GGP Ground Truth for Satellite Gravity Missions

The launch of the satellite GRACE heralds a new era of high precision, time-varying, satellite measurements of the Earth's gravity field. An important aspect of the mission is to consider how the predicted high-accuracy gravity data might be validated. Two kinds of validation have been proposed; the first is internal, whereby the data are modeled for self – consistency to verify the internal accuracy of the spherical harmonic coefficients. The second is external validation using data from independent measurements, for example ocean bottom pressure gauges or continental land surface gravity measurements. Here we consider the latter possibility using the European sub-array of the Global Geodynamics Project (GGP) as 'ground truth' for comparison with GRACE. As a pilot study, we use 190 days of 1-hour data from the beginning of GGP (1 July 1997), at 8 European stations. We remove local tides, polar motion and local air pressure and filter the data to 6-hour samples and find that there are large variations between some stations, but also some stations show high correlations over periods of several weeks. For each time sample, the 8 stations are used to interpolate a minimum curvature (gridded) surface that extends over the geographical region. Although promising, no conclusions can yet be made on the accuracy that might be achieved in future comparisons with GRACE.