Spectral-Fidelity-Preserving Recalibration of Reflective Solar Bands for MERSI-II Onboard Fengyun-3D Satellite

Since its 2017 launch, the Medium-Resolution Spectral Imager-II (MERSI-II) aboard the China Fengyun-3D (FY-3D) polar-orbiting satellite has delivered global observations of the atmosphere, oceans, and terrestrial vegetation, yet its reflective solar bands (RSBs) exhibit gradual on-orbit radiometric degradation and temporal fluctuations. We introduce a new spectral-fidelity-preserving (SFP) cross-calibration framework that uses AQUA/moderate resolution imaging spectroradiometer (MODIS) as the reference. Unlike conventional approaches that homogenize spectral responses, the SFP model explicitly retains MERSI-II's intrinsic spectral observation characteristics while correcting radiometric bias. When applied to FY-3D/MERSI-II observations (January 2018-March 2025), the seven recalibrated RSBs achieve a mean relative bias near 1% and a 2%-3% improvement relative to the operational calibration coefficients, remaining stable throughout the 7-year record. Mann-Kendall (MK) tests identify distinct degradation breakpoints, with B05 (1.38μ m) and B06 (1.64μ m) changing earlier around October 2021, and other bands fluctuating after May 2024. We also produce daily calibration coefficients, substantially improving upon current monthly or semiannual updates. The related algorithm and daily calibration coefficients are publicly released on GitHub to support community uptake and operational use.