The THAAD system is a land-based missile defense system that includes a truck-mounted launcher, a complement of interceptor missiles, an AN/TPY-2 tracking radar and an integrated fire control system. U.S. Strategic Command intends to deploy and employ THAAD, a rapid response weapon system, to protect critical assets worldwide. Commanders will use the THAAD kill vehicle to intercept an incoming threat ballistic missile in the endo-atmosphere or exo-atmosphere, limiting the effects of weapons of mass destruction on battlefield troops and civilian populations.

The Terminal High Altitude Area Defense (THAAD) system will provide extended coverage for a greater diversity and dispersion of forces and the capability to protect population centers. In early 2004, MDA changed the name of the THAAD element from "Theater High Altitude Area Defense" to "Terminal High Altitude Area Defense." The principal additional capability provided by this system is the ability to deal with longer-range theater missile threats as they begin to emerge. THAAD also reduces the number of missiles that the lower-tier systems must engage and provides a shoot-look-shoot capability--the ability to engage incoming missiles more efficiently. With a range of over 200 km and a maximum altitude of 150 km, THAAD is designed to intercept ballistic missiles at long ranges above the atmosphere.

THAAD is designed to defend against medium- to longrange theater ballistic missiles. It will constitute the upper tier of a two-tier theater defense system, with PAC-3 as the lower tier. The peak reentry velocity of the missiles THAAD is to counter is proportional to their maximum range and increases as the range increases. In developing countries, there are more than 30 types of theater ballistic missiles either operational or under development. In addition, the number of countries that possess longer range theater missiles is expected to increase.

"Upper- tier" systems would intercept incoming long- and medium-range missiles during their flight in or above the outer atmosphere. "Lower-tier" systems defend at short to medium ranges against missiles in their late or final flight stages. Both upper- and lower-tier systems work in conjunction with space-based sensors -- the same sensors that will be used for surveillance and early warning against missiles targeted at the United States.

THAAD is the first weapon system with both endo-atmospheric (inside the atmosphere) and exo-atmospheric (outside the atmosphere) capability developed specifically to defend against short, medium and intermediate range ballistic missiles. The THAAD system will provide high-altitude missile defense over a larger area than the complementary Patriot system, and, like the Patriot, intercepts a ballistic missile target in the "terminal" phase of flight--the final minute or so when the hostile missile falls toward the earth at the end of its flight. THAAD uses "hit-to-kill" technology, using only the force of a direct impact with the target to destroy it.

MDA's THAAD element is being developed in incremental, capability-based blocks to provide a ground-based missile defense system able to defend against short-and medium-range ballistic missile attacks. THAAD will include missiles, a launcher, an X-band radar, and a command and control/battle management system. Its launcher is in a mobile, tactical fire unit with eight missiles per launcher and three launchers per fire unit. Its radar provides early warning to the specific location threatened by a ballistic missile and precise tracking of the missile, including in-flight data updates, plus an accurate determination of the missile launch point.

In a potential sequence of operations, an external early warning sensor, if available, would detect the target and cue the THAAD system for an interceptor launch before the Theater Missile Defense-Ground Based Radar (TMD-GBR) could acquire the target. With or without the external sensor, the TMD-GBR would eventually acquire and track the target. After receiving target identification and guidance information from the radar, the THAAD interceptor would engage the target, and a kill assessment would be conducted by the TMD-GBR and tactical operations center. Then, if necessary, a second THAAD interceptor would be launched. If the subsequent kill assessment again shows that the target was not destroyed, the TMD-GBR would cue the PAC-3 system to engage the missiles that evaded THAAD.

The THAAD focal plane array is a heat-sensitive device that performs thermal imaging for tracking, discrinkation, and aim point selection of targets to achieve hit-to-kill engagements. In the 1990s DOD was testing interceptors with a platinum silicide (PtSi) seeker, while planning to produce interceptors with an indium antimonide (InSb). The Army planned to begin using an indium antimonide (InSb) focal plane array in the interceptor's seeker component beginning with test flight 8 scheduled for June 1997. The decision to award the UOES option was based upon confidence in the extensive testing conducted to date on both the PtSi and InSb seekers and their high degree of component commonality. The InSb seeker is less, not more complex than the PtSi seeker. The InSb seeker components are approximately 95% common with the PtSi seeker. The platform, optics, and gimbals are identical, while other components, such as the seeker electronics assembly and the dewar, are nearly identical. The fabrication, calibration, and integration of an InSb focal plane is also less complex than with PtSi. Although the InSb focal plane required different signal processing software (SW), by 1997 the development and coding was on schedule. At that time, Hardware-In-The-Loop (HWIL) testing is nearing completion and had demonstrated the ability of an InSb seeker, software, and the flight test computer to successfully perform target acquisition. The similarity between the PtSi and the InSb seekers and their demonstrated performance in stand-alone seeker testing provided confidence in the decision to exercise the UOES option. The schedule risk for the InSb seeker is lower than for the PtSi seeker. InSb focal planes are two to three times more producible than PtSi. The InSb focal plane arrays are off-theshelf items and, therefore, have a shorter delivery lead time than PtSi focal planes. In addition, the fact that they are off-theshelf allows for individual selection of the highest quality focal planes from the on-hand supply.

In November 1992, the Under Secretary of Defense for Acquisition - who is responsible for BMDO treaty compliance - expressed concern over whether the THAAD program's design and flight tests were in compliance with the ABM Treaty. Accordingly, the Under Secretary amended authorization for the demonstration and validation phase. He directed that BMDO not proceed with the current development contract beyond the final design review that is scheduled for November 1993, unless the program's design and flight tests were certified as treaty compliant [they were].

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