The mid-course phase -- where intercepts take place in space (not inside the earth's atmosphere) -- allows the largest opportunity to intercept an incoming missile. At this point the missile has stopped thrusting, so it follows a more predictable path. Since the interceptor has longer to engage, fewer interceptor sites are needed to defend larger areas. Unfortunately, a longer period in space provides an attacker the opportunity to deploy countermeasures against a defensive system. However, the defensive system also has more time to observe and discriminate countermeasures from the warhead.
An intercontinental ballistic missile should reach a velocity on the order of 6 to 7 km/s at burnout depending on the distance of its target from the launch site. After burnout, the target starts decelerating at an approximate height of 250 km until it reaches an apogee of approximately 1,600 km. Later, it starts descending and accelerating due to gravity. Missiles with ranges under 300 km remain in the atmosphere for their entire trajectory (and travel slower than longer range missiles), thus reducing the abruptness of the reentry transition. Such a 300-km range Scud missiles reach speeds of more than 1,500 m/see and altitudes around 30 km before burnout.
A common approach is to detect the target by using space-based IR sensors, which can sense the huge amount of energy emitted from the rocket plume. After detection, a target track is established by ground-based RF sensors (radars) in coordination with IR sensors to generate useful and accurate target position data to guide an interceptor. Since an intercontinental ballistic missile should travel roughly 10,000 km and fly 30-40 minutes, it makes perfect sense to look for the capability to intercept the ICBM for the midcourse or terminal phase before it hits the target.
A midcourse seeker must detect relatively small, cold warheads moving above the atmosphere and distinguish them from decoys that might be deployed to fool missile defenses. A partial answer to the problem of discriminating between warheads and decoys before reentry could be to design an ABM nuclear warhead to be effective, exo-atmospherically, against whole swarms of incoming targets, say by making its yield large enough to kill ICBMs at great distances.
Although mid-course intercept of ballistic missiles has several advantages, such as the ability to locate assets at home and adequate time for detection, decision and interception, it has more drawbacks. The disadvantages can be summarized as being more susceptible to electronic attack, probability of debris landing in friendly territory if the warhead is not completely destroyed and possibility of the defense system being overwhelmed by utilization of submunitions instead of a single warhead.
The major technical challenge in the midcourse layer is to develop a capability to discriminate RVs from accompanying decoys or other penetration aids. For example, using sensors in space to observe the operation of a PBV as it starts to release its payload could permit early identification of RVs among the clouds of decoys. This early identification in turn, could mitigate the problems associated with tracking and intercepting RVs from either space or the surface.
The MIT Countermeasures Report emphasized that there is no reason to believe that a country capable of building and launching a ballistic missile can also exploit an electronic attack. An electronic attack might very likely affect, overwhelm or fail the planned NMD system. An electronic attack may include submunitions, false targets including replica decoys, decoys using signature diversity, and decoys using anti-simulation (metallized balloons, shrouds, chaff, electronic decoys), radar signature reduction, infrared stealth, hiding the warhead, and maneuver.
Historical lessons learned show that the attacker does not need to possess the sophisticated technology as the defender to defeat the defense system. During the 1991 Gulf War, probably unintentional breakup and tumbling of al-Husayn missiles resulted in the almost total failure of Patriot defenses. All solutions associated with the post-boost-phase defense must consider a common fact that when the acceleration of the missile ends, the possibility is great for the deployment of different electronic attacks. For long-range ballistic missiles, each deployed particle from the main payload follows the same trajectory regardless of its mass. Thus, a small chaff dipole weighing on the order of grams is not different from a heavy warhead in outer space where atmospheric effects can be neglected.
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