Operation Desert Storm demonstrated that the U.S. military and other allied forces have limited capability against theater ballistic missiles. In fact, U.S. defensive capability is limited to weapons that defend against missiles nearing the end of their flight, such as the Patriot. No capability currently exists to destroy missiles in the boost phase. Consequently, DOD is expending considerable resources to develop the ABL’s capability to intercept missiles in their boost phase. In simple terms, the ABL program will involve placing various components, including a powerful multimegawatt laser, a beam control system, and related equipment, in a Boeing 747-400 aircraft and ensuring that all the components work together to detect and destroy enemy missiles in their boost phase. In November 1996, the Air Force awarded a 77-month program definition and risk reduction contract to the team of Boeing, TRW, and Lockheed Martin. Under the contract, Boeing is to produce and modify the 747-400 aircraft and integrate the laser and the beam control system with the aircraft, TRW will develop the multimegawatt Chemical Oxygen Iodine Laser (COIL) and ground support systems, and Lockheed Martin will develop the beam control system. The various program components are in the early phases of design and testing. One prototype ABL will be produced and used in 2002 to shoot down a missile in its boost phase. If this demonstration is successful, the program will move into the engineering and manufacturing development phase in 2003. Production is scheduled to begin about 2005. Initial operational capability of three ABLs is scheduled for 2006; full operational capability of seven ABLs is scheduled for 2008. The ABL is a complex laser weapon system that is expected to detect an enemy missile shortly after its launch, track the missile’s path, and destroy the missile by holding a concentrated laser beam on it until the beam’s heat causes the pressurized missile casing to crack, in turn causing the missile to explode and the warhead to fall to earth well short of its intended target. The ABL’s opportunity to shoot down a missile lasts only from the time the missile has cleared the cloud tops until its booster burns out. That interval can range from 30 to 140 seconds, depending on missile type. During that interval, the ABL is expected to detect, track, and destroy the missile, as shown in figure 1. The first step—detection—is to begin when the ABL’s infrared search sensor detects a burst of heat that could be fire from a missile’s booster.Because clouds block the view of the infrared search sensor, the sensor cannot detect this burst of heat until the missile has broken through the cloud tops—assumed to be at about 38,500 feet. The sensor detects the heat burst about 2 seconds after the missile has cleared the cloud tops. (In the absence of clouds, detection can occur earlier.) The ABL would then use information from the sensor to verify that the heat burst is the plume of a missile in its boost phase and would then move the telescope located in the nose of the aircraft toward the coordinates identified by the infrared sensor. The second step—tracking—is to be performed sequentially and with increasing precision by several ABL devices. The first of these tracking devices, the acquisition sensor, is to take control of the telescope, center the plume in the telescope’s field of view, and hand off that information to the next device, the plume tracker. The plume tracker, having taken control of the telescope, is to track and determine the shape of the missile plume and use this information to estimate the location of the missile’s body and project a beam from the track illuminator laser to light up the nose cone of the missile. The plume tracker is then to hand its information, and control of the telescope, to the final tracking device, the fine tracker. The fine tracker is to measure the effects of turbulence and determine the aimpoint for the beacon laser and, ultimately, for the COIL laser. The reflected light from the illuminator laser provides information that is to be used to operate a sophisticated mirror system (known as a fast-steering mirror) that helps to compensate for optical turbulence by stabilizing the COIL beam on the target. The reflected light from the beacon laser provides information that is to be used to operate deformable mirrors that will further compensate for turbulence by shaping the COIL beam. With the illuminator and beacon lasers still operating, the fine tracker is to determine the aimpoint for the COIL laser. The COIL laser is to be brought to full power and focused on the aimpoint. At this point, the final step in the sequence—missile destruction—is to begin. During this final step, a lethal laser beam is held on the missile. The length of time that the beam must dwell on the missile will depend on turbulence levels and the missile type, hardness, range, and altitude. Throughout the lethal dwell, the illuminator and beacon lasers are to continue to operate, providing the information to operate the fast-steering and deformable mirrors. Under the intense heat of the laser beam, which is focused on an area