Heike Peter (PosiTim UG, Seeheim-Jugenheim, Germany); Ulrich Meyer (University of Bern, Switzerland)

Accurate time-variable Earth’s gravity field information is essential for precise orbit determination (POD) of low Earth orbiters (LEO). In particular the POD of altimeter satellites relies on accurate modelling of the time-variable gravity field. The Combination Service for Time-variable Gravity Fields (COST-G) provides monthly gravity fields based on a combination of GRACE/GRACE-FO derived monthly gravity field solutions from different analysis centers. These monthly solutions are available with a latency of 2-3 months. They serve as base for a deterministic signal model (DSM) of time-variable gravity that enables a few months prediction and therefore application in operational LEO POD. The combined monthly solutions themselves, as well as several variants of the DSM are used for GPS-based POD of Copernicus Sentinel-2, -3 and -6 satellites. The orbit results show a significant improvement when using both, the COST-G monthly and the new DSM gravity field models compared to results using existing long-term static gravity field models, including seasonal and trend estimates, as well. RMS values of the GPS carrier phase residuals and orbit overlaps are significantly reduced. Orbit validation performed by Satellite Laser Ranging (SLR) also show that SLR residuals can be lowered. The DSMs show a similar performance as the monthly gravity fields. In particular they provide more realistic trend estimates in river basins with strong non-seasonal inter-annual variations than the predicted trends of outdated long-term gravity fields. The DSM thus complements the monthly gravity fields as a new COST-G product. It is foreseen to update the COST-G DSM quarterly using a sliding window approach.