The military has been forced into damage control after Defense Week magazine revealed that the warhead was carrying a global positioning satellite beacon that made it easy to track.

In fact, all four of the dummy warheads used in tests so far carried beacons, and even with this head start the "hit-to-kill" interceptor missiles managed to bring down only two.

Last month's test was held up by the Bush Administration as proof that its $US100 billion missile defense plan could work.

But critics now say the missiles would have had little chance of pinpointing the warhead's path without the help of the beacons.

Mr Phillip Coyle, who ran the missile shield program until last year, said the beacons had been "a big help" to the interceptor missiles, acting "like a pinger saying, 'Here I am'."

A Pentagon official, Rear-Admiral Craig Quigley, conceded the obvious - the system would not have the benefit of such helpful beacons in the case of a real nuclear attack.

"The beacon tells [the interceptor] roughly where in space to start looking," he said.

"We will get to the point where we develop real final systems. But you can't go any faster now."

Admiral Quigley said the tests should still be seen as successes because the missiles had used their on-board navigation systems to home in on and destroy the warheads in the final phases.

Defense Week reported that the beacon helped the defense missiles compensate for deficiencies in US radar tracking technology on the ground.

In the July 14 test, an interceptor missile launched from the Marshall Islands destroyed a dummy warhead fired 7,700 kilometers away at California's Vandenberg Air Force Base.

News of the successful interception had been seen as a much needed fillip for the White House as it struggled to persuade doubting legislators that the hugely expensive "Son of Star Wars" defense shield could work.

"They hit a bullet with a bullet, and it does work," the Republican Leader in the Senate, Senator Trent Lott, boasted.

The director of the Ballistic Missile Defense Organization, Air Force Lieutenant General Ronald Kadish, told a congressional committee that the Pentagon had been spurred on by its success and wanted to increase the pace of testing.

He said about 20 more tests were scheduled over the next five years, beginning in October.

The revelations about the beacons threatens to take much of the political gloss off the successful interception and will ensure close scrutiny of future tests.

The original Reagan era "Star Wars" program ran into similar controversy when a target was found to have been secretly heated to help the interceptor rocket's sensors pick it up.

Copyright © 2001 Sydney Morning Herald

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