Editing Global Positioning System (section)

=== Error sources and analysis ===
{{Main|Error analysis for the Global Positioning System}}

GPS error analysis examines error sources in GPS results and the expected size of those errors. GPS makes corrections for receiver clock errors and other effects, but some residual errors remain uncorrected. Error sources include signal arrival time measurements, numerical calculations, atmospheric effects (ionospheric/tropospheric delays), [[ephemeris]] and clock data, multipath signals, and natural and artificial interference. Magnitude of residual errors from these sources depends on geometric dilution of precision. Artificial errors may result from jamming devices and threaten ships and aircraft<ref>Attewill, Fred. (February 13, 2013) [http://metro.co.uk/2013/02/13/vehicles-that-use-gps-jammers-are-big-threat-to-aircraft-3474922/ Vehicles that use GPS jammers are big threat to aircraft] {{Webarchive|url=https://web.archive.org/web/20130216014922/http://metro.co.uk/2013/02/13/vehicles-that-use-gps-jammers-are-big-threat-to-aircraft-3474922/ |date=February 16, 2013 }}. Metro.co.uk. Retrieved on August 2, 2013.</ref> or from intentional signal degradation through selective availability, which limited accuracy to ≈ {{cvt|6-12|m||-1|}}, but has been switched off since May 1, 2000.<ref>{{cite web
 | url = http://www.gps.gov/systems/gps/modernization/sa/faq/
 | title = Frequently Asked Questions About Selective Availability
 | publisher = National Coordination Office for Space-Based Positioning, Navigation, and Timing (PNT)
 | quote = Selective Availability ended a few minutes past midnight EDT after the end of May 1, 2000. The change occurred simultaneously across the entire satellite constellation.
 | date = October 2001
 | access-date = June 13, 2015
| archive-url = https://web.archive.org/web/20150616044948/http://www.gps.gov/systems/gps/modernization/sa/faq/
 | archive-date = June 16, 2015
| url-status = live
 }}</ref><ref>{{Cite web|url=https://blackboard.vuw.ac.nz/bbcswebdav/pid-1444805-dt-content-rid-2193398_1/courses/2014.1.ESCI203/Esci203_2014_GPS_1.pdf|title=Blackboard}}</ref>