ANCHORAGE, Alaska (AP) — Defense contractor Lockheed Martin says it will team with Alaska's state-owned aerospace corporation to pursue a U.S. Missile Defense Agency contract to maintain and improve the country's ground-based missile defense system.

The system is designed to defend against a limited attack by intermediate- and long-range ballistic missiles. It includes interceptor missiles stationed at Fort Greely, Alaska, about 100 miles south of Fairbanks, and at Vandenberg Air Force Base in California.

Boeing was picked over Lockheed as the system's original prime contractor. But the Missile Defense Agency on May 14 issued an amended draft request for proposals.

A final request is expected this summer, with a five-year contract awarded early next year. The military has valued the contract at about $600 million per year.