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Global Positioning System: Difference between revisions
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SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, '''[[Differential GPS]]''' was used by many civilian GPS receivers to greatly improve accuracy. | SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, '''[[Differential GPS]]''' was used by many civilian GPS receivers to greatly improve accuracy. | ||
During the | During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying it to the enemy. But since SA was also denying the same accuracy to thousands of friendly troops, turning it off or setting it to an error of 0 meters (effectively the same thing) presented a clear benefit. | ||
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 military resisted for most of the 1990s, but SA was eventually "discontinued"; the amount of error added was "set to zero"<ref>[[Office of Science and Technology Policy]]. [http://www.ostp.gov/html/0053_2.html Presidential statement to stop degrading GPS]. [[May 1]], [[2000]].</ref> in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the [[FAA]],<ref>[[FAA]], [http://gps.faa.gov/gpsbasics/SA-text.htm ''Selective Availability'']. Retrieved Jan. 6, 2007.</ref> have stated that it is not intended to be reintroduced. | 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 military resisted for most of the 1990s, but SA was eventually "discontinued"; the amount of error added was "set to zero"<ref>[[Office of Science and Technology Policy]]. [http://www.ostp.gov/html/0053_2.html Presidential statement to stop degrading GPS]. [[May 1]], [[2000]].</ref> in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the [[FAA]],<ref>[[FAA]], [http://gps.faa.gov/gpsbasics/SA-text.htm ''Selective Availability'']. Retrieved Jan. 6, 2007.</ref> have stated that it is not intended to be reintroduced. | ||