North Korea’s recent series of missile tests have wrought on many questions about ballistic missile defense. This article provides a detailed overview of all deployed American ballistic missile defense systems, including their components, test history, and planned upgrades.

But first, an explanation of some important terms. Ballistic missiles are similar to the rockets which launch astronauts and satellites, except instead of placing a payload into orbit, they carry a warhead which falls back to earth at extreme speed. Ballistic missiles can travel very long distances in a short period of time, making them well-suited for delivering nuclear weapons. Unlike airplanes or cruise missiles, which are powered by engines for the vast majority of their flight, ballistic missiles use rocket motors during their boost phase only and rely on that energy to reach their target. Some more complex ballistic missiles feature warheads that separate from the propulsion stack, reducing drag during descent.

Ballistic missiles (or just their warheads, if separating) reach extreme speeds, from 3,000 to 15,000 miles per hour, as they fall back to earth. This makes them extremely difficult to track and destroy once they have begun their descent. The only currently viable approach is to obliterate the incoming warhead with another highly maneuverable missile, referred to as an interceptor. Some interceptors use explosives that detonate close to the target, while others slam directly into the target and destroy it using kinetic energy (a technique known as hit-to-kill). Alternative systems which use lasers or high-velocity guns are being researched but will not be covered here since they are not deployed.

The following table contains important specifications for American-made ballistic missile defense systems. In-depth explanations follow the table.

Patriot

The Raytheon-manufactured Patriot missile system is the oldest, most thoroughly tested, and most widely deployed component of American ballistic missile defense. It was originally fielded in the 1980s as an anti-aircraft system but was later upgraded to perform ballistic missile defense as well. In this role, Patriot has a very short range of about 12 miles. As such, it is used for point defense of cities, airfields, bases, etc. Patriot is optimized against tactical and short-range ballistic missiles, but future upgrades will enhance its capabilities against medium-range missiles as well.

Note that Patriot is not used for defense of the US homeland since it can only intercept missiles with relatively low ranges; any nuclear ballistic missile attack on the US would be launched with either intermediate range submarine-launched missiles or intercontinental missiles.

However, Patriot’s small size and relatively low complexity come with benefits — compared to other missile defense systems, Patriot is cheaper, simpler, very mobile, and can defend against aircraft. That’s why Patriot is operated by a diverse group of countries, while other ballistic missile defense systems are used exclusively by America and a select few wealthy allies.

Each Patriot battery consists of a number of discrete components, including command and control vehicles, communications vehicles, power generators, a trailer-based radar, and a number of trailer-based missile launchers. A typical Patriot battery has eight launchers but can handle a maximum of 16. The launchers and radar must be emplaced and connected to the other equipment prior to operation. Two types of passive electronically scanned array radars are used with the Patriot system. The AN/MPQ-53 is the original radar, while the upgraded AN/MPQ-65 offers higher performance against small, fast-moving targets.

The most recent versions of Patriot use both PAC-2 and PAC-3 interceptors. The larger PAC-2 has an explosive warhead and a longer range against airbreathing targets such as airplanes. Its warhead is proximity-fused (explodes near the target) and has a number of small tungsten penetration rods used to increase lethality. The small, highly maneuverable, hit-to-kill PAC-3 is optimized for ballistic missile interception and has no explosive warhead. Each Patriot launcher carries a maximum of either four PAC-2 missiles or 16 PAC-3 missiles (but not a combination of the two).

Patriot has seen combat in various settings and has been extensively tested. During the Gulf War, Patriot batteries performed relatively poorly against Scud-type ballistic missiles — this experience lead to the development of enhanced PAC-2 variants and ultimately the PAC-3. More recently, Patriot was used in combat by the Saudis, where it achieved a success rate of 100% against ballistic missiles according to Raytheon.

To enhance Patriot’s range and lethality, especially against medium-range ballistic missiles, a new upgrade called PAC-3 Missile Segment Enhancement (MSE) is currently being evaluated — this version has been successfully tested four times. Raytheon is also developing a new active electronically scanned array gallium nitride radar to improve performance and reliability while providing 360-degree coverage.

