Vicarious calibration of SeaWiFS

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has been providing daily global imagery of the world's oceans since September 1997. SeaWiFS is an eight-band, visible and near-infrared satellite radiometer with a spatial resolution of 1.1 km at nadir. The goal of the SeaWiFS Project is to produce a five-year ocean color data set with a 5% absolute and 1% relative radiometric accuracy on the water-leaving radiances. The SeaWiFS atmospheric correction algorithm must remove the atmospheric signal to yield the water-leaving radiances. The atmospheric correction estimates the aerosol radiances at 765 nm and 865 nm and aerosol radiances to the other SeaWiFS bands. Uncertainties in the atmospheric correction algorithm and possible variations in the instrument calibration with time require a mission-long vicarious calibration program to monitor the performance of the sensor system and to meet the radiometric constraints on the ocean color data set. The Calibration/Validation (Cal/Val) group of the SeaWiFS Project is using the NASA/NOAA Marine Optical Buoy (MOBY), deployed off of Lanai, Hawaii, as the primary vicarious calibration site for SeaWiFS. The Cal/Val group has performed the initial vicarious calibration of the SeaWiFS data by comparing water-leaving radiances measured by MOBY with water-leaving radiances from contemporaneous SeaWiFS images of overflights of the buoy site. The group has derived a set of system gains and offsets which, when applied to the SeaWiFS instrument calibration, yields values for water leaving radiances measured by SeaWiFS and MOBY that agree to within 1%. The Cal/Val group has verified this vicarious calibration by comparing matchup data sets between SeaWiFS and various ship cruises.