foreseen. Any systematic errors in their computed orbits could possibly adversely affect scientific products used in climate change studies, such as sea level and ice sheet heights. DORIS residuals, which can be interpreted as a measure of orbit determination performance, often show systematic errors. We have therefore analyzed long time series of DORIS range-rate residuals in order to investigate possible systematic errors common to all DORIS analysis strategies and software packages, either on a satellite or on a station basis. In particular, the investigation has focused on global DORIS data of six satellites (TOPEX, Jason, Envisat and SPOT-2, -4 and -5) and station-specific data for Fairbanks, Easter Island and Syowa Base. Large measurement errors when crossing the South Atlantic Anomaly are easily detected in the DORIS residuals of Jason, while Envisat residuals show the most prominent evidence of multipath interference and the effect of a flight software update. Particularly, large errors were also found in low-elevation data.