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{{About|the American
{{redirect|GPS|GPS devices|Satellite navigation device|other uses}}
{{Use mdy dates|date=January
{{Use American English|date=July 2020}}
{{Infobox navigation satellite system
| name = Global Positioning System (GPS)
| image =
| country = United States
| type = Military, civilian
| status = Operational
| operator = [[United States Space Force|US Space Force]]<br>([[Mission Delta 31]])
| coverage = Global
| precision = {{convert|30|-|500|cm|ft|sigfig=2|abbr=on}}
| satellites_nominal = 24
| satellites_current = 31 operational
| first_launch = {{start date and age|1978|2|22}}
| last_launch =
|
| regime = 6 [[Medium Earth orbit|MEO]] planes
| orbit_height = {{convert|20180|km|mi|abbr=on}}
| cost = {{Tree list}}
* Initial constellation:
** {{Tree list/branching}}
** $12 billion<ref name="time-gps-cost"/>
{{Tree list/end}}
{{Tree list}}
* Operating cost:
** {{Tree list/branching}}
** $1.84 billion per year (2023)<ref name="time-gps-cost">{{Cite web|date=April 27, 2022|title=Fiscal Year 2023 Program Funding
|url=https://www.gps.gov/policy/funding/2023/|access-date=September 24, 2023}}</ref>
{{Tree list/end}}
| website = {{URL|https://www.gps.gov|gps.gov}}
| orbit_period = {{Fraction|1|2}} [[Sidereal day|sd]] or 11 hours and 58 minutes
| revisit_period = 1 sidereal day
}}
{{multiple image
| perrow = 2
| width = 175
| image1 = GPS
| image2 = Magellan GPS Blazer12.jpg | caption2 =
| image3 = KyotoTaxiRide.jpg | caption3 = [[Automotive navigation system]] in a taxicab
| image4 = 2nd Space Operations Squadron (7033686).jpeg
| caption4 = A [[United States Space Force]] officer operates the Global Positioning System in 2022.}}
{{Geodesy}}
The '''Global Positioning System''' ('''GPS''')
== Overview ==
The GPS project was started by the [[United States Department of Defense|U.S. Department of Defense]] in 1973.<ref name=":1">{{Cite book |last=Lee |first=Eunsu |url=https://books.google.com/books?id=0A9vEAAAQBAJ&dq=The+GPS+project+was+started+by+the+U.S.+Department+of+Defense+in+1973.&pg=PA3 |title=Geographic Information Systems for Intermodal Transportation: Methods, Models, and Applications |date=March 21, 2023 |publisher=Elsevier |isbn=978-0-323-90130-7 |pages=3 |language=en}}</ref> The first prototype spacecraft was launched in 1978 and the full constellation of 24 satellites became operational in 1993.<ref name=":1" /> After [[Korean Air Lines Flight 007]] was shot down when it mistakenly entered Soviet airspace, President [[Ronald Reagan]] determined that the GPS system would be made available for civilian use as of 1988;<ref name=":2">{{Cite web |title=Statement by Deputy Press Secretary Speakes on the Soviet Attack on a Korean Civilian Airliner {{!}} Ronald Reagan |url=https://www.reaganlibrary.gov/archives/speech/statement-deputy-press-secretary-speakes-soviet-attack-korean-civilian-airliner-1 |access-date=2025-05-27 |website=www.reaganlibrary.gov |language=en}}</ref> however, initially this civilian use was limited to an average accuracy of {{convert|100|m|spell=us}} by use of [[Selective Availability]] (SA), a deliberate error introduced into the GPS data that military receivers could correct for.
As civilian GPS usage grew, there was increasing pressure to remove this error. The SA system was temporarily disabled during the [[Gulf War]], as a shortage of military GPS units meant that many US soldiers were using civilian GPS units sent from home. In the 1990s, [[Differential GPS]] systems from the [[US Coast Guard]], [[Federal Aviation Administration]], and similar agencies in other countries began to broadcast local GPS corrections, reducing the effect of both SA degradation and atmospheric effects (that military receivers also corrected for). The U.S. military had also developed methods to perform local GPS jamming, meaning that the ability to globally degrade the system was no longer necessary. As a result, United States President [[Bill Clinton]] signed a bill ordering that Selective Availability be disabled on May 1, 2000;<ref name="gpsgovaccurary">{{cite web |author1=((National Coordination Office for Space-Based Positioning, Navigation, and Timing)) |date=March 3, 2022 |title=GPS Accuracy |url=https://www.gps.gov/systems/gps/performance/accuracy/ |url-status=live |archive-url=https://web.archive.org/web/20220412092629/https://www.gps.gov/systems/gps/performance/accuracy/ |archive-date=April 12, 2022 |access-date=April 12, 2022 |website=GPS.gov}}</ref> and, in 2007, the US government announced that the next generation of GPS satellites would not include the feature at all.
Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of [[GPS Block III]] satellites and Next Generation Operational Control System (OCX)<ref name="losangelesmil">{{cite web|url=https://www.losangeles.spaceforce.mil/?id=18676 |title=Factsheets: GPS Advanced Control Segment (OCX) |publisher=Losangeles.af.mil |date=October 25, 2011 |access-date=November 6, 2011 |url-status=live |archive-url=https://web.archive.org/web/20120503181621/http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=18676 |archive-date=May 3, 2012 }}</ref> which was authorized by the [[United States Congress|U.S. Congress]] in 2000. When Selective Availability was discontinued, GPS was accurate to about {{convert |5 |m |spell=us}}. GPS receivers that use the L5 band have much higher accuracy of {{convert|30|cm|sp=us|0|}}, while those for high-end applications such as engineering and land surveying are accurate to within {{convert|2|cm|abbr=on|frac=4}} and can even provide sub-millimeter accuracy with long-term measurements.<ref name="gpsgovaccurary" /><ref>{{Cite news|url=https://www.theverge.com/circuitbreaker/2017/9/25/16362296/gps-accuracy-improving-one-foot-broadcom|title=GPS will be accurate within one foot in some phones next year|work=The Verge|access-date=January 17, 2018|archive-url=https://web.archive.org/web/20180118113646/https://www.theverge.com/circuitbreaker/2017/9/25/16362296/gps-accuracy-improving-one-foot-broadcom|archive-date=January 18, 2018|url-status=live |last1=Kastrenakes |first1=Jacob |date=September 25, 2017 }}</ref><ref>{{cite news |last1=Moore |first1=Samuel K. |title=Superaccurate GPS Chips Coming to Smartphones in 2018 |url=https://spectrum.ieee.org/superaccurate-gps-chips-coming-to-smartphones-in-2018 |access-date=January 17, 2018 |work=IEEE Spectrum |date=September 21, 2017 |archive-url=https://web.archive.org/web/20180118011412/https://spectrum.ieee.org/tech-talk/semiconductors/design/superaccurate-gps-chips-coming-to-smartphones-in-2018 |archive-date=January 18, 2018 |url-status=live }}</ref> Consumer devices such as smartphones can be accurate to {{convert|4.9|m|abbr=on}} or better when used with assistive services like [[Wi-Fi positioning system|Wi-Fi positioning]].<ref>{{cite journal |url=https://www.nist.gov/how-do-you-measure-it/how-do-you-measure-your-___location-using-gps|title=How Do You Measure Your Location Using GPS? |date=March 17, 2021 |access-date=March 7, 2022 |journal=NIST |publisher= National Institute of Standards and Technology}}</ref>
{{As of|2023|July}}, 18 GPS satellites broadcast L5 signals, which are considered pre-operational prior to being broadcast by a full complement of 24 satellites in 2027.<ref>{{cite web |title=New Civil Signals |url=https://www.gps.gov/systems/gps/modernization/civilsignals/ |website=GPS.gov |access-date=November 22, 2023}}</ref>
== History ==
[[File:AFSC Film, NAVSTAR GPS-Circa 1977.ogv|thumb|Air Force film introducing the Navstar Global Positioning System, circa 1977]]
The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems,<ref>{{cite book|title=The global positioning system: a shared national asset: recommendations for technical improvements and enhancements|last1=National Research Council (U.S.). Committee on the Future of the Global Positioning System|last2=National Academy of Public Administration|publisher=National Academies Press|year=1995|isbn=978-0-309-05283-2|page=16|url=https://books.google.com/books?id=Za8RBP5iTYoC|access-date=August 16, 2013}}</ref> combining ideas from several predecessors, including classified engineering design studies from the 1960s. The [[U.S. Department of Defense]] developed the system, which originally used 24 satellites, for use by the United States military, and became fully operational in 1993. Civilian use was allowed from the 1980s. [[Roger L. Easton]] of the [[Naval Research Laboratory]], [[Ivan A. Getting]] of [[The Aerospace Corporation]], and [[Bradford Parkinson]] of the [[Applied Physics Laboratory]] are credited with inventing it.<ref name="DarrinO'Leary2009">{{cite book|author1=Ann Darrin|author2=Beth L. O'Leary|title=Handbook of Space Engineering, Archaeology, and Heritage|url=https://books.google.com/books?id=dTwIDun4MroC&q=Roger+Easton&pg=PA239|date=June 26, 2009|publisher=CRC Press|isbn=978-1-4200-8432-0|pages=239–240|access-date=July 28, 2021|archive-date=August 14, 2021|archive-url=https://web.archive.org/web/20210814192242/https://books.google.com/books?id=dTwIDun4MroC&q=Roger+Easton&pg=PA239|url-status=live}}</ref> The work of [[Gladys West]] on the creation of the mathematical geodetic Earth model is credited as instrumental in the development of computational techniques for detecting satellite positions with the precision needed for GPS.<ref>{{cite news |last=Butterly |first=Amelia |date=May 20, 2018 |title=100 Women: Gladys West – the 'hidden figure' of GPS |url=https://www.bbc.com/news/world-43812053 |url-status=live |archive-url=https://web.archive.org/web/20190213112200/https://www.bbc.com/news/world-43812053 |archive-date=February 13, 2019 |access-date=January 17, 2019 |work=BBC News}}</ref><ref>{{Cite news |last=Mohdin |first=Aamna |date=November 19, 2020 |title=Gladys West: the hidden figure who helped invent GPS |language=en-GB |work=The Guardian |url=https://www.theguardian.com/society/2020/nov/19/gladys-west-the-hidden-figure-who-helped-invent-gps |access-date=2023-11-29 |issn=0261-3077}}</ref>
The design of GPS is based partly on similar ground-based [[radio-navigation]] systems, such as [[LORAN]] and the [[Decca Navigator System]], developed in the early 1940s. In 1955, [[Friedwardt Winterberg]] proposed a test of [[general relativity]]—detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predicted that the clocks on GPS satellites, as observed by those on Earth, run 38 microseconds faster per day than those on the Earth. The design of GPS corrects{{Citation needed|reason=The provided reference is not about what is really implemented, because that source only explains what would happen under this model. The clocks are synchronized at times, but we don't have a source if actual relativity correction formulas are used or just some correction for the various clock drift reasons.|date=August 2025}} for this difference; because without doing so, GPS calculated positions would accumulate errors of up to {{convert|10|km/day|sp=us|mi/day|0}}.<ref>{{cite book |url=http://bourabai.kz/winter/satelliten.htm |title=Relativistische Zeitdilatation eines künstlichen Satelliten (Relativistic time dilation of an artificial satellite |publisher=Astronautica Acta II (in German) (25). Retrieved October 19, 2014 |access-date=October 20, 2014 |archive-url=https://web.archive.org/web/20140703080406/http://bourabai.kz/winter/satelliten.htm |archive-date=July 3, 2014 |url-status=live }}</ref>
=== Predecessors ===
When the [[Soviet Union]] launched its first artificial satellite ([[Sputnik 1]]) in 1957, two American physicists, William Guier and George Weiffenbach, at [[Johns Hopkins University]]'s [[Applied Physics Laboratory]] (APL) monitored its radio transmissions.<ref name="guier-weiffenbach">{{cite journal|last1=Guier|first1=William H.|last2=Weiffenbach|first2=George C.|title=Genesis of Satellite Navigation |journal=Johns Hopkins APL Technical Digest|volume=19|issue=1|pages=178–181|year=1997|url=http://www.jhuapl.edu/techdigest/td/td1901/guier.pdf|access-date=April 9, 2012|archive-url=https://web.archive.org/web/20120512002742/http://www.jhuapl.edu/techdigest/td/td1901/guier.pdf|archive-date=May 12, 2012}}</ref> Within hours they realized that, because of the [[Doppler effect]], they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their [[UNIVAC I]] computer to perform the heavy calculations required.
