Platform and Environmental Effects on Above-Water Determinations of Water-Leaving Radiances

Abstract

A comparison of above- and in-water spectral measurements in Case-1 conditions showed the uncertainty in above-water determinations of water-leaving radiances depended on the pointing angle of the above-water instruments with respect to the side of the ship. Two above-water data processing methods were used to create a diagnostic variable (formulated for Case-1 waters only) to quantify the presence of superstructure reflections that degraded the above-water intracomparisons of water-leaving radiances by as much as 13%?27% (for far-to-near viewing distances, respectively). The primary conclusions of the above- and in-water intercomparison of water-leaving radiances were as follows: (a) the SeaWiFS 5% radiometric objective was achieved with the in-water instruments; (b) the above-water approach produced agreement to within 5%, but reliably for about half the data, and only with well-controlled procedures and severe filtering to remove glint contamination; (c) a decrease in water-leaving radiance values was seen in the presence of swell, although, wave crests were radiometrically brighter than the troughs; and (d) standard band ratios used in ocean color algorithms remained severely affected, because of the relatively low signal at 555 nm and, thus, proportionally significant ship contamination at this wavelength. Suggestions for a more precise above-water measurement protocol are tentatively proposed.