A GOCE-only global gravity field model by the space-wise approach

The global gravity field model computed by the spacewise approach is one of three official solutions delivered
by ESA from the analysis of the GOCE data. The model consists of a set of spherical harmonic coefficients and
the corresponding error covariance matrix. The main idea behind this approach is to exploit the spatial
correlation of the gravity field to estimate grids of potential and its second order radial derivatives at mean
satellite altitude; from these grids, spherical harmonic coefficients are then derived by numerical integration.
The filtering strategy includes also a Wiener filter along the orbit to reduce the noise variance and correlation
before gridding the data. In the first release of the space-wise approach, based on a period of about two months, some prior information coming from existing gravity field models entered into the solution especially at low degrees and low orders. In this work, the strategies adopted to remove these dependencies on external data are described. Mainly two modifications have been implemented. The first is an improved modelling of the error covariance matrix of the estimated along-track gravitational potential to remove the dependency on EGM08 at very low degrees; the second is an internally computed GOCE-only prior model to be used in place of the official quick-look model, thus removing the dependency on EIGEN5C especially in the polar gaps. Once the procedure to obtain a GOCE-only solution has been outlined, a new global gravity field model has been computed by applying the space-wise approach to eight months of GOCE orbit and gradiometer data. The results are presented and discussed in this paper, also comparing them with respect to the previous solution.