Towards a Consistent Estimation of the Earth’s Gravity Field by Combining Normal Equation Matrices from GRACE and SLR

Since 2002, the satellite mission GRACE observes the Earth’s static gravity field and its natural changes. The estimation of degree 1 and 2 coefficients is difficult, especially the accuracy of the flattening coefficient C2, 0 is weak. In temporal GRACE gravity field models, C2, 0 is therefore often replaced by a value based on Satellite Laser Ranging measurements. In this study, we combine both techniques to get a consistent normal equation matrix, which allows us to study correlations between spherical harmonic coefficients. Unlike common practice, we use 8 SLR satellites and set-up normal equations up to degree 20. The combination is done using different weighting factors to investigate the influence of both techniques on the combined normal equation matrix. Our results show that especially the coefficient C2, 0 benefits from SLR data, but also (near-) sectorial coefficients and coefficients which correspond to resonance frequencies of SLR satellites. Moreover, we find that high correlations in the SLR normal equation between zonal coefficients are reduced by the combination.