Atmospheric Compensation and Tracking Using Active Illumination

■ The U.S. Air Force is developing the airborne laser (ABL), whose mission is to engage and destroy theater ballistic missiles such as the SCUD while these missiles are in their boost phase. This mission capability requires high-energy laser propagation over long horizontal paths (200 to 300 km) through the upper atmosphere. To be effective in the presence of atmospheric turbulence, the ABL must utilize precision tracking and adaptive-optics compensation. Although the strength of atmospheric turbulence at ABL altitudes (35,000 to 45,000 feet) is relatively weak compared to sea level, the long horizontal laser-propagation paths create severe challenges for the adaptive-optics and tracking systems. An equally difficult challenge is created because the missile provides no beacon for the adaptive-optics and tracking systems. The target missile must be actively illuminated so that backscatter from the missile body can be used to form an image for the tracking system and provide a beacon for the adaptive-optics system. To understand this problem better and to improve system performance, we conducted propagation experiments at the Firepond telescope facility on Millstone Hill in Westford, Massachusetts. These tests utilized a 5.4-km horizontal propagation range between Millstone Hill and a fire tower in the town of Groton, Massachusetts. These experiments, which demonstrated for the first time active tracking and adaptive compensation under ABL conditions, suggest that the ABL can meet its mission goals and perform at levels required for effective theater missile defense.