Atmospheric compensation and tracking using active illumination

The USAF 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 (AO) compensation. Although the strength of atmospheric turbulence at ABL altitudes (35,000 to 45,000 ft) is relatively weak compared to sea level, the long horizontal laser-propagation paths create severe challenges for the AO and tracking systems. An equally difficult challenge is created because the missile provides no beacon for the AO 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 AO 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, MA. These tests used a 5.4-km horizontal propagation range between Millstone Hill and a fire tower in the town of Groton, MA. These experiments, which demonstrated 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. (Author)