CIVILIAN users of the US military鈥檚 Global Positioning System got an unexpected and tantalising treat last month. For about 19 hours on 20 April, while the US Air Force was searching for the wreckage of an A-10 ground attack aircraft missing in the Rocky Mountains, the satellite navigation system suddenly became far more accurate than usual鈥攊ndeed, apparently more accurate than the Pentagon admits it can be.
GPS satellites broadcast two signals. One is available to anyone, and the other is encrypted so that it is accessible only to the military. By measuring the time delay between signals from different satellites, civilian GPS receivers can generally calculate location to within 100 metres鈥攁lthough if signals are rebroadcast from a fixed receiver at a known position, it is possible to narrow this down to a few metres. According to the Pentagon, military GPS receivers are accurate to within about 20 metres, even without this refinement, known as differential GPS.
However, Stan Huntting, the author of a program called SA Watch, which estimates the error in the GPS signals, says that on 20 April civilian GPS receivers were providing locations with an accuracy of as little as 2 metres without the help of rebroadcasting.
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The US Air Force Space Command, which operates the GPS satellites, confirms that the usual intentional inaccuracy in the civilian signal鈥攁 feature known as 鈥渟elective availability鈥濃�攚as deactivated on 20 April. But Don Miles, a lieutenant-colonel and spokesman for the space command, refuses to say why.
One strong possibility, however, is that the satellites were being used to aid the search for an A-10 ground attack aircraft that disappeared during a training exercise in Arizona earlier in the month. Both civilian and military planes were scouring a mountainous region of Colorado for wreckage, and giving all of the aircraft involved access to the same high-quality satellite navigation signal would have helped speed the search.
Indeed, Huntting notes that shortly after the publicly available signal was returned to its usual level of accuracy the Pentagon announced that the wreckage had finally been located.
The Pentagon, which has had to rely in part on 鈥渇uzzy鈥� civilian GPS receivers because of a shortage of equipment that can receive the military broadcasts, also switched off selective availability at the start of the Gulf War to improve the civilian signal. 鈥淚n all the major conflicts we鈥檝e had to date, they鈥檝e turned it off,鈥� says Laurence Adams, a former president of the aerospace company Lockheed.
Adams also chaired a National Research Council panel which argued in 1995 that selective availability should be permanently deactivated (This Week, 10 June 1995, p 8). President Bill Clinton鈥檚 administration has promised to do this, but has not yet set a date.
Civilian GPS users were eagerly discussing the 20 April incident last week on the Internet, and are arguing that the Pentagon should be more forthcoming about the technical capability of its navigation system.