A complete set of algorithms and models for the level_2 processing of the European CZCS historical data was integrated in the OCEANcode software package. The OCEANcode allows the calibration of the sensor-recorded signal taking into account the instrument sensitivity loss; the correction of the calibrated signal for atmospheric contamination and derive sub-surface re¯ectances; and then the estimation of the concentration of water constituents. The atmospheric correction is performed on the basis of a re¯ectance-model-based algorithm. The Rayleigh correction is applied consistently for all water pixels, using a multiple scattering approach, and introducing atmospheric pressure and Ozone concentration data in the computation. The marine aerosol correction uses a pixel-by-pixel iterative procedure, allowing successive estimates of both the marine re¯ectance in the red spectral region (670 nm) and the A Ê ngstrù m exponent, which links simple wavelengths ratios to re¯ectance ratios. For case 1 waters, the optical properties of which are essentially dominated by planktonic pigments, the interrelations between marine re¯ectances and re¯ectance ratios at various wavelengths are derived from modelled calculations. For identi® ed case 2 waters, where water constituents other than planktonic pigments (i.e. dissolved organics and suspended sediments) dominate the water optical properties, the evaluation of marine re¯ectances is approximated by means of interpolated A Ê ngstrù m exponent values computed over case 1 water pixels and of empirical relationships derived from in situ measurements. The computation of chlorophyll-like pigments is performed with algorithms based on blue/green (443± 550 nm) re¯ectance ratios, for lower pigment concentration, or on green/green (520± 550 nm) re¯ectance ratios, for higher pigment concentration. As for the case of atmospheric corrections, the inter-relations between pigment concentration and re¯ectance ratios are model-derived for case 1 waters, and empirically determined for case 2 waters.
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