[[File:Managers for the Timation program.jpg|thumb|The [[Naval Research Laboratory]]'s managers for the Timation program and, later, the GPS program: [[Roger L. Easton]] (left) and [[Al Bartholemew]].]]
Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem: pinpointing the user's ___location, given the satellite's. (At the time, the Navy was developing the submarine-launched [[UGM-27 Polaris|Polaris]] missile, which required them to know the submarine's ___location.) This led them and APL to develop the [[Transit (satellite)|TRANSIT]] system.<ref>{{citation |author=Johnson |first=Steven |title=Where good ideas come from, the natural history of innovation |year=2010 |place=New York |publisher=Riverhead Books}}</ref> In 1959, ARPA (renamed [[DARPA]] in 1972) also played a role in TRANSIT.<ref>{{cite book |last1=Worth |first1=Helen E. |url=http://space50.jhuapl.edu/pdfs/book.pdf |title=Transit to Tomorrow. Fifty Years of Space Research at The Johns Hopkins University Applied Physics Laboratory |last2=Warren |first2=Mame |year=2009 |access-date=March 3, 2013 |archive-url=https://web.archive.org/web/20201226045330/http://space50.jhuapl.edu/pdfs/book.pdf |archive-date=December 26, 2020 |url-status=live}}</ref><ref name="Alexandrow">{{cite web |author=Alexandrow |first=Catherine |date=April 2008 |title=The Story of GPS |url=http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565 |archive-url=https://web.archive.org/web/20130224065525/http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565 |archive-date=February 24, 2013}}</ref><ref name=gap>{{cite book
|url=http://www.darpa.mil/about/history/first_50_years.aspx|title=DARPA: 50 Years of Bridging the Gap|date=April 2008|archive-url=https://web.archive.org/web/20110506103713/http://www.darpa.mil/About/History/First_50_Years.aspx|archive-date=May 6, 2011}}</ref>
TRANSIT was first successfully tested in 1960.<ref>{{cite web|last=Howell|first=Elizabeth|title=Navstar: GPS Satellite Network|url=http://www.space.com/19794-navstar.html|publisher=SPACE.com|access-date=February 14, 2013|archive-url=https://web.archive.org/web/20130217140737/http://www.space.com/19794-navstar.html|archive-date=February 17, 2013|url-status=live}}</ref> It used a [[satellite constellation|constellation]] of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the [[Timation]] satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.<ref>{{Cite web |title=NRL Launched First Time-Based Navigation Satellite in 1967 |url=https://www.nrl.navy.mil/Media/News/Article/3411925/nrl-launched-first-time-based-navigation-satellite-in-1967/ |access-date=2025-01-05 |website=U.S. Naval Research Laboratory |language=en-US}}</ref>
In the 1970s, the ground-based [[Omega (navigation system)|OMEGA]] navigation system, based on phase comparison of signal transmission from pairs of stations,<ref>{{cite web |author=Proc |first=Jerry |title=Omega |url=http://www.jproc.ca/hyperbolic/omega.html |url-status=live |archive-url=https://web.archive.org/web/20100105155410/http://www.jproc.ca/hyperbolic/omega.html |archive-date=January 5, 2010 |access-date=December 8, 2009 |publisher=Jproc.ca}}</ref> became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.
Although there were wide needs for accurate navigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation of a constellation of navigation satellites. During the [[Cold War]] [[arms race]], the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded.{{citation needed|date=September 2024}} It is also the reason for the ultra-secrecy at that time. The [[nuclear triad]] consisted of the United States Navy's [[submarine-launched ballistic missile]]s (SLBMs) along with [[United States Air Force]] (USAF) [[strategic bomber]]s and [[intercontinental ballistic missile]]s (ICBMs). Considered vital to the [[nuclear strategy|nuclear deterrence]] posture, accurate determination of the SLBM launch position was a [[force multiplication|force multiplier]].
Precise navigation would enable United States [[ballistic missile submarine]]s to get an accurate fix of their positions before they launched their SLBMs.<ref>{{cite web |url=http://www.trimble.com/gps/whygps.shtml#0|archive-url=https://web.archive.org/web/20071018151253/http://www.trimble.com/gps/whygps.shtml#0|archive-date=October 18, 2007|title=Why Did the Department of Defense Develop GPS?|publisher=Trimble Navigation Ltd|access-date=January 13, 2010}}</ref> The USAF, with two-thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The U.S. Navy and U.S. Air Force were developing their own technologies in parallel to solve what was essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (comparable to the Soviet [[RT-23 Molodets|SS-24]] and [[RT-2PM Topol|SS-25]]) and so the need to fix the launch position had similarity to the SLBM situation.
In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN System. A follow-on study, Project 57, was performed in 1963 and it was "in this study that the GPS concept was born
[[File:Navigation Technology Satellite – II.jpg|thumb|Navigation Technology Satellite – II (Timation IV): NTS-II, the first satellite completely designed and built by NRL under GPS Joint Program funding. Launched June 23, 1977.]]
