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

TL;DR

This paper presents a new calibration methodology for a polarimetric GNSS-R sensor on an aircraft, significantly reducing measurement uncertainty and enhancing land and ocean surface property retrieval accuracy.

Contribution

A novel calibration approach for Level-1 power and antenna cross-pol gain in polarimetric GNSS-R systems is introduced, validated, and shown to improve measurement precision.

Findings

34% reduction in reflectivity retrieval uncertainty

Effective calibration validated across diverse land and water surfaces

Enhanced accuracy in polarimetric measurements for remote sensing

Abstract

Polarimetric 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 (LHCP and 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 on-board antenna cross-pol gain by comparing measurements from inland lakes and ocean 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 multi-land surface types and incidence angles. These results validate the effectiveness of the calibration method and pave the way for future terrestrial science applications.