On-orbit cross-calibration of the DPC and POSP sensors onboard the GaoFen-5 (02) satellite

The directional polarimetric camera (DPC) and the Particulate Observing Scanning Polarimeter (POSP) sensors onboard the satellite constitute the polarization crossfire suite (PCF). It is designed for the global monitoring of atmospheric fine particulate matter from space. The DPC is a wide field-of-view (FOV) multi-angular polarization imager. The POSP is a cross-track scanning polarimeter equipped with an onboard radiometric and polarimetric calibration system. A cross-calibration strategy from POSP to DPC has been designed to ensure their in-flight radiometric consistency. In this study, a detailed analysis of the potential error sources in the cross-calibration process was conducted, including the pixel spatial matching method, the spectral response function (SRF), and viewing geometry discrepancies. Results indicate that higher site homogeneity leads to smaller errors caused by pixel-level spatial mismatch. When site inhomogeneity is constrained to 1%, the calibration error can be limited to 0.05% at nadir and 0.32% at the image edge. Errors caused by viewing geometry discrepancies decrease with wavelength over desert scenes but increase with wavelength over ocean scenes. The errors caused by viewing geometry and SRF discrepancies range from 0.25% to 0.29% over the desert and from 2.03% to 2.91% over the ocean. An optimized PCF cross-calibration workflow is then proposed. It includes the improved strategy for pixel spatial matching and viewing geometry matching. A radiative transfer model is also employed to correct the spectral and viewing geometry differences. This method yields radiometric calibration coefficients highly consistent with those from other vicarious calibration methods and exhibits different spectral performances over ocean and desert scenarios. The radiometric performance of DPC onboard GaoFen-5(02) was further evaluated over its first two and a half years in orbit through cross-calibration with POSP. The 670 nm band of DPC shows the smallest relative radiometric response across the FOV and the highest temporal stability. The 865 nm band shows approximately 1.5% relative radiometric variation and 1.0% temporal degradation. In contrast, the 443 nm band exhibits the largest variation in both relative radiometric response and temporal stability. The relative radiometric difference between the center and edge of the FOV was 5.8% in March 2022. It increased to 9.4% by June 2024. Meanwhile, the absolute radiometric response decreased by about 5.5% over the same period. The 490 nm band performs better than 443 nm. Its relative radiometric response remains within 4.5%. The temporal degradation is about 3.5%. DPC/GaoFen-5(02) exhibits significantly improved in-flight radiometric performance compared to the earlier DPC/GaoFen-5.