The boost phase is defined from ballistic missile launch until it stops accelerating under its own power. It is the most forward-based defense. General Fogleman stated, "Developing the capability to destroy a ballistic missile in the boost phase is vital.TBMs are best targeted in the boost phase when they are large (intact missile with a very large infrared signature), vulnerable, and highly stressed targets." Intercepting a missile in boost phase is the "ideal" solution; a large area can be defended and negates most countermeasures. It is also the most technically challenging mission.
A ballistic missile trajectory from launch to impact goes through the boost phase, the mid-course and the terminal. The boost phase part of the trajectory is when the rocket is under powered flight and closest to its launch point. Then it goes ballistic, and coasts on its very high speed trajectory to an apogee in mid-course, and then a descent into the terminal phase to re-enter the atmosphere and hit the target. When the missile boosters are spent, the missile continues its ascent into the midcourse part of flight (which lasts nominally 20 minutes for a long-range missile). In this stage of flight, a ballistic missile releases its payload warhead(s), sub-munitions, and/or penetration aids it carried into space. The mid-course has ascent phase and a descent phase. It is like throwing a ball in the air; it will come down. The missile enters the terminal phase when the missile or the elements of its payload, for example, its warheads, reenter the atmosphere. This is a very short phase, lasting from a few minutes to less than a minute.
The boost phase is that part of flight when the ballistic missile's rocket motors are ignited and propel the entire missile system towards space. The boost phase is the portion of a missile's flight in which it is thrusting up through the atmosphere seeking the velocity needed to reach its target. The boost phase lasts for a relatively short time with respect to the overall flight time of the ballistic missile. It lasts roughly 3 to 5 minutes for a long-range missile and as little as 1 to 2 minutes for a short-range missile. The trajectory occurs all the way from a few hundred kilometers to many thousands of kilometers with intercontinental ballistic missiles. So there is not only a trajectory associated with every ballistic missile, there is a range associated with it.
There are opportunities and challenges to engage a threat missile in each of these phases. The layered defense, or defense-in-depth, approach will increase the chances that the missile and its payload will be destroyed. A defense that can destroy a ballistic missile in its boost phase can protect the world against a specific threat location, because in the boost phase it doesn't matter where it was going. Defense in the terminal phase protects a specific area or a point from any threat direction.
The US has pursued a multi-layered approach, consisting of lower tier, upper tier, and boost phase intercept systems. The lower tier systems provide point defense around specific areas to intercept missiles that leak through or cannot be engaged by the upper tier or boost phase systems. The upper tier and boost phase systems bring wide area coverage to the missile defense architecture by reaching out to engage enemy missiles early in flight even over their own territory. Together these systems provide early and repeated engagements to achieve the level of protection required by the warfighting commanders-in-chief. Missile defense architectures include a wide spectrum of capabilities, with contributions from air, sea, and land based systems.
Intercepting a missile in its boost phase is the ideal solution for a ballistic missile defense. For multiple ballistic missiles with multiple RVs, the region that potentially has the highest defense payoffs is the boost/postboost layer. Viable technical approaches exist for intercepting from space a ballistic missile during the boost portion of its flight. Inclusion of boost-layer defense would substantially discount the value of ballistic missiles with multiple independently targetable reentry vehicles (MIRV) and provide the threatening forces with incentives to accomplish the long-standing arms control objective of reducing MIRVed ICBMs.
The intercept of a missile in its boost phase has numerous benefits. The boosting missile, still under power from its rocket motor(s), is vulnerable due to its slower speed, large cross-section and still-attached fuel tanks. Also, if a missile is successfully attacked during the boost phase, it can be destroyed prior to release of any decoys and/or countermeasures. Finally, in the event of a successful intercept, the missile and its payload of weapons of mass destruction-nuclear, chemical or biological-may fall back on the country from which it was launched.
Intercepting a missile in the boost phase results in the defense of any target that the missile might be aimed at and can destroy a missile regardless of its design range. A midcourse intercept capability provides wide coverage of a region or regions, while a terminal defense protects a localized area. Intercepting a missile near its launch point is always preferable to intercepting that same missile closer to its target. Another advantage of the layered approach is that it complicates an adversary's plans. Countermeasures, for example, will always be a challenge for the defense. But because countermeasures have to be tailored to the specific phase of a missile's flight, layered defenses pose major challenges to an aggressor.
Intercepts in the boost phase also offer multiple engagement opportunities to ensure high levels of defense effectiveness. The synergism provided by layers of the defense significantly increases the task of designing and deploying effective offensive countermeasures. If missiles have fast-burn boosters to counter initial boost-layer defenses, the task of releasing decoys is more complicated, mitigating the requirement to design means of discrimination in the midcourse layer. Furthermore, follow-on defensive system concepts could block the fast-burn approach. Intercepts in the boost/postboost layer can also destroy the post-boost vehicle (PBV) before it releases decoys and other penetration aids designed to confuse the defenses, should such decoys and penetration aids be present.
Having the objective of intercepting the target during the boost phase introduces significant detection and decision time limitations on the defense system. The technical and operational challenges of intercepting ballistic missiles are unprecedented. Reliability will be realized, in part, through redundancy in the system. Effectiveness is partly a function of the number of opportunities the system provides to intercept an in-flight missile and how early and how often those opportunities occur in the missile's flight. Because of the need redundancy, whatever BMD systems are deployed should allow multiple engagement opportunities in the boost, midcourse, and terminal phases of a ballistic missile's flight.
