System Vicarious Calibration of the PACE Ocean Color Instrument

System vicarious calibration (SVC) of satellite ocean-color sensors involves comparing retrievals of water-leaving radiance (Lw) with in-situ measurements at the time of overpass and adjusting the calibration coefficients to ensure agreement between retrieved and measured quantities. This approach is designed to reduce uncertainties associated with purely radiometric calibration techniques, which lack the accuracy required for science applications, and to minimize biases introduced by atmospheric correction. For the recently launched PACE Ocean Color Instrument (OCI), the methodology utilizes hyperspectral Lw measurements from HyperNav radiometer systems deployed at various locations (Crete, Moorea, Puerto Rico, Hawaii) and from the Marine Optical Buoy (MOBY) near Lanai. Match-ups are rigorously selected based on criteria for atmospheric, surface, water, and geometry conditions. Top-of-atmosphere (TOA) radiance derived from onboard calibration techniques is compared to TOA radiance calculated from in-situ Lw measurements, resulting in calibration adjustment gains. The application of these adjusted gains to OCI imagery in diverse oceanic regions demonstrates more realistic values for water reflectance, enhancing the accuracy of retrieved ocean color data for scientific analyses.