Gain 3 Drift Correction

The lunar calibration data are collected at non-standard gains (Gain 4 for Band 1 and Gain 3 for the other bands) to keep the radiances below the knees of the bilinear gains on the lunar disk. The SeaWiFS calibration strategy requires that the gain ratios (4:1 for Band 1, 3:1 for the other bands) for the gains used in the lunar calibrations to be constant over time or that the on-orbit calibration incorporate corrections for any drifts in these gain ratios. Potential drifts in the gain ratios over time are evaluated using the on-orbit gain calibrations, where on a daily basis a constant voltage source is introduced for a brief period of time into the post-detector electronics for each band, allow the electronic gains to be monitored.

Prior to this reprocessing, the on-orbit gain calibration data analysis for the operational ocean data processing extened from the start of the mission through the end of 2006. Based on that analysis, linear corrections to the 3:1 gain ratios were computed over time for Bands 7 and 8. These corrections were applied to the lunar time series, then the long-term radiometric correections for these two bands were recomputed. The revised radiometric corrections for Bands 7 and 8 were incorporated into the operational calibration.

For the current reprocessing, the the strategy for the Gain 3 drift correction has been revised. The extension of the gain calibration data analysis through April of 2009 has shown that the only band/gain combination that shows any drift over time is Band 7 Gain 3. The following figures show the four electronic gains over time Bands 1, 2, 7, and 8.
Band 1

Band 2

Band 7

Band 8


The plots shown above for Bands 1 and 2 are typical for Bands 1 through 6. All four electronic gains show comparable behavior over the mission lifetime. The plot for Band 8 shows that what had been interpreted as a drift in the 3:1 gain ratio at the time of the past reprocessing is really digitization noise in the Gain 3 response for that band. The plot for Band 7 shows a significant "out of family" response for Gain 3 over the whole mission. Band 7, Gain 3 is the only band/gain combination that shows such outlier behavior. The figure below shows the 3:1 gain ratio data for Band 7, normalized to the valaue of the gain ratio at the start of the mission. The blue curve is a quadratic fit to the gain ratio over time, where the 2nd order term of the fit has been modified slightly so that the curve has a slope of zero at the end of the mission. This curve is the Gain 3 drift correction that is applied to the Band 7 data in the revised instrument calibration. For this reprocessing, the gain drift correction has been added to the SeaWiFS calibration table as a separate function distinct from the long-term radiometric correction for the instrument.