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DGPS services are widely available from both commercial and government sources. The latter include WAAS and the [[US Coast Guard|U.S. Coast Guard's]] network of [[Low frequency|LF]] marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.
During the 1990-91 [[Gulf War]], the shortage of military GPS units caused many troops and their families to buy readily available civilian units.
In the 1990s, the [[Federal Aviation Administration|FAA]] started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own [[radio navigation]] systems. The amount of error added was "set to zero"<ref name="OSTP">{{cite web|url=http://www.ngs.noaa.gov/FGCS/info/sans_SA/docs/statement.html|publisher=[[Office of Science and Technology Policy]]|title=Statement by the President regarding the United States' Decision to Stop Degrading Global Positioning System Accuracy|date=May 1, 2000|accessdate=2009-02-02}}</ref> at midnight on May 1, 2000 following an announcement by U.S. President [[Bill Clinton]], allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the U.S. military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.<ref name="OSTP" />
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