Modelling The Long-period Temporal Variation of The Gravity Field In Context of The Goce Satellite Mission

In the framework of the GOCE satellite mission, the modelling of the long-period temporal variation of the earth's gravity field is investigated. The temporal variations of the ocean topography and land hydrology (snow coverage and soil moisture) have been considered in this investigation. The POCM model (four years monthly aver- ages) has been used to represent the ocean topography, and the SIB2 model (two years monthly averages) has been chosen to model the snow coverage and soil moisture. An approximation of both fields has been carried out using a harmonic series expansion up to degree and order 180. Both FFT and finite element algorithms have been used to recover the harmonic coefficients. In order to investigate the effect of the inhomo- geneous data distribution (Gibbs' phenomenon), a local study at the satellite level for the potential and its first and second derivatives has been carried out using both the de- rived harmonic series expansion and the local TC program. This study demonstrates that the global approximation performed by the harmonic series expansion is quite well. Examining the frequency behaviour of the temporal variation phenomena shows a dominating annual periodicity for both fields apart of a stationary component. This annual periodicity has been modelled using the FFT analysis of the harmonic coeffi- cients for both fields. The omission errors (signal contents of non-annual periods) are checked and proved to be relatively small.