Updates from the Navy TRANSIT system were too slow for the high speeds of Air Force operation. The [[United States Naval Research Laboratory|Naval Research Laboratory]] (NRL) continued making advances with their [[Timation]] (Time Navigation) satellites, first launched in 1967, second launched in 1969, with the third in 1974 carrying the first [[atomic clock]] into orbit and the fourth launched in 1977.<ref>{{cite web|url=http://support.radioshack.com/support_tutorials/gps/gps_tmline.htm|title=A Guide to the Global Positioning System (GPS) – GPS Timeline|publisher=Radio Shack|access-date=January 14, 2010|archive-url=https://web.archive.org/web/20100213100725/http://support.radioshack.com/support_tutorials/gps/gps_tmline.htm|archive-date=February 13, 2010}}</ref>
Another important predecessor to GPS came from a different branch of the United States military. In 1964, the [[United States Army]] orbited its first Sequential Collation of Range ([[SECOR]]) satellite used for geodetic surveying.<ref>{{cite web|title=Geodetic Explorer – A Press Kit|date=October 29, 1965|access-date=October 20, 2015|publisher=NASA |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660002550_1966002550.pdf|archive-url=https://web.archive.org/web/20140211071631/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660002550_1966002550.pdf|archive-date=February 11, 2014|url-status=live}}</ref> The SECOR system included three ground-based transmitters at known locations that would send signals to the satellite transponder in orbit. A fourth ground-based station, at an undetermined position, could then use those signals to fix its ___location precisely. The last SECOR satellite was launched in 1969.<ref>{{cite encyclopedia|url=http://www.astronautix.com/craft/secor.htm|title=SECOR Chronology|encyclopedia=Mark Wade's Encyclopedia Astronautica|access-date=January 19, 2010|archive-url=https://web.archive.org/web/20100116213013/http://astronautix.com/craft/secor.htm|archive-date=January 16, 2010}}</ref>
=== Development ===
With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program. Satellite orbital position errors, induced by variations in the [[gravity field]] and [[Refraction|radar refraction]] among others, had to be resolved. A team led by Harold L. Jury of Pan Am Aerospace Division in Florida from 1970 to 1973, used real-time data assimilation and recursive estimation to do so, reducing systematic and residual errors to a manageable level to permit accurate navigation.<ref>Jury, H. L., 1973, Application of Kalman Filter to Real-Time Navigation using Synchronous Satellites, Proceedings of the 10th International Symposium on Space Technology and Science, Tokyo, Japan, pp. 945–952.</ref>
During Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagon discussed the creation of a ''Defense Navigation Satellite System (DNSS)''. It was at this meeting that the real synthesis that became GPS was created. Later that year, the DNSS program was named ''Navstar.''<ref>{{cite web |url=http://www.au.af.mil/au/cadre/aspj/airchronicles/aureview/1981/may-jun/garwin.htm |title=MX Deployment Reconsidered |website=au.af.mil |archive-url=https://web.archive.org/web/20170625123356/http://www.airuniversity.af.mil/ |archive-date=June 25, 2017 |access-date=June 7, 2013}}</ref> Navstar is often erroneously considered an acronym for "NAVigation System using Timing And Ranging" but was never considered as such by the GPS Joint Program Office (TRW may have once advocated for a different navigational system that used that acronym).<ref>{{Cite book|url=https://history.nasa.gov/sp4801-chapter17.pdf|title=Societal Impact of Spaceflight|last1=Dick|first1=Steven|last2=Launius|first2=Roger|publisher=US Government Printing Office|year=2007|isbn=978-0-16-080190-7|___location=Washington, DC|page=331|access-date=July 20, 2019|archive-url=https://web.archive.org/web/20130303214202/http://history.nasa.gov/sp4801-chapter17.pdf|archive-date=March 3, 2013|url-status=live}}</ref> With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites, ''Navstar-GPS''.<ref>{{cite book |author1=Rip |first=Michael Russell |url={{google books|plainurl=y|id=mB9W3H90KDUC}} |title=The Precision Revolution: GPS and the Future of Aerial Warfare |author2=James M. Hasik |publisher=Naval Institute Press |year=2002 |isbn=978-1-55750-973-4 |page=65 |access-date=January 14, 2010}}</ref> Ten "[[GPS Block I|Block I]]" prototype satellites were launched between 1978 and 1985 (an additional unit was destroyed in a launch failure).<ref name="ieee2008">{{cite journal | title = Evolution of the Global Navigation SatelliteSystem (GNSS) | first1 = Christopher J. | last1 = Hegarty | first2 = Eric | last2 = Chatre | journal = Proceedings of the IEEE | date = December 2008 | pages = 1902–1917 | doi = 10.1109/JPROC.2008.2006090 | volume=96| issue = 12 | s2cid = 838848 |issn = 0018-9219 }}</ref>
The effect of the ionosphere on radio transmission was investigated in a geophysics laboratory of [[Air Force Cambridge Research Laboratory]], renamed to Air Force Geophysical Research Lab (AFGRL) in 1974. AFGRL developed the Klobuchar model for computing [[ionosphere|ionospheric]] corrections to GPS ___location.<ref>{{cite web |title=ION Fellow – Mr. John A. Klobuchar |url=https://www.ion.org/awards/2003-ionfellow-Klobuchar.cfm |url-status=live |archive-url=https://web.archive.org/web/20171004140058/https://www.ion.org/awards/2003-ionfellow-Klobuchar.cfm |archive-date=October 4, 2017 |access-date=June 17, 2017 |website=www.ion.org}}</ref> Of note is work done by Australian space scientist [[Elizabeth Essex-Cohen]] at AFGRL in 1974. She was concerned with the curving of the paths of radio waves ([[atmospheric refraction]]) traversing the ionosphere from NavSTAR satellites.<ref>{{cite web |url=http://harveycohen.net/crcss |title=GPS Signal Science |website=harveycohen.net |archive-url=https://web.archive.org/web/20170529200107/http://harveycohen.net/crcss/ |archive-date=May 29, 2017}}</ref>
