the Smart Rock is the interceptor vehicle. This design has been evolving and maturing since the Brilliant Pebbles program back in the 1980s. Instead of having the interceptors stationed in orbit like BP called for, the current architecture launches them on demand on a high speed long range missile. That new missile was recently tested for the first time. It's supposed to have a range of 3,000 miles compared to the 200-400 mile range of the old Safeguard Spartan missile. That long range is how a single site in Alaska can reach missiles coming in from almost anywhere. Take a globe and stretch a string between possible launch sites in the Middle East or Asia and you'll find the tragectories will pass within 3,000 miles of the interceptor site.

The US Army has been at the forefront of missile defense from the beginning. Their headquarters for missile defense is located in Huntsville, AL, my hometown. Of course, the Air Force is heavily involved as well, especially on the sensor systems (ground and space).