Validation of Using SWARM to Fill-in the GRACE/GRACE-FO Gap: Case Study in Africa

The American/German missions Gravity Recovery and Climate Experiment (GRACE) and the GRACE Follow-On (GRACE-FO) and the European mission (Swarm) play an important role in study of the Earth's gravity field with unprecedented high-precision and high-resolution measurements. The aim of this study is to use Swarm data to fill-in the data-gap between GRACE and GRACE-FO missions from July 2017 to May 2018, and evaluate the new datasets in Africa. We used the available data from the triple GRACE processing centers CSR, GFZ and JPL, in addition to the Swarm TVGF data provided by the Czech Academy of Sciences (ASU) and the International Combination Service for Time-variable Gravity (COST-G). The GRCAE and Swarm date have been tested in the frequency and space domains. For the frequency domain, the data assessed in two different levels: the potential degree variances and the harmonic coefficients themselves. The results show consistency between GRACE/GRACE-FO and Swarm for all processing centers. In the space domain, a comparison between GRACE/GRACE-FO and Swarm for the TWS, gravity anomaly, and the potential/geoid have been carried out. For the TWS, an artificial gap (AG) - simulating the gap between GRACE and GRACE-FO – has been artificially made in the GRACE data from July 2015 to May 2016. The GRACE AG has been filled by the two sets of the Swarm data for CSR, GFZ and JPL. The results indicated that the best agreement has been achieved between GRACE-CSR and Swarm COST-G. For the gravity anomaly and the potential/geoid, a better agreement between GRACE and Swarm data has been concluded. Eventually, we chose Swarm COST-G data to fill-in the gap between GRACE and GRACE-FO CSR in order to be used, among others, to estimate the TWS in Africa for the period from April 2002 to October 2020. This study is supported by the National Natural Science Foundations of China (NSFC) under Grants Nos. 42030105, 41721003, 41804012, 41631072, and 41874023.