Comparison of SeaWiFS On-Orbit Lunar and Vicarious Calibrations

摘要

The NASA Ocean Biology Processing Group's Calibration and Validation Team has used monthly lunar calibrations of SeaWiFS to establish and maintain the on-orbit radiometric stability of instrument at the 0.1% level over its 9-year mission. The Cal/Val Team has compared the SeaWiFS lunar observations with the USGS ROLO photometric model of the Moon to verify the long-term stability of the SeaWiFS radiometric calibration. This stability has allowed the Team to apply a system-level vicarious calibration of the sensor/atmospheric calibration algorithm that is independent of time, yielding a single gain per band. SeaWiFS bands 1-6 (412-670 nm) are calibrated against water-leaving radiances measured by the Marine Optical Buoy (MOBY) that have been propagated to the top of the atmosphere. Band 7 (765 nm) is calibrated relative to band 8 (865 nm) so that the atmospheric correction algorithm selects maritime aerosol models over open ocean scenes. The long-term radiometric stability of SeaWiFS allows the Cal/Val Team to directly compare the mean residuals of the lunar observations from the ROLO model with the vicarious gains. A linear fit of the vicarious gains vs 1 - (mean ROLO residual) for bands 1-6 gives a slope of 1.084 with a correlation of 0.980. The predicted mean ROLO residual for band 7, computed from the observed mean residual for band 8 and the vicarious gain for band 7, agrees with the observed mean residual for band 7 to within 0.5%. The radiometric stability of SeaWiFS allows the comparison of the prelaunch calibration of SeaWiFS, the calibration of MOBY, and the calibration of the USGS ROLO model. Such a comparison is of interest to other Earth-observing instruments which use the Moon as a calibration reference, such as MODIS, VIIRS, and ABI.