Matt A. King, Philip Moore, Peter John Clarke, David Lavallee

One of the initial challenges of the GRACE mission is to validate the accuracy of the time-variable gravity fields. These gravity fields contain both spatially correlated (systematic) and random noise and hence spatial averaging needs to be implemented. Before the fields may be interpreted, optimum averaging radii need to be determined through comparison with independent data. We compare time series of vertical solid Earth deformations computed from 22 approximately monthly GRACE gravity fields (covering 2002.3 until 2004.6) with vertical displacements measured using a global GPS network of 63 sites, about half of which are located on small islands. The GPS data were processed using a Precise Point Positioning approach using fiducial free orbits and attempting to minimise propagated systematic errors. The optimum mean correlations were obtained at 500 km averaging radii for continental sites (R=0.55), >=2000 km for island sites (R=0.3). Subtracting the GRACE displacement time series from the GPS ones revealed a mean Variance reduction of ~14%. To supplement the GPS data at the island sites, we also computed displacements based on TOPEX/POSEIDON (T/P) sea surface height data between 2002.3 and 2003.96 corrected for steric effects. Correlations reached a maximum at 2000 km with a correlation of 0.38, increasing to 0.47 after the removal of six outlying sites. Overall, we conclude that optimum averaging radii are ~500 km for continental sites and >=2000 km over the oceans, but that the measurement precision of GPS and T/P may be inflating these values.