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In-orbit vicarious calibration for ocean color and aerosol products

Abstract : It is well known that, to accurately retrieve the spectrum of the water-leaving radiance and derive the ocean color products from satellite sensors, a vicarious calibration procedure, which performs sensor in-orbit calibration for a whole system (the sensor and algorithms) is necessary. Both Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) have employed in-orbit vicarious calibration procedure that uses the in situ measurements with the Marine Optical Buoy (MOBY) in the waters off Hawaii. Such method can also be applied to vicarious inter-calibrate other sensors. In addition to the ocean color products, aerosol optical property data over ocean are routinely retrieved from both SeaWiFS and MODIS measurements. The aerosol retrieval algorithm uses radiances measured at two near-infrared (NIR) wavelengths, at which the ocean appears black due to strong absorption by water, to estimate the aerosol optical properties and extrapolate these into the visible. The spectral information from two band measurements is used to retrieve the most appropriate aerosol models. With the derived aerosol models, the aerosol optical thickness can then be estimated using the measured signal at 865 nm. In this paper, I outline the procedure for the in-orbit sensor vicarious calibration for the ocean color and aerosol products. Simulations that demonstrate the effectiveness of the vicarious calibration method on the derived ocean color and aerosol products are presented and discussed. Results of sensitivity studies that show effects of the calibration error at 865 nm, appropriateness of aerosol models, and the solar-sensor viewing geometry on the accuracy of the retrieved ocean color and aerosol optical properties are presented.