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Trouble aboard latest Global Positioning System satellite

Posted: June 17, 2009

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The U.S. military's newest Global Positioning System navigation satellite is suffering from unusual signal distortions that will keep it out of service for at least six months, the Air Force announced Tuesday.

An artist's concept shows a GPS Block 2R satellite orbiting Earth. Credit: Lockheed Martin
The GPS 2R-20 satellite, a modernized version of the current GPS series, blasted off aboard a Delta 2 rocket March 24 from Cape Canaveral, Fla.

Engineers discovered problems with the spacecraft's navigation signals during standard checkout activities a few weeks later, according to the Air Force.

"The signal distortion was initially observed as an elevation-dependent bias in ranging measurements from GPS monitor stations," the Air Force said in a written statement from the Space and Missile Systems Center.

GPS satellites transmit military and civilian signals on two L-band frequencies.

The "out of family" measurements were first detected April 9, according to a presentation last month to the European Navigation Conference by a senior GPS official.

GPS 2R-20 was scheduled to enter service in April or May, but officials have kept the suspect satellite out of the operational constellation as they continue to troubleshoot the problem, the Air Force statement said.

"Worldwide GPS users are not affected since this satellite is still in early orbit checkout and has not been introduced into the operational constellation," Air Force officials said.

GPS satellites were valued at $75 million as of last year. The Air Force could not respond to an updated cost figure Wednesday.

The Air Force and contractors established a dedicated response team, which is currently concluding its investigation into the cause of the anomaly. The team has also identified corrective measures to eliminate the bias and bring the signals within Air Force performance standards.

Engineers have linked the poor signal quality to the interface with the L5 demonstration payload, a new navigation frequency being tested for the aviation industry. The payload consists of two additional boxes weighing about 40 pounds.

The L5 signal is not affected by the problem, according to the Air Force.

The new frequency was turned on April 10 and early tests indicate it is performing well.

Officials had to demonstrate the L5 signal before an August deadline imposed by the International Telecommunications Union, which oversees frequency filings for satellites.

The L5 demonstration was also planned to make sure the new signal did not interfere with other navigation frequencies broadcast by GPS satellites. The signal distortion on GPS 2R-20 is "clearly" related to L5, the Air Force statement said.

The response team will spend the next few months attempting to solve the signal distortion and make sure the fix will not inadvertently impact military and civil GPS users.

Officials predict the satellite could enter service by October if no problems develop during the test campaign this summer.

The Air Force has "high confidence" the problem will not affect plans to launch the next GPS satellite in August.

The August mission will launch the last member of the current series of GPS spacecraft. The next-generation GPS 2F satellite line will begin launching late this year or in early 2010.

GPS 2R-20 was slated to replace the GPS 2A-27 satellite, which has been in orbit since 1996. The aging satellite still occupies its operational orbital location while engineers work on its replacement.

GPS satellites fly 11,000 miles above Earth in six orbital groupings to provide global navigation coverage. Each group consists of five operational satellites, reaching a total of 30 spacecraft in the whole constellation.

The Government Accountability Office last month identified potential future gaps in GPS service due to delays and cost overruns in the GPS 2F series and concerns about the timeliness the follow-on GPS 3A system to be launched in the next decade.

The Air Force says the GPS constellation is still running at full capacity and officials expect it to remain healthy.

"The Air Force remains committed to ensuring a continued high level of GPS service and capability, maintaining GPS as the premier provider of positioning, navigation, and timing for military and civilian users throughout the world," the statement read.