The launch of the missile is spotted by a space based early warning radar and its path is tracked by a network of space and ground based radars.
The Fylingdales Ballistic Missile Early Warning System (BMEWS) in North Yorkshire is one of the ground-based "phased-array" radars. Fylingdales will be able to track the missile in flight. The Early Warning Satellites are currently part of the "Defense Support Programme" but the US is developing a series of more high-tech satellites called the "Space-Based Infra-Red System" (SBIRS). Menwith Hill in North Yorkshire is one of the bases that would collect data from these new satellites and relay it on to the USA.