Tracking and prediction of the future location of missile targets is at the forefront of antiballistic missile defense. Existing methods, such as Interactive Multiple Model (IMM) use multiple hypotheses, and use pattern recognition schemes which compare kinematic information about the target, such as time history of altitude, ground path, and flight angle versus time, with a template. This prior art requires large numbers of templates, as for each target there are altitude, flight path, ground path. The templates are based on the target taking a nominal path, but the target motors may burn long or short, thereby not matching the templates. The enemy may change any of the parameters of the flight path, such as lofting or depressing the flight path, so as to avoid matching a template. A template technique, to be effective in view of these possible changes, would require a very large number of templates, including a large number for each type of target. When there are very large numbers of templates, there will likely be an overlap between the targets, so that it becomes difficult to distinguish between a given target that is lofted and another target type that is depressed. For example, a target may have an engine which runs "hot" or "cold," and it may be directed along a flight path which is either depressed or lofted, thus requiring a very large number of templates to handle even one target. When the target has multiple stages, each stage may itself run hot or cold, and there are even more variations. The large number of variations makes the use of kinematic templates very complex. In addition, the large number of templates may result in overlap of the kinematic features among various different templates, thereby leading to indeterminate results. The potential inaccuracies are exacerbated by variations of the kinematics attributable to inaccurate time after lift-off (TALO).
A study on boost-phase defense commissioned by MDA [Battleson, Kirk, et al., Phase One Engineering Team (POET), Parameters Affecting Boost Phase Intercept System (February 2002)] focused on selected issues of high risk, including methods for early launch detection of missile launches, interceptor divert requirements, and discrimination of the missile's body from its luminous exhaust plume [Plume-to-hardbody handover]. The study concluded that there are no fundamental reasons why an interceptor cannot hit a boosting target with sufficient accuracy to kill the warhead. However, the study identified several challenges, including understanding the plume phenomenology well enough to have confidence in the Plume-to-hardbody handover.
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. In contrast, a boost-phase seeker has the seemingly simple task of homing in on a very hot, bright ballistic missile rocket. During the boost phase, however, a ballistic missile's signature comprises both the missile body itself and the large rocket plume. At high altitudes, the plume "blooms" around the missile -- in effect, creating a smoke screen of hot exhaust gas that, depending on the kill vehicle's angle of approach, can obscure the body of the rocket. A kill vehicle must be able to detect and hit the missile within the plume.
That so-called plume/hard-body problem could be solved in several ways, with current emphasis on the use of a multicolor seeker sensitive to two or more wavelength bands. Although both a missile and its plume are hot and bright, they have different spectral characteristics. If signal processors could operate in two or more wavelength bands, they might be able to subtract the plume's contribution to the image seen by the seeker from that of the missile, leaving behind only the missile's characteristics for targeting. However, a multicolor seeker suitable for use in boost-phase interceptors would be more difficult and costly to produce than a one-color seeker.
Beginning in 1989, research efforts to locate the hardbody center-of-mass used a low energy laser to actively illuminate the hardbody. A low-energy scan would originate at the estimated position of the target plume's intensity centroid, and continue along the target's estimated velocity vector to intercept the hardbody. The hardbody's dimensions would be apparent from the low-energy laser speckle return of the hardbody, upon which the location of the center-of-mass can be derived. However, plume phenomenology experiments in 1990 uncovered the existence of plume speckle reflectance emanating from the exhaust of a solid-propellant rocket motor due to the presence of metallic particulates in the plume. This impeded attempts, that are dependent upon the speckle return of an actively illuminated target hardbody, to discern the plume/hardbody interface.
The Doppler return frequency spectra of the plume and the hardbody respectively possess distinct properties and are differentiable from each other. The plume and missile hardbody have opposite velocity vectors which result in opposite Doppler shifts. In addition, the frequency spectrum bandwidth of the plume-induced Doppler return has been observed to be significantly broader than that of the hardbody-induced Doppler return spectrum, due to the diverse velocity orientations of the numerous particulates in the plume. Hence, these differences can be exploited in the attempt to discern the plume/hardbody interface.
An important aspect of missile defense is the multiband spectral characterization of plume radiation during the boost phase of a missile. This has led to study of the utility of a dual-mode ultraviolet (UV) and mid-wave infrared (MWIR) seeker. Combining the conventional MWIR sensor with shorter wavelengths provides increased information content for the image and can aid in optical target characterization. However, even dual-mode seekers have potential problems. Onboard optical seekers are subject to some vehicle self-interference. Sources of optical interference include outgassing of vehicle contaminants, and by-products of the vehicle plume and attitude control systems, especially if solid aluminized propellants are used.
Carbon particles are commonly present in the exhaust plume of kerosene liquid-oxygen (LOX) motors. Once formed, carbon may contribute a continuum-like feature to the optical radiation of a rocket exhaust plume, especially in the near-UV. A carbon monoxide-oxygen chemiluminescence mechanism may also be a source of radiation for the propellant because carbon dioxide is a large plume exhaust species and atomic oxygen is formed in the shear layer of the plume where the ambient oxygen molecules are dissociated. Such optical interference effects lead to an increased background radiation level for the seeker in all spectral bands, but are most problematical in the infrared. Sensor confusion may also be caused by deliberate countermeasures. Therefore, multi-spectral imaging is important for ground-based imagery for optical signature characterization and onboard seekers.