Enhancing GRACE-FO Orbit Accuracy Using OpenIFS Weather Model through Accelerometer Transplant and Tropospheric Delay Correction

Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) is a gravity field retrieval mission consisting of two identical satellites orbiting the Earth since 2018. Each satellite is equipped with a Global Positioning System (GPS) receiver, a microwave ranging system, and an accelerometer (ACC) to measure non-gravitational accelerations. The data from these instruments, alongside the estimations of gravitational force models, are used to determine the satellites' orbits. Soon after launching, one of the satellite’s ACC degraded, and its data was replaced with ACC data transplant, a synthetic data derived from simulations and the other twin satellite’s ACC data. The default data used for orbit determination of GRACE-FO satellites includes Clouds and Earth's Radiant Energy Systems (CERES) climatology for ACC data transplant and Vienna Mapping Function 3 (VMF3) for tropospheric error correction in GPS observation processing. In this study, we propose a novel approach that uses the Open Integrated Forecasting System (OpenIFS) weather model to account for both ACC transplant and tropospheric delay correction, and in the end, assessed the accuracy of GRACE-FO satellites’ orbits. Orbit determination for GPS satellites was conducted with tropospheric slant delays derived from OpenIFS, using data from 256 stations. Then, the precise orbits of GRACE-FO satellites were estimated using the GPS precise orbits as well as the gravitational and non-gravitational forces acting on the satellites. The ACC transplant was performed by the Technical University of Graz, using the OpenIFS-derived simulated non-gravitational accelerations. This method demonstrates an overall 4 cm improvement in orbit accuracy compared to the traditional method, as it offers a more realistic ACC simulation by explaining the rapid changes of atmosphere affecting the GRACE-FO orbit. OpenIFS provides a greater temporal resolution compared to VMF3 and compensates for the asymmetric atmosphere; thus, the GPS orbit is more precise. The largest accuracy improvement occurred in the along-track direction. This is likely due to a more accurate representation of the radiative effects caused by the satellite entering and exiting Earth's shadow, which typically has the most significant effect on along-track accelerations and, consequently, on the satellite's orbit in this direction.