Soil Moisture Retrieval Using Single-Frequency Dual-Polarization GNSS-R Data From Airborne GLORI Experiment

In global navigation satellite system reflectometry (GNSS-R) research, polarization was historically overlooked. However, it has garnered increasing attention in recent years. This article focuses on analyzing the first publicly available dual-polarization, single-frequency dataset from the airborne global navigation satellite system reflectometry instrument (GLORI), collected during the summer of 2021. Based on theoretical analysis, multiple soil moisture retrieval algorithms were implemented. Initially, only the surface reflectivity of LHCP-receiving/RHCP-transmitting (LR) and RHCP-receiving/RHCP-transmitting (RR) polarizations was investigated. As additional surface parameters, such as surface roughness and vegetation, were integrated into the algorithm, the retrieval accuracy, measured by root mean square error (RMSE), improved significantly from approximately 0.07 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to 0.03 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>. The retrieval accuracy of RR polarization is slightly better than that of LR polarization. Nevertheless, when both dual polarizations were considered, the retrieval accuracy was comparable to that of using only one polarization. When surface roughness, leaf area index, and incidence angle are taken into account, the soil moisture retrieval accuracy, indicated by RMSE, reaches 0.0344 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>. This clearly demonstrates the great potential of dual polarization in soil moisture estimation. GLORI data is the first publicly available dual-polarization GNSS-R data that encompasses not only coherent and but also noncoherent scattering scattering information. This article further discusses the noncoherent scattering properties of LR and RR polarizations. In the context of coherent scattering, it is found that the scattering properties at LR polarization are stronger than those at RR polarization. Conversely, for noncoherent scattering, the scattering properties at LR polarization are weaker than those at RR polarization for corresponding land surface types. By employing both cohrent and noncoherent scattering properties with the consideratons of LR and RR polarizations, the soil moisture retrieval accuracy indicated by RMSE and correlation coefficients (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i>) will be 0.0316 and 0.9043, respectively. The analysis of dual-polarization data incorporating both coherent and noncoherent scattering mechanisms holds significant potential for advancing soil moisture retrieval accuracy through future data mining efforts, while also informing the design of next-generation polarimetric GNSS-R payloads, such as ESA’s HydroGNSS and China’s GNSS-ReSAR.