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Endo-Atmospheric

The missile defense battle space is divided between intercepts in the atmosphere, what is called endo-atmospheric, and out of the atmosphere, what is called exo-atmospheric. An endoatmospheric missile is one that remains within the earth's atmosphere, i.e., at an altitude below 100 kilometers. For endo-atmospheric missions, three desirable sensor characteristics are a wide field of regard that includes radiation detection when the missile is at a low angle of attack (i.e., forward-looking detection), uncooled optics, and detection within the MWIR spectrum.

At first the theory of ICBM interception called for destruction of the incoming missile as far away as possible, while it was still outside the atmosphere. Then the development of chaff and decoys in increasing numbers and complexity led to a need for waiting until the atmospheric drag on reentry provided a way to distinguish the real warhead from the false images. Endo-atmospheric interception thus became firm U.S. doctrine, absorbing an enormous investment in the development of terminal defense radars, exotic computers, new high-performance missiles, and solutions for the complex problems of reentry physics.

Current and planned ground-based ballistic missile defense systems rely on optical seekers with a hit-to-kill (HTK) strategy or blast fragmentation warhead to impact the incoming warhead. For this, the kinetic kill vehicle (KKV) must sense and track the target candidate and carry out the maneuvers required for the HTK intercept. For endo-atmospheric interceptors flying at hypersonic velocities, the aerodynamic heating loads will significantly increase temperatures on external surfaces, including optical windows. Further, thermal excitation of atmospheric species occurs in the flow field around the KKV. Given that the seekers operate in the infrared spectrum, emissions from hot optics and/or excited constituents in the sensor's field of regard could lead to sensor blinding in some regions of the spectrum.

The endoatmospheric vehicles depend on the ability of a passive infrared system operating within the Earth's atmosphere to provide the vehicle with adequate signal to noise in order to perform the homing and intercept of the incoming warhead. This system suffers from extensive IR seeker cooling requirements, and thus the performance will be degraded unless advanced cooling methods are developed. Two basic approaches to endoatmospheric non-nuclear destruction of an incoming missile or aircraft are 1) hit-to-kill by directly impacting the target with a large, heavy interceptor mass at high velocity, and 2) blast-fragmentation involving multiple impacts of small fragments at very high velocities and strike angles (from the interceptor's nose) resulting from the explosion of a high explosive warhead in the interceptor in the vicinity of the ballistic missile.

The hit-to-kill or kinetic energy technology approach is based on the fact that when one object strikes another object at high speeds, a tremendous amount of destructive energy is released. The impact of an interceptor missile with an incoming tactical ballistic missile, aircraft, or cruise missile, can result in the total disintegration of both vehicles. Such impact can literally vaporize even metals. In contrast, blast-fragmentation warheads may only redirect or break up the target vehicle. However, even with a large hit-to-kill interceptor, the effective impact window is relatively small.

Most known warheads have been designed for either intercepting targets with high relative closing velocities or low relative closing velocities. One embodiment provides a single warhead capable of generating fragment patterns for intercepting targets over a wide range of relative velocities, including velocities above and below 15,000-20,000 feet per second. A warhead having such a combination of features would be useful in endo-atmospheric ballistic missile interceptors with altitude requirements of 5,000-150,000 feet, and in space defense missile systems that intercept at co-orbital or anti-co-orbital velocities.

Ideally, 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.

Stripping can be made possible by providing a dual mode IR and LADAR seeker on a kill vehicle which analyses various target characteristics when the kill vehicle is within range. On board computers use these characteristics to discriminate a threat target from decoys and direct the kill vehicle to the target. First a dual mode IR and LADAR seeker is used to obtain precise target dynamic measurements of the objects. Atmospheric drag is then determined from these measurements to separate the objects into threat objects and decoy objects. After this determination, the track to the threat object is calculated and the kill vehicle is directed to intercept the identified threat.

The cluttered battle space includes booster debris, previous intercepts and raid attacks, results in a situation in which there are numerous closely spaced objects, CSOs. One needs to be able discriminate between targets which are removed from one another by as little as five meters. Incorrect target discrimination, during these stressing conditions, will substantially reduce the lethality of hit-to-kill defensive weapon systems. Using a dual mode seeker involving both IR and LADAR detection systems for precise target dynamics measurement, extends stripping to a much higher altitude than ground based radar. Initial results show that this approach works to altitudes in excess of 100 km. Stripping becomes a reliable way to discriminate lighter weight decoys from reentry vehicles at altitudes and ranges consistent with planned divert capabilities for terminal phase missile defense.




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