How Precise Are Seawifs Ocean Color Estimates? Implications of Digitization-Noise Errors

Various subtle but important digitization round-off and noise errors are found in SeaWiFS imagery. These errors often cause large relative errors at a pixel and cause pixelization or "speckling" across the image, which is particularly obvious in the SeaWiFS chlorophyll standard product. Using simulations and current SeaWiFS algorithms, we show the effect of digitization-noise errors on the SeaWiFS ocean-color data products. It is found that the SeaWiFS mission goals, namely to estimate water-leaving radiances to within ±5% and chlorophyll-a concentrations to within ±35% for Case I waters, cannot always be met. For maritime aerosol conditions the errors in the estimated chlorophyll concentrations over oligotrophic waters can easily reach ±60% and the absolute values over adjacent pixels can vary 2-3 fold. Several schemes can be used to reduce this type of error, among which the spatial smoothing of the atmospheric-correction bands is recommended. This is because the errors in those bands are propagated and exaggerated to the visible bands and to the chlorophyll product through the atmospheric-correction process. The smoothing scheme, however, will cause more pixels to be discarded over cloud edges. Ultimately, a significant increase in both digitization bits and sensitivity is essential to achieve the stated mission goals. Such improved specifications are available on MODIS.