Here is an article from TIME magazine discussing the significance of the planned US test of the Ground-based Mid-course Defense system (GMD):

Shahab-3 Missile In Iran

Fake “North Korean” missiles have been hurtling over the Pacific toward the U.S. for years, providing test fodder for the Pentagon’s missile-defense systems. But next month, the fake enemy missiles flying over the same ocean are going to be “Iranian.” The timing of the test, however, has nothing to do with a missile test-fired by Iran on Tuesday. That was a medium-range Sajjil-2 missile capable of targeting Israel or U.S. bases in the Persian Gulf. Next month’s U.S. interceptor test will, instead, be aimed at the as-yet-hypothetical threat of an Iranian intercontinental ballistic missile (ICBM), even though such a threat has been deemed by the Obama Administration to be unlikely in the immediate future.

The Administration announced earlier this year that the long-range Iranian threat isn’t advancing as quickly as once feared. Recent U.S. intelligence assessments have concluded that “the threat of a potential Iranian ICBM had been slower to develop than previously estimated,” Ellen Tauscher, the State Department’s arms-control chief, told Congress. Some intelligence estimates say an Iranian ICBM might not happen until 2020. That assessment prompted the President, with Pentagon support, to scrap a land-based interceptor system based in Poland and the Czech Republic and instead to deploy ships capable of shooting down the short- and mid-range Iranian missiles that U.S. intelligence believes pose a more imminent threat — like the sort of missile test-fired by Iran this week.

Next month’s missile test not only will be aimed at a threat deemed less than urgent but will also involve tougher technical challenges. Destroying a “North Korean” missile involves hitting it as it zooms from left to right across an interceptor’s field of view, but the locations of the “Iranian” missile and the U.S. interceptors require more of a head-on collision. That means the closing speed between the two projectiles will be faster than in previous tests: close to 18,000 m.p.h., compared with 15,000 m.p.h. in prior exercises.  [TIME Magazine]