The cost of deploying Aegis Ashore, the land component of the ship-based missile defense system, to a site in Europe as well as for a testing and development in Hawaii has doubled, according to a report by the Government Accountability Office.

The Aegis Ashore site planned for Romania and the Hawaii site has climbed from a 2010 estimate of $813 million to a cost now expected to be $1.6 billion, the congressional oversight agency said, citing information provided by the Pentagon’s Missile Defense Agency (MDA).

The GAO said the higher cost came because the 2010 estimate was made before program requirements were fully understand and because the acquisition strategy had not been completely ironed out.

“Since that time, the program has repeatedly added and moved a significant amount of content to both its resource and schedule baselines to respond to acquisition strategy changes and requirements that were added after the baseline was set,” the GAO said in Friday’s report (GAO-13-432).

The Pentagon plans to deploy the Romania site in 2015 as part of the Obama administration’s European Phased Adapted Approach (EPAA) for missile defense on the continent. Under the plan, the United States will set up Aegis Ashore at a second location in Poland in 2018 as part of the second phase of the program. That portion is anticipated to cost $746 million.

“Instability in the Aegis Ashore program’s resource baseline makes it impossible to understand annual or longer term progress by comparing the latest reported estimates to the prior year baseline or the original baseline,” the GAO said.

The agency said baselines were “significantly affected” by new requirements to pay for the deckhouse construction costs with development money instead of military construction funds.

“Despite this changes, the resource baseline still does not include all costs associated with Aegis Ashore–such as for the Aegis Ashore adjustments needed to the Aegis (Ballistic Missile Defense) modernized weapons system software,” GAO said.

GAO said the MDA expected to report more concrete costs in its Baseline Acquisition Report this year, because “more contracts will have been negotiated.”

Aegis Ashore is an adaptation of the Aegis Ballistic Missile Defense system now fielded on Navy cruisers and destroyers, and has been one of the more successful BMD programs, having scored 24 hits in 30 attempts since testing began in 2002. The most recently successful test was in February.

The Obama administration in 2009 abandoned plans by his predecessor’s White House to field a long-range ballistic missile defense system in the Czech Republic and Poland in favor of the EPAA policy that began with missile defense destroyers deploying to Europe and will expand to land.

NATO has adopted the EPAA, but the Obama administration’s approach has done little to quell criticism from Russian officials who believe the system is a threat to Moscow’s strategic deterrent.

Lockheed Martin [LMT] builds the Aegis system while Raytheon [RTN] supplies the SM-3 interceptor missile.