WASHINGTON — Boeing is studying a two-stage variant of the Ground-based Midcourse Defense System interceptor that would be used as a part of a Europe-based missile shield, a company official said.

Boeing is prime contractor on the Ground-based Midcourse Defense System, which uses a three-stage, long-range interceptor to defend the U.S. mainland against missile attacks. The interceptors are being installed in silos at Fort Greely, Alaska, and Vandenberg Air Force Base, Calif.

The U.S. Missile Defense Agency (MDA) hopes to deploy 10 interceptors in Poland, along with a tracking radar in the Czech Republic, to engage missiles launched from Iran. The agency has targeted 2011 for deployment of such a system, whose interceptors would not need the range of those based in the United States.

Scott Fancher, vice president and program director for the Ground-based Midcourse Defense program at Boeing Missile Defense Systems of Arlington, Va., said the company is engaged in a several-month study of the two-stage booster under a contract with the MDA.

Rick Lehner, a spokesman for the MDA, said the study contract was awarded Feb. 23 and that its value is relatively small. The MDA has requested $42 million in 2008 to order long-lead components for the Europe-based interceptors and has budgeted $412 million for the effort through 2013, he said. The MDA plans to spend some $550 million on the Czech radar facility, he added.

Meanwhile, the MDA has three flight tests of the Ground-based Midcourse Defense System scheduled for this year, two of which will involve attempts to intercept a target missile launched from Vandenberg. The first of those, scheduled to take place during the 2007 quarter ending March 31, will not be an intercept but rather a test of the shield’s command-and-control and sensor elements, Fancher told reporters March 12 during a missile defense media roundtable.

It will be the first test in which the new Sea-Based X-band Radar is fully integrated with the missile shield’s fire-control system, Fancher said. The radar, which sits atop a converted floating oil-drilling platform, is designed to track targets and distinguish between warheads and decoys, he said.

The first intercept test, slated for the second quarter of the calendar year, will be different from a similar exercise conducted in September in two key respects, Fancher said. First, the MDA and its contractors have made improvements to the navigation and flight-control systems on the interceptor’s kill vehicle based on data from the previous test, he said. Second, the Sea-Based X-band Radar will participate in what Fancher characterized as a shadow mode: it will be integrated with the other elements of the system but will not have input into the actual intercept, he said.

During the second intercept test, scheduled for third quarter of the calendar year , the Sea-Based X-band R adar will have input into the intercept attempt, Fancher said.

Fancher added that Boeing is making upgrades to the Ground-based Midcourse Defense System that by 2009 will enable the MDA to conduct tests without taking it out of defensive service.