THEATRE MISSILE DEFENCE

The United States is developing several theater missile defense (TMD) systems, which are intended to defend against shorter-range missiles than the national missile defense (NMD) system is. While NMD is designed to defend the entire United States, these theater systems are intended to defend smaller areas. They are designed to be mobile so that they can be deployed with troops or moved as needed to defend US allies. (See Table 2.)

Theater missile defenses fall into three categories: "lower-tier" (or "low-altitude") defenses, "upper-tier" (or "high-altitude") defenses, and "boost-phase" defenses.

Lower-Tier Defenses
As their name implies, lower-tier defenses are designed to intercept missiles low in the atmosphere (at altitudes less than approximately 20 kilometers). The interceptors must intercept their targets in the atmosphere because they maneuver to their target by using fins to steer through the air (in the same way that a sailor steers a boat through water by using a rudder). Lower-tier defenses have relatively slow-flying interceptors that cannot fly very far before intercepting their targets; therefore, lower-tier defenses can cover only relatively small areas. Lower-tier defenses are designed to intercept short-range ballistic missiles, with ranges of up to roughly 600 to 1,500 kilometers, depending on the system. In addition, these defenses are designed to shoot down aircraft and cruise missiles.

The United States has one lower-tier theater defense in operation:

Patriot PAC-2, a transportable, truck-mounted system designed to defend small areas against aircraft and ballistic missiles with ranges of up to about 600 kilometers. The interceptor uses a "blast fragmentation" warhead that is designed to explode once it gets within several meters of its target. The current version is an upgraded version of the Patriot Pac-2 defense that was used against Iraqi Scud missiles during the 1991 Gulf War but failed to destroy all but a few, if any, Scud warheads.

Lower-tier defenses currently under development by the United States are:

Patriot PAC-3, a transportable, truck-mounted system designed to defend small areas against ballistic missiles with ranges up to about 1,500 kilometers. Unlike the PAC-2, this system does not use an explosive warhead. Rather, PAC-3 uses a "hit-to-kill" interceptor (based on the earlier Erint missile), which is designed to destroy its target by hitting it directly.

Navy Area Defense, a ship-based system designed to defend small areas against aircraft, and ballistic missiles with ranges up to 600-1,000 kilometers. Like Patriot Pac-2, this system will use an explosive warhead.

Medium-range Extended Air Defense System (MEADS), a truck-mounted system designed to be more mobile than the Patriot systems and to be deployed with ground troops as they move in the field. By contrast, Patriot is designed to be operated from a single location for days at a time or longer. MEADS uses a hit-to-kill warhead and is designed to intercept ballistic missiles with ranges of up to perhaps 1,500 kilometers. MEADS is an international program under joint development with Germany and Italy.

Upper-Tier Defenses
Upper-tier defenses are designed to intercept missiles high in the atmosphere or above the atmosphere. This permits large ground areas to be covered. At the same time, upper-tier defenses are designed to intercept longer-range theater missiles, with ranges of up to 3,500 kilometers. Upper-tier defenses are also intended to be used as the first layer of a layered defense against short-range missiles, with the lower-tier theater defenses providing the second layer of defense.

The United States has two upper-tier defenses under development. Both use hit-to-kill interceptors that maneuver to their target by using small thrusters to change their trajectory. (These interceptors operate at high altitudes where there is not enough air to enable them to maneuver by using fins.) The interceptors use infrared sensors to detect the target and home on it. Such sensors, which detect heat, will be blinded if they are used at low altitudes where the air resistance causes heating of the fast-flying sensor. Thus, upper-tier interceptors have a minimum intercept altitude below which they cannot intercept a target. Because they are designed to intercept their targets high in or outside the atmosphere, these defenses--like the national missile defense system--are vulnerable to countermeasures that use lightweight decoys.

The two systems are:

THAAD (Theater High-Altitude Area Defense), a land-based system intended to defend large areas against missiles with ranges of up to 3,500 kilometers. THAAD is designed to be transportable by aircraft. It is designed to intercept missiles high in the atmosphere (at altitudes above about 40 kilometers) or above the atmosphere. The THAAD interceptor has a top speed of about 2.5 kilometers per second.

Navy Theater-Wide, a ship-based system intended to defend large areas against missiles with ranges of up to 3,500 kilometers. Navy Theater-Wide is designed to intercept only above the atmosphere; it will use the LEAP (lightweight exo-atmospheric projectile) kinetic kill vehicle, which cannot intercept at altitudes below about 80-100 kilometers. Thus, the system could not intercept short-range missiles with a range less than about 300-400 kilometers, since they never reach an altitude of 80-100 kilometers. Navy Theater Wide is intended to intercept targets in the middle of their trajectory (in mid-course) or--if the ship can position itself near the missile launch site--in the beginning of their trajectory shortly after the missile engine finishes burning (in ascent phase). The interceptor has a speed of about 4.5 kilometers per second. The system will be deployed on Aegis ships, which use the SPY radar system.

A second generation system--Navy Theater Wide Block II--is also planned for deployment after 2010. This system would use a faster interceptor and a more powerful radar.

