Sky-Radiance Measurements for Ocean-Color Calibration-Validation

The calibration of an ocean-color sensor or validation of water products is generally based on ground-based extinction measurements from which the aerosol products (optical thickness t(alpha) and aerosol type) are deduced. Sky-radiance measurements complement the extinction measurements mainly in the aerosol-model characterization. Our basic goal is to promote calibration-validation activities based on the radiative properties of the aerosols rather than their chemical or physical properties. A simple method is proposed (and evaluated) to convert sky radiances measured in the principal plane into atmospheric phase functions P . Indeed t(alpha) and P are the required inputs to a radiative-transfer code for predicting the top-ofthe-atmosphere radiances. The overall error in this prediction is a few percent. This method can operate on a worldwide network on ground-based sun radiometers and then be used to achieve a statistical analysis for validating satellite products.