Missile Defence, Deterrence and Arms Control - UNIDIR

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threshold strike for which a midcourse system could be effective is
estimated to be in the order of tens of missiles, or roughly China’s
current ICBM arsenal. But even with an 80 per cent attrition
rate—generous according to former Defense Secretary William Perry—a
midcourse defence could become oversaturated at even lower
thresholds.
Fourth, a fixed ground-based midcourse system would leave key
components such as the X-band radars vulnerable to attack. A ballistic
missile attack from any potential aggressor would most likely be
preceded by some form of conventional strike against the system’s key
detection and tracking components, rendering it impotent against the
impending ballistic missile launch. Anecdotally, America’s early warning
for an impending missile attack would be when a rain of missiles fell at
Thule and Flyingdales.
Finally, a midcourse ground-based system would only suffice to
protect the United States homeland. The architecture for such a system
would be a poor defence in theatres beyond the United States homeland
since ballistic missile flight times in those theatres would be short and the
interceptors quite distant from the point of attack. Although a midcourse
system would only protect the United States homeland, it could do so
regardless of where the missile strike originated—it is not threat specific.
Hence Russian and Chinese concern that any “limited” midcourse
national missile defence, once the technology had been mastered, could
easily be upgraded to defend against their existing arsenals. Clearly, once
the United States is capable of deploying 100 interceptors to effectively
intercept ballistic missiles in the midcourse phase, there is little to stop
Washington from deploying 1,000 interceptors to defend against a
Russian strike.
Boost Phase Interception
Because of the technical hurdles to targeting a ballistic missile in the
midcourse of its trajectory, many experts are moving towards the
consensus that pursuing a ballistic missile defence for the boost phase is
the most feasible option. Two types of boost phase defences have been
suggested: sea-based platforms, most likely on Aegis cruisers, and
airborne laser systems either on manned or unmanned aerial vehicles.