The polarization instrument of Fengyun-3 precipitation satellite is the first Polarization and Multi-Angle Imager (PMAI) with short-wave infrared channel in China, aiming to accurately measure the radiation characteristics of clouds and aerosols in the atmosphere. The accuracy of radiometric measurement is an important technical index of instrument performance, which is of great significance for the inversion of high-precision quantitative parameters of satellite remote sensing. For the non-polarization channel of a polarization imager, polarization is a kind of interference information, and the polarization sensitivity of the instrument needs to be inverted and quantitatively removed to improve the accuracy of radiation calibration. A method of least squares fitting response value of complete linear polarization incident light based on different polarization angles is proposed to measure the polarization sensitivity of the non-polarization channel full field of view. According to the measured polarization rate of each channel and the polarization characteristics of the incident light, the polarization sensitivity of each channel is calibrated based on the polarization calibration model. The results show that the polarization sensitivity of the non-polarization channel shows obvious edge effect, gradually increasing from the center of the focal plane to the edge, and has obvious spectral differences, the smaller the wavelength having the higher the polarization sensitivity. The maximum polarization sensitivity occurs in the edge field of view of the non-polarization channel in the 1030nm band, close to 1.6%, which has a great impact on the accuracy of radiometric calibration. After polarization sensitivity calibration, the polarization sensitivity of the edge field of view is within 0.5%. The results show that by calibrating the polarization sensitivity of the full field of view of the non-polarization channel, the radiometric calibration can be effectively improved, which provides strong support for high-precision quantitative remote sensing.
Normalized Differential Vegetation Index (NDVI), usually calculated by surface reflectance at red and near-infrared bands (NDVI_Surf), which is an essential index in remote sensing. NDVI_Surf is generally used to discriminate different surface cover types and adopted in many surface models as a vital adjustable parameter to estimate the surface reflectance in remote sensing aerosol retrieval. However, NDVI_Surf is challenging to obtained directly and usually calculated by the red and near-infrared reflectance at the top of atmosphere (NDVI_TOA). NDVI_TOA is very susceptible to the atmosphere with different angles. If NDVI_Surf is replaced by the NDVI_TOA, it will cause an error of surface reflectance estimation and then make the wrong aerosol retrieval. In this study, Second Simulation of a Satellite Signal in the Solar Spectrum, Vector version (6SV) radiative transfer code was used to analyze the effects of NDVI_TOA on a surface Bidirectional Polarization Distribution Function (BPDF) model under different atmosphere and multi-angles conditions. The results display that the NDVI_TOA decreases with the rise of AOD. Within scattering angle (SA) of 60° to 115°, the influences of NDVI_TOA on BPDF are great and increases with the AOD reduces. Within the SA between 115° to 180°, the effects of NDVI_TOA on BPDF are small and remain unchanged with the AOD decreases. The simulation and analysis results have a great significance for polarized aerosol retrieval.
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