The GFZ-GRGS EIGEN-6S3/C3 Gravity Field Models
Förste, Christoph1; Abrykosov, Oleh2; Flechtner, Frank2; Dahle, Christoph2; Bruinsma, Sean3; Marty, Jean-Charles3; Lemoine, Jean-Michel3; Biancale, Richard3
1GFZ German Research Centre for Geosciences, GERMANY; 2GFZ, GERMANY; 3CNES/GRGS, FRANCE

The European GOCE Gravity consortium (EGG-C), under ESA contract, computes gravity field models based on GOCE data. GFZ Potsdam and GRGS Toulouse are partners within the EGG-C and have a long-standing close cooperation in the field of global gravity field determination from the GRIM models of the eighties-nineties to the present EIGEN models. In the context of the EGG-C activities GFZ and GRGS compute gravity field models based on the combination of GOCE with GRACE data. The so-called combined models, in which both satellite and surface data (gravimetry on land and mean sea surface minus mean dynamic topography on the oceans) are used, are constructed outside the ESA framework. The latest release of GFZ'S and GRGS' EIGEN models is called EIGEN-6S3/C3, which is based on 2 years of GOCE and 8 years of GRACE data processing, to which 25 years of LAGEOS-1/2 data are added in order to model J2 in particular with high accuracy. The various data used for both EIGEN models were combined at normal equation level. In the case of the combined model, DTU10 surface gravity data sets were added, which allows the model resolution to be augmented from approximately 77 km (degree and order 260) of EIGEN-6S3 to nearly 10 km (degree and order 1949) for EIGEN-6C3.
A special feature of the latest EIGEN models is the representation of time-variable gravity in the form of drift, annual and semiannual coefficients to degree and order 50. These variations in gravity are due to mass redistribution on the Earth's surface, which can be estimated using GRACE data. For better results, precise orbits of altimeter satellites are computed with such a time-variable gravity field model (EIGEN-GL04S).
Compared to the first release of EIGEN-6 this new release shows a general improvement (orbit computation tests, GPS/leveling) due to the assimilation of more data and the better quality of the reprocessed GOCE data. Oceanographic validation of the new EIGEN-models in comparison to other global gravity field models was performed through the analysis of differences between mean geostrophic currents derived from the tested gravity field models and inferred from drifter data as a function of resolution (down to 80 km).