Using Single-Frequency Dual-Polarization GNSS-R Based on Airborne GLORI Data to Retrieve Soil Moisture

摘要: In Global Navigation Satellite System - Reflectometry (GNSS - R) studies, polarization was once overlooked. However, in recent years, it has attracted growing attention. This paper focuses on exploring dual polarization for single - frequency data from the airborne GLORI experiment.
Based on theoretical analysis, several retrieval algorithms for soil moisture estimation were applied. Initially, only the surface reflectivity of Left - Right (LR) and Right - Right (RR) polarizations was examined. As additional surface parameters, such as surface roughness and vegetation, were integrated into the algorithm, the retrieval accuracy, measured by RMSE, improved significantly from approximately 0.07 to 0.03. 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 (LAI), and incidence angle are taken into account, the retrieval accuracy, indicated by RMSE, reaches 0.0344. 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 both coherent and non - coherent scattering. This paper further discusses the non - coherent 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 non - coherent scattering, the scattering properties at LR polarization are weaker than those at RR polarization for corresponding land surface types.
The analysis of dual - polarization data will contribute to future data mining for more accurate soil moisture retrieval and the design of future polarization GNSS - R payloads. The retrieval accuracy considering non - coherent scattering properties implies that both coherent and non - coherent scattering should be incorporated into future GNSS - R data sets, as they are comparable for future soil moisture retrieval algorithms.