FengYun-3 B Satellite Medium Resolution Spectral Imager Visible On-Board Calibrator Radiometric Output Degradation Analysis

The output of the FengYun-3 B (FY-3B) satellite's medium resolution spectral imager (MERSI) visible on-board calibrator (OBC) degrades with time, causing concerns regarding its reliability in radiometric calibration. Special efforts have been made to extract the variation in information of the interior lamps. The specifications include: 1) digital count retrieval from the OBC files and an exponential plus linear fit applied to determine the relative change in the MERSI response; 2) anchoring to cross calibration with MODIS to acquire the absolute radiometric calibration coefficients; 3) a time series calibration tracking method based on the pseudoinvariant target site applied to monitor the degradation of the FY-3B MERSI instrument; and 4) “net” interior lamp radiometric output (ILRO) obtained by subtracting long-term pseudoinvariant tracking results from the scaled visible on-board calibrator-based model gain coefficients. The results show that the decay rates of the ILRO are wavelength dependent and the shortwave outputs are found to experience a large degradation, particularly at 470, 550, 412, 443, 490, 520, and 565 nm. The largest annual degradation rate is greater than 15% in the first year. The implementation of the current procedure combined with a detailed analysis will provide a basis for further determining lamp changes, leading to improved absolute calibration accuracy of MERSI products.