Prior to this reprocessing, the counts-to-radiance conversion coefficients used in the SeaWiFS calibration were the coefficients derived by NIST in 1997 as part of the recalibration of SeaWiFS which took place prior to launch during the integration of the instrument with the spacecraft. In assessing the calibration of SeaWiFS on orbit, it was observed that the 1997 coefficients give rise to trends with wavelength in both the lunar calibration residuals from the USGS photometric model of the Moon and in the vicarious gains computed from MOBY data. To mitigate these trends, the conversion coefficients have been reverted to the values derived by SBRS, the instrument builder, in 1993 duirng the initial calibration of SeaWiFS. This change to the SeaWiFS calibration had the desired effect on the lunar residuals, as is shown below.
Figure 1: Lunar calibration residuals for the SBRS and NIST prelaunch coefficients.
This change in the counts-to-radiance conversion coefficients does not impact the on-orbit calibration of SeaWiFS over time. For the ocean color data, the change in coefficients for Bands 1 through 7 are offset by corresponding changes in the vicarious gains, as is shown here:
| 412 nm | 443 nm | 490 nm | 510 nm | 555 nm | 670 nm | 765 nm | 865 nm | |
|---|---|---|---|---|---|---|---|---|
| NIST Gains | 1.0368 | 1.0132 | 0.9918 | 0.9982 | 0.9993 | 0.9729 | 0.9716 | 1.00 |
| SBRS Gains | 1.0087 | 0.9998 | 0.9898 | 0.9932 | 1.0069 | 0.9821 | 0.9676 | 1.00 |
