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An intercept test of a missile-destroying laser aboard a converted Boeing Co 747 aircraft has been postponed for a fourth time because of technical problems, the Pentagon's Missile Defense Agency said.

"Trouble shooting indicates that a hot bypass valve on the aircraft is in an abnormal condition, thus not allowing for proper component cooling," the agency said in a statement on its website. "The team is evaluating the potential causes."

The test involving the Airborne Laser Test Bed was to have taken place on Sunday morning off the California coast. A new date for the experiment will be set for the coming week.

The goal is to destroy a simulated enemy ballistic missile in flight more than 161 km (100 miles) away, or about twice the range demonstrated in a maiden test on Feb 11 using a chemical oxygen iodine laser aboard the jumbo jet. The laser heats the boosting ballistic missile's skin, weakening it and causing failure from high-speed flight stress.

The previous postponements took place over roughly the past three weeks. They were attributed by the agency, in turn, to a problem with a stand designed to hold the target before launch, a tracking system software glitch and, last Tuesday, a problem with a tracking camera's cooling system that prompted a system reboot.

Army Lieutenant General Patrick O'Reilly, the agency chief, said the first two delays reflected safety concerns and a limited pool of target ballistic missiles that represent the perceived threat from countries such as Iran and North Korea.

Boeing provides the aircraft, battle management and overall systems integration for the flying ray run. Northrop Grumman Corp supplies the megawatt-class laser and Lockheed Martin Corp supplies the beam control.

The successful test in February prompted calls on the Pentagon to restore funding for further development of the system. Defense Secretary Robert Gates cut it to a research effort last year from a development program headed for possible deployment.

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（中国日报网英语点津 Helen 编辑）

Todd Balazovic is a reporter for the Metro Section of China Daily. Born in Mineapolis Minnesota in the US, he graduated from Central Michigan University and has worked for the China daily for one year.