FY-3F MERSI-III On-Orbit Calibration Methodology and Performance of Reflective Solar Bands

The Medium Resolution Spectral Imager-III (MERSI-III), the third generation of the MERSI series, began observations from the Feng Yun-3F (FY-3F) in a sun-synchronous morning orbit on August 3, 2023. MERSI-III collects data from 25 spectral bands, including 19 reflective solar bands (RSBs) and six thermal emissive bands (TEBs) with wavelengths ranging from 0.4 to $2.15~\mu $ m and from 3.7 to $12.5~\mu $ m, respectively. Compared with its predecessors, MERSI-III features an enhanced on-orbit calibration capability for RSBs thanks to the incorporation of a newly designed visible onboard calibrator (VOC). This VOC is a solar diffuser (SD)-based onboard calibration system consisting of an SD panel, a solar attenuation plate positioned in front of the SD panel, and an onboard SD degradation monitor (SDDM). We provide an overview of the MERSI-III instrument, its VOC functions, and the corresponding on-orbit calibration methodologies. The performance of the on-orbit calibration is also assessed using a wide dynamic-integrated vicarious calibration (WD-IVC) technique, which combines multiple vicarious calibration (VC) methods, including intercomparison using invariant pixels, absolute calibration with pseudo-invariant calibration sites (PICSs), and Rayleigh scattering calibration. Based on three months of validation results, the on-orbit calibration demonstrates strong performance in both absolute accuracy and radiometric stability. Among the 14 validated RSBs, 13 bands meet the basic calibration accuracy requirement of a calibration bias of less than 5%, while seven bands meet the desired requirement of a calibration bias of less than 3%.