Improved modelling of surface forces in the orbit determination of ERS and ENVISAT

Precise orbit determination of the European remote sensing satellites ERS-2 and ENVISAT, a prerequisite for the processing of their altimeter data, requires highly accurate force models. Inaccuracies in the modelling of non-gravitational forces, caused by interaction of photons, molecules, and atoms with the spacecraft surfaces, form a major error source. A highly detailed new non-gravitational force modelling package named ANGARA was implemented and tested for ERS-2 and ENVISAT. The resulting orbits for ERS-2 were compared with those from an existing box-wing panel model and a simple constant-area model. In addition, the effects on orbit accuracy of three thermospheric density models (DTM94, MSIS-86, and NRLMSISE-00) and a horizontal wind model (HWM-93) were evaluated. Force model parameters, estimated from tracking data during the precise orbit determination process, absorb most of the differences between the considered models. At the same time, an analysis of these parameters, and orbit determination runs with a limited parameterization, can give important clues about modelling accuracy. Atmospheric density is the dominant error source at high solar activity, degrading the radial orbit accuracy. Radiation pressure forces, for which uncertainties in material properties are the largest error source, play an equally important role at low solar activity.