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Terminal Phase Defense

Terminal phase is the last one or two minutes of ICBM flight. The warhead, along with any decoys or chaff, reenters the atmosphere. Aerodynamic drag then produces dramatically differing behavior for light as opposed to heavy objects. Decoys decelerate significantly and may burn up, but the warhead does neither. Thus at re-entry the defense can discriminate the warhead unambiguously.

On the other side of the coin, terminal defense presents severe challenges resulting from the very high speed of the offensive warhead and the very short time in which terminal defense can operate.

The most likely terminal phase defenses are either one based on the Exoatmospheric/Endoatmospheric Interceptor (E2I) or a version of the ground-based mid-course defense redeployed as an early terminal-phase system.

The E2I program was terminated in 1993, though much of it was migrated to the THAAD theatre defense system. It was the successor of the High Endoatmospheric Interceptor (HEDI) program and competed unsuccessfully against the Ground Based Interceptor (GBI) as the interceptor for the NMD system. Chief among its technical challenges was developing an infrared sensor system that would work in the extremely hot environment of a rocket traveling at very high speed inside the atmosphere.

Deploying the ground-based mid-course NMD (currently under development) as a terminal-phase system would involve other challenges. Assuming that the currently used sensor suite, which is not designed for endoatmospheric operation, would be retained, the intercept would have to occur very high in the atmosphere. However, there would also be an upper limit on the intercept altitude in order for atmospheric filtering to be effective. This would constrain the intercept to occur roughly 150-250 km above the earth. At this altitude only one X-band radar would be able to view the threat complex, and hence the accuracy to which the radar could locate both the warhead and the decoys would not be particularly good. It would be difficult to make the target map passed to the interceptor sufficiently accurate to allow a successful intercept. It remains to be studied whether the X-band radar in concert with the SBIRS-Low system would be sufficient for that task.

Several countermeasures are available to combat a terminal-phase defense:

The administration's FY02 Budget Estimate devotes $988 million to terminal defenses, with the majority of the funds going to THAAD.

Maintained by Michael Levi