On-Orbit Characterization of FY-4B AGRI Calibration Components for Solar Diffuser Degradation Estimation

The Advanced Geostationary Radiation Imager (AGRI) of the FY-4B satellite uses a set of reflective solar band (RSB) onboard calibration components, with a solar diffuser (SD) and an SD reflectance degradation monitor (SDRDM). The SDRDM is primarily used to monitor the degradation of the SD bidirectional reflectance distribution function (BRDF) during the satellite’s on-orbit period and to provide degradation correction coefficients. In this work, the degradation coefficients of the SD BRDF in the visible near-infrared (NIR) band are calculated on the basis of the observation dataset obtained with the SDRDM from August 2021 to March 2023. The time series of calculated SD degradation coefficients exhibits notable anomalous fluctuations due to the cosine error in the incidence angle associated with the placement of the calibration device and the variation in stray light with the illumination angle. A systematic degradation coefficient evaluation method based on the optical mechanical structure characteristics and working principle of the SDRDM is proposed. The SD BRDF degradation coefficient is obtained by combining time-series data with the same geometric lighting conditions and interband degradation coefficient ratios and removing the relative angular changes and associated effects. With the stable CH3 band as the reference band, geometric factors are reconstructed for other bands for subsequent SD degradation monitoring. Finally, the monitoring results are applied to observed SD data for verification, and from August 2021 to March 2023, the SD BRDF degradation was 2.3% at 0.45–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.55~\mu $ </tex-math></inline-formula>m and almost no degradation at 0.55–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.90~\mu $ </tex-math></inline-formula>m.