Platform Pertubations in Above-Water Radiometry

A comparison of above- and in-water spectral measurements in coastal (but predominantly Case-1) conditions showed the uncertainty in above-water determinations of water-leaving radiances made from an offshore tower depended on the proximity of the above-water measurement with respect to the side of the platform. For the purposes of this study, the proximity of the sampling platform is parameterized as the perpendicular distance (denoted here as x) from the side of the sampling platform to the center of the area on the sea surface observed by the sea-viewing sensor, the so-called surface spot, which is set by the field of view of the radiometer (or the overlapping fields of view of a multi-aperture sensor). Two above-water data processing methods were used to create a diagnostic variable (formulated for Case-1 waters only, but also applicable to Case-2 conditions over short time scales) to quantify the presence of superstructure reflections. Based on the height of the tower, H, the analyses were partitioned into near- and far-field data sets (x H, respectively). The primary conclusions of the radiometric intercomparisons were as follows: a) the maximum perturbations occur very close to the tower (small values of x/H), and as x/H increases and approaches 1 (i.e., as the surface spot becomes as far away as the platform is high), the platform perturbations converge towards smaller and smaller values; and b) within the far field (x > H), the platform perturbation is negligible, and a remote sensing 5% absolute accuracy objective can be satisfiedJRC.H.5-Rural, water and ecosystem resource