Stokes Parameters Retrieval and Calibration of Hybrid Compact Polarimetric GNSS-R Signals

Global Navigation Satellite System-Reflectometry (GNSS-R) is a promising field with a diverse set of applications. Polarimetric GNSS-R has been shown to be sensitive to different land parameters, such as freeze/thaw, crop growth, or soil moisture. In this article, the polarimetric scenario of a GNSS reflection is defined, and the theoretical basis and the algorithm to retrieve the Stokes parameters of a GNSS-R signal are proposed. The presented algorithms are validated using data collected by the two linear orthogonal polarization channels of the Soil Moisture Active Passive (SMAP) radar antenna working as GNSS-R, known as SMAP-Reflectometry (SMAP- $R$ ). The Stokes parameter retrieval is presented for the SMAP- $R$ case, and the receiver phase offset and intensity are calibrated. The same calibration procedure proposed by the CYclone GNSS (CYGNSS) mission is applied to the SMAP- $R$ first Stokes parameter, allowing the retrieval of the normalized bistatic radar cross Section (NBRCS) of such measurement. Finally, the SMAP- $R$ data are compared to the CYGNSS NBRCS over the ocean, showing an unbiased root-mean-square error of 3.3 dB and a small bias.