GPS accuracy tweaked to within three metres during war

The US Department of Defence improved the performance of its GPS satellite navigation system to provide accuracy within three metres for precision guidance systems for munitions, aircraft and ground forces during the Iraq war.

The US Department of Defence improved the performance of its GPS satellite navigation system to provide accuracy within three metres for precision guidance systems for munitions, aircraft and ground forces during the Iraq war.

John Clark, principal systems engineer at The Aerospace Corp, an El Segundo, California -based not-for-profit company that supports the Defence Department's GPS Joint Program Office in Los Angeles, says the Global Positioning System (GPS) enhancement represented an eightfold increase from normal accuracy of 16 metres.

Clark, speaking in Albuquerque, New Mexico, at the annual Institute of Navigation conference, says the US Air Force's Second Space Operations Squadron, based at Schriever Air Force Base in Colorado, was able to enhance the precision of GPS in the Iraq war by precisely timing software uploads to positioning satellites in view over the Mideast.

The GPS constellation consists of 24 satellites arranged in clusters of six planes. Each plane is aligned with four satellites, which circle the Earth in a race-track orbit, with each plane rising and setting over a particular spot throughout the day. Clark says the space operations squadron was able to improve the accuracy of GPS satellites by timing uploads of satellite ephemeris (precisely plotted positions of the GPS satellites) and timing information just as a particular plane was scheduled to "rise" over the Mideast.

The squadron uploaded the ephemeris and timing information from a worldwide network of ground control stations that includes those in the Azores island chain in the Atlantic and Diego Garcia Island in the Pacific. Clark says the network was configured in a day to handle the demands of the Iraq war.

During the war, the squadron provided GPS accuracies of 2.66 metres "over Baghdad," according to the squadron's commander, Air Force Lt Col Scott Henderson. Henderson says that as individual satellites came up on the horizon over the Mideast, the squadron would upload to the satellites the latest navigation parameters. He says this improved the accuracy of the satellites by 35%. The precise location and timing information helped better determine the position of the satellites. This in turn allowed the satellites to provide more accurate location information to receivers in precision weapons and aircraft as well as receivers used by ground forces.

In another development, Mike Shaw, director of radio navigation and positioning in the Office of Science and Technology at the US Department of Transportation, says the US has some concerns that China has started to develop its own satellite navigation system. He says China has launched test navigation satellites but has not asked for a spectrum allocation through the International Telecommunications Union (ITU).

Shaw says the US is concerned about how a Chinese satellite navigation system would fit into the same frequency bands used by GPS and the Galileo satellite navigation system used by the European Union. "Spectrum is a finite resource," he says.

In addition, Shaw says the US has concerns that the EU will require US aircraft operating in European airspace to use its Galileo satellite system.

"We hope that the EU does not mandate the use of Galileo, but this is an issue that has not been resolved," Shaw says. The US delegation to the World Radiocommunications Conference held under the auspices of the ITU is addressing a variety of Galileo/GPS issues at the international policy level.

Though no incidents of GPS jamming during the Iraq war have been publicly identified, the Defence Department remains concerned about that possibility and has started to develop GPS antennas that can avoid jamming, says Tony Abbott, an Aerospace Corp engineer. He says these improvements include digital antennas that use adaptive signal processing techniques, which filter signals from GPS satellites from the noise created by jammers.

Abbott says the Defence Department has also started work on multibeam antennas that would automatically seek out and lock on to the signal from each satellite in the GPS constellation to prevent interference by a jamming system.

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