On-Orbit Intrinsic Characterization and Radiometric Performance of the Primary Sensor Onboard the Chinese New-Generation Ocean Color Observation Satellite

The Chinese new-generation ocean color observation satellite (HY-3A), launched on November 16, 2023, carries the 2nd Chinese Ocean Color and Temperature Scanner (COCTS2) as its primary sensor. COCTS2 offers a swath width of over 3000 km, a spatial resolution of 500 m, and 15 reflective solar bands (RSBs) spanning 360 to 1640 nm. This study reviews COCTS2 intrinsic characterization and radiometric performance during its first year on-orbit and assesses the accuracy of global ocean color products. It demonstrates a high signal-to-noise ratio (SNR) under typical ocean radiance, outperforming specification values by a significant margin of 50%. Meanwhile, the image non-uniformity is better than 1%, and the linear response remains close to 1 within the radiance dynamic range under oceanic ocean color conditions. Notably, COCTS2 is the first Chinese sensor equipped with an onboard calibration system, achieving solar calibration accuracy within 2%. Through solar calibration, we established a radiometric degradation model that maintains stable performance throughout nine months, with radiometric degradation across RSBs below 1%. Subsequently, we derived absolute calibration gain factors using cross calibration approaches, achieving consistency with the solar calibration gain factors. On this basis, we established a relative calibration gain factors by vicarious calibration, and after application, the radiometric accuracy was better than 95%. Ultimately, in validating global ocean color products, we compare COCTS2-derived remote sensing reflectance (Rrs) with Sentinel-3A OLCI and AERONET-OC data, confirming the high quality of the ocean color products. And compared with PACE OCI, although there is a certain deviation in ultraviolet band, it still has a high correlation.