System vicarious calibration of GCOM-C/SGLI visible and near-infrared channels

Vicarious calibration coefficients (k(v)) of Second-generation GLobal Imager (SGLI) for ocean color processing were derived using in-situ radiometric buoy measurements from the Marine Optical BuoY (MOBY) and the BOUee pour l'acquiSition d'une Serie Optique a Long termE (BOUSSOLE). Two aerosol-model look up tables (LUTs) used in the GCOM-C aerosol retrieval algorithm (LUT-A) and in the previous version of ocean color atmospheric correction algorithm (LUT-B) were tested in the procedures to calculate k(v) and retrieve remote sensing reflectance (R-rs) and aerosol optical thickness (AOT). Bias of the processed R-rs compared to AERONET-OC R-rs was reduced by applying the determined k(v) (i.e., corrected SGLI radiance = original SGLI radiance/k(v)). LUT-A yielded smaller AOT bias compared to AERONET-OC AOT; on the other hand, LUT-B gave smaller R-rs noise due to gentle slope of the aerosol reflectance even though it caused AOT overestimation. When k(v) was derived by adjusting to the AOT measurements, k(v) was about 1.1 by LUT-A and 1.2 by LUT-B in the near-infrared (NIR) channel. However, the k(v) in the NIR channel was close to 1.0 when AOT and land surface reflectance measurements of Radiometric Calibration Network (RadCalNet) were used. The LUT-A with k(v) from MOBY and BOUSSOLE are currently adopted for the SGLI standard ocean color processing. Improvement is needed, however, to design an optimal LUT suitable for both aerosol and ocean color purposes.

Automated summary

This study derived vicarious calibration coefficients (k(v)) for Second-generation GLobal Imager (SGLI) using in-situ radiometric buoy measurements, and tested two aerosol-model look up tables (LUTs) in the calculation of k(v) and retrieval of remote sensing reflectance (R-rs) and aerosol optical thickness (AOT). The results showed that LUT-A had smaller AOT bias and LUT-B had smaller R-rs noise. AOT overestimation was observed, but the NIR channel's k(v) was close to 1.0 when using AOT and land surface reflectance measurements. Currently, the results are applied for the standard ocean color processing of SGLI, but further improvement is needed to design an optimal LUT suitable for both aerosol and ocean color purposes.