Changes in the radiometric sensitivity of SeaWiFS determined from lunar and solar-based measurements

We report on the lunar and solar measurements used to determine the changes in the radiometric sensitivity of the Sea-viewing Wide Field-of-view Sense s (SeaWiFS). Radiometric sensitivity is defined as the output from the inst rument (or from one of the instrument bands) per unit spectral radiance at the instrument's input aperture. Knowledge of the long-term repeatability o f the SeaWiFS measurements is crucial to maintaining the quality of the oce an scenes derived from measurements by the instrument. For SeaWiFS bands 1- 6 (412-670 nm), the change in radiometric sensitivity is less than 0.2% for the period from November 1997 through November 1998. For band 7 (765 nm), the change is approximately 1.5% and for band 8 (865 nm) approximately SE. The rates of change of bands 7 and 8, which were linear with time for the f irst eight months of lunar measurements, are now slowing. The scatter in th e data points about the trend lines in this analysis is less than 0.3% for all eight SeaWiFS bands. These results are based on monthly measurements of the moon. Daily solar measurements using an onboard diffuser show that the radiometric sensitivities of the SeaWiFS bands have changed smoothly durin g the time intervals between lunar measurements. Because SeaWiFS measuremen ts have continued past November 1998, the results presented here are consid ered as a snapshot of the instrument performance as of that date.