On-Orbit Analysis and Correction of the Inconsistency in the Response Characteristics of TG-2/MAI CCD Pixels

The difference in the response characteristics of charge-coupled device(CCD)pixels is one of the main factors restricting the quality of the Multi-angle Polarization Imager(MAI)and its quantitative application. In order to improve the quality of CCD imaging,this paper used a total of 104,403frames of observational data from September 2016to March 2018,based on the full-range multi-section analysis and correction method,to realize the analysis and correction for the inconsistency in the pixel response characteristics of MAI polarized and non-polarized channels.And the results were verified using the Global Ozone Monitoring Experiment 2(GOME-2)and Moderate-Resolution Imaging Spectroradiometer(MODIS)data.Firstly,assuming there are sufficient observational samples,that is,the objects observed by each pixel have the same ergodicity,and the average Digital Number(DN)value of all samples corresponding to each CCD pixel could represent the response characteristics of each pixel of the CCD.Secondly,constructing reference images for each channel by using 104 403frames observation data,and the 5×5pixels in the CCD center are used as standard DN values corresponding to each reference image.Next,the response characteristics of the MAI polarized and non-polarized channels are analyzed respectively.The results show that there is significant inconsistency in the response characteristics in each MAI pixel and each channel.The inconsistency in each channel falls roughly between 4%and 10%.And for polarized channels,the inconsistencies in the pixel response characteristics between the three polarized channels in the same polarization band have some similarities,but there are certain differences,and the difference in pixel response inconsistency is basically within 1%.Then,the observational data for two years are divided into two periods for comparative analysis.The result shows that the CCD pixel response characteristics do not decay over time.This also shows that the amount of reference image data is sufficient,which further rerifies the rationality of the above assumptions.Therefore,the full-range multi-section correction method can be used to correct the inconsistency of each pixel's response characteristics on a channel-bychannel basis.After correction was performed based on this method,the image quality of MAI is significantly improved.The dependence on the observational zenith angle of the CCD pixel response is significant.The image is smoother,and the graininess is basically eliminated.Also,the scenes of some areas have changed-especially targets with a reflectivity between low and medium reflectivity,such as broken cloud and so on.Compared with GOME-2,the average absolute deviations between the reflectance of the MAI 565,670,and 763 nm bands and the GOME-2reference reflectance are reduced from 1.6%,4.2%,and 2.2%to 0.5%,2.6%,and 0.4%after correction,respectively.In addition,the cloud detection result based on the multi-band cloud identification method shows that,compared with the MODIS cloud detection product at a similar time,the corrected MAI cloud detection result looks more accurate.Therefore,the full-range multi-section analysis and correction method can realize the monitoring and correction of the inconsistency in the response characteristics of the MAI CCD pixels,which significantly improves the quality of the on-orbit observations of this instrument. And this method can also be applied to the on-orbit calibration of other CCD instruments.