GPS accuracy tweaked to within three metres during Iraq war

The US Department of Defense improved the performance of its GPS satellite navigation system to provide accuracy within three...

The US Department of Defense 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 Corporation, a not-for-profit company which supports the Defense Department's GPS Joint Program Office in Los Angeles, said the GPS enhancement represented an eightfold increase from normal accuracy of 16 metres. 

Clark said 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. 

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 said 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, which includes those in the Azores island chain in the Atlantic and Diego Garcia Island in the Pacific. Clark said 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 said that as individual satellites came up on the horizon, the squadron would upload to the satellites the latest navigation parameters, improving 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. 

Bob Brewin writes for Computerworld

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