The Boeing Co. has been awarded a seven-year, $3.5 billion contract to continue development and operations of the U.S. Missile Defense Agency's ground-based midcourse defense system, including the interceptor missile field at Fort Greely in Alaska, the Defense Department announced Friday.

The new contract, in the works for two years, extends Boeing's connection to the nation's missile-defense system, where it's been a prime contractor since 2001. Northrop Grumman Corp. will share part of the Boeing contract as its partner overseeing ground-system elements and providing support in engineering and testing.

Boeing Strategic Missile & Defense Systems spokeswoman Jessica Carlton said Boeing has 80 employees based at Fort Greely, while Northrop has 30. Most of Boeing's missile division employees are based in Huntsville, Ala., she said.

More than 20 operational interceptor missiles are kept in silos at Fort Greely and Vandenberg Air Force Base, Calif. The system is designed to defend the United States and Canada against a small-scale missile attack from a place like North Korea, not the kind of massive strike that could emanate from Russia. Critics have said the system is a waste of money because of the difficulty in shooting down a ballistic missile, but advocates say that the most recent tests demonstrate the interceptors can hit their targets in space.

Ground-based midcourse defense is one of a number of experimental and developing components of a broad missile-defense network. Ground-based means the interceptors are stationed on land, and midcourse means the enemy warheads are targeted while they are midway to their targets, usually in near space. The system uses radars, satellites and other sensors, communications terminals and a 20,000-mile fiber-optic communications network to help determine whether interceptors should be launched and to quickly set their targets.

Other missile-defense systems would be based on ships, aircraft and in space, and could attack enemy missiles at launch or as they approach their targets.