(Reuters) - Boeing Co is the prime contractor for U.S. Ground-Based Midcourse Defense (GMD) weapons system designed to destroy incoming ballistic missiles launched by North Korea or Iran. The system is facing a key test on Sunday.

Following are key facts about the system, which has failed to intercept a dummy missile in the three last tests and has not demonstrated a successful missile intercept since 2008.

CURRENT SYSTEM

The Pentagon's Missile Defense Agency (MDA) has 30 ground-based interceptors at military bases in California and Alaska. Twenty of the interceptors carry an early version of the "kill vehicle" that separates from the rocket and hits the incoming missile. The other 10 carry the new version being tested Sunday.

Defense Secretary Chuck Hagel announced plans to add 14 more ground-based interceptors in March 2013 after North Korea threatened to launch a nuclear attack against the United States.

Those plans, and deliveries of replacement interceptors already on order from Boeing, are on hold until after Sunday's test. The government is likely to reassess its plans, if the test fails, according to the MDA director, Navy Admiral James Syring.

NEXT INTERCEPT TEST

In Sunday's test, slated to occur between noon and 4 p.m. EDT (1600 and 2000 GMT), a target simulating an intermediate-range missile will be launched from the Kwajalein Atoll in the Marshall Islands.

The test calls for U.S. satellites to detect the launch and radar to track the target while a ground-based interceptor is launched from Vandenberg Air Force Base in California. Radar systems on a U.S. Navy Aegis destroyer near the Marshall Islands and a large floating platform will track the target and relay that data to the ground-based interceptor, to be launched from Vandenberg Air Force Base in California.

Once the interceptor is launched, a kill vehicle or warhead built by Raytheon Co is supposed to separate from the rocket, detect the target and destroy it.

The kill vehicle being tested on Sunday is known as EKV Capability Enhancement II, or CE-II, which failed in both previous intercept tests conducted in 2010.

PREVIOUS TESTS

In July 2013, MDA tested the system using the initial kill vehicle, known as CE-I, but it failed to separate from the third stage of the rocket.

In January 2010, the agency tested the follow-on CE-II kill vehicle, but the test failed because of a quality control problem that occurred during production.

In December 2010, the test failed again with the cause was later traced to a design flaw in the guidance system.

COSTS

The total cost of the system is projected to reach $41 billion, with about $4.5 billion of that total to be spent between fiscal years 2013 and 2017, according to the Government Accountability Office, the investigative arm of Congress.