Terminal High-Altitude Area Defense (THAAD)

THAAD is a long-range, roadmobile, hit-to-kill ballistic missile defense system built by Lockheed Martin. Like Patriot, THAAD’s components are mounted to a series of trailers and trucks. Unlike Patriot, THAAD was designed from the ground up as a high-performance ballistic missile defense system. As a result, its range is much greater (around 120 mi versus 12 mi for Patriot) and it can intercept intermediate-range ballistic missiles. THAAD’s interceptors are powerful enough to reach the edge of space, allowing them to engage both endo-atmospheric (in atmosphere) and exo-atmospheric (in space) threats. Destroying ballistic missiles at high altitudes has a number of benefits, such as reducing the likelihood that the warhead’s contents will fall back to earth intact and allowing for more for follow-up should the first interceptors fail.

The passive electronically scanned array AN/TPY-2 radar used with THAAD is very powerful and can track targets hundreds of kilometers away, feeding the information to other missile defense systems. In addition to performing terminal detection as part of a THAAD battery, the AN/TPY-2 is also operated as a forward-based early warning radar by a number of US allies. The AN/TPY-2 can be used to cue the Patriot’s shorter-range AN/MPQ-53/65 set.

Of course, THAAD’s performance comes at a price — relative to Patriot, it is more expensive and less mobile. THAAD is also unable to destroy aircraft and cruise missiles. Generally, the two systems are deployed in conjunction, with Patriot providing point defense and anti-aircraft capabilities while THAAD defends against high-performance ballistic missiles and provides broader coverage.

The main components of a THAAD battery are the launchers, one AN/TPY-2 radar, and two tactical operations centers. Various support units such as power generators and refrigeration units are also required, given the AN/TPY-2’s formidable power consumption and heat generation. Six launchers, each with eight interceptors, is the standard battery configuration, but up to nine launchers can be integrated.

While THAAD has not yet seen combat, the system has performed well in testing, with a perfect record of 14/14. Five batteries have been activated so far, with plans for more. One battery is deployed in South Korea and another permanently in Guam.

Aegis Ballistic Missile Defense (Aegis BMD)

While the Patriot and THAAD are mobile land-based systems, Aegis BMD is used primarily by warships. It is an upgrade to the base Aegis Combat System, which equips all American Arleigh Burke-class destroyers and Ticonderoga-class cruisers. As of early 2017, 34 American cruisers and destroyers use Aegis BMD, while the Japanese Maritime Self-Defense Force (JMSDF) has six Aegis BMD destroyers. In addition to the steady stream of older ships being upgraded to the Aegis BMD standard, a large number of newly-built Aegis BMD destroyers will be entering the fleet in the coming years.

The system is centered around the AN/SPY-1 radar and the Mk 41 Vertical Launching System (VLS), which houses the missile interceptors. Power generation, refrigeration, communications links, and computing power are all organic to the ship. There is also a program called “Aegis Ashore,” which transplants Aegis BMD equipment to a static land installation.

Three missile interceptors are used with Aegis BMD: the RIM-161 Standard Missile-3 (SM-3), the RIM-156 Standard Missile-2 (SM-2), and the RIM-174 Standard Missile-6 (SM-6). The Standard Missile-3, an exo-atmospheric interceptor designed to intercept medium and intermediate range ballistic missiles, is the largest and most expensive of the bunch. The SM-3’s range and altitude ceiling are far superior to THAAD, but the SM-3 is not capable of performing endo-atmospheric intercepts because it uses an exo-atmospheric separating kill vehicle.

That task is left to the SM-2 Block IV and SM-6 Dual I+. The SM-2 Block IV is a modification of the common SM-2 naval air defense missile, adding enhanced range and guidance capabilities to help defeat ballistic threats. The SM-2 Block IV was canceled after a limited run and replaced by the SM-6, which keeps the extended-range airframe but adds the active radar seeker from the AIM-120 AMRAAM air-to-air missile. This allows the SM-6 to engage targets without relying on the launching ship’s illumination radars, which have limited range, for guidance. Only SM-6 variants Dual I and newer (Dual I+) are equipped to intercept ballistic missiles.

As of 2017, US has a stockpile of 213 SM-3 missiles, 72 SM-2 Block IV missiles, and an unknown number of SM-6 Dual I+ missiles. Over time, the SM-3 and SM-6 stockpiles will increase substantially as new missiles are purchased.

