An ICBM begins its flight under rocket power, accelerating to its final speed within several minutes during its boost phase. For early-technology liquid fuel ICBMs such as those that might be developed by a rogue state, the boost phase lasts about five minutes. Acceleration begins at about 4 Gs and increases to about 30 Gs at final-stage burnout.
During the boost phase, the fiery rocket plume makes the missile easy for infrared sensors to identify and track. As well, unlike in other phases, there is a single, large target for the defense to intercept. These qualities make boost phase defense, at base, technically simpler than other types of defense.
While boost phase intercept has certain advantages, it also has some drawbacks. Chief among these is the short timeline for intercept. Since the boost phase lasts only a few minutes, decisions regarding intercept must be made very quickly. Certainly, there would not be enough time for high level officials to be involved in the command/control structure. This means that a very sophisticated automated command/control system would be required.
Concerns about the final fate of the missile are most significant for boost phase defense. While defenses at other stages target the warhead directly, boost phase defenses target the booster rocket, making survival of the warhead highly likely. One can make a good estimate of how short the warhead will land by simply assuming the rocket thrust is terminated early. For example, with a liquid-fuel missile of 8000 km range, thrust termination ten seconds early would result in a shortfall of roughly 5000 km Further details depend on the specific defense technology.
One countermeasure common to all boost phase systems is available. Fast-burning solid-fuel boosters would shorten the boost phase. This countermeasure is of considerable technical difficultly and would likely not be available to states of concern. Specifically, North Korea’s ICBM program is based on liquid fueled Scud technology and would have to be severely altered to pursue solid fuel systems.
Several boost-phase defenses are possible. Some are based on existing TMD programs, while others are at a more primitive stage. The links below give detailed technical descriptions of these systems.
Sea Based Defense Derived From Existing Systems
Defense Based on the Airborne Laser TMD System
Space Based Interceptor (Brilliant Pebbles)
Space Based Laser
Other Recent Boost-Phase Proposals (Garwin/Postol System)