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7 September 2006SeaWiFS long-term solar diffuser reflectance trend analysis
The NASA Ocean Biology Processing Group's Calibration and Validation (Cal/Val) Team implemented daily
solar calibrations of SeaWiFS to look for step-function changes in the instrument response and has used these
calibrations to supplement the monthly lunar calibrations in monitoring the radiometric stability of SeaWiFS
during its first year of on-orbit operations. The Team has undertaken an analysis of the mission-long solar
calibration time series, with the lunar-derived radiometric corrections over time applied, to assess the long-term
degradation of the solar diffuser reflectance over nine years on orbit. The SeaWiFS diffuser is an aluminum
plate coated with YB71 paint. The bidirectional reflectance distribution function of the diffuser was not fully
characterized before launch, so the Cal/Val Team has implemented a regression of the solar incidence angles and
the drift in the node of the satellite's orbit against the diffuser time series to correct for solar incidence angle
effects. An exponential function with a time constant of 200 days yields the best fit to the diffuser time series.
The decrease in diffuser reflectance over the mission is wavelength-dependent, ranging from 9% in the blue (412
nm) to 5% in the red and near infrared (670-865 nm). The degradation of diffuser reflctance is similar to that
observed for SeaWiFS radiometric response itself from lunar calibration time series for bands 1-5 (412-555 nm),
though the magnitude of the change is four times larger for the diffuser. Evidently, the same optical degradation
process has affected both the telescope optics and the solar diffuser in the blue and green. The Cal/Val Team has
developed a methodology for computing the signal-to-noise ratio (SNR) for SeaWiFS on orbit from the diffuser
time series. The on-orbit change in the SNR for each band over the nine-year mission is less than 7%. The
on-orbit performance of the SeaWiFS solar diffuser should offer insight into the long-term on-orbit performance
of solar diffusers on other instruments, such as MODIS, VIIRS, and ABI.
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Robert E. Eplee Jr., Frederick S. Patt, Robert A. Barnes, Charles R. McClain, "SeaWiFS long-term solar diffuser reflectance trend analysis," Proc. SPIE 6296, Earth Observing Systems XI, 62960Q (7 September 2006); https://doi.org/10.1117/12.679170