Relative to THAAD and Patriot, Aegis BMD warships have the advantage of being able to inconspicuously loiter offshore without the need for a large patch of land from which to operate. Given the politically charged nature of land-based American missile defense deployments in foreign countries, this capability is highly valuable. The SM-3 also has a much longer range than THAAD’s interceptors, hence the decision to use Aegis Ashore instead of THAAD for European missile defense. However, Aegis Ashore sites are not mobile, so their construction is a more substantial commitment than a THAAD deployment.

As far as reliability, the SM-3 has succeeded in 29/36 intercept tests, the SM-2 Block IV in 4/4 intercept tests, and the SM-6 Dual I in 2/2 intercept tests. The SM-3’s relatively low success rate reflects the inherent complexity of the separating kill vehicle configuration.

Aegis BMD is being aggressively upgraded to meet future threats. The new Air and Missile Defense Radar (AMDR), officially designated AN/SPY-6, will equip all Flight III Arleigh Burke-class destroyers and offer a 30x increase in sensitivity and a 15 dB increase in power. The AN/SPY-6 is an active electronically scanned array radar, allowing it to also provide illumination, jamming, and potentially even a communications. The interceptors are under ongoing development as well; the newest SM-3 variant, Block IIA, achieves sizeable performance gains and may even be capable of intercepting intercontinental ballistic missiles, a capability which would have massive strategic implications.

Ground-based Midcourse Defense (GMD)

The most ambitious and controversial element of American ballistic missile defense is GMD. Unlike the other systems, GMD has the explicit goal of defending the American homeland against a strategic ballistic missile attack. If North Korea or Iran were to launch a nuclear-armed intercontinental missile at the American homeland, GMD would be the system tasked with destroying it (although that may change as the SM-3 is further improved).

The GMD system uses the Ground-Based Interceptor (GBI), a large hit-to-kill missile housed in a silo. GBI has a multi-stage booster with a separating kill vehicle that is released once the missile reaches space. Boeing manufactures the airframe and Raytheon produces the current kill vehicle. Because intercontinental missiles travel very quickly, even relative to other ballistic missiles, the kill vehicle needs to be exceedingly accurate. The inherent difficulties of the task combined with an ambitious schedule have lead to repeated developmental problems and test failures (GMD has only succeeded in 10/18 intercepts.) Fortunately for the Missile Defense Agency, GMD’s most recent test — the first ever attempted intercept of an intercontinental ballistic missile — ended in success.

GMD does not have its own radar per se. Instead, it uses information from any sensors available, including the AN/TPY-2, AN/SPY-1, early warning radars such as PAVE PAWS, and satellites. Lockheed Martin is currently developing the Long Range Discrimination Radar (LRDR), an actively scanned gallium nitrite radar which is designed for ballistic missile tracking and will feed precise targeting data to GMD. One main goal of LRDR is to help discriminate between actual warheads and decoys. It will also aid in discerning whether the interceptors have successfully destroyed the target or not, an important task given GMD’s relatively low per-interceptor success rate.

There are currently 36 GBIs at two bases: Fort Greely in Alaska and Vandenberg Air Force Base in California. The interceptors are controlled by command centers at Fort Greely and in Colorado Springs.

GMD’s lacking performance is being addressed by two separate programs. The Redesigned Kill Vehicle (RKV) program has been established to improve kill vehicle reliability and performance while hopefully reducing complexity and cost. The RKV is being co-developed by engineers from the Missile Defense Agency and America’s three largest defense companies: Lockheed Martin, Boeing, and Raytheon. In the long run, the MDA hopes to replace the RKV with the Multi-object Kill Vehicle (MKV). This payload would feature multiple kill vehicles, each of which would be independently targeted. This would drastically increase efficiency, as one GBI airframe could be used to defeat multiple warheads.

The future:

All the aforementioned missile defense systems have the disadvantage of relying on expensive interceptors that are destroyed with each interception. Going forward, the US military is investing heavily in technologies such as lasers and hypervelocity projectiles, which would drastically reduce costs per interception and increase magazine capacity. However, these technologies are still developmental and have some issues, especially concerning power storage and barrel wear (in the railgun’s case). Until those engineering challenges can be overcome, the interceptor-based systems described above will remain America’s best solution to the ballistic missile defense challenge.