Proceedings PaperWind And Nadir Angle Effects On Airborne Lidar Water "Surface" Returns
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The reflection of optical energy from the air/sea interface and by the volume backscattering immediately below involves a number of complex phenomena which have critical effects on the design and performance of airborne laser hydrography systems. Both of these reflections can be considered as "surface" return, because when the interface return is weaker than the volume return, the latter will be detected, although at a somewhat biased location, as found during lidar/sonar depth measurement intercomparisons. The character of the surface return depends on the ratio of the peak volume backscatter power and the peak interface reflection power. Analytic expressions are derived for mean values of these quantities. The functionalities of the volume-to-interface peak power ratio on wind speed and direction, off-nadir beam incidence angle, and water clarity parameters are examined to determine the parameter ranges for which the a priori origin of surface returns is uncertain. Depth measurement error magnitudes are calculated for the case of a volume return being mistaken for an interface return. The error model sucessfully predicts the shoal bias observed in field data. Potential methods for reducing this error are discussed. An expression for the temporal profile of a volume backscatter return is presented, and a potential method of estimating a key water clarity parameter from the airborne data is reported.