The base at Adak, Alaska was deemed to be the most suitable of six sites considered because it was closest to the anticipated operating area of the new sea-based X-Band radar in the eastern Pacific, said Rick Lehner, a spokesman for the Missile Defense Agency.

"Adak is a naval base that was closed in 1996," he said. "So all the infrastructure is there and nobody is using it."

The radar, which is scheduled to begin operations in 2005, is designed to track the trajectory of incoming long-range missiles.

The tracking data is then relayed to ground-based interceptor missiles at Fort Greely, Alaska and Vandenberg Air Force Base in California.

The Pentagon originally planned to build an X-band radar on land at Shemya island on the westernmost end of the Aleutian chain.

But after withdrawing from the 1972 ABM treaty, which barred sea-based missile defenses, the Bush administration opted for a mobile, sea-based radar whose position could be moved depending on the threat.

"When it's out at sea, it gives the command and control system for missile defense a longer period of time to assess the missiles coming toward us," said Lehner.

A self-propelled, semi-submersible oil rig is being modified at shipyards in Brownsville and Corpus Christi, Texas, to carry the 50,000-ton radar, which is about 130 meters (390 feet) long and 83 meters (250 feet) high.

The Missile Defense Agency plans to have a limited missile defense system in place by next year with four interceptors at Vandenberg Air Force Base and another six at Fort Greely.

Up to 20 interceptor missiles are supposed to be deployed at Fort Greely by the end of 2005.

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