After [[Korean Air Lines Flight 007]], a [[Boeing 747]] carrying 269 people, was shot down by a Soviet [[interceptor aircraft]] after straying in [[prohibited airspace]] because of navigational errors,<ref>{{cite web|url=http://www.icao.int/cgi/goto_m.pl?icao/en/trivia/kal_flight_007.htm |title=ICAO Completes Fact-Finding Investigation |publisher=International Civil Aviation Organization |access-date=September 15, 2008 |archive-url=https://web.archive.org/web/20080517005421/http://www.icao.int/cgi/goto_m.pl?icao%2Fen%2Ftrivia%2Fkal_flight_007.htm |archive-date=May 17, 2008 }}</ref> in the vicinity of [[Sakhalin]] and [[Moneron Island]]s, President [[Ronald Reagan]] issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good.<ref name="KAL007">{{cite news|url=http://iipdigital.usembassy.gov/st/english/article/2006/02/20060203125928lcnirellep0.5061609.html|archive-url=https://web.archive.org/web/20131009161500/http://iipdigital.usembassy.gov/st/english/article/2006/02/20060203125928lcnirellep0.5061609.html|archive-date=October 9, 2013|access-date=June 17, 2019|title=United States Updates Global Positioning System Technology|publisher=America.gov|date=February 3, 2006}}</ref> The first Block II satellite was launched on February 14, 1989,<ref>{{cite book|last1=Rumerman|first1=Judy A.|title=NASA Historical Data Book, Volume VII|date=2009|publisher=NASA|page=136|url=https://history.nasa.gov/SP-4012v7ch2.pdf|access-date=July 12, 2017|archive-url=https://web.archive.org/web/20171225230629/https://history.nasa.gov/SP-4012v7ch2.pdf|archive-date=December 25, 2017|url-status=live}}</ref> and the 24th satellite was launched in 1994. The GPS program cost at this point, not including the cost of the user equipment but including the costs of the satellite launches, has been estimated at US$5 billion (equivalent to ${{Inflation|US|5|1994|fmt=c}} billion in {{Inflation/year|US}}).<ref>Scott Pace, Gerald P. Frost, Irving Lachow, David R. Frelinger, Donna Fossum, Don Wassem, Monica M. Pinto. The Global Positioning System Assessing National Policies, Rand Corporation, 1995, [https://www.rand.org/content/dam/rand/pubs/monograph_reports/MR614/MR614.appb.pdf Appendix B]. {{Webarchive|url=https://web.archive.org/web/20160304094441/https://www.rand.org/content/dam/rand/pubs/monograph_reports/MR614/MR614.appb.pdf|date=March 4, 2016}}, GPS History, Chronology, and Budgets.</ref>
Initially, the highest-quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded, in a policy known as [[Selective Availability]]. This changed on May 1, 2000, with U.S. President [[Bill Clinton]] signing a policy directive to turn off Selective Availability to provide the same accuracy to civilians that was afforded to the military. The directive was proposed by the U.S. Secretary of Defense, [[William J. Perry|William Perry]], in view of the widespread growth of [[differential GPS]] services by private industry to improve civilian accuracy. Moreover, the U.S. military was developing technologies to deny GPS service to potential adversaries on a regional basis.<ref>{{cite web|url=http://ngs.woc.noaa.gov/FGCS/info/sans_SA/docs/GPS_SA_Event_QAs.pdf |title=GPS & Selective Availability Q&A |publisher=NOAA |access-date=May 28, 2010 |archive-url=https://web.archive.org/web/20050921115614/http://ngs.woc.noaa.gov/FGCS/info/sans_SA/docs/GPS_SA_Event_QAs.pdf |archive-date=September 21, 2005 }}</ref> Selective Availability was removed from the GPS architecture beginning with GPS-III.
GPS is owned and operated by the United States government as a national resource. The Department of Defense is the steward of GPS. The ''Interagency GPS Executive Board (IGEB)'' oversaw GPS policy matters from 1996 to 2004. After that, the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems.<ref>{{cite web|last=
The U.S. Department of Defense is required by law to "maintain a Standard Positioning Service (as defined in the federal radio navigation plan and the standard positioning service signal specification) that will be available on a continuous, worldwide basis
=== Timeline and modernization ===
{{prose|section|date=July 2023}}
[[File:GPS-0012 San Diego Air & Space Museum.jpg|thumb|[[Test article (aerospace)|Qualification vehicle]] for GPS Block II on display in [[San Diego Air & Space Museum|San Diego]] – the only vehicle on public display.<ref>{{Cite web |last=Czopek |first=Frank |title=GPS 12 |url=https://www.ion.org/museum/item_view.cfm?cid=4&scid=15&iid=23 |access-date=October 14, 2024 |website=Institute of Navigation – Navigation Museum}}</ref>]]
{|class="wikitable"
|+ Summary of satellites<ref>[http://www.insidegnss.com/node/918 GPS Wing Reaches GPS III IBR Milestone] {{Webarchive|url=https://web.archive.org/web/20130523204537/http://www.insidegnss.com/node/918 |date=May 23, 2013 }} in ''[[Inside GNSS]]'' November 10, 2008</ref><ref>{{cite web |url=http://www.navcen.uscg.gov/?Do=constellationStatus |title=GPS Constellation Status for 08/26/2015 |access-date=August 26, 2015 |archive-url=https://web.archive.org/web/20150905082039/http://www.navcen.uscg.gov/?Do=constellationStatus |archive-date=September 5, 2015 |url-status=live }}</ref><ref>{{cite web|url=http://spaceflightnow.com/2015/10/31/recap-story-three-atlas-5-launch-successes-in-one-month/|title=Recap story: Three Atlas 5 launch successes in one month|date=October 31, 2015 |access-date=October 31, 2015|archive-url=https://web.archive.org/web/20151101182626/http://spaceflightnow.com/2015/10/31/recap-story-three-atlas-5-launch-successes-in-one-month/|archive-date=November 1, 2015|url-status=live}}</ref>
! rowspan="2" | Block || rowspan="2" | Launch <br />period || colspan="4" | Satellite launches || rowspan="2" | Currently<br /> in orbit<br /> and healthy
|-
!
|-
! [[GPS Block I|I]]
Line 128 ⟶ 130:
|-
! [[GPS Block IIF|IIF]]
| 2010–2016 || 12 || 0 || 0 || 0 ||
|-
! [[GPS Block IIIA|IIIA]]
| 2018– ||
|-
! [[GPS Block IIIF|IIIF]]
Line 137 ⟶ 139:
|-
! colspan="2" | Total
|
|-
| colspan="7" style="font-size: smaller;" | (Last update:
[[USA-203]] from Block IIR-M is unhealthy<br /><ref>{{cite web|url=http://www.navcen.uscg.gov/?pageName=gpsAlmanacs|title=GPS almanacs|publisher=Navcen.uscg.gov|access-date=October 15, 2010|archive-url=https://web.archive.org/web/20100923053920/http://www.navcen.uscg.gov/?pageName=gpsAlmanacs|archive-date=September 23, 2010|url-status=live}}</ref> For a more complete list, see ''[[List of GPS satellites]]''
|}
* In 1972, the
* In 1978, the first experimental Block-I GPS satellite was launched.<ref name="ieee2008" />
* In 1983, after Soviet
* By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
* Beginning in 1988, command and control of these satellites was moved from [[Onizuka AFS]], California to the [[2nd Space Operations Squadron|2nd Satellite Control Squadron]] (2SCS) located at
* On February 14, 1989, the first modern Block-II satellite was launched.
