|
The NASA Ocean Biology Processing Group's Calibration and Validation Team uses SeaWiFS on-orbit lunar
calibrations to monitor the radiometric response of the instrument over time. With almost eleven years of
lunar measurements (more than 124 monthly observations) available for this analysis, the Cal/Val Team
has undertaken an investigation of the optimum function to use in fitting the time series and the fidelity of
resulting radiometric corrections that are applied to the ocean data. Two aspects of the on-orbit behavior
of SeaWiFS show changes over time: the long-term radiometric response for each band and the dependence
of the individual detector response in each band on the varying focal plane temperatures. Since band 8 (865
nm) shows the greatest changes in response over time, the analysis has concentrated on that band.
The initial goal of the SeaWiFS on-orbit calibration effort has been to use a single function to fit the
mission-long lunar time series. To date, that goal has been met by using a pair of simultaneous decaying
exponential functions with short-period and long-period time constants. As late mission observations were
added to the time series (beyond seven years into the mission), the long-term radiometric trend has been
approaching a linear function of time. Consequently, the long-term trend is starting to bias the fit for the
first three years of the mission. The Cal/Val team has addressed this issue by introducing a radiometric
epoch into the time series fitting functions, where the best fit for the early mission is provided by exponential
functions with periods of 200 and 2500 day and the best fit for the late mission is provided by an exponential
with a 400-day time constant and a linear function (or an exponential with a 40,000-day time constant). A
complication in optimizing these fits is that the dependence of the detector response on varying focal plane
temperatures began changing approximately seven years into the mission.
Analyses of periodic residuals in the lunar calibration time series in the latter part of the mission show
that either the temperature-dependence of the detector response or the overall thermal environment of the
instrument is changing over time. The Cal/Val Team has used correlations between these residuals and
the focal plane temperatures to evaluate revisions to the temperature corrections for the detector response.
Complications in computing these revised temperature corrections are that the behavior of the temperature
corrections is not readily described by an analytical function and that the long-term radiometric fits
compensate, to an extent, for changes in the temperature corrections.
In order to develop an improved calibration model for SeaWiFS, the Cal/Val Team has developed a
methodology for simultaneously fitting the long-term radiometric trend of each band and the change in
the temperature-dependence of the individual detector responses. This work shows the increased fidelity of
the calibration derived simultaneously for the long-term radiometric trend and the focal plane temperature
response compared to the sequential derivations of these corrections.
|