about the size of a basketball, the missile’s pressurized casing fractures, and then explodes, destroying the missile. The ABL is expected to operate from a central base in the United States and be available to be deployed worldwide. The program calls for a seven-aircraft fleet, with five aircraft to be available for operational duty at any given time. The other two aircraft are to be undergoing modifications or down for maintenance or repair. When the ABLs are deployed, two aircraft are to fly, in figure-eight patterns, above the clouds at about 40,000 feet. Through in-flight refueling, which is to occur between 25,000 and 35,000 feet, and rotation of aircraft, two ABLs will always be on patrol, thus ensuring 24-hour coverage of potential missile launch sites within the theater of operations. The ABLs are intended to operate about 90 kilometers behind the front line of friendly troops but could move forward once air superiority has been established in the theater of operations. When on patrol, the ABLs are to be provided the same sort of fighter and/or surface-to-air missile protection provided to other high-value air assets, such as the Airborne Warning and Control System and the Joint Surveillance Target Attack Radar System. A key factor in determining whether the ABL will be able to successfully destroy a missile in its boost phase is the Air Force’s ability to predict the levels of turbulence that the ABL is expected to encounter. Those levels are needed to define the ABL’s technical requirements for turbulence. To date, the Air Force has not shown that it can accurately predict the levels of turbulence the ABL is expected to encounter or that its technical requirements regarding turbulence is appropriate. The type of turbulence that the ABL will encounter is referred to as optical turbulence. It is caused by temperature variations in the atmosphere. These variations distort and reduce the intensity of the laser beam. Optical turbulence can be measured either optically on non-optically. Optical measurements are taken by transmitting laser beams from one aircraft to instruments on board another aircraft at various altitudes and distances. Non-optical measurements of turbulence are taken by radar or by temperature probes mounted on balloons or on an aircraft’s exterior. The Air Force’s ABL program office has not determined whether non-optical measurements of turbulence can be mathematically correlated with optical measurements. Without demonstrating that such a correlation exists, the program office cannot ensure that the non-optical measurements of turbulence that it is collecting are useful in predicting the turbulence likely to be encountered by the ABL’s laser beam. Concern about turbulence measurements was expressed by a DOD oversight office nearly 1 year ago. In November 1996, during its milestone 1 review of the ABL program, the Defense Acquisition Board directed the program office to develop a plan for gathering additional data on optical turbulence and present that plan to a senior-level ABL oversight team for approval. The Board also asked the program office to “demonstrate a quantifiable understanding of the range and range variability due to optical turbulence and assess operational implications.” This requirement was one of several that the Air Force has been asked to meet before being granted the authority to proceed with development of the ABL. That authority-to-proceed decision is scheduled for June 1998. In February 1997, the program office presented to the oversight team a plan for gathering only non-optical data. The oversight team accepted the plan but noted concern that the plan was based on a “fundamental assumption” of a correlation between non-optical and optical measurements. If that assumption does not prove to be accurate, according to the oversight team, the program office will have to develop a new plan to gather more relevant (i.e., optical rather than non-optical) measurements. Accordingly, the oversight team required that the program office include in its data-gathering plan a statement agreeing to demonstrate the correlation between the non-optical and optical measurements. Program officials said they plan to demonstrate that correlation in the summer of 1997. To establish that a correlation exists, the program office plans to use optical and non-optical turbulence measurements taken during a 1995 Air Force project known as Airborne Laser Extended Atmospheric Characterization Experiment (ABLE ACE). Optical measurements were made by transmitting two laser beams from one aircraft to instruments aboard another aircraft at distances from 13 to 198 kilometers and at altitudes from 39,000 to 46,000 feet. These measurements provided the data used to calculate the average turbulence strengths encountered by the beams over these distances. The ABLE ACE project also took non-optical measurements of turbulence using temperature probes mounted on the exterior of one of the aircraft. Rather than taking measurements over the path of a laser beam between two aircraft, as with the optical measurements, the probes measured temperature variations of the air as the aircraft flew its route.