First Demonstration of an Onboard Polarimetric Calibration and Ice-Cloud-Based Validation Framework for the Particulate Observing Scanning Polarimeter (POSP)

Abstract

Accurate global measurements of atmospheric aerosols and clouds require satellite polarimetric sensors to maintain stable radiometric and polarimetric performance throughout their on-orbit operations. However, monitoring and correcting on-orbit degradation remains a challenge, as most current polarimetric sensors lack active onboard polarimetric calibration capabilities and rely heavily on vicarious calibration methods constrained by atmospheric uncertainties. This study presents a comprehensive evaluation of the Particulate Observing Scanning Polarimeter (POSP) onboard the GaoFen-5(02) [GF-5(02)] and DaQi-1 (DQ-1) satellites, which features a unique onboard polarimetric calibration system. Analyzing data from the first one to two years of operation, we identified significant temporal drifts in the calibration coefficients for the short-wave infrared (SWIR) bands (1610 and 2250 nm). These SWIR drifts are consistent with the high thermal sensitivity of the detectors in the on-orbit environment. In addition, the calibration coefficients of the two independent instruments exhibit coherent periodic fluctuations across multiple bands, suggesting the influence of common external thermal and radiative conditions. To independently validate the onboard calibration updates, high-altitude ice clouds observed at scattering angles larger than 170° were used as approximately unpolarized natural References. The validation results demonstrate that the dynamically updated onboard calibration coefficients substantially improve the SWIR polarimetric accuracy. After calibration, the median degree of linear polarization (DoLP) residuals of the SWIR bands agree with the simulated intrinsic ice-cloud DoLP to within one standard deviation of the simulated ensemble. The daily DoLP time series further confirm that the upward drift in the SWIR bands is effectively removed after calibration. Moreover, the validation results reveal a clear latitude dependence: mid-to-high-latitude ice clouds provide more stable and lower uncertainty references than low-latitude ice clouds, which exhibit noticeable seasonal DoLP increases. Overall, this study provides the first on-orbit evaluation of an active onboard polarimetric calibration system, quantifies SWIR-band calibration drifts and cross-platform periodic variations, validates the onboard updates with large scattering angle ice clouds, and establishes a transferable calibration and validation (Cal/Val) framework for future spaceborne polarimetric missions.