IN-ORBIT CALIBRATION STRATEGY FOR OCEAN COLOR SENSORS

To recover the ocean water-leaving radiance and derive biophysical par ameters from observations of space-borne ocean color sensors, the requ ired uncertainty in the measured top-of-atmosphere radiance is at pres ent impossible to achieve prior to launch. A methodology and strategy for achieving the required uncertainty in the post-launch era is prese nted here. The method consists of combining direct measurements of the water-leaving radiance, white-cap radiance, and aerosol optical thick ness made simultaneously with satellite overpasses, with radiative tra nsfer theory to reduce the calibration uncertainty of the visible band s 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 clea r with most of the aerosol generated by local processes such as breaki ng waves, for example, the aerosol optical thickness in the visible si milar to 0.05-0.10. In this case, the radiative transfer process is co nsiderably simplified and molecular scattering is the dominant atmosph eric component in the visible. It is shown that such a procedure alone is sufficient to reduce the calibration uncertainty to required level s. Further reduction is possible by reducing the uncertainty in the NI R calibration by measuring sky radiance from island locations (or a sh ip), and using these to predict the at-sensor radiance. For the most p art, this NIR calibration is limited by the uncertainty in the calibra tion of the radiometer used to measure the sky radiance. Finally, the sensory calibration is maintained by monitoring the actual water-leavi ng radiance continuously at a single location, where the atmosphere is sufficiently clear that atmospheric correction introduces only a smal l error, and directly comparing the true and the sensor-derived water- leaving radiances. (C) Elsevier Science Inc., 1998.