On the co-estimation of static and monthly gravity field solutions from GRACE Follow-On data

Temporal gravity field modelling from GRACE Follow-On data is usually performed by computing monthly snapshots of spherical harmonic coefficients representing the state of the Earth’s gravity field. Associated to this, the spherical harmonic series has to be truncated at a certain point, commonly at
degree/order 96. Higher degrees and orders are fixed to the a priori used background gravity field model.
We present an investigation on the influence of the high degrees and orders of different a priori background gravity field models on monthly gravity field model computations from GRACE Follow-On data. Furthermore, we extend the temporal gravity field modelling to additionally co-estimate a static gravity field for the GRACE Follow-On satellite mission along with the monthly snapshots to provide for a consistent handling of correlations between temporal and static gravity field coefficients.
Moreover, we model the stochastic noise of the data with an empirical description 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, to further study the influence of the high degrees and orders of the a priori background gravity field model on such noise models.
We compare and validate the monthly solutions with the models 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 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.