The development of HJ SAR soil moisture retrieval algorithm

Four satellites with S-band synthetic aperture radar (SAR) onboard, along with four optical satellites, form the Huan Jing (HJ) constellation, which consists of the Chinese Environment and Disaster Monitoring Satellites that are designed for fast monitoring of the dynamics of soil moisture and other environmental variables. A 24-hour revisit time of the future four HJ SAR satellites enables an estimation of soil moisture change by using multi-temporal observations. The algorithm for estimating soil moisture change has been developed for HJ SAR in four steps: (a) theoretical simulation of radar backscattering from vegetated and bare soil with first-order scattering model and the Advanced Integral Equation Method (AIEM); (b) vegetation correction based on a simplified scattering model and ancillary optical observations of the HJ constellation; (c) simplification of a radar backscattering model for bare soil to separate the effects of surface roughness and soil moisture; and (d) estimation of soil moisture change from multi-temporal SAR observations. The comparison between field measurements and soil moisture retrieved by the proposed algorithm demonstrates good accuracy.