Simulation of whitecaps and their radiometric properties in the SWIR
A 3D simulation of the dynamic sea surface populated with whitecaps is presented. The simulation considers the dynamic evolution of whitecaps depending on wind speed and fetch. It is suitable for imaging simulations of maritime scenarios. The calculation of whitecap radiance is done in the SWIR spectral band by considering wave hiding and shadowing, especially occurring at low viewing angles. Our computer simulation combines the 3D simulation of a maritime scene (open sea/clear sky) considering whitecaps with the simulation of light from a light source (e.g. laser light) reflected at the sea surface. The basic sea surface geometry is modeled by a composition of smooth wind driven gravity waves. The whitecap generation is deduced from the vertical acceleration of the sea surface, i.e. from the second moment of the wave power density spectrum. To predict the view of a camera, the sea surface radiance must be calculated for the specific waveband with the emitted sea surface radiance and the specularly reflected sky radiance as components. The radiances of light specularly reflected at the windroughened sea surface without whitecaps are modeled by considering an analytical statistical sea surface BRDF (bidirectional reflectance distribution function). A specific BRDF of whitecaps is used by taking into account their shadowing function. The simulation model is suitable for the pre-calculation of the reflected radiance of a light source for near horizontal incident angles where slope-shadowing of waves has to be considered. The whitecap coverage is determined from the simulated image sequences for different wind speeds and is compared with whitecap coverage functions from literature. A SWIR-image of the water surface of a lake populated with whitecaps is compared with the corresponding simulated image. Additionally, the impact of whitecaps on the radiation balance for a bistatic configuration of light source and receiver is calculated for different wind speeds.