Note: allow me to qualify my background and expertise to speak on this subject. I spent 15 years in Naval Intelligence during the cold war year under the Regan, Bush, and Clinton administrations. I spent considerable time studying, analyzing, teaching, and drafting in-depth reports about weapons and weapons related technologies. I left the service in late 1994, but I continue to follow weapons trends and technologies. It's no secret that the developers of the proposed MDS have encountered major problems in its design and functionality. The guidance scheme seems to be the main sticking point. So lets take a look...
The best time to intercept a Intercontinental Ballistic Missile (ICBM) is while it is still high in the Earth's atmosphere, before it can release its Multiple Independently Retargetable reentry Vehicles (MIRV), in which the actual nuclear weapon resides. The reasons for wanting to intercept the missile in this stage are as follows:
*1. A solid ICBM is a much larger and makes for a more temping target than a MIRV.
*2. A single Russian ICBM or SLBM can carry upwards to 12 MIRV's, thus increasing exponentially the number of targets that must be tracked and destroyed.
*3. A MIRV by its design is small and hard for radar to pick up and track.
However, the range of the missiles used to intercept the incoming ICBM's is limited by a number of factors, chief among them, range, so I believe the developers of the MDS have decided to concentrate their efforts on tracking and destroying the incoming MIRV's. Tracking to MIRV's as they arch down towards their targets is hard enough, but if we factor in their speed (in excess of Mach 1), the challenge of destroying one let alone hundreds become a daunting task.
I believe Planner Array Pulse Doppler radar is being employed to track the incoming MIRV's. Once the target is acquired and the interceptor missile deployed, it needs to guided to its target. There are two methods practical for doing so: infrared and semi-active homing, or guidance. Infrared works by homing in on the heat signature of the incoming missile. A sensor is placed in the interceptor missiles nose, and once it is guided to the general vicinity of the incoming target, the heat sensor takes over guiding the missile the rest of the way in. With infrared guidance intercepting the incoming ICBM before they release their MIRV's would be the guidance scheme of choice. Once the MIRV's are released however, infrared for all practical purposes cannot be used, because MIRV's are un-powered; they free fall to the earth on a preplanned trajectory. So once the MIRV's have been released semi-active guidance has to be employed.
