Correction of specular reflections at the water surface

Abstract

For remote sensing of water constituents it is very important to correct specular reflections at the water surface with high accuracy since these have the same order of magnitude as the reflectance of the water body. If the geometry is well-defined, e.g. at satellite observation, the specular reflections can be calculated reliably, but frequently the geometric details are not known with sufficient accuracy. This is the case in particular for airborne and shipborne measurements which are affected by the slopes of individual water waves or which are performed at cloudy sky. Erroneous correction of specular reflections produces characteristic spectral patterns in the derived sub-surface reflectance spectrum. These patterns can be used as fingerprints to identify erroneously corrected measurements. They indicate that a wrong spectrum of the downwelling irradiance has been taken. From measurements a simple empirical model for the downwelling spectral irradiance has been derived. It approximates the irradiance in the spectral range from 400 to 800 nm by a sum of four base spectra which represent the light sources sun, blue sky, aerosols and clouds. This model is suited to reconstruct spectrally highly resolved (1 nm) irradiance measurements at all weather conditions. It allows an improved correction of specular reflections: the relative contributions of the four light sources are changed iteratively until the fingerprint patterns in the reflectance spectrum are at minimum.