Why not use the E-10A

http://globalsecurity.org/org/news/2...tary767-01.htm

to spot incoming ballistic missiles and then have an interceptor
dropped from a B-52 hardpoint to engage the target?

As the B-52 will already be several miles high it reduces the rocket
booster power needed in an interceptor and an airborne radar can see a
bit further than one on the surface giving a bit of extra warning.

The entire operation can fly at 500+ mph to defend any spot on Earth,
limited only by midair refueling.

-HJC