Calibration Update

On December 1, 2000 the SeaWiFS Calibration and Validation Team (CVT) updated the calibration table and vicarious calibration coefficients used in the SeaWiFS operational processing. This update implements three changes in the SeaWiFS calibration, two incorporated into the calibration table and one incorporated into the vicarious gains.

The first update to the calibration table is the addition of a second piecewise linear time correction segment to the time corrections for both bands 7 and 8. The time corrections for bands 7 and 8 now incorporate two quadratic segments and two linear segments. This update is based on the continued analysis of the lunar calibration time series. Figure 1 shows the lunar observations, along with the fitted time correction segments.

The second update to the calibration table is the incorporation of a set of revised half-angle mirror side reflectances. During ocean observations, adjacent LAC or HRPT scan lines are observed by different sides of the half-angle mirror. The level-1b algorithm corrects for the difference in the relative reflectance of the two mirror sides.Figure 2 and Figure 3 show the relative reflectances of the two mirror sides, as measured on orbit. The sloping lines are linear fits to the measured reflectances, while the horizontal lines show the prelaunch values of the reflectances that the level-1b algorithm has been using prior to this update. The differences between the measured reflectances and the prelaunch values now occasionally give rise to one-count differences between adjacent scan lines in the ocean data. To correct these differences without changing the level-1b algorithm, the CVT has determined the value of the mirror side reflectances from the linear fits for the starting times of each of the time correction segments in the calibration table. The mirror side reflectances contained in the calibration table are updated with the appropriate values at the start of each time segment. This provides a series of step-function adjustments to the mirror side reflectances that will keep the differences in the adjacent scan lines to less than one count. During the next reprocessing, the level-1b algorithm will be modified to apply mirror side reflectances that vary smoothly with time.

The update to the vicarious gains incorporates an improved implementation of the out-of-band correction to the SeaWiFS data during the vicarious calibration. The modified out-of-band correction resulted in small adjustments to the vicarious gains. The effect of these gain changes on the retrieved normalized water-leaving radiances and chlorophyll concentrations are shown in Figures 4-7. Figure 4 shows the time series of global mean clear-water radiances computed from the operational 8-day timebin files over the course of the mission, with the values from two 8-day timebins computed with the adjusted gains represented by asterisks. Clear-water radiances are radiances computed for waters with a minimum depth of 1 km and a maximum chlorophyll concentration of 0.15 mg/m^3. The corresponding chlorophyll time series is shown in Figure 5, along with a typical error bar on the chlorophyll concentration. The changes in the radiances and in the chlorophyll concentrations due to the adjusted gains are well within the errors of the mean values and within the timebin-to-timebin variations.Figure 6 and Figure7 show how the adjusted gains affect the chlorophyll concentrations on a regional basis, with Figure 6 showing data for the Sargasso Sea and Figure 7 showing data in the vicinity of Easter Island. Again, the changes in the chlorophyll concentrations are small. During the next reprocessing, the improved out-of-band correction will be applied to the entire mission data set.

The new SeaWiFS calibration table, SEAWIFS_SENSOR_CAL.TBL-200007, is available through the Goddard DAAC. The new vicarioius gains, which are input parameters for MSl12, are listed in Table 1.