The board's report also said engineers have not been able to demonstrate that the system's sensors can tell whether an interceptor has destroyed a warhead because the sensors cannot distinguish between a warhead and other objects, such as a piece of a destroyed rocket.
That concern, which also applies to other U.S. missile defense systems, would back up the findings of a longtime critic of missile defense, MIT scientist Ted Postol. He has argued that adversaries could overcome the U.S. shield by using decoy warheads, such as balloons, when they launch a missile.
"If you can't tell the difference between a warhead and pieces of debris from an attempted intercept, how are you going to identify a decoy that's designed to fool you?" Postol said.
The board's report said that if the system is firing its limited number of interceptors at junk or decoys, the result would be "dramatic and devastating."
Lehner acknowledged that identifying warheads remains difficult but said the current technology is adequate to counter the threat from "rogue nations" and is continuing to improve.
Coyle, who oversaw weapons testing in 1990s and recently was a science adviser to Obama, said it is not clear how or whether engineers can address the problem of decoys.
"Clearly, from their report, the DSB believes that these systems require major changes," he said.
A renowned physicist who has read the report, Richard Garwin, said the problems identified in the report appear too overwhelming to overcome.
"If you would only replace the radars by real radars and you replace the interceptors by faster interceptors and you find some way of discriminating between a warhead and a decoy, then yes, it's a good foundation for moving forward," he said sarcastically.
Congressional investigators said the Defense Department is committing to technologies before they are proven and that the administration is risking "performance shortfalls, unexpected cost increases, schedule delays and test problems."
Key parts of the system are already experiencing problems. For instance, the GAO report said pressure to meet Obama's 2015 deadline to field the second phase of the system has led the Defense Department to order dozens of interceptors even though it's not clear they will work. Testing isn't expected to be completed until next year.
The report raised larger concerns for the interceptors planned for the last two stages.
On the third-stage interceptor, the report says critical technologies in its rocket motors had problems during testing that may require redesign. The report also criticizes the administration for setting out a timeline that calls for scheduling a test for the final interceptor a year before the military can confirm whether its design can work.
Similarly, the report says the military plans to install its radar in Romania before it is tested, and that the radar could interfere with Romanian cellphone, data, television and radio broadcasts unless it is altered.
Lehner, the Missile Defense Agency spokesman, said that claim is being examined. If there is a problem, the U.S. would work to resolve it, he said. "This is something we take very seriously."
Missile Defense Agency: http://www.mda.mil/system/system.html
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