In general, ballistic missiles share a common, fundamental element -- they follow a ballistic trajectory that includes three phases. These phases are the boost phase, the mid-course phase, and the terminal phase. Traditionally, missile defenses -- like the former U.S. Safeguard system, the Russian Moscow ABM (Anti-Ballistic Missile) system, and today's Patriot system -- have operated in the terminal phase. The terminal phase of a ballistic missile's flight is normally less than one minute long, depending on the threat range.
Therefore, defensive systems must be very close to the missile's target in order to defend against the attack, and only a small area can be defended. Countermeasures are less of a challenge in this phase. Defensive systems designed for the terminal phase are most effective in protecting smaller target areas such as fixed installations, posts, and airfields, or troop concentrations and staging areas.
In the terminal layer, the atmosphere helps the defense discriminate because atmospheric drag would decelerate heavy RVs less than their accompanying lighter penetration aids. The key technical challenges for endoatmospheric interceptors are accommodating the severe heating caused by friction with the atmosphere and achieving a high degree of maneuverability. Terminal defense could benefit from the easier discrimination of RVs from decoys by atmospheric slowdown, but only, at the expense of requiring a more complicated interceptor that could withstand the heating and mechanical stress caused by operating in the upper atmosphere. Midcourse interceptors are inherently simpler and could be used much more flexibly throughout the long midcourse portion of the RVs' flight trajectory. However, the defense must have confidence in its ability to discriminate RVs in midcourse in the expected threat environment.
Ideally, the seekers discriminate between warheads and decoys in the threat cloud, and destroy the warheads. Such target discrimination is not trivial, and presents significant technical challenges for hit-to-kill missile defense systems. Each object in a threat cloud must be effectively discriminated from the actual warhead-bearing reentry vehicles. Further complicating this task are the high altitudes (e.g., 50 kilometers or more) at which the discrimination takes place. The bulk of this discrimination task is carried out by the ground-based radars using a number of discrimination techniques, one of which is called stripping, thereby enabling the seekers.
In more detail, stripping relies on atmospheric drag to separate objects based on their ballistic coefficient .beta.=M/C.sub.d A, where M is the object mass, C.sub.d is the coefficient of drag, and A is the object area. Stripping is dependent on trajectory and object dynamics (spin and precession). Due to the coarseness of ground-based radar measurements, a significant atmospheric drag is required (altitudes of 50 to 70 km) to produce a measurable amount of relative velocity or separation. At extremely high altitudes (e.g., 100 kilometers or more), such atmospheric drag is substantially reduced or otherwise lacking, thereby impeding early discrimination. The density of objects and the deployment of chaff complicates and delays extraction of target dynamics and subsequent discrimination. In addition, the poor signal quality (e.g., low SNR) can contribute to late discrimination. Early discrimination is desirable for enabling a shoot-look-shoot engagement, appropriate interceptor commitment to threat load, and increased range of destroyed warhead from the targeted or otherwise defended assets.
Using all discrimination means available, the ground-based radar hands over assumed target position information to the seeker/kill vehicle. Typical seekers use a single color, passive IR sensor. Such seekers have less than optimal performance in situations where a potential target is in close proximity to other objects, (e.g., decoys and booster debris), which is typical of a threat cloud. Furthermore, additional error occurs because of the viewing ambiguity associated with a two dimensional (2D) IR seeker. Moreover, the residual effects of prior interceptions, such as the bright light called "flash," limit the effectiveness of the system. In particular, flash temporarily forms an alternate light source, thereby blinding the IR sensor. A laddering effect results, where a next incoming reentry vehicle after an intercept can approach even closer to the defended target due to the flash recovery time required by the interceptor's IR sensors.
In short, systems employing ground-based radar in conjunction with 2D IR seeker configurations have difficulty discriminating between legitimate targets and clutter, particularly at exo-atmospheric altitudes. Incorrect target discrimination, during high altitude stressing conditions, will substantially reduce the effectiveness of hit-to-kill defensive weapon systems.
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