AIUB-CHAMP02S: The influence of GNSS model changes on gravity field recovery using spaceborne GPS Original Research Article

The gravity field model AIUB-CHAMP02S, which is based on six years of CHAMP GPS data, is presented here. The gravity field parameters were derived using a two step procedure: In a first step a kinematic trajectory of a low Earth orbiting (LEO) satellite is computed using the GPS data from the on-board receiver. In this step the orbits and clock corrections of the GPS satellites as well as the Earth rotation parameters (ERPs) are introduced as known. In the second step this kinematic orbit is represented by a gravitational force model and orbit parameters. In order to ensure full model consistency the GPS satellite orbits and clock corrections, which have been used for the generation of the kinematic LEO trajectories, were taken from the Center for Orbit Determination in Europe (CODE), located at AIUB (). In recent years many changes have taken place in the processing chain of global navigation satellite system (GNSS) data, e.g., the implementation of absolute antenna phase center modeling. Therefore a reprocessing of the GPS data to obtain state-of-the-art GPS satellite orbits and clock corrections was performed. From these updated GPS products new kinematic orbits of the CHAMP satellite were derived for the years 2002–2007. From the updated CHAMP trajectories spherical harmonic (SH) coefficients of the Earth’s gravity field were determined in exactly the same way as from the original LEO orbit. This allowed us to study the impact of the improved LEO orbits on the derived gravity field parameters and the generation of the multi-year gravity field model AIUB-CHAMP02S. The change of the IGS standards creates an inconsistency to existing global gravity field models, which mainly affects the zonal coefficients of low even degrees. The inconsistency is caused by the change to the absolute antenna phase center model and can be reduced by estimating the phase center variation of the CHAMP GPS antenna.