Determining pseudo-invariant calibration sites for comparing inter-mission ocean color data

Comparing inter-mission space instrument performance is crucial to accurate satellite measurements which guarantees the quality of ocean color products. However, comparing inter-mission instrument performance is limited by strong dispersion, which clearly originates from the spatial and temporal variability inside the oceanic sampling sites. We designed a novel comprehensive score metric (CSM) to quantitatively identify the candidate pseudo-invariant calibration sites (PICS) for inter-mission comparisons. We calculated the CSM from a year of ocean color and meteorological products from 2018 using a pixel-by-pixel method with a simple average of a temporal meteorological metric, a spatial aerosol metric, a temporal optical metric, a spatial optical metric, a data quality metric, a spectral shape metric, and a directional homogeneity metric. When we filtered the data with the threshold CSM > 0.6, we found that atmospheric and oceanic conditions from two smooth belts in the low latitude open ocean were more clear, stable, and homogeneous than other regions, and we suggested these smooth belts as candidate PICS. With image data from the Visible Infrared Imaging Radiometer (VIIRS) and Medium Resolution Spectral Imager II (MERSI II), we found that our candidate PICS were more effective than regions with low CSM in providing stable synchronous data for inter-mission comparisons. The MERSI II instrument, however, experienced significant degradation in radiance measurements from January to April in 2021, while the VIIRS instrument performed well. These results suggested that the CSM values were "experimental" but, under restrictive conditions, were sufficient for an inter-mission comparison and calibration application.