Research on the monitoring method of BRDF degradation for the solar diffuser on HY-1C.

To address BRDF (bidirectional reflectance distribution function) degradation of the HY-1C satellite's calibration solar diffuser (CSD) due to long-term space radiation, which compromises the radiometric calibration accuracy of the satellite calibration spectrometer (SCS), a reference solar diffuser (RSD) is used to monitor CSD's BRDF degradation. The study derives an SCS radiometric calibration model based on CSD and combines it with the relationship between on-board calibration-time BRDF and laboratory measurements to obtain the BRDF degradation factor's original expression. A dual-reference band-ratio time-series model is developed by coupling BRDF degradation with wavelength and on-orbit cumulative exposure time, normalizing against the degradation-insensitive 900 nm BRDF of SCS Band 118 and the first on-board calibration BRDF. This model mitigates errors from observation angular variations, Solar attenuation screen (SAS) transmittance, spectral response mismatches, and data fluctuations. Analysis of nearly five years of in-orbit data shows BRDF degradation strongly depends on wavelength, with shorter wavelengths degrading more severely: UV1 (355 nm) and UV2 (385 nm) exhibit a degradation factor of 0.945, exceeding Band 18 (399.5 nm, 1.000→0.967, 3.3%) and Band 30 (459.5 nm, 1.0000→0.986, 1.4%), while longer wavelengths like Band 100 (810.5 nm) remain stable. Cross-validation using TERRA MODIS shows enhanced consistency: the determination coefficient (R2) between SCS-corrected and MODIS Band 1 reflectance reaches 0.9945, with absolute reflectance deviation in visible near-infrared channels decreasing by up to 52.69%, and 95% confidence intervals narrowing. This study provides a high-precision BRDF degradation monitoring and correction model for long-term in-orbit radiation calibration, meeting high-precision cross-calibration requirements for the same and cross-platform remote sensors.