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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 th...
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 th...
Ocean color climate data records require water-leaving radiances with 5% absolute and 1% relative accuracies as input. Because of the amplification of any sensor calibration errors by the atmospheric correction, the 1% relative accuracy requirement translates into a 0.1% long-term radiometric stability requirement for top-of-theatmosphere radiances. The rigorous on-orbit calibration program developed and implemented for SeaWiFS by the NASA Ocean Biology Processing Group (OBPG) Calibration and Validation Team (CVT) has allowed the CVT to maintain the stability of the radiometric calibration of SeaWiFS at 0.13% or better over the mission. The uncertainties in the resulting calibrated top-of-the-atmosphere (TOA) radiances can be addressed in terms of accuracy (biases in the measurements), precision (scatter in the measurements), and stability (repeatability of the measurements). The calibration biases of lunar observations relative to the USGS RObotic Lunar Observatory (ROLO) photometric model of the Moon are 2-3%. The biases from the vicarious calibration against the Marine Optical Buoy (MOBY) are 1-2%. The precision of the calibration derived from the solar calibration signal-tonoise ratios are 0.16%, from the lunar residuals are 0.13%, and from the vicarious gains are 0.10%. The long-term stability of the TOA radiances, derived from the lunar time series, is 0.13%. The stability of the vicariouslycalibrated TOA radiances, incorporating the uncertainties in the MOBY measurements and the atmospheric correction, is 0.30%. These results allow the OBPG to produce climate data records from the SeaWiFS ocean color data.
The NASA Ocean Biology Processing Group's Calibration and Validation Team has analyzed the mission-long Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) on-orbit gain and detector calibration time series to verify that lunar calibrations, obtained at nonstandard gains and radiance ranges, are valid for Earth data collected at standard gains and typical ocean, cloud, and land radiances. For gain calibrations, a constant voltage injected into the postdetector electronics allows gain ratios to be computed for all four detectors in each band. The on-orbit lunar gain ratio time series show small drifts for the near infrared bands. These drifts are propagated into the ocean color data through the atmospheric correction parameter epsilon, which uses the 765/865 nm band ratio. An anomaly analysis of global mean normalized water-leaving radiances at 510 nm shows a small decrease over the mission, while an analysis of epsilon shows a corresponding increase. The drifts in the lunar time series for the 765 and 865 nm bands were corrected. An analysis of the revised water-leaving radiances at 510 nm shows the drift has been eliminated, while an analysis of epsilon shows a reduced drift. For detector calibrations, solar diffuser observations made by the individual detectors in each band allows the response of the detectors to be monitored separately. The mission-long time series of detector calibration data show that the variations in the response of the individual detectors are less than 0.5% over the mission for all bands except the 865 nm band, where the variations are less than 1%.
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