Calibration of GOCE SGG data using high-low SST, terrestrial gravity data and global gravity field models

It is the aim of the GOCE mission to determine a model of the Earth's gravity field with high accuracy and resolution. For this purpose, gravity gradients will be measured in combination with high-low satellite-to-satellite tracking. The gravity gradients are derived from pair-wise differenced accelerations as determined by the six three-axes accelerometers that form the GOCE gradiometer. Since the measured accelerations suffer from errors of a random and systematic nature, the gravity gradients may suffer from random and systematic errors as well. Systematic errors are, for example, a scale factor and a bias. The common accelerations of the paired accelerometers also are contaminated with such errors. The common accelerations are used in the drag-free control of the satellite and are important for the separation of the gravitational and non-gravitational forces in the gravity field determination. The checking of the gravity gradients and the common accelerations against independent data (i.e. external to the GOCE satellite) in order to free the observations as well as is possible from systematic errors is called external calibration. The possibilities and limitations of using terrestrial gravity data and global gravity models for external calibration of the gravity gradients are reviewed. It turns out that the determination of a gravity gradient scale factor and bias using just the accurate knowledge of the central term and the flattening (J(2)) of the Earth's gravity field is not good enough. When global gravity field models are used for the calibration, higher degrees and orders should be taken into account as well. With today's existing global models it seems to be possible to remove the greater part of the systematic errors of the GOCE gradients. A gravity gradient bias can accurately be recovered using terrestrial gravity data in a regional approach with least squares collocation. However, since regional data are used it may not be possible to determine calibration parameters valid for the whole (global) gravity gradient data set. Nevertheless, regional terrestrial gravity data could be used to validate the measured and calibrated gravity gradients. In addition, a possible use of GOCE high-low satellite-to-satellite tracking data to calibrate the common accelerations is explored; it is shown that this approach fails. If more accurate gravity field information becomes available then such a calibration may become feasible.