Analysis of polarimetric GNSS-R Stokes parameters of the Earth's land surface

Abstract

Polarimetric Global Navigation Satellite System – Reflectometry (GNSS-R) has proven to be sensitive to different land parameters, such as vegetation or soil moisture. In this manuscript, the correlation of the Stokes parameters of the GNSS reflections collected by the Soil Moisture Active Passive - Reflectometry (SMAP-R) with SMAP soil moisture (SM), SMAP vegetation optical depth (VOD), and the Global Forest Canopy Height (CH) are analyzed. SMAP-R is the first hybrid compact polarimetric reflectometer in space, and it provides a valuable dataset useful to design future GNSS-R mission that target polarimetric retrievals. The SMAP-R total-intensity reflectivity is presented, estimated from the bistatic radar cross section assuming coherent reflection. The calibrated intensity and the normalized Stokes parameters of the received wave are compared for a 1-year period to the SMAP VOD, the SMAP SM, the Global Forest CH, and the standard deviation of the digital elevation model. To provide a first assessment of the performance of full-Polarimetric GNSS-R to estimate SM and VOD, a linear regression approach is implemented using the four Stokes parameters and the child Stokes parameters. The two approaches show a correlation coefficient of R = 0.73 and R = 0.55 to estimate SM and VOD, respectively, enabling the estimation of both variables using polarimetric GNSS-R data without the need for ancillary data. Finally, the differences between the polarimetric signature at different incidence angles and polarizations are discussed.