The U.S. Missile Defense Agency (MDA) tentatively plans to begin design work next year on an operational constellation of missile tracking satellites, development of which would cost $2.7 billion through 2011, according to agency officials and documents.

The MDA is requesting $535,000 in 2006 for an operational Space Tracking and Surveillance System (STSS), according to agency budget justification documents sent to Congress in March. The documents indicate that funding for the effort, if approved, would rise dramatically in 2007, to $167 million, and reach $440 million the following year.

Whether that funding actually would materialize is a big question mark, however.

“Anytime you’re talking about money that is six years out, it’s much more of a direction than stating a requirement for specific dollars,” said Jeremiah Gertler, a senior fellow at the Center for Strategic and International Studies here. “Santa Claus and the Easter Bunny live in the out years.”

Gertler is a former House Armed Services Committee staffer who oversaw the STSS program.

Northrop Grumman Space Technology of Redondo Beach, Calif., is the MDA’s designated STSS prime contractor and is finishing work on a pair of demonstration satellites slated to launch in 2007. But the MDA is considering holding a competition for the operational STSS system, according to the Pentagon officials.

The STSS program, previously known as the Space Based Infrared System Low, is aimed at developing satellites that would track missiles during the middle portion of flight as they coast through space. The MDA and its predecessor agencies have spent well over $2 billion developing the concept under various programs dating back at least to the early 1990s.

In mid- 2002, with Northrop Grumman and Spectrum Astro of Gilbert, Ariz., competing for a contract to build an operational constellation consisting of some 24 missile tracking satellites in low Earth orbit, the MDA revamped the program. Plans for an operational constellation were shelved, and the MDA awarded Northrop Grumman a contract worth $868 million to finish building two experimental missile tracking satellites on which construction was halted years before because of delays and cost overruns. The MDA added $750 million to that contract last year but declined to provide any details other than to acknowledge that another satellite had been ordered.

The Pentagon officials said work on the two demonstration satellites is progressing, noting that testing on the satellite platforms — which will feature infrared sensors built by Raytheon Co. of Waltham, Mass. — was completed in March. Software for operating the satellites was tested successfully in January, the officials said.

But the U.S. Government Accountability Office paints a different picture. In a March 31 report titled “Defense Acquisitions: Assessments of Major Weapons Programs,” the congressional watchdog agency said quality and schedule problems due to a “lack of experience” on the part of Raytheon engineers, along with vague systems engineering procedures, brought work to a halt for two months at one point. This led Northrop Grumman to increase its presence at Raytheon’s facilities, the report said. Raytheon also has added technicians with more experience on satellite programs to the effort, the report said.

Sabrina Steele, a spokeswoman for Raytheon, deferred to Northrop Grumman for comment.

Bob Bishop, a spokesman for Northrop Grumman, deferred to the Pentagon .

The two experimental satellites have yet to be mated with their payloads, according to Defense Department officials. The first sensor is expected to be ready for integration at some point this summer, while the second is expected to be ready in late 2005 or early 2006.

The integration work likely will be a challenge, Gertler said.

“It’s easy to build pieces and to say what the pieces will do when linked together,” Gertler said. “It’s arranging the marriage that’s difficult.”

The $2.7 billion budgeted through 2011 for the operational STSS system is separate from the spending intended for the demonstration satellites. The MDA divides funding for its various programs into two-year blocks that correspond roughly to when the system would be deployed. The operational STSS system is funded under Block 2012 in accordance with its planned initial launch date, for example.

The initial two demonstration satellites are designated Block 2006. According to the MDA budget justification documents, the total funding for STSS Block 2006 from 2004 through 2011 exceeds $1 billion.

There also are Block 2008 and Block 2010 funding lines for the STSS program. The Block 2010 effort, which accounts for roughly $60 million in 2004 and 2005, covers an unspecified number of classified demonstration satellites that will follow the 2007 flight experiment . Block 2008, funding for which starts in 2007 and totals $126 million by 2011, includes upgrades to ground-system hardware and software.