Spatio-temporal characteristics of solar diffuser observation in GOCI-II radiometric calibration

The Geostationary Ocean Color Imager-II (GOCI-II) performs on-orbit radiometric calibration (RC) using absolute radiometric gain values derived from solar calibration. The current RC model applied a spatially uniform gain across all sensor bands, potentially neglecting spatio-temporal variations in sensor response. To address this limitation, we conducted an analysis of two-dimensional gain matrices between January 2021 and January 2023. Normalized gain values were derived to assess both spatial and temporal variations. The analysis revealed consistent radiometric degradation, with average gain decreases of 1.6% during summer and 2.1% during winter. These seasonal differences are attributed to changes of observational geometry. Spatial analysis showed regional variation, with 1.2 % differences in a single observation. Our findings highlight the limitations of the uniform gain assumption and emphasize the need for pixel- or region-specific correction strategies to enhance the radiometric accuracy and temporal stability of GOCI-II ocean color products, thereby ensuring consistent data quality throughout the satellite mission.