GRAVITY FIELD AND OCEAN TIDES MODELING FOR PRECISE ORBIT DETERMINATION OF DORIS SATELLITES

Gravitational forces are the major sources of perturbing accelerations acting on satellites in low Earth orbits. Recently, the modeling of the global Earth gravity field strongly benefited from the satellite gravity missions CHAMP, GRACE and GOCE. Besides the static gravity models also the time-variable models are now available, including the seasonal and linear (or piecewise linear) terms. We focus on the gravity modeling for precise orbit determination (POD) using DORIS (Doppler Orbitography and Radiopositionning Integrated by Satellite) data. First we show the relative effect of various gravitational and non-gravitational perturbation forces on orbits of Earth artificial satellites at several different altitudes (460-5900 km; satellites Swarm A, SPOT-5, Jason-2, Lageos-1). Then we study the impact of a particular setting (maximum degree, time-variable terms) of gravity field and ocean tide models on the quality of the determined orbits. For DORIS satellites SPOT-5 and Jason-2, we optimized the geopotentical coefficient truncation degree to meet the limit of 1 mm radial orbit error and 2 mm cross-track and along-track orbit error. A minimum limit for the geopotential coefficient truncation degree is 75 for SPOT-5 and 50 for Jason-2, when using the common dynamic orbit settings and daily orbit arc. The minimum limit for the application of the gravity changes due to the ocean tides is 25 for SPOT-5 and 20 for Jason-2. However, we also demonstrate that these limits depend on an orbit parametrization. Our experiments with SPOT-5 and Cryosat POD show a significant impact of the piecewise linear modeling, applied in the time-varying part of the gravity field model EIGEN-6S2, indicated by the effect on the RMS of the orbit fit. A similar effect of the annual and semiannual gravity terms application on the RMS of the orbit fit was not found, but the arc overlap RMS decreased by 0.6-2 %.