Calibration of the Polarimetric GNSS-R Sensor in the Rongowai Mission

Polarimetric global navigation satellite system reflectometry (GNSS-R) systems, equipped with an additional polarization channel, offer enhanced capabilities for separating vegetation and surface scattering effects, thereby improving GNSS-R land remote sensing applications such as soil moisture retrieval in vegetated and forested areas and biomass estimation. However, the effectiveness of these applications relies on accurate calibration of the polarimetric GNSS-R sensor. In the Rongowai mission, a newly developed next-generation GNSS-R receiver (NGRx) is installed on a domestic Air New Zealand airplane to collect data during its commercial flights. The NGRx processes multi-GNSS satellite signals simultaneously and utilizes a dual-channel [left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP)] antenna, thereby improving spatial coverage and retrieval accuracy. The dual-polarized antenna also provides the possibility to examine the polarimetric GNSS-R system. In this article, a new methodology is developed to calibrate the Level-1 power measurement and the onboard antenna cross-pol gain by comparing measurements from inland lakes and oceans with modeled results. The calibration results in a 34% decrease in the uncertainty in co-pol reflectivity retrieval. The retrieved cross-pol and co-pol reflectivity after calibration are examined by their statistical distribution and spatial mapping with 1.5-km resolution, with multiland surface types and incidence angles. These results validate the effectiveness of the calibration method and pave the way for future terrestrial science applications.