practical limitation on the altitude at which a very massive laser may be flown and kept aloft for many hours at a time. This limit is about 12 km for the 747-400F airframe and the load that it must carry.

Minimizing the atmosphere is not sufficient, however, and other measures are required. Astronomers have used AO to at least partially cancel out atmospheric disturbances, achieving great improvements in imaging. These phase distortions can be applied using a deformable mirror, but there are limitations on how well this works.

It is convenient to distinguish between two classes of AO errors and corrections: (1) first order and (2) higher order. These describe the variation of the phase error across the aperture. First-order (i.e., linear) errors cause an apparent movement of the whole image with time as the atmosphere changes, because the phase error is linear across the aperture. This is a prism distortion. Left uncorrected, the object is not positioned where it appears to be. Because of the time variation, this blurs the image over the time of exposure. Usually, the first-order error is the most serious. It can be compensated for by a simple, flat mirror that tips or tilts, keeping the image in the same place. Second-order (quadratic) and higher errors are defocus and two types of astigmatism. They are corrected using a deformable mirror whose local shape can be varied in compensation. These corrections are also time dependent. The higher order corrections are much more complex, but also important.

The ABL makes its AO corrections using two different systems. To make a first-order correction, the nose of the target missile is illuminated with a TILL and the edges of the nose image are used to define the position of the target. The TILL is a kilowatt-class solid-state laser that instead of being focused on the target, illuminates it with a meters-wide beam to see the geometrical edges. In turn, the first-order phase distortion derived from the nose image is applied to a planar mirror for the HEL beam to correct this error. To deal with higher order errors, a second kilowatt-class laser (BILL) provides a beacon on the body of the target. The wavefront of this image is processed, and the phase corrections are applied to a deformable mirror to correct the HEL beam.

For various reasons, one of which was the ABM Treaty, the ABL was designed for use against short-range theater missiles, not long-range missiles like ICBMs. Because the long-range missiles burn longer and burn out at higher altitudes, more of the optical path goes through less of the turbulent atmosphere. Consequently, the AO problem is much easier to solve. In turn, this means that the effective range of the ABL can be greater for long-range missiles than for short-range missiles.

In principle an advantage of the ABL is that aircraft could be deployed to respond to an evolving threat. This could be simpler than deploying an entire ground-based interceptor base. However, the redeployment of an ABL CAP would require enough aircraft to maintain an aircraft on station at the previous threat area(s). In addition, ABLs require substantial infrastructure on the ground for supply and maintenance. Thus, such a redeployment is by no means trivial.

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