"Although MDA delivered regional capabilities as scheduled, DOT&E (Director Operational Test and Evaluation) concluded that testing was insufficient and the capabilities, as delivered, likely do not provide robust defense against medium-range ballistic missiles as planned," the report said.
The Government Accountability Office assessed a variety of MDA-developed systems, known as elements, that when woven together can defend against incoming enemy ballistic missiles.
The systems assessed include the Aegis Ballistic Missile Defense Weapon System, the Army Navy/Transportable Radar Surveillance and Control Model 2, the Ground-based Midcourse Defense and the Terminal High Altitude Area Defense.
The US Missile Defense Agency is developing such systems for delivery in the United States, as well as Asia-Pacific, the Middle East and Europe.
But the GAO report noted that testing carried out on the systems did not resemble realistic conditions.
"All four tests used to support the delivery of these capabilities included limitations that deviated from how the warfighter would use them under realistic conditions," the report said.
It added that testing had uncovered weaknesses in the systems that could cause them to miss incoming targets.
"In addition to the testing limitations, the tests revealed performance issues that could lead to missed or multiple attempts to defeat a single threat. For example, there were difficulties tracking targets, discriminating the lethal object in the targets, and mitigating interference from any debris," the report said.
On Tuesday, the Missile Defense Agency expected to conduct a high-profile test of its ability to shoot down incoming intercontinental ballistic missiles.
The Ground-based Midcourse Defense system has been tested 17 times in the past, but it has failed on eight of those occasions, giving it only a 53 percent success rate, according to the Missile Defense Agency's website.
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