SeaWiFS On-orbit Calibration Changes Derived from Four Years of Lunar Measurements

Lunar measurements are an integral part of the calibration of the Sea-viewing Wide Field-of View Sensor (SeaWiFS). The first measurements of the moon were made on 14 November 1997. Regular, monthly lunar measurements have continued since then, providing a four year data set. The SeaWiFS Project uses the moon as a stable reflectance target over the lifetime of the mission. However, corrections must be made to the lunar measurements for geometric factors, such as the lunar phase angle. The four year lunar data set shows the greatest calibration change to occur at 865 nm. The changes decrease with decreasing wavelength to a minimum near 500 nm. There is a small increase in the calibration change in the blue at 412 and 443 nm. The sensitivity of the lunar technique is sufficient to detect small annual oscillations in some of the bands due to improper correction factors for the temperatures of the focal planes. Over time, the rates of the calibration changes in the SeaWiFS bands are decreasing, based on the lunar measurement set. At the next SeaWiFS reprocessing, these changes will be fitted to exponential time series.