Prior to the SeaWiFS launch, two separate calibrations were performed, one by SBRS in 1993 and the other by NIST in 1997. The NIST 1997 gains have been used for all processing to date, modified by the vicarious gains determined using the MOBY data (bands 1 through 6) and model assumptions near MOBY (band 7). Recent analysis has shown that the vicarious gains are more consistent with the SBRS 1993 calibration. Also, the SBRS gain for Band 8 is about 4% lower than NIST, which is consistent with other analyses of Band 8 which show its output with the NIST gain to be high by several percent (see item 2.4). In addition, the reflectance-based calibration performed by Barnes (using the various solar irradiance models described in 2.6) is also more consistent with SBRS 1993. The switch to the at-launch gains is under consideration, to either the SBRS values or an average of several sets of gains, along with updated Band 1 through 7 vicarious gains for consistency. (Note that for ocean data, the vicarious calibration largely cancels out any effect on the Band 1-6 nLw, but the reflectances will be affected. Also vicarious gains are not used for land data processing.)
There is a growing set of evidence that SeaWiFS band 8 may produce top-of-the-atmosphere radiances that are too large. The error in these radiances appears to be on the order of 5% to 10%. There is no single comparison reference for band 8, such as MOBY (which is used for bands 1 through 6). However, the set of comparison measurements, both of radiances and derived products such as aerosol column amounts, show a consistent difference. Changing the calibration coefficient for band 8, based on these comparisons, is a judgement call by the project.
The result of a lowering of the calibration coefficient for band 8 will be a decrease in the aerosol column amounts in the SeaWiFS data set. At present, there is a very limited number of negative aerosol column amounts in the SeaWiFS data set. A lowering of the calibration coefficient for band 8 will increase the number of these results. Consequently, it may be prudent to limit the calibration change (if a change is to be made at all) to a smaller amount, such as 5%, rather than a larger amount, such as 10%. In addition, it may also be prudent to understand the impact of the calibration change on the number of negative aerosol column amounts in the data set before a change is made. This effect will be at least partly offset by the change for very clear atmospheric conditions described above (1.6).
When we calibrate at MOBY, we assume an "average" maritime model to determine the band 7 gain. The average Angstrom coefficient for the m50, m70, m90, and m99 models is 0.31. However, according to AERONET results from Lanai, the most common Angstrom coefficient is more like 0.7, which actually corresponds to c50 in our model suite. Until we have confidence in a NIR calibration based on in situ aerosol match-ups, perhaps we should consider tuning band 7 to a c50 model at Lanai. Alternatively, we might consider using a different region, such as the western equatorial Pacific, where measurements indicate that the assumption of angstrom=0.31 is valid. The most common angstrom coefficient measured at Nauru is 0.3, with half the variability seen at Lanai.
A technique has been developed and tested whereby in situ aerosol optical thickness measurements can be used to derive the expected aerosol path radiances in the 8 SeaWiFS bands. Using this method, it is possible for the first time to obtain a direct calibration of the NIR bands. A description of the approach is available here:
http://orca.gsfc.nasa.gov/seawifs/doc/aot_based_calibration.pdfInitial results suggest that the 865 nm calibration is 4% high, and that we can significantly improve the AOT match-ups by applying the NIR calibration. However, we have found no significant benefit with regard to oceanic optical property retrievals, and questions remain as to whether we should really expect SeaWiFS AOT retrievals to match sun photometer measurements.
The use of in situ sun photometers to constrain the models used in the vicarious calibration of band 7 was also investigated. The results of these analyses also proved inconclusive. The uncertainties inherent in the in situ instrumentation were too large for the proposed methods to be robust. It is recommended that the current NIR vicarious calibration method be maintained.
The position of MOBY in the lee of the Hawaiian Islands makes the use of ancillary winds on a 5 minute grid for calibration purposes questionable. The MOBY buoy recently added a meteorological package, which allows for the true wind speed to be used in the vicarious calibration procedure. Comparisons of SeaWiFS ancillary wind fields to those measured at MOBY show large deviations. Preliminary results indicate that the uncertainty in the gains resulting from the use of the coarsely gridded ancillary wind fields is indistinguishable from the uncertainty resulting from the atmospheric correction. Therefore, the use of ancillary wind fields in the calibration procedure is adequate given our current understanding of the inherent uncertainties.
