Characterization of High Altitude Turbulence for Air Force Platforms

The Department of Defense (DoD) has an urgent need to both understand and predict the effects of high-altitude (z > 10 km) turbulence (HAT) on systems that operate at or propagate through those altitudes. Examples of systems affected by HAT include surveillance aircraft such as the U-2 and the unmanned Global Hawk, developing weapons systems such as the airborne laser (ABL), and developing communication systems such as the transformational communication satellites (TSAT). The surveillance aircraft are primarily affected by mechanical turbulence, which is the fluctuation of wind velocity and is usually caused by velocity gradients, flow over terrain, or convection. High frequency gravity waves can affect the aircraft the same way and are caused by the same forcing mechanisms as actual mechanical turbulence and are considered as part of the HAT forecast problem. The effects of mechanical turbulence include upsetting the autopilot, degrading the quality of the surveillance observations, and in the worse case jeopardizing the aircraft itself. Laser-based systems such as the ABL and TSAT are affected by optical turbulence, which is the fluctuation of the index of refraction and is caused by mechanical turbulence in the presence of temperature gradients. For directed-energy weapon systems such as the ABL, optical turbulence causes the laser to wander, spread, and scintillate which will degrade its power, result in increased dwell time to destroy a target, and decrease the effective range. For laser-based communication systems such as TSAT, optical turbulence can result in significant data dropouts.