Bridging data gaps in Earth's gravity field from integrating GRACE, GRACE-FO, and Swarm data: Case study in Africa

The GRACE and GRACE Follow-On (GFO) missions, led by American and German teams, along with the European mission Swarm, have revolutionized the study of Earth's dynamic gravity field through precise measurements. Our objective is to fill the data GRACE gaps and the gap between GRACE and GFO missions using Swarm data, focusing on Africa. We utilized data from GRACE processing centers (CSR, GFZ, and JPL), Swarm data from the Czech Academy of Sciences (ASU) and the International Combination Service for Time-variable Gravity (COST-G), QF, as well as IGG data. Both frequency and space domains were examined, evaluating Potential Degree Variances (PDV), harmonic coefficients, Terrestrial Water Storage (TWS), gravity anomaly, and potential/geoid using GRACE, GFO, and Swarm. Results indicated agreement among processing centers for potential degree variances, gravity anomaly, and geoid undulation. However, discrepancies were observed in harmonic coefficients and TWS. To address this, we employed parametric least square adjustment to estimate new Swarm-modified coefficients, selecting Swarm ASU and GRACE/GFO CSR data. Comparison of Singular Spectrum Analysis method (SSA), IGG, and Swarm-modified SHCs during the data gap period exhibited correlation coefficients exceeding 0.86. Overall, the new coefficients significantly improved agreement between original GRACE coefficients and modified coefficients in all aspects.