* The [[Gulf War]] from 1990 to 1991 was the first conflict in which the military widely used GPS.<ref>[https://www.rand.org/pubs/monograph_reports/MR614.html The Global Positioning System: Assessing National Policies] {{Webarchive|url=https://web.archive.org/web/20151230101234/http://www.rand.org/pubs/monograph_reports/MR614.html |date=December 30, 2015 }}, p.245. RAND corporation</ref>
* In 1991,
* In 1991, [[TomTom]], a Dutch sat-nav manufacturer was founded.
* In 1992, the [[2nd Space Wing]], which originally managed the system, was inactivated and replaced by the [[50th Space Wing]].
* By December 1993, GPS achieved [[initial operational capability]] (IOC), with a full constellation (24 satellites) available and providing the Standard Positioning Service (SPS).<ref name="IOCFOC">{{cite web|url=http://tycho.usno.navy.mil/gpsinfo.html|title=USNO NAVSTAR Global Positioning System|publisher=U.S. Naval Observatory|access-date=January 7, 2011|archive-url=https://web.archive.org/web/20110126200746/http://tycho.usno.navy.mil/gpsinfo.html|archive-date=January 26, 2011}}</ref>
* Full Operational Capability (FOC) was declared by [[Air Force Space Command]] (AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS).<ref name="IOCFOC" />
* In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President [[Bill Clinton]] issued a policy directive<ref>[[National Archives and Records Administration]]. [http://clinton4.nara.gov/textonly/WH/EOP/OSTP/html/gps-factsheet.html U.S. Global Positioning System Policy] {{Webarchive|url=https://web.archive.org/web/20060406125528/http://clinton4.nara.gov/textonly/WH/EOP/OSTP/html/gps-factsheet.html |date=April 6, 2006 }}. March 29, 1996.</ref> declaring GPS a [[dual-use]] system and establishing an [[Interagency GPS Executive Board]] to manage it as a national asset.
Line 157 ⟶ 160:
* On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally.
* In 2004, the United States government signed an agreement with the European Community establishing cooperation related to GPS and Europe's [[Galileo (satellite navigation)|Galileo system]].
* In 2004, United States President [[George W. Bush]] updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.<ref>{{cite web|url=http://pnt.gov/ |title=National Executive Committee for Space-Based Positioning, Navigation, and Timing |publisher=Pnt.gov |access-date=October 15, 2010
* In November 2004, [[Qualcomm]] announced successful tests of [[assisted GPS]] for [[mobile phones]].<ref>{{cite web|url=http://www.3g.co.uk/PR/November2004/8641.htm|title=Assisted-GPS Test Calls for 3G WCDMA Networks|date=November 10, 2004|publisher=3g.co.uk|access-date=November 24, 2010
* In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.<ref>{{cite web|title=Press release: First Modernized GPS Satellite Built by Lockheed Martin Launched Successfully by the U.S. Air Force – Sep 26, 2005|url=http://news.lockheedmartin.com/2005-09-26-First-Modernized-GPS-Satellite-Built-by-Lockheed-Martin-Launched-Successfully-by-the-U-S-Air-Force|publisher=Lockheed Martin|ref=September 26, 2005|language=en|access-date=August 9, 2017|archive-url=https://web.archive.org/web/20170810090450/http://news.lockheedmartin.com/2005-09-26-First-Modernized-GPS-Satellite-Built-by-Lockheed-Martin-Launched-Successfully-by-the-U-S-Air-Force|archive-date=August 10, 2017|url-status=live}}</ref>
* On September 14, 2007, the aging mainframe-based [[Ground segment
* On May 19, 2009, the United States [[Government Accountability Office]] issued a report warning that some GPS satellites could fail as soon as 2010.<ref>{{cite news|url=https://www.theguardian.com/technology/2009/may/19/gps-close-to-breakdown|title=GPS system 'close to breakdown'|last=Johnson|first=Bobbie|newspaper=The Guardian|date=May 19, 2009|access-date=December 8, 2009|___location=London|archive-url=https://web.archive.org/web/20130926155833/http://www.theguardian.com/technology/2009/may/19/gps-close-to-breakdown|archive-date=September 26, 2013|url-status=live}}</ref>
* On May 21, 2009, the [[Air Force Space Command]] allayed fears of GPS failure, saying: "There's only a small risk we will not continue to exceed our performance standard."<ref>{{cite news|url=https://abcnews.go.com/Technology/AheadoftheCurve/story?id=7647002&page=1|title=Air Force Responds to GPS Outage Concerns|last=Coursey|first=David|date=May 21, 2009|work=ABC News|access-date=May 22, 2009|archive-url=https://web.archive.org/web/20090523175214/http://abcnews.go.com/Technology/AheadoftheCurve/story?id=7647002&page=1|archive-date=May 23, 2009|url-status=live}}</ref>
* On January 11, 2010, an update of ground control systems caused a software incompatibility with 8,000 to 10,000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale,
* On February 25, 2010,<ref>{{cite web|url=
* July 24, 2020, operation of the GPS constellation is transferred to the newly established [[United States Space Force|U.S. Space Force]] as part of its establishment.<ref name="2nd Space Operations Squadron">{{Cite web |title=2nd Space Operations Squadron |url=https://www.petersonschriever.spaceforce.mil/About-Us/Fact-Sheets/Display/Article/2814232/2nd-space-operations-squadron/ |access-date=2024-10-15 |website=Peterson and Schriever Space Force Base |language=en-US}}</ref> [[File:2nd Space Operations Squadron emblem.png|thumb|266x266px|Emblem of the [[2nd Space Operations Squadron]] – the unit responsible for operating the constellation ]]
* On October 13, 2023, the Space Force activated [[Positioning, Navigation, and Timing Integrated Mission Delta (Provisional)|PNT Delta (Provisional)]] to manage US [[Satellite navigation|navigation warfare]] assets. [[2nd Space Operations Squadron|2SOPS]] and GPS operations were realigned under this new [[Structure of the United States Space Force|Delta]].<ref name="2nd Space Operations Squadron"/>
=== Awards ===
[[File:Dr Gladys West.jpg|alt=Air Force Space Commander presents
On February 10, 1993, the [[National Aeronautic Association]] selected the GPS Team as winners of the 1992 [[Collier Trophy|Robert J. Collier Trophy]], the US's most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the
Two GPS developers received the [[United States National Academy of Engineering|National Academy of Engineering]] [[Charles Stark Draper Prize]] for 2003:
* [[Ivan Getting]], emeritus president of [[The Aerospace Corporation]] and an engineer at [[Massachusetts Institute of Technology
* [[Bradford Parkinson]], professor of [[aeronautics]] and [[astronautics]] at [[Stanford University]], conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel.
