Bye Bye, SA!

2003.03.20: The United States has not reenabled SA with the commencement of military action in Iraq.

For years, the U.S. Government has used a technique called Selective Availability (SA) to degrade the accuracy of the GPS signals that are accessible by civilians. One major reason for this was to prevent enemies from using the GPS to accurately target U.S. or allied assets.

The most noticeable effect of SA on civilian applications was that the accuracy of computed locations was degraded by nearly an order of magnitude. Without SA, most consumer-grade GPS receivers can compute positions to within 15 meters (about 49 feet) or so, 95 percent of the time. With SA, these receivers' accuracy couldn't be trusted to any better than 100 meters (about 328 feet).

Latitude error around 0400
UTC, 02 May 2000On May 1, 2000, however, U.S. President Bill Clinton did the public a good thing. He announced the decision to immediately discontinue the use of SA.

Just after 0400 UTC on May 2, 2000, many GPS users saw an abrupt (and welcome) improvement in the accuracy of the positions reported by their equipment--proof positive of the end of SA.


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Chuck Taylor -- (Copyright) -- (Contact)