Measurement and prediction of optical turbulence effects as function of altitude in the marine boundary layer

Abstract

The impact of atmospheric turbulence in the marine boundary layer on the performance of ship-borne and coastal optical and infrared surveillance sensors is well known. On the one hand, the detection process may benefit from scintillation of signals from point-like targets at long range, while target identification is hampered by turbulence induced blurring and beam wander. According to the commonly used turbulence model, based on the Monin-Obukhov similarity theory for the marine boundary layer (the so-called bulk model), the structure function for refractive index C n 2 decreases with altitude. This height dependence of C n 2 and the associated blurring, scintillation and beam wander, has consequences for the range performance of sensors against incoming small targets. In the paper, predictions of the height-dependence of turbulence, based upon the bulk model, will be given for a number of scenarios from previous measurement campaigns, such as POLLEX and SAPPHIRE. In these experiments, taking place in a variety of weather conditions (e.g. Air-Sea Temperature Difference (ASTD)), images were collected from vertical arrays of sources at long range. In addition imagery was collected of helicopters, acting as point-targets at ranges up-to more than 30 km. The imagery is used to determine turbulence effects, which are compared with the model predictions. The results of this comparison, showing details on the validity of the model, are presented in the paper.