Investigation of optical turbulence from an unmanned aerial system

Wave propagation of electro-optical systems, lasers or imaging depends on the state of the atmosphere their beams are passing through. Fluctuations in the refractive index of air are responsible for signal attenuation and image degradation. This atmospheric effect is called optical turbulence and its strength is quantified by the structure function parameter of the refractive index 𝐶2𝑛 . In the atmospheric surface layer it is highly variable. Usually 𝐶2𝑛 decreases with height. In non-uniform terrain, big horizontal variations can arise. We developed a mobile airborne system for monitoring 𝐶2𝑛 to investigate the three-dimensional character of optical turbulence. Therefore the dodecacopter system HORUS (Hovering Remote controlled Ultra-light Sensor-platform (AIRCLIP /Dresden/Germany)) was chosen as mobile platform. An ultrasonic anemometer was mounted on a boom for high-resolution measurements of temperature and wind speed. Analyzing the time series of temperature, 𝐶2𝑛 values were derived from time averages of several minutes. The measurements took place in the surface layer over land in the vicinity of an 80 m high tower, equipped with ultrasonic anemometers at four discrete heights. Comparison measurements were performed. The minimum length of the boom outside the turbulent influence of the rotors was investigated. The comparison of the 𝐶2𝑛 values shows a good agreement. A second, smaller quadrocopter system in combination with a new very small and light-weight ultrasonic anemometer was also tested for turbulence measurements. The system is introduced and the applicability shown. Results from first field trials are presented and discussed.