Boeing Co.’s difficulties with an interim generation of GPS satellites have raised concerns among some congressional aides about a possible gap in navigation, positioning and timing coverage after the current fleet begins to degrade around the end of the decade.

Due to what these staffers characterize as the uncertainty surrounding the Boeing-designed GPS 2F satellites, the U.S. Air Force cannot afford to keep pushing back its target deployment date for the next-generation system, dubbed GPS 3. Meanwhile, design studies suggest that the GPS 3 development program will prove challenging, compounding concerns about a coverage gap, sources said.

Candrea Thomas, a spokeswoman for Air Force Space and Missile Systems Center in Los Angeles, which procures military satellites, said concerns about an erosion of navigation capabilities before the GPS 3 satellites start launching in 2013 are misplaced. But in a written response to questions, Thomas acknowledged that Boeing has encountered snags with the interim GPS 2F satellites, which are supposed to start launching in 2008.

Delays and test failures have driven up the cost of the GPS 2F satellites by more than $100 million, from $1.82 billion to $1.99 billion, Thomas said May 10 in a written response to questions submitted April 20. The problems forced Boeing to repair, redesign and retest components on the satellites, Thomas said.

Boeing is building nine GPS 2F satellites under a contract that includes an option for three more , Thomas said. The cost of each satellite was estimated at $152 million in 2002, but now is expected to be $166 million , she said.

Joe Tedino, a spokesman for Boeing Integrated Defense Systems, St. Louis , deferred to the Air Force for comment on this story.

The Air Force originally envisioned launching up to 33 GPS 2F satellites beginning around 2002. But those plans changed as the Air Force began looking ahead to GPS 3. In 2000, the service scaled back the size of the procurement and directed Boeing to modify the satellites to give them more power and additional signals for military and civilian users. The initial launch date for the satellites shifted to 2005.

The GPS 2F program has had other disruptions. In 2002, for example, Boeing kicked Computer Sciences Corp., its major GPS 2F subcontractor, off of its team due to problems encountered developing the system’s software.

The GPS satellites being launched today are built by Lockheed Martin Space Systems of Sunnyvale, Calif. At around the same time the Air Force ordered the upgrades to the GPS 2F satellites, the service directed Lockheed Martin to make similar modifications to the eight GPS 2R satellites still awaiting launch.

The first of the modernized Lockheed Martin GPS 2RM satellites launched in 2005, and the second is expected to launch in late 2006.

Boeing and Lockheed Martin are competing for the contract to build the GPS 3 satellites and are in the midst of design studies that began in 2000 . The Air Force had hoped at one time to start launching the GPS 3 satellites around 2009, but has pushed that date out several times due in part to the longevity of the current constellation.

Some congressional staffers now warn that the Air Force is losing the luxury of schedule flexibility on GPS 3 due to the problems with GPS 2F. They have even raised the possibility of purchasing additional GPS 2RM satellites from Lockheed Martin, which might require restarting that production line .

But Air Force Lt. Col. Eric Schnaible, a spokesman for Space and Missile Systems Center, dismissed that idea, saying the Air Force is not weighing the possibility of buying more GPS 2RM satellites.

Steve Tatum, a Lockheed Martin spokesman, referred questions on GPS acquisition plans to the Air Force.

Meanwhile, the GPS 3 development effort is looking more complicated than originally thought, according to the congressional aides, as well as another source close to the program.

One issue is that the advanced anti-jam features and powerful signals envisioned for the new system are driving designers toward larger satellite platforms, which could throw a wrench in the Air Force’s GPS 3 launch plans, said the source close to the program.

GPS satellites today are launched one-at-a-time aboard Delta 2 rockets, but the GPS 3 satellites are expected to be lofted in pairs aboard larger vehicles such as the Delta 4 or Atlas 5. But this source said that the GPS 3 designs are growing to the point that the dual-launch strategy may not be feasible.

Another factor complicating the GPS 3 program is the Air Force’s new satellite acquisition philosophy, which calls for developing and deploying new technologies and capabilities in a more incremental fashion. Under that approach, designed to reduce program risks, the full capabilities envisioned for the GPS 3 satellites would not be included on the initial units but would come on line over time with successive launches.

Because GPS 3 design work has been under way since 2000, the new philosophy, unveiled earlier this year, raises the question of whether portions of the Boeing and Lockheed Martin efforts to date have been wasted, the sources said. On the other hand, the new approach could be the ticket to keeping the program on schedule since the development work is proving to be so difficult, the sources said.