Cross-Calibration of S-NPP/VIIRS and Tiangong-2/MAI Visible Channels Using the SNO Method

摘要

The space laboratory "Tiangong-2" (TG-2) was launched on 15 September 2016. The Multi-Angle Polarization Imager (MAI), onboard TG-2, is the first visible and near-infrared multi-angle polarization imager developed in China. MAI can accomplish polarization observation from up to 14 different viewing angles, which is of great significance for relevant research on the microphysical characteristics of clouds and aerosols. Aimed at the problem of a lack of an onboard calibration device for MAI, to correct TG-2's calibration coefficients in time, the Simultaneous Nadir Overpass (SNO) cross-calibration method is proposed as a reasonable and feasible method for MAI 565- and 670-nm channels. Data from the M4 and M5 channels of the Visible Infrared Imaging Radiometer Suite (VIIRS), with good calibration accuracy, are used as reference data, and a series of matching conditions and a spectral adjustment algorithm are specified. To apply the SNO method, eight groups of examples from December 2016 to February 2017 are selected, and a large number of matching pixel samples covering land, ocean surface, and cloud layers are obtained. The resulting cross-calibration curves between MAI 565-nm and VIIRS M4 channels, and between MAI 670-nm and VIIRS M5 channels, show correlation coefficients of reflectivity of 0.986 and 0.994, mean biases of 2.48% and 5.90%, and RMSEs of 3.56% and 6.95%, respectively. Overall, the cross-calibration method achieved good results, which are of reference significance to correct the laboratory calibration coefficient of MAI. The proposed method effectively solves the problems of MAI's subsequent on-orbit monitoring and vicarious calibration, and also lays a foundation for later applications of MAI data in research on cloud and aerosol microphysical parameters.