Simulation of the GOCE gravity field mission

GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) is one of the four selected ESA Earth Explorer Missions. The main objective of GOCE is the determination of the Earth's gravity field with high spatial resolution and with high homogeneous accuracy. For this purpose, two observation concepts will be realised. Satellite-to-Satellite Tracking (SST) in high-low mode will be used for the orbit determination and for the retrieval of the long-wavelength part of the gravity held. Satellite Gravity Gradiometry (SGG) will be employed for the derivation of the medium/short-wavelength parts of the gravity field.. For the realisation, a GPS receiver, a 3-axis gradiometer and auxiliary instruments are needed; e.g, star trackers to control the orientation of the spacecraft or thrusters for attitude and drag-free control. Each instrument exhibits, its own error behaviour which affects the measurements and the final products in some typical way. The SID consortium consists of three organisations: the Institut fur Astronomische und Physikalische Geodasie (IAPG), Delft Institute for Earth-Oriented Space Research (DEOS) and the Space Research Organization Netherlands (SRON). They have the goal to provide a reliable error budget for the GOCE system, identify it's weaknesses and limitations and make st complete error analysis of the whole mission. A complete simulator of the gradiometer has been developed. The simulator describes the test mass motion as amass-spring system. The output are gravity gradients (V-ij(out)) as if they were measured, based on input gravity accelerations, SIC position, orientation and disturbing forces. The corresponding error PSD's (Power Spectral Densities) representing the effect of different error sources are computed. The PSD's are propagated to spherical harmonic error spectra, geoid heights and gravity anomaly accuracies to assess effects of instrument and measurement errors on gravity field determination. The different tasks within the consortium are described by means of a now chart.