Refraction and Long-Range FLIR Images

Abstract

With the advent of focal plane arrays, ER sensors are increasingly being used for long-range applications with increased resolution to allow object identification. For sea applications the detection of near surface objects is often limited by the horizon and substantial image distortions due to atmospheric refraction are encountered. In some cases, targets can be detected well beyond the geometrical horizon limit while under different conditions, the horizon limit can be much smaller than the geometrical value and mirages can occur. For the prediction of the performance of the IR sensors, it is essential to take these effects into account. Ship-borne IR sensors to detect near sea-level targets, such as ships or low-altitude aircraft, are particularly sensitive to the refraction phenomena. This is why accurate models have been developed to cover the Marine Boundary Layer (MBL) which is the atmospheric layer immediately above the water surface. The predictions of these models are so significant that a sea-shore measurement program was undertaken to verify the validity of the models and to give a higher degree of confidence in its application to a variety of weather scenarios. The present report summarizes the model development, its experimental validation and the system implications of the effects of refraction.