Lab measurements to support modeling terahertz propagation in brownout conditions

Brownout, the loss of visibility caused by dust and debris introduced into the atmosphere by the downwash of a helicopter, currently represents a serious challenge to U.S. military operations in Iraq and Afghanistan, where it has been cited as a factor in the majority of helicopter accidents. Brownout not only reduces visibility, but can create visual illusions for the pilot and difficult conditions for crew beneath the aircraft. Terahertz imaging may provide one solution to this problem. Terahertz frequency radiation readily propagates through the dirt aerosols present in brownout, and therefore can provide an imaging capability to improve effective visibility for pilots, helping prevent the associated accidents. To properly model the success of such systems, it is necessary to determine the optical properties of such obscurants in the terahertz regime. This research attempts to empirically determine, and measure in the laboratory, the full complex index of refraction optical properties of dirt aerosols representative of brownout conditions. These properties are incorporated into the AFIT/CDE Laser Environmental Effects Definition and Reference (LEEDR) software, allowing this program to more accurately assess the propagation of terahertz radiation under brownout conditions than was done in the past with estimated optical properties.