Gravity Field Analysis from the Satellite Missions CHAMP and GOCE

This work describes global gravity field modeling from observations of the CHAMP and the GOCE satellite mission. In the case of CHAMP the so-called Energy Balance Approach is applied to measurements of GPS high-low tracking of the low orbiting CHAMP spacecraft. In the case of GOCE gravity gradiometry is the main observable. The experience gained processing real data of the CHAMP satellite mission is incorporated in the development of operational quick-look software for the GOCE mission. The task of global gravity field analysis is to derive from measurements along the satellite orbit a model of the Earth’s gravitational field expressed in spherical harmonic coefficients on the Earth’s surface. The theoretical background needed to relate the observations to the gravity field and compute a global model from satellite observations is explained in detail. The CHAMP mission has been launched in 2000. It is the first dedicated gravity field mission to carry a GPS receiver in space. The main observables are the observations from the GPS satellites to CHAMP, the so-called satellite-to-satellite tracking (SST). A gravity field model is computed with the Energy Balance Approach using kinematic orbit positions. It is based on the law of energy conservation, which states, that the sum of potential and kinetic energy is constant in a conservative force field. The kinetic energy is computed from the orbit positions via the orbit velocities. The unknown energy constant can be estimated which allows for the computation of the potential. The satellite is subject to non-conservative forces which are corrected for. These forces are partly measured by the onboard accelerometer and partly computed by models. The spherical harmonic coefficients are then derived from the potential values along the orbit in a least squares adjustment. The solution is validated using GPS-leveling data and compared to other global gravity field solutions. The GOCE mission is scheduled for launch in early 2009. It is the first gravity gradiometer in space. The bandlimited satellite gravity gradiometry (SGG) observations are supported by SST. Due to the complexity of the mission and the data processing final results will only be available several weeks after the end of the mission. As there is a need for an analysis of the data in parallel to the mission, a quicklook gravity field analysis has been developed and implemented. It uses the semi-analytical approach which is based on the simplifying assumption of a circular orbit. This simplification allows to set up normal equations using fast Fourier transform (FFT) techniques. The normal equation matrix becomes block-diagonal and the least squares adjustment can be solved very efficiently. The gain in speed comes at the cost of accuracy. The method can be divided in two sub-approaches: the 1D-FFT approach and the 2D-FFT or (torus-) approach. Both approaches are applied to real CHAMP data and simulated GOCE data in order to asses their possibilities and limitations.