ZARM monthly GRACE-FO gravity field solutions utilizing GRACE-D data from high-precision environment modelling

The Center for Applied Space Technology and Microgravity (ZARM) has recently developed monthly gravity field solutions from GRACE Follow-On data, which we present here including generation specifications and comparisons to solutions from other well known institutions.

Because the GRACE-D accelerometer data deteriorated shortly after launch, we developed a procedure to reproduce the accelerometer signal as precisely as possible using high-precision environment modelling. The modelling approach is extended by a physically-motivated and minimalistic transplant to considerably improve the precision of the acceleration data. In this context transplant means that properties from one GRACE satellite can be used for the other GRACE satellite due to the close proximity of both satellites. The transplant property is a thermospheric density value, which is estimated at positions of GRACE-C and applied by a time-offset to the GRACE-D satellite. This is necessary due to the insufficient accuracy of thermospheric density models and improves the artificial data in along-track direction. This is crucial because this is also the the main measuring direction of the inter-satellite microwave ranging instrument.

The data generation procedure includes accelerometer calibration parameter obtained from tailored precise orbit determination (POD) to reduce the influence of simulation errors and GRACE-C characteristics on the GRACE-D data.

This procedure provides artificial accelerometer data, which enables us to generate monthly gravity field solutions of high accuracy. We show how our solutions compare to solutions using GRACE-D transplant data from TU Graz or JPL, for different periods of the mission duration and present the limitations and potential improvement capabilities.

The GRACE-D accelerometer and other auxiliary data from our process is published as additional material to the monthly gravity field solutions on a publicly accessible data server. We encourage the community to use this data for comparisons and further research.