WASHINGTON — The U.S. Missile Defense Agency (MDA), which had already scaled back testing plans for a multibillion-dollar pair of experimental missile tracking satellites launched last year, is warning of more changes as it tries to align the program with the funding Congress made available in 2010.

The two Space Tracking and Surveillance System (STSS) demonstration satellites, designed to track ballistic missiles during every stage of flight, were launched Sept. 25 after years of delay, and encountered technical issues from the start. The originally planned three-month checkout period for the spacecraft was extended due to problems with their attitude control subsystems that required new software to be uploaded late last year, said Gabe Watson,  vice president for missile defense and warning programs at STSS prime contractor Northrop Grumman Aerospace Systems of Los Angeles.

The satellites are now functioning properly, Watson said in a Jan. 21 interview.

Each has two sensors — one for target acquisition and one for tracking — that are now being calibrated. It is not known when the satellites will be ready to begin tracking missiles as part of a test program, MDA spokeswoman Debra Christman said.

“Much of the basic functional testing is completed and we continue to methodically activate the on-board systems and examine the performance of the satellites in different environments,” Christman said Jan. 28 in an e-mailed response to questions. “Our approach is investigatory in nature and we are not working to a specific timeline.”

Even more uncertain is what the MDA will do with the satellites once they have been fully calibrated.  The MDA’s 2010 budget request included funds to conduct two dedicated missile launches for STSS testing, and the satellites also were to observe at least two launches of other agency systems, such as the Aegis Ballistic Missile Defense or Ground-based Midcourse Defense interceptors. But an Integrated Master Test Plan completed by the MDA in mid-2010 eliminated the launches dedicated to STSS testing, Christman said.

That change was made before Congress trimmed $139 million from the MDA’s $966.7 million request for tests and targets in the 2010 Defense Appropriations Act, which was signed into law in December. The report accompanying the bill said most of that reduction was aimed at an unspecified “premature request.” The report also  eliminated at least one test involving the STSS satellites, dubbed FTS-01, from the MDA’s budget.

The STSS satellites will take part in as many other missile defense tests as practical, Christman said.

“Congress in its 2010 appropriations bill reduced some of the funding for some of the targets this year, so we are working with MDA to restructure the program for this year,” Watson said. “That’s all in a state of flux at the moment.

“We were very disappointed Congress removed the funding for FTS-01. … This is a first-of-a-kind system, and it’s bringing new capabilities to the country. To have two satellites flying in formation with these suites of sensors both capable of tracking cold bodies in space, that’s a capability the country hasn’t had before.”

Northrop Grumman originally built the STSS satellites for an experiment dubbed the Flight Demonstration System that was canceled in 1999. The program was given a new lease on life in 2002 when the MDA awarded Northrop Grumman an $868 million contract to refurbish the satellites for launch. The total spent on the program since 2002 has been $1.35 billion, the MDA said last year. In 2003, the U.S. Government Accountability Office, citing Pentagon estimates, said the STSS program was expected to cost $3.1 billion through 2009.

Congress has repeatedly turned down MDA funding requests to begin work on an operational missile tracking constellation pending the results of the STSS demonstration, and the agency requested no funding for such a system in 2010.  The agency has not revealed detailed plans for the Precision Tracking and Surveillance System other than saying it likely would hold an open competition to build it rather than make a sole-source award to Northrop Grumman.