COST-G models of time-variable gravity for precise orbit determination

The Combination Service for Time-variable Gravity fields (COST-G) provides models of the time-variable Earth gravity field that may be used for precise orbit determination (POD). While the monthly snapshot solutions based on GRACE or GRACE-FO GPS and microwave ranging interferometer data are the best available information on the time-variable gravity field, their latency of about 2-3 months prevents their use in operational POD. COST-G also provides a signal model fitted to the monthly GRACE-FO gravity fields that is updated quarterly and allows for the prediction of time-variable gravity, e.g. for use in applications that rely on short latency like operational POD.

We present POD results for LEO and MEO satellites based on the COST-G fitted signal model (FSM), where we put special attention on the impact of the C₂₀ gravity field coefficient, which is poorly defined from the GRACE/GRACE-FO observations, but may be replaced by SLR-only or SLR+GRACE/GRACE-FO combined solutions. We moreover present an extension of the COST-G FSM, also covering the GRACE-period and therefore suited for reprocessing campaigns that rely on a consistent model of time-variable gravity.