Improved GOCE Gradiometer Processing - Wiener-Method for Angular Rate Determination

One of the most important and likewise most complex parts of the GOCE gradiometer processing is the determination of the angular velocities of the spacecraft about the gradiometer axes. In the nominal GOCE processing this step is called angular rate reconstruction. It is done by combining gradiometer and star sensor attitude information. The gradiometer measurements are best within a frequency range from 5 to 100 mHz, the so-called gradiometer measurement band width (MBW). Below, the accelerometer measurements suffer from a low frequency drift. The star sensors, on the other side, are very accurate for low frequencies. Therefore, to find the best angular rate estimation over the entire frequency range, a combination between the two sets of attitude information, one coming from star sensor and one coming from gradiometer has to be done. In this paper, an improved method for angular rate determination is presented. Within the new method the optimal combination between star sensor and gradiometer angular rate is found by spectrally weighting the angular rate components with the weights of a Wiener filter, according to their corresponding noise. The improvement in the achievable gravity field accuracy due to the Wiener method for angular rate determination is illustrated with several numerical scenarios, starting from the gradiometer control voltages up to the level of gravity field solutions.