Defense against missile attack has been important for centuries. In the distant past, fortified structures were used to protect against missiles such as projectiles from catapults. With the advent of projectile-firing cannon, fortified structures became less useful, and the inadequacy of fortifications was exacerbated by the introduction of bomb-carrying aircraft.
More recently, rocket-propelled missiles have become very important, because of their ability to quickly transport extremely destructive payloads to distant locations. The payloads that are now of importance include nuclear, chemical, and biological weapons, known generally as weapons of mass destruction (WMD). These payloads when carried by rocket-propelled missiles are potentially so destructive that a great deal of attention has been directed toward attempts to neutralize the threat.
As a response to the perceived threat to the Unites States of inter-continental ballistic missiles (ICBMs) launched from distant locations and carrying WMD, programs have been instituted to investigate and produce ballistic missile defense systems. Ballistic missiles have an extremely limited time between launch and impact, so defense systems must very quickly identify and destroy the threat.
The ballistic missile goes through several distinct phases during its operation. The first phase is launch, in which a rocket engine lifts the missile and propels it upward. The missile is relatively vulnerable at this stage, but there are substantial difficulties in identifying it at this stage. For example, the launch is liable to be in a hostile territory. Furthermore, while the launch may produce a heat (infrared) and light signature that would be identifiable if viewable, there may not be a line-of-sight between sensors and the missile launch that might identify the situation. Spacecraft may be able to view the region, but the communications between the spacecraft and defense systems tend not to provide long warning times of missile launch. Moreover, sensor systems such as space based sensors which detect heat or light, can only track the missile while those signature characteristics are present and detectable (i.e. during launch and/or boost phases).
Following launch, the rocket-propelled missile passes through a boost stage, in which the rocket engine propels the missile through a principal portion of the atmosphere. This phase also produces a heat signature. Since the missile is at a significant altitude in this phase, it may be observable by land-based, air-based, or space-based infrared sensors. The missile may also be observable on land-based radar systems. Thus, a missile may be identifiable when in its boost phase. At some time, the rocket engine stops operating, so boost thrust goes to zero. Following the termination of thrust, the missile enters a mid-course phase, in which the missile proceeds along a ballistic trajectory to its apogee, carried by its own inertia. The missile in its ballistic mode proceeds toward its target. In the mid-course phase, the heat signature is much reduced. Therefore, infrared sensors become less effective; however, the missile may be clearly viewed by radar.
At some point after the apogee of the missile's path, as the missile approaches its target, it begins to re-enter dense portions of the atmosphere. This re-entry may be at a location essentially above the target. Destruction of the missile during the re-entry phase may still result in damage to the target, since the payload weapon may still be effective and active. Despite the missile being damaged and kept from properly functioning, the constituent parts may still be very harmful to the target region.
Thus, it is desirable to identify and destroy missiles early in flight, to allow time for repeated tries at destruction and so that the destroyed missile falls short of its target, preferably in the hostile territory. In particular, early intercept (EI), which is a launch of a countermeasure during the threat's boost phase with intercept occurring after burnout but before the threat reaches its apogee, provides for increased forward engagement battlespace with potential shoot-look-shoot (SLS) opportunities. Early intercept also allows for interception of the threat before it deploys its payload and before the threat is able to deploy a countermeasure package.
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