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.
