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GPS chief dismisses concerns about health of system

Posted: June 19, 2009

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The manager of the Global Positioning System network responded Friday to growing concerns about potential coverage gaps in the navigation service used to guide missiles to targets and tell drivers what's around the next corner.

An artist's concept shows a GPS Block 2R satellite orbiting Earth. Credit: Lockheed Martin
Air Force Col. Dave Madden, commander of the GPS program, said the military-run system is healthy and has "ample" satellites to make it until next-generation spacecraft are ready to enter service.

In a press conference Friday, Madden addressed a recent report by an internal government watchdog group and news this week that the GPS fleet's newest satellite could not enter service as scheduled because of technical trouble.

The GPS 2R-20 satellite, launched in March, is being kept out of service for now because of signal distortions that are degrading the accuracy of its navigation measurements, the Air Force announced this week.

The distortions are being caused by the interface between the satellite and a new civilian signal for the aviation industry, according to the Air Force.

Madden said engineers will work around the issue by adjusting other parameters to correct the inaccuracies. A fix should be ready in time to introduce the satellite into the constellation by September or October.

The interference is affecting the standard L1 and L2 signals broadcast by the spacecraft, but the new L5 demo payload is working fine, officials said.

Engineers traced the problem to the connection between the L5 instrument and an auxiliary port on the satellite, dismissing concerns that the issue would cause delays in the new GPS 2F program that will also host the new signal.

The L5 payload was added late to GPS 2R-20 to meet an August deadline imposed by the International Telecommunication Union, which oversees frequency filings for satellites.

GPS 2F satellites will use a different interface because the L5 signal has always been planned for that series.

"Everything about this signal and how it was integrated on the satellite really has nothing to do with the development of future satellites," Madden said.

The last member of the current group of GPS satellites will launch in August. That craft will not have a similar problem because it does not carry L5.

The GPS 2R-20 announcement came one month after the Government Accountability Office released a report saying the GPS program "faces potential gaps or decreases in positioning, navigation and timing capabilities."

The GAO report blamed the potential gaps on delays of the GPS 2F program and questions about the schedule of the next-generation GPS 3A series.

Madden said he was concerned that the tone of the GAO report would damage consumer confidence in the GPS program, which is used by the U.S. military and millions of civilians around the world.

The space-based navigation network has met performance requirements since 1993, Madden said.

Madden said the GPS constellation has 30 operational satellites and three in-orbit spares, well above the threshold of 24 satellites to provide uninhibited global coverage. Older GPS satellites have been operating for up to 16 years, and officials expect new spacecraft to last just as long.

Plans call for 12 Boeing-built GPS 2F satellites to begin launching in February 2010 to keep the system working until GPS 3A satellites are scheduled to enter service in 2014.

But the GAO reported the GPS 3A schedule is aggressive, saying any delays would put the system at risk of service gaps.

The current crop of GPS spacecraft should be operating until at least 2016, keeping the constellation at full strength even if the GPS 3A series is pushed back, Air Force officials said.

"We have extremely high confidence that we're not going to approach the 24-satellite requirement," Madden said.