Scattering Matrix Retrieval Using Full Polarimetric GNSS-R

This manuscript presents the mathematicalbackground and modeling for full-polarimetric Global NavigationSatellite System - Reflectometry (GNSS-R) receivers for nonnegligiblecross-polar component in the scattered signal. A signalmodel is presented to retrieve the Stokes parameters using theMironov model to estimate the soil surface's dielectric constant.The manuscript compares data collected by the SMAPReflectometry(SMAP-R) receiver and the proposed signal model,emphasizing the need to consider the cross-polar component (𝑺𝒉𝒗).Simulations obtained without considering the cross-polarcomponent have poor agreement in all Stokes parameters. Amodel is implemented using a complex cross-polar componentresulting in notable improvements in the bias and unbiased rootmean square difference (ubRMSD) between modeled andmeasured SMAP-R Stokes parameters. Furthermore, amethodology is proposed for estimating the 𝑺𝒉𝒗 component usingSMAP-R and a soil moisture reference dataset. An analysis of 𝑺𝒉𝒗reveals a moderate correlation with vegetation water content(VWC) and surface roughness (𝝈𝒔𝒍𝒑). We developed a soil moisturemodel by estimating 𝑺𝒉𝒗 using both VWC and 𝝈𝒔𝒍𝒑, showing thepotential of full-polarimetric GNSS-R to provide soil moistureestimates upon proper characterization of the cross-polarcomponent. Results show an ubRMSD of 0.09 m3/m3 with respectto the In-situ Soil Moisture Network (ISMN). Finally, this studyestablishes that an RMSD better than 0.02 when estimating 𝑺𝒉𝒗 isrequired for a soil moisture product accuracy better than0.07 m3/m3 for polarimetric GNSS-R SM retrievals.