ESA’s Release 5 Gravity Field Model by the Direct Approach and its Part in the Combined Model EIGEN-6C4
Abstract:GFZ Potsdam and GRGS Toulouse have a close cooperation in the field of global gravity field determination and both are members of the European GOCE Gravity Consortium and compute gravity fields on behalf of the European Space Agency. Here we present release 5 of the GOCE gravity field model by means of the Direct Approach (GOCE-DIR5), which was made with the GOCE data from the entire mission (Nov09 - Oct13) as well as with LAGEOS and GRACE data. Compared to release 4, GOCE data at the lower mission altitudes are included and the max d/o has been increased to 300.
The 3-axes GOCE gradiometer gave gravity gradients that are measured with a high accuracy only within its measurement bandwidth of ~ 0.005 to 0.1 Hz. Thus, the gradient observation equations must be filtered which was done by a band pass filter of 8-120 sec. The GOCE GPS-SST data are only used to geolocate the gradients. The low-to-medium degree spherical harmonic coefficients of the gravity field are determined using GRACE as well as LAGEOS data from GRGS Release 3. All data are combined at normal equation level and a regularized solution is obtained by Cholesky decomposition.
The GOCE-DIR5 data were used to make our latest combined model EIGEN-6C4. This is the 4th release of EIGEN-6C which was the very first combined gravity field model using GOCE data. EIGEN-6C4 is complete to d/o 2190 and was made from the GOCE-DIR5 data combined with DTU10 and EGM2008 surface gravity data. The combination of the different data types has been done on the basis of full normal equations up to d/o 370. In contrast to GOCE-DIR5 the satellite normal equation for EIGEN-6C4 contains 5 types of time variable parameters for each coefficient up to d/o 80. The coefficients beyond deg 370 were obtained from a block diagonal normal equation from the terrestrial data only.
The comparison of test results (orbit computation, GPS leveling, geostrophic current speeds based on SVP buoy velocity data) of GOCE-DIR5 and EIGEN-6C4 with previous gravity field models like EGM2008, EIGEN-6C3 and others demonstrates the gain in accuracy at high degrees, while their performance is identical to recent GRACE-only model for the low degrees. Compared to the earlier releases of EIGEN-6 this new release shows a general improvement. GOCE-DIR5 and EIGEN-6C4 are available at the ICGEM database at GFZ Potsdam: http://icgem.gfz-potsdam.de