GOCE gravity gradients: a new satellite observable

Abstract

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) is one of the flagships in ESA’s Living Planet Programme. With the help of the on-board, very precise gravitational gradiometer the Earth’s gravity field is to be determined with unprecedented accuracy. The gradiometer measures gravity field differences that are contained in six gravity gradient (GG) tensor components. Because of the instrument characteristics, four out of six tensor elements are very accurate, whereas the other two are less accurate. We will concentrate on the noise characteristics of the accurate GGs by a spectral analysis of the residuals of a semi-analytical gravity field solution for the first measurement phase (November - December 2009). Formal errors of spherical harmonic coefficients are estimated in a semi-analytical way by means of a GOCE combined gravity field solution. In addition they are then compared with formal errors of recent GOCE-only models. As in the measured GG tensor there are two less accurate components the rotation of the GG tensor from the instrument frame to a local Earth related frame is not straightforward. A possible solution is to replace the less accurate components with model gradients. We will discuss the trade-off between measured and model GGs and assess the information content of the rotated gradients in different local frames, specifically the local northoriented frame and the local orbital frame. Furthermore we will present a quality assessment of the GOCE GGs over time. We will show that the errors of Vxx andVxz are constant over time, but that the errors in Vyy increase after each calibration event, which is due to a drift in the differential scale factor associated withVyy. Finally, the Vzz error level seems to have decreased by 25% after the switch from CPU A to B in February 2010.