The THAAD (Terminal High Altitude Area Defense) and Patriot air defense systems were both developed and manufactured by Lockheed, which made it easier to modify their fire control systems so both could use the superior radar and fire control system of THAAD. In February 2022 this was successfully tested when a Patriot PAC-3 anti-missile missile intercepted an incoming target detected by the THAAD radar.
THAAD is a step up from the Patriot PAC-3 anti-missile, which is an anti-aircraft missile adapted to take out incoming missiles. PAC-3 works, but has a limited (35 kilometers) range. THAAD was developed to defend against longer-range ballistic missiles that come at their targets at higher speeds than Patriot PAC-3 can handle. The changes to the THAAD fire control system enable THAAD to have Patriot handle targets it was designed to intercept while the more expensive THAAD missiles concentrate on missiles PAC-3 cannot deal with.
Each THAAD battery has 48 missiles and six launchers (an 8x8 truck carrying eight missiles in storage/firing containers) plus an AN/TPY-2 radar (on a tractor-trailer vehicle) and two truck mounted mobile tactical control stations. Total cost for each battery is about a billion dollars. The 6.2- meter (18 foot long) THAAD missiles are 340mm in diameter and weigh 900 kg (1,980 pounds). This is about the same size as the Patriot anti-aircraft missile, but twice the weight of the smaller PAC-3 anti-missile version of the Patriot.
The range of THAAD is over 200 kilometers, max altitude is 150 kilometers, and top speed is 2,800 meters (9,000 feet) a second. THAAD is intended for short (like SCUD) or medium range (up to 2,000 kilometer) range ballistic missiles. THAAD began development in the late 1980s and entered service in 2006. Originally the U.S. Army planned to buy at least 18 launchers, 1,400 missiles, and 18 radars. That goal has been adjusted as the number of export customers increases.
The United States has another BMD (Ballistic Missile Defense) system; the U.S. Navy RIM-161A, also known as the Standard Missile 3 (or SM-3). This missile has a longer range than THAAD (over 500 kilometers) and max altitude of 160 kilometers. The Standard 3 is based on the Standard 2, and costs over three million dollars each. The Standard 3 has four stages. The first two stages boost the interceptor out of the atmosphere. The third stage fires twice to boost the interceptor farther beyond the earth's atmosphere. Prior to each motor firing, it takes a GPS reading to correct the course for approaching the target. The fourth stage is the nine kg (20 pound) LEAP kill vehicle, which uses infrared sensors to close on the target and ram it. In addition to Patriot, Aegis and THAAD there is GBI (ground-based interceptor), a system specifically designed for intercepting ICBM warheads and only stationed in North America.
THAAD and Patriot are often used in close proximity to each other and operated by the same country. Patriot entered service in 1984 but PAC-3 was not available until 1995, with updates in 1996 and 2000. Any Patriot battery can be upgraded to handle PAC-3 by adding some software and hardware upgrades. The Patriot launcher can carry and launch four canisters each with one PAC-2 anti-aircraft missile or four of the smaller PAC-3 missiles.