Field and laboratory validation of surface layer optical turbulence and off-axis irradiance

Abstract

The effects of optical turbulence on high energy laser propagation have been well documented. The optical turbulence is typically characterized using the index of refraction structure parameter, C_n². The value and 3-D variation of C_n² can be diagnosed for the surface boundary layer (lowest 50 m of the atmosphere) from values of temperature, pressure, humidity and wind velocity using meteorological similarity theory. Examples of such similarity theory C_n² calculators include the Tunick model for overland applications and the Navy Surface Layer Optical Turbulence (NSLOT) model for ocean scenarios, both of which are implemented in the AFIT CDE's (Center for Directed Energy) HELEEOS (High Energy Laser End-to-End Operational Simulation) and LEEDR (Laser Environmental Effects Definition and Reference) models where they can be assessed from 400 nm to 8.6 m, continuously. The HELEEOS model further allows for the calculation of the irradiance from within a HEL beam that is scattered by molecules and particulates in the atmosphere to an off-axis observation point, while incorporating the spreading effects of the turbulence and thermal blooming. Field and laboratory experiments conducted at Wright-Patterson AFB, Ohio in summer 2009 allowed for validation measurements for the surface layer optical turbulence and off-axis algorithms to be collected. Turbulence strength measurements were made at a wavelength of 1.55 μm using a state of the art bistatic turbulence profiler for both horizontal and vertical paths. Pressure, wind speed, wind direction, relative humidity and aerosol loading data were collected simultaneously with the C_n² measurements. As part of the experiment, the profiler's beams were imaged offaxis with a calibrated camera array and the received irradiance of the off-axis scattering was quantified. Characterization of the aerosol distribution along the laser path and the path to the observer is accomplished by determining the visibility and climatological aerosols for southwestern Ohio. Comparisons between predicted and measured C_n² and off-axis irradiance are made. Additionally an experimental technique to derive C_n² measurements from weather radar is described and compared to the turbulence profiler data.