Another successful test of the Ground-based Midcourse Defence (GMD) system has kept the USA on track to field a ballistic missile defence testbed in Alaska in 2004.

In the 15 March test, the prototype interceptor destroyed a ballistic-missile target over the Pacific, marking the third successive hit-to-kill intercept and the fourth success in six tests conducted since October 1999.

The test was the most complex yet, involving three balloon decoys and the re-entry vehicle target. Two smaller balloons were added to the one large balloon used in previous tests, otherwise the geometry of the intercept was similar.

The modified Minuteman II ballistic-missile target launched from Vandenberg AFB, California; 20min later and 7,700km (4,800 miles) away, the interceptor was launched from Kwajalein Atoll in the Marshall Islands. Space-based sensors detected the launch and cued ground-based radars in California and Kwajalein to provide target data to the interceptor's exo-atmospheric kill vehicle (EKV), which separated from its booster more than 2,200km from the target.

After separation, the EKV used onboard infrared and visual sensors, augmented by discrimination data from a prototype X-band radar on Kwajalein, to locate and track the target. The onboard sensors selected the target warhead and not the balloons, says the US Missile Defense Agency (MDA), stressing that a C-band transponder on the warhead did not provide any targeting information to the interceptor after its launch. Body-to-body impact occurred 10min after interceptor launch, at a closing speed exceeding 24,000km/h (13,000kt) and an altitude of more than 220km.

The flight was the fourth system-level test of the GMD, for which Boeing is prime contractor. Raytheon supplies the EKV and ground radars, TRW the battle management command and control system and inflight interceptor communications system. Lockheed Martin provided the booster and Orbital Sciences the target for this test. At least 19 more flight tests are planned by 2007.

Source: Flight International