In-Orbit Calibration Strategy for Ocean Color Sensors

To recover the ocean water-leaving radiance and derive biophysical parameters from observations of space-borne ocean color sensors, the required uncertainty in the measured top-of-atmosphere radiance is at present impossible to achieve prior to launch. A methodology and strategy for achieving the required uncertainty in the post-launch era is presented here. The method consists of combining direct measurements of the water-leaving radiance, whitecap radiance, and aerosol optical thickness made simultaneously with satellite overpasses, with radiative transfer theory to reduce the calibration uncertainty of the visible bands with respect to the near-infrared (NIR). This procedure is possible over the open ocean, where, in the absence of aerosol transported from land over long distances by the wind, the atmosphere can be very clear with most of the aerosol generated by local processes such as breaking waves, for example, the aerosol optical thickness in the visible ∼0.05–0.10. In this case, the radiative transfer process is considerably simplified and molecular scattering is the dominant atmospheric component in the visible. It is shown that such a procedure alone is sufficient to reduce the calibration uncertainty to required levels. Further reduction is possible by reducing the uncertainty in the NIR calibration by measuring sky radiance from island locations (or a ship), and using these to predict the at-sensor radiance. For the most part, this NIR calibration is limited by the uncertainty in the calibration of the radiometer used to measure the sky radiance. Finally, the sensor calibration is maintained by monitoring the actual water-leaving radiance continuously at a single location, where the atmosphere is sufficiently clear that atmospheric correction introduces only a small error, and directly comparing the true and the sensor-derived water-leaving radiances.