Sea Surface Simulation in the Infrared Modeling and Validation

A physics based 3D simulation of sea surfaces is presented. The simulation is suitable for the pre-calculation of detector images for an IR camera. Synthetic views of a maritime scenario are calculated in the MWIR and LWIR spectral bands and the images are compared with data collected in a field trial. In our computer simulation the basic sea surface geometry is modeled by a composition of smooth wind driven gravity waves. Sea surface animation is introduced by time dependent control of the basic statistics. Choppy waves are included into the model to improve the realism of the rough sea. To predict the view of a thermal camera the sea surface radiance must be calculated. This is done with respect to the emitted sea surface radiance and the reflected sky radiance, using either MODTRAN or a semi-empirical model. Slope-shadowing of the sea surface waves is considered, which strongly influences the IR appearance of the sea surface near the horizon. MWIR and LWIR simulations are shown of sun glint as well as of whitecaps which depend upon wind velocity. For validation purposes appropriate data sets (images and meteorological data) were selected from field measurements. A simple maritime scenario including a floating foreground object has been prepared and views of two different thermal imagers, similar to those used in the field trials, have been simulated. The validation is done by visual inspection of measured and simulated images and in addition by numerical comparison based on image statistics. The results of the comparison are presented. For an accurate reflectance calculation it is necessary to consider the maritime sky. The model is improved by inclusion of a static two-dimensional cloud layer. The cloud distribution adjusted to measured data with respect, e.g. to power spectral density and temperature distribution.