GPS developer [[Roger L. Easton]] received the [[National Medal of Technology]] on February 13, 2006.<ref>{{cite web |url-status=live |agency=[[United States Naval Research Laboratory]]
On October 4, 2011, the [[International Astronautical Federation]] (IAF) awarded the Global Positioning System (GPS) its 60th Anniversary Award, nominated by IAF member, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognized the uniqueness of the GPS program and the exemplary role it has played in building international collaboration for the benefit of humanity.<ref>{{cite web|url=https://www.gps.gov/news/2011/10/IAC-award/|title=GPS Program Receives International Award|date=October 5, 2011|website=GPS.gov |first1=Richard A. |last1=Williams Jr. |archive-url=https://web.archive.org/web/20170513140254/http://www.gps.gov/news/2011/10/IAC-award/|archive-date=May 13, 2017|access-date=December 24, 2018}}</ref> On December 6, 2018, Gladys West was inducted into the Air Force Space and Missile Pioneers Hall of Fame in recognition of her work on an extremely accurate geodetic Earth model, which was ultimately used to determine the orbit of the GPS constellation.<ref>{{Cite web|title=Mathematician inducted into Space and Missiles Pioneers Hall of Fame|url=https://www.afspc.af.mil/News/Article-Display/Article/1707464/mathematician-inducted-into-space-and-missiles-pioneers-hall-of-fame/|access-date=August 3, 2021|website=Air Force Space Command |date=December 7, 2018 |language=en-US|archive-date=June 3, 2019|archive-url=https://web.archive.org/web/20190603171222/https://www.afspc.af.mil/News/Article-Display/Article/1707464/mathematician-inducted-into-space-and-missiles-pioneers-hall-of-fame/|url-status=dead}}</ref><ref>{{Cite web |title=dinh vi gps |url=https://dinhvigps.vn/ |access-date=2025-01-05 |website= |language=en}}</ref> On February 12, 2019, four founding members of the project were awarded the Queen Elizabeth Prize for Engineering with the chair of the awarding board stating: "Engineering is the foundation of civilisation; ...They've re-written, in a major way, the infrastructure of our world."<ref>{{cite news|url=https://www.bbc.com/news/science-environment-47212151|title=Queen Elizabeth Prize for Engineering: GPS pioneers lauded |first=Jonathan|last=Amos|work=BBC News|date=February 12, 2019|access-date=April 6, 2019|archive-url=https://web.archive.org/web/20190406234539/https://www.bbc.com/news/science-environment-47212151|archive-date=April 6, 2019|url-status=live}}</ref>
== Principles ==
{{more citations needed section|date=March 2015}}
The GPS satellites carry very stable [[atomic clocks]] that are synchronized with one another and with the reference atomic clocks at the ground control stations; any drift of the clocks aboard the satellites from the reference time maintained on the ground stations is corrected regularly.<ref>{{Cite web |last=Nelson |first=Jon |date=June 19, 2019 |title=What Is an Atomic Clock? |url=http://www.nasa.gov/feature/jpl/what-is-an-atomic-clock |access-date=2023-04-04 |website=NASA |url-status=live |archive-url=https://web.archive.org/web/20230405131119/https://www.nasa.gov/feature/jpl/what-is-an-atomic-clock |archive-date= April 5, 2023 }}</ref> Since the speed of [[radio wave]]s ([[speed of light]])<ref>{{Cite web |title=Radio wave {{!}} Examples, Uses, Facts, & Range |url=https://www.britannica.com/science/radio-wave |access-date=2023-04-04 |website=Britannica |language=en}}</ref> is constant and independent of the satellite speed, the time delay between when the satellite transmits a signal and the ground station receives it is proportional to the distance from the satellite to the ground station. With the distance information collected from multiple ground stations, the ___location coordinates of any satellite at any time can be calculated with great precision.
=== More detailed description ===
Line 201 ⟶ 195:
* A message that includes the time of transmission (TOT) of the code epoch (in GPS time scale) and the satellite position at that time
Conceptually, the receiver measures the TOAs (according to its own clock) of four satellite signals. From the TOAs and the TOTs, the receiver forms four [[time of flight]] (TOF) values, which are (given the speed of light) approximately equivalent to receiver-satellite ranges plus time difference between the receiver and GPS satellites multiplied by speed of light, which are called
In practice the receiver position (in three dimensional [[Cartesian coordinate system|Cartesian coordinates]] with origin at the Earth's center) and the offset of the receiver clock relative to the GPS time are computed simultaneously, using the [[#Navigation equations|navigation equations]] to process the TOFs.
The receiver's Earth-centered solution ___location is usually converted to [[latitude]], [[longitude]] and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to the [[geoid]], which is essentially mean
=== User-satellite geometry ===
{{further|#Geometric interpretation}}
Although usually not formed explicitly in the receiver processing, the conceptual time differences of arrival (TDOAs) define the measurement geometry. Each TDOA corresponds to a [[hyperboloid]] of revolution (see [[Multilateration]]). The line connecting the two satellites involved (and its extensions) forms the axis of the hyperboloid. The receiver is located at the point where three hyperboloids intersect.<ref name="Abel1">{{cite journal |
It is sometimes incorrectly said that the user ___location is at the intersection of three spheres. While simpler to visualize, this is the case only if the receiver has a clock synchronized with the satellite clocks (i.e., the receiver measures true ranges to the satellites rather than range differences). There are marked performance benefits to the user carrying a clock synchronized with the satellites. Foremost is that only three satellites are needed to compute a position solution. If it were an essential part of the GPS concept that all users needed to carry a synchronized clock, a smaller number of satellites could be deployed, but the cost and complexity of the user equipment would increase.
