DORIS time bias estimated using Jason-1, TOPEX/Poseidon and ENVISAT orbits

Abstract

DORIS is a globally distributed, all-weather satellite tracking system providing near-continuous precise Doppler coverage of the TOPEX/Poseidon (T/P), Jason-1, ENVISAT, and SPOT series of satellites. The DORIS system, which has been critical in establishing the high-precision orbit determination standards now enjoyed by these missions, continues to evolve and improve. There is a small, 5 to 10 μs, discrepancy between the DORIS time-tag computed by Centre National d’Etudes Spatiales (CNES) using the DORIS timing data, and a time-tag estimated using DORIS range-rate data with respect to orbits referred to Satellite Laser Ranging (SLR) or Global Positioning System (GPS) time. This discrepancy is evaluated using DORIS time biases estimated over T/P, Jason-1, and ENVISAT orbits, which are computed using SLR or GPS tracking and reference SLR or GPS time. Although DORIS is installed on other satellites, these are the only three where the DORIS time bias can be observed with the help of an alternate tracking system – SLR or GPS. For T/P, Jason-1 and ENVISAT, this DORIS time bias quantities to 5–10 μs. Over the span of the T/P mission following cycle 92, this time bias has ranged from +10 to − 10 μs. This paper addresses the precision and nature of the estimated time biases by evaluating such estimates over orbits computed with various gravity field models, computed using SLR + DORIS and GPS tracking, and computed by the Goddard Space Flight Center (GSFC), Jet Propulsion Laboratory (JPL), and CNES analysis centers. The paper includes descriptions of the DORIS measurement, time-tag processing, expected time-tag error, and time bias estimation. In describing the estimated DORIS time bias, the paper offers a new approach for evaluating the DORIS resolution capability.