COST-G gravity field models for precise orbit determination of Low Earth Orbiting Satellites

Abstract

Accurate time-variable Earth’s gravity field information is important for precise orbit determination (POD) of low Earth orbiters (LEO). In particular the POD of altimeter satellites benefits from 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. The monthly GRACE-FO gravity fields from June 2018 to June 2021 serve as base for a fitted signal model (FSM) 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 FSM are used for GPS-based POD of Copernicus Sentinel-2 and −3 satellites flying at altitudes of 786km and 814,5km, respectively. The orbit results show a significant improvement when using both, the COST-G monthly and the new COST-G FSM 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 are reduced up to 20%, orbit overlaps between 30–50%. Orbit validations performed by Satellite Laser Ranging (SLR) also show that SLR residuals are reduced by about 10%. The COST-G FSMs show a similar performance as the monthly gravity fields. The LEO POD in particular benefits from the more realistic mass trend estimates in river basins with strong non-periodic inter-annual variations compared to the predicted trends of outdated long-term gravity fields. The FSM thus complements the monthly gravity fields as a new COST-G product. It is foreseen to update the COST-G FSM quarterly.