Strategic Capability of Upper-Tier Defenses
Although upper-tier theater defenses are nominally designed to intercept theater missiles with ranges of up to only 3,500 kilometers, if such defenses can be made to work they will also have a capability against intercontinental-range ballistic missiles (ICBMs) with range up to 10,000 kilometers.

In fact, a 1998 Pentagon study titled "Report to Congress on Utility of Sea-Based Assets to National Missile Defense" (the unclassified summary of which was sent to Congress on August 10, 1999) concluded that integrating the planned Navy Theater Wide system into the planned ground-based NMD system would result in a far superior system. Doing so would add some additional 600 Navy Theater Wide interceptors to the ground-based NMD system.

The dual-capability of upper-tier theater defenses is a consequence of the fact that these defenses are designed to intercept their targets high in or outside the atmosphere, where the probability of making an intercept depends on the closing speed of the interceptor and the target (in this case the missile warhead). While a theater ballistic missile with a range of 3,500 kilometers has a reentry speed of roughly 5 kilometers per second, a ballistic missile with a much longer range of 10,000 kilometers has only a slightly higher reentry speed of roughly 7 kilometers per second (which is 40% greater than 5 kilometers per second). (There is a direct relationship between the range of a ballistic missile and its speed at the end of its flight, when it reenters the atmosphere. The longer the range of the missile, the greater its speed upon reentry.) If two targets have reentry speeds that differ by 40%, their closing speeds relative to an interceptor will differ by less than 40%. Thus, unless the capability of an upper-tier theater defense is already marginal against theater missiles with a range of 3,500 kilometers (which the defense is designed to intercept), it will have an inherent capability against ICBMs. On the other hand, if such defenses are not capable of intercepting an ICBM, then they would have no or very limited capability against long-range theater ballistic missiles.

Because upper-tier defenses have an inherent capability against ICBMs, their development and deployment is restricted by the original 1972 Anti-Ballistic Missile Treaty. In order to permit deployment of these systems to proceed legally, the United States sought to modify the treaty beginning in Fall 1993, and reached partial agreement with Russia in September 1997. Russia agreed to consider deployment of the US THAAD system legal under the treaty, but not the deployment of Navy Theater Wide. That issue remains unresolved .

However, the United States plans to go forward with deployment of Navy Theater Wide. The Pentagon argues that Navy Theater Wide would have no capability against strategic missiles, but this assertion is based on the limitations of the SPY radar that is carried on the Aegis cruisers on which Navy Theater-Wide would be deployed. If the interceptor is instead given information from more capable sensors, such as the ground-based radars that will be deployed as part of the NMD system or the SBIRS-low space-based missile tracking system, then there is little doubt that it would have strategic capability.

In fact, the SBIRS-low missile tracking system (described in the NMD section above) is intended for use with both NMD and TMD. In principle, this system could greatly increase the defended areas of upper-tier theater defenses, since their coverage is limited more by the system radars than the interceptors. In particular, it could allow an upper-tier theater defense--especially Navy Theater Wide--to become a wide-area, ICBM-capable system that could augment or serve as a national missile defense. These upper-tier systems would no longer be limited by the capabilities of their ground- or ship-based radars, which would otherwise be needed to track the missile targets and guide the interceptors near their targets. Not surprisingly, whether deployment of SBIRS-low is permitted under the Anti-Ballistic Missile Treaty is under contention between Russia and the United States.

Boost-Phase Defenses
In addition to the defenses discussed above, which are designed to intercept their targets in the middle or at the end of the missile's flight, the United States is also developing systems to intercept missiles during the early, powered part of flight when the rocket booster is burning. This part of the missile's trajectory is called the "boost phase," and such ballistic missile defenses are termed "boost-phase defenses." The advantage of boost phase defenses is that they are designed to destroy the missile before the warhead and any decoys are released, so there would be only one target to destroy rather than potentially dozens or hundreds. A boost phase defense would also be able to destroy submunitions before they were released. Deploying chemical or biological weapons on numerous submunitions would be the best way for an attacker to distribute the agents, and would simply overwhelm any mid-course and terminal defense system. The disadvantage of boost-phase defenses is that the boost phase lasts for only a few minutes, and the interceptor must be able to make its intercept close to the launch site.

The United States has one boost-phase theater defense under development:

Airborne Laser (ABL), designed to attack short- and medium-range missiles during their boost phase with a laser based on a. modified Boeing 747 airplane. The laser would be targeted on the missile, and if it shined on the same spot for long enough, it would weaken the metal surface by heating it to its structural-failure temperature--where the strength of the metal falls dramatically. For theater missiles, the airplane must be within several hundred kilometers of the missile the laser is attacking. It is generally assumed that the airplane would need to fly outside the borders of a country to avoid being shot down by air defenses; thus, this system is presumed to be incapable of attacking missiles launched from relatively large countries. The exception would be in a conflict in which the United States had already established air superiority. In principle, the airborne laser would also be capable of causing a long-range missile to fail but Russian and Chinese land-based missiles are not assumed to be vulnerable to the ABL since they are based far inland. However, the airborne laser could in principle threaten Russia's long-range submarine-launched ballistic missiles.

From: Union of Concerned Scientists Fact Sheet