Time-variable gravity field determination from GRACE Follow-On data usingthe Celestial Mechanics Approach extended by empirical noise modelling

We study gravity field determination from GRACE Follow-On satellite-to-satellite tracking using the inter-satellite K-band link and kinematic positions of the satellites as observations and pseudo-observations respectively. A key component of any model is the accurate specification of its quality. In the case of gravity field modelling from satellite data with the Celestial Mechanics Approach (CMA) a least-squares adjustment is performed to obtain a monthly solution of the Earth’s gravity field. However, the jointly estimated formal errors usually do not reflect the error level that could be expected but provides much lower error estimates.
We present gravity field solutions computed with the CMA and extend it by an empirical modelling of the noise based on the post-fit residuals between the final GRACE Follow-On orbits, that are co-estimated together with the gravity field, and the observations, expressed in position residuals to the kinematic positions and in K-band range-rate residuals.
We compare and validate the solutions that use empirical modelling with solutions from the operational GRACE Follow-On processing at AIUB by examining the stochastic  behaviour of the respective post-fit residuals, by investigating areas where a low noise is expected and by inspecting the mass trend estimates in certain areas of global interest. Finally, we investigate the influence of the empirically weighted solutions in a combination of monthly gravity fields based on other approaches as it is done by the Combination Service for Time-variable Gravity fields (COST-G) and make use of noise and signal assessment applying the quality control tools routinely used in the frame of COST-G.