Systematic Error Mitigation in DORIS Derived Geocenter Motion

The relative orbit-centering stability between the different Jason POD analysis centers (JPL, GSFC, ESOC and CNES) is usually assessed by comparing orbits in the North-South direction (Z-component of the terrestrial reference frame). Any miscentering of orbit in this direction is of primary interest since it impacts significantly studies of global and regional Mean Sea Level (Global MSL error = -0.16 x DZ, where DZ is the mean orbit error in Z). The main contribution to this miscentering effect on the orbits comes from the tracking measurements. Indeed, even though satellites ideally orbit around the center-of-mass of the total Earth system (CM or geocenter), the strength of the tie to the reference network origin is dependent on the tracking measurement used in the process of orbit determination: 100% for SLR-only orbits, 75% for DORIS-only orbits, and 30% for GPS-derived orbits (depending on the ambiguity fixing strategy, and relative to the reference given by the GPS orbits/clocks solution). The well-known seasonal signature in Z ( 5 mm) observed between DORIS/SLR and GPS-based orbits may in part be due to the un-modeled non-tidal component of the geocenter motion, as of yet there is no consensus model for non-tidal geocenter motion. Thus, we will examine strategies to mitigate sensitivity to miscentering effects on the orbit coming from the DORIS tracking measurements; in this way the use of a model of the motion of the CF with respect to the CM won't be needed. Estimations of the geocenter motion have already been successfully achieved using the SLR network, but the DORIS network derived geocenter motion has been reported to be noisier with larger systematic errors. Yet, due to the more numerous and better uniformly distributed DORIS stations across the globe, it could have the potential to yield competitive results, once the systematic errors are identified and mitigated, as will be shown in this paper. The obtained orbit parameterization will be tested on different altimeter satellites and the DORIS-derived geocenter motion will be compared to independent SLR-based estimates.