多角度偏振成像仪(MAPI)可获得多光谱、多角度的偏振信息,用于反演气溶胶与云的微物理特性。偏振的高精度探测需要精确的偏振定标。由于MAPI未配置星上定标器,因此采用自然景物作为偏振定标源。建立了全视场在轨偏振定标模型,分析了水云偏振特性。选用散射角为100°的水云像元作为无偏的定标源。求解了偏振响应矩阵所需的偏振片-滤光片组合的相对透过率、镜头起偏度以及低频相对透过率,最终计算出全像面偏振响应矩阵分布。分析了偏振片-滤光片组合的相对透过率三年平均值的变化,采用实验室定标结果进行验证,得到的相对误差为0.71%。采用最小二乘法拟合了镜头起偏度、低频相对透过率随视场角的变化。采用实验室偏振定标结果进行验证,得到的中心视场相对误差为1.22%。分析了算法的不确定度,得到中心视场合成不确定度为1.27%,边缘视场合成不确定度为2.19%,满足设计指标。所提定标方法对宽视场偏振成像仪的在轨定标有一定的借鉴作用,为气溶胶反演应用提供质量保证。

A multi-angle polarization imager (MAPI) can obtain multi-spectral and multi-angle polarization information, which is used to retrieve the microphysical properties of aerosols and clouds. Polarization high-precision detection requires accurate polarization calibration. Since MAPI is without an on-orbit calibrator, we use natural scenes as the sources of polarization calibration. A full-field on-orbit polarization calibration model is established, and the polarization characteristics of water clouds are analyzed. Water cloud pixels with a scattering angle of 100°are selected as unpolarized calibration sources. The relative transmittance of the polarizer-filter combination, the polarization degree of the optical lens, and the low frequency relative transmittance required by the polarization response matrix are solved, and finally the polarization response matrix of the full image is calculated. The average value of relative transmittance of the polarizer-filter combination for three years is analyzed and verified by laboratory calibration results. The relative error is 0.71%. The least squares method is used to fit the variations of the polarization degree of the optical lens and the low frequency relative transmittance with the angle of view. The laboratory polarization calibration results are used to verify that the relative error of the central field of view is 1.22%. The uncertainty of the algorithm is analyzed. The synthetic uncertainty of the central field of view and the marginal field of view is 1.27% and 2.19%, respectively, which meet the design specifications. The calibration method can be used as a reference for the on-orbit calibration of wide-field polarized imaging devices, and provides quality assurance for aerosol inversion.