Ballistic Missile Defense Intercept Technology
For purposes of this report, ballistic missile defense intercept can occur in three phases of flight: boost phase, midcourse phase, and terminal phase. This terminology is defined below:
“Boost-phase intercept” (BPI) will be used exclusively for intercept of the threat missile prior to the end of powered flight of the main stages of the missile. Intercept during this phase is noteworthy because, if successful, the missile’s payload cannot reach its intended target. Whether the payload itself survives boost-phase intercept depends on where on the target missile the intercept occurs. The degree of payload shortfall depends on when during the target missile’s boost phase the intercept occurs. The main challenge associated with boost-phase intercept is the short time associated with powered flight, typically between 60 and 300 seconds depending on the missile’s range and propellant type.
“Midcourse intercept” refers to exoatmospheric intercept after threat booster burnout. During this phase, all objects follow ballistic trajectories under the sole influence of Earth’s gravitational field. The midcourse phase is noteworthy because it is the longest phase of a missile’s flight (for those missiles that leave the atmosphere), thereby providing more time for observing and reacting to the threat. However, it is also the phase where decoys may be most effective because all objects follow ballistic trajectories regardless of their mass. The terms “ascent phase intercept” and “early intercept” are redundant because they refer to intercept after the end of the boost phase of flight but prior to apogee, which makes them part of midcourse intercept. Intercepting threat missiles as early as possible during the midcourse phase increases battle space and defends large footprints from a single forward site, thereby adding shot opportunities that use interceptors more efficiently.
“Terminal defense intercept” refers to endoatmospheric intercept after the midcourse defense opportunity. The presence of substantial dynamic forces make this phase unique as far as ballistic missile defense is concerned because light objects such as decoys, which slow down faster due to atmospheric drag, follow substantially different trajectories than heavy objects such as reentry vehicles. The altitude at which the transition from midcourse to terminal defense occurs is somewhat ambiguous, with light decoys being slowed appreciably relative to reentry vehicles at altitudes between 70 and 100 km and appreciable aerodynamic forces on the reentry vehicle occuring at altitudes below approximately 40 km.
NOTE: Postboost, predeployment intercept (PBDI) refers to intercept of a missile’s postboost vehicle (PBV) or payload deployment module, if any, after the main rocket engines burn out and prior to the complete deployment of multiple objects contained in the missile’s payload (reentry vehicles, decoys, and other countermeasures). This distinction is important because intercepts during the PBDI phase potentially eliminate some objects depending on how early in the PBDI phase the intercept occurs, PBVs are more easily detected and tracked, and PBVs may undergo lower power maneuvers as they deploy their multiple objects. The duration of the PBDI phase depends on PBV design and mission. However, it can be very or vanishingly short as noted in a recent Defense Science Board report entitled Science and Technology Issues of Early Intercept Ballistic Missile Defense Feasibility (September 2011).