ABM site- Grand Forks- 1975

http://en.wikipedia.org/wiki/Anti-ballistic_missile

Anti-Ballistic Missile (ABM) systems have always been controversial. Expensive, prone to technical glitches. They seemed to motivate the opposing cold-war side to just build more missiles. Early systems relied on atomic warheads to zap the incoming missiles and only had to get close for a kill. Later and present day technology pushes for a 'hit and kill' non-nuclear defense. The defensive missile must hit or get very close to the incoming warhead.