Improved alternative GRACE-FO accelerometer transplant product computed at Graz University of Technology (TUG)

Gravity field recovery using GRACE-FO observations requires the reduction of non-conservative forces usually measured by the onboard accelerometer in the center of mass of each spacecraft. The accelerometer of GRACE-D degraded significantly in June 2018 showing increased noise levels by orders of magnitude and common mode errors on all acceleration components. However, the quality of the accelerometer measurements is vital for accurately estimating the Earth’s gravity field. The standard approach to account for the degraded data of GRACE-D is to replace the measurements with synthetic data, the so-called transplant data. At Graz University of Technology (TUG) a very effective procedure to estimate such an accelerometer transplant was developed adopting a remove-restore approach (Behzadpour, 2021). Hereby, data from GRACE-C and non-conservative force models are used to substitute GRACE-D data. Since then the effort was made to further improve the TUG transplant product.

Effects that need to be considered are on the one hand the increased solar activity as we are approaching the peak of solar cycle 25 which makes it harder to estimate an accurate transplant product by using force models and on the other hand influences such as thruster leakage which occurs on both GRACE-C and GRACE-D. Investigations that were carried out comprise the adaption of the physical properties of the satellite’s surface elements and the consideration of bias jumps at each thruster firing. The bias jumps for GRACE-C were corrected before the transplant process and with the use of actual GRACE-D data thruster leakage and some residual drag acceleration was restored for GRACE-D.