Ramon Lopez/WASHINGTON DC
Degradation of the global positioning system (GPS) satellite navigation signal by the US Department of Defense was discontinued on 1 May by order of President Clinton. The move allows civil GPS users to enjoy the same accuracy that the US military has enjoyed and will also help the US Federal Aviation Administration in its commitment to adopting a GPS-based sole-means navigation system for civil aviation in the USA.
Known as "selective availability" (SA), the GPS signal time-dithering error was a national security measure allowing only the US military to use the system's full accuracy. Civil users will now also benefit from 10m (33ft) accuracy or better compared with the previous 100m. The DoD, however, has White House approval to reapply SA on a regional basis if national security dictates the need. The DoD is developing precision GPS jamming techniques that will allow for the system's continued civil use outside any theatre of operations.
The 24-satellite GPS constellation is operated by the US Air Force and provides signals worldwide free of charge. As part of a 1996 presidential directive, Clinton committed to discontinue SA degradation in 2006, but the White House has decided to shut it off six years ahead of schedule.
• Since 1 May, when SA was switched off, the Swiss-based Federation Aeronautique Internationale has carried out GPS signal accuracy tests in the south-east of the UK, revealing a 100% probability of fix accuracy within 32m, with average fixes to 13.8m accuracy. Tests were carried out from a moving vehicle against accurately surveyed ground points.
Source: Flight International