Comprehensive assessment of Fengyun-3 satellites derived soil moisture with in-situ measurements across the globe

Soil moisture plays a pivotal role in terrestrial water circulation, energy transfer, and mass exchange. The X-band radiometer on-board Fengyun-3B and -3C satellites have been used for soil moisture retrieval since 2011. In this study, the spatial and temporal integrity of all available Fengyun-3 series soil moisture products were evaluated, and the data accuracy was validated against four in-situ measurements ascertained globally. The validation results reveal that: (1) The ascending Fengyun-3B soil moisture retrievals express higher spatial coverage percentages than others, and a higher temporal coverage percentage appears at 20-60 degrees N and 20-50 degrees S in summer. (2) The soil moisture products derived from both Fengyun-3B and -3C satellites reveal superior goodness of fit and smaller bias in the northern hemisphere in-situ measurements than in the southern hemisphere. In particular, the satellite retrieved pixel scale soil moisture datasets possesses a better correlation to small scale networks with densely distributed stations. In the southern hemisphere, the bias was positively correlated with the vegetation index and negatively correlated to land surface temperature, and this regularity may act as a reference for Fengyun-3 soil moisture correction. Moreover, the Fengyun-3 soil moisture products reveal positive responses to precipitation, and the memory of soil moisture to precipitation could last for at least 5 days. (3) The X-band signals were gradually impacted by increasing vegetation optical depth, leading to obvious overestimations. Further corrections, including hardware and software, are urgently required to enhance the anti-interference and robustness of the X-band signal.