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.
THAAD system development was initiated in 1987 with concept studies performed by the High Endoatmospheric Defense Interceptor Project Office at the U.S. Army Strategic Defense Command. These studies investigated the application of key strategic defense technologies (infrared seeker, fast response control system and advanced cooled seeker window designs) to the theater missile defense problem.
The Terminal High Altitude Area Defense 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 was 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 was designed to intercept ballistic missiles at long ranges above the atmosphere.
THAAD was 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 was to counter was 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 was 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 was 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 was 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 was 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 was a heat-sensitive device that performs thermal imaging for tracking, discrimination, 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 was 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 was 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 was 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 was lower than for the PtSi seeker. InSb focal planes are two to three times more producible than PtSi. The InSb focal plane arrays were 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.
Endoatmospheric interceptors using infrared seekers require a protective window for operation within the atmosphere. Theater and Terminal defense interceptors operate at altitudes where atmospheric absorption/attenuation in the visible and infrared regions is not significant. Therefore, it is desired to retain the window during exoatmospheric operation as well, and that the window exhibit high transmission from 500 nm through 11 microns. The THAAD seeker window is sapphire, which is extremely expensive [sapphire costs in excess of $30K per window], difficult to fabricate and limits seeker operation to the MWIR.
Although the IR seeker window is protected by a shroud during early flight, the window still experiences moderate convective heating after shroud removal for low altitude, short range intercepts. In 2007 a team at the Arnold Engineering Development Center's (AEDC) Hypervelocity Wind Tunnel 9 in Silver Spring, MD subjected a full-scale model of a THAAD missile nosecone with a new infra-red (IR) seeker window to hypervelocity airflow conditions exceeding what a missile would experience in the final stage of flight. All seven of the flight-quality full-scale IR seeker windows survived the testing.
During flyout, the seeker window is protected from the severe flight environment by a two-piece clamshell shroud. The shroud is ejected just prior to seeker acquisition by inflating metal bladders in the nose cone to impart the required separation velocity. Shroud technology, used in THAAD, was developed by the U.S. Army under the auspices of BMDO. The seeker window is a rectangular sapphire plate mounted in the forecone. Again, the seeker window technology is a legacy of the seeker windows developed for the HEDI program.
The seeker design includes an all-reflective Korsch optical system and platinum silicide staring focal plane array. The mid-wave infrared seeker is mounted on a 2-axis stabilized platform to isolate the seeker measurements from vibration and other disturbances. The seeker design includes an all-reflective optical system and platinum sillicide staring focal plane array. This sensitive staring focal plane array, which serves as the "eyes" for the THAAD interceptor, emerged from BMD sensor technology efforts over the last ten years. A ring-laser gyro inertial measurement unit (IMU) is mounted on the platform to measure and stabilize the platform motion and serve as a reference for seeker measurements.
In November 1992, the Under Secretary of Defense for Acquisition - who was 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 was scheduled for November 1993, unless the program's design and flight tests were certified as treaty compliant [they were].
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