Error analysis for the Global Positioning System: Difference between revisions

Revision as of 16:15, 9 May 2025

Revision as of 16:27, 9 May 2025

Line 101:

SA errors are actually [[Pseudorandomness|pseudorandom]], generated by a cryptographic algorithm from a classified ''seed'' [[key (cryptography)|key]] available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.

Before it was turned off on May 2, 2000, typical SA errors were about 50&nbsp;m (164&nbsp;ft) horizontally and about 100&nbsp;m (328&nbsp;ft) vertically.<ref>{{Cite book |last=Grewal (2001)|first=Mohinder S. |title=Global positioning systems, pinertial navigation, and integration |last2=Weill |first2=Lawrence Randolph |last3=Andrews |first3=Angus P. |date=2001 |publisher=Wiley |isbn=978-0-471-20071-0 |___location=New York, NY |pages=103.}}</ref> Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called [[Differential GPS]] (DGPS). DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President [[Bill Clinton|Clinton]] in 2000.<ref>{{Cite web |title=President Clinton Orders the Cessation of GPS Selective Availability |url=https://clintonwhitehouse4.archives.gov/WH/EOP/OSTP/html/0053.html }}</ref>

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.