Improved Ers and Envisat Precise Orbit Determination

For over a decade, the Delft precise ERS-1 and ERS-2 orbits have been used in many scientific investigations based on ERS altimeter and interferometric SAR data. This orbit computation is now continued for Envisat, with an unprecedented accuracy due to better tracking data and lower force model errors. Orbit errors due to inaccuracies in the Earth’s gravity model, which have been the main focus of research during the 1990s, have been reduced to a lower level than those incurred by errors in measurement modelling and non-gravitational force modelling. We have compared several models and methods in order to investigate and where possible reduce each of the error sources. When reprocessing the tracking data over the solar maximum period of the past years, frequent large and unpredictable changes in thermospheric density form the dominant remaining source of orbit error, for which accurate modelling remains a difficulty. Nevertheless, the quality of our Envisat precise orbits already rivals that of TOPEX/Poseidon and Jason. The orbit quality will likely further improve as solar activity continues to drop towards solar minimum.