Line 217 ⟶ 211:
The description above is representative of a receiver start-up situation. Most receivers have a [[track algorithm]], sometimes called a ''tracker'', that combines sets of satellite measurements collected at different times—in effect, taking advantage of the fact that successive receiver positions are usually close to each other. After a set of measurements are processed, the tracker predicts the receiver ___location corresponding to the next set of satellite measurements. When the new measurements are collected, the receiver uses a weighting scheme to combine the new measurements with the tracker prediction. In general, a tracker can (a) improve receiver position and time accuracy, (b) reject bad measurements, and (c) estimate receiver speed and direction.
The disadvantage of a tracker is that changes in speed or direction can be computed only with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the [[random error]] of position measurement. GPS units can use measurements of the [[Doppler shift]] of the signals received to compute velocity accurately.<ref>{{cite book |title=Global Positioning Systems, Inertial Navigation, and Integration |edition=2nd |first1=Mohinder S. |last1=Grewal |first2=Lawrence R. |last2=Weill |first3=Angus P. |last3=Andrews |publisher=John Wiley & Sons |year=2007 |isbn=978-0-470-09971-1 |pages=
=== Non-navigation applications ===
{{for|a list of applications|#Applications}}
GPS requires four or more satellites to be visible for accurate navigation.<ref>{{Cite web |last= |first= |last2= |first2= |last3= |first3= |title=The Global Positioning System: Global Positioning Tutorial |url=https://oceanservice.noaa.gov/education/tutorial_geodesy/geo09_gps.html#:~:text=It%20takes%20four%20GPS%20satellites,error%20in%20the%20receiver's%20clock. |access-date=2025-01-05 |website=oceanservice.noaa.gov |language=EN-US}}</ref> The solution of the [[#Navigation equations|navigation equations]] gives the position of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day, thereby eliminating the need for a more precise and possibly impractical receiver based clock. Applications for GPS such as [[time transfer]], traffic signal timing, and [[IS-95#Physical layer|synchronization of cell phone base stations]], [[#Timekeeping|make use of]] this cheap and highly accurate timing. Some GPS applications use this time for display, or, other than for the basic position calculations, do not use it at all.
<!--This paragraph seems to be in the wrong section, or possibly the section heading needs changing to reflect its current content.-->Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship
== Structure ==
{{more citations needed section|date=March 2015}}
The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment.<ref
=== Space segment ===
{{See also|GPS satellite blocks|List of GPS satellites}}
[[File:160921-F-0000U-001.jpg|thumb|GPS II underwent a four-month series of qualification tests in the AEDC Mark I Space Chamber to determine whether the satellite could withstand extreme heat and cold in space, 1985.]]
[[File:GPS24goldenSML.gif|thumb|upright=1.35|A visual example of a 24
The space segment (SS) is composed of 24 to 32 satellites, or Space Vehicles (SV), in [[medium Earth orbit]], and also includes the payload adapters to the boosters required to launch them into orbit. The GPS design originally called for 24 SVs, eight each in three approximately circular [[orbital plane (astronomy)|orbits]],<ref>{{cite journal |first=P. |last=Daly |date=December 1993 |title=Navstar GPS and GLONASS: global satellite navigation systems |journal=Electronics & Communication Engineering Journal |volume=5 |issue=6 |pages=349–357 |doi=10.1049/ecej:19930069|doi-broken-date=July 6, 2025 }}</ref> but this was modified to six orbital planes with four satellites each.<ref>{{cite web|last=Dana|first=Peter H.|format=GIF|url=http://www.colorado.edu/geography/gcraft/notes/gps/gif/oplanes.gif|title=GPS Orbital Planes|date=August 8, 1996|access-date=February 27, 2006|archive-url=https://web.archive.org/web/20180126111533/https://www.colorado.edu/geography/gcraft/notes/gps/gif/oplanes.gif|archive-date=January 26, 2018
Orbiting at an altitude of approximately {{convert|20200|km|mi|abbr=on}}; orbital radius of approximately {{convert|26600|km|mi|abbr=on}},<ref>{{cite book|title=Global Positioning: Technologies and Performance |first1=Nel |last1=Samama |publisher=John Wiley & Sons |year=2008 |isbn=978-0-470-24190-5 |page=
{{As of|2019|2}},<ref>{{cite web |url=https://www.gps.gov/systems/gps/space |title=Space Segment |publisher=GPS.gov |access-date=July 27, 2019 |archive-url=https://web.archive.org/web/20190718190908/https://www.gps.gov/systems/gps/space/ |archive-date=July 18, 2019 |url-status=live }}</ref> there are 31 satellites in the GPS [[satellite constellation|constellation]], 27 of which are in use at a given time with the rest allocated as stand-bys. A 32nd was launched in 2018, but as of July 2019 is still in evaluation. More decommissioned satellites are in orbit and available as spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve accuracy but also improves reliability and availability of the system, relative to a uniform system, when multiple satellites fail.<ref>{{cite journal|last=Massatt|first=Paul|author2=Wayne Brady|url=http://www.aero.org/publications/crosslink/summer2002/index.html|title=Optimizing performance through constellation management|journal=Crosslink|date=Summer 2002|pages=17–21|archive-url=https://web.archive.org/web/20120125065043/http://www.aero.org/publications/crosslink/pdfs/CrosslinkV3N2.pdf|archive-date=January 25, 2012 }}</ref> With the expanded constellation, nine satellites are usually visible at any time from any point on the
=== Control segment ===
[[File:GPS monitor station.jpg|right|thumb|upright=0.8|Ground monitor station used from 1984 to 2007, on display at the [[Air Force Space and Missile Museum]]
The control segment (CS) is composed of:
Line 250 ⟶ 244:
# six dedicated monitor stations.
The MCS can also access [[Satellite Control Network]] (SCN) ground antennas (for additional command and control capability) and NGA ([[National Geospatial-Intelligence Agency]]) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in
OCS replaced the 1970s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported U.S. armed forces.
{{anchor|OCX}}OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System<ref name="losangelesmil" /> (OCX), is fully developed and functional. The U.S. Department of Defense has claimed that the new capabilities provided by OCX will be the cornerstone for enhancing GPS's mission capabilities, enabling U.S. Space Force to enhance GPS operational services to U.S. combat forces, civil partners and domestic and international users.<ref>{{Cite web |title=DoD Decision Breathes New Life into Critical OCX Satellite Program |url=https://www.defense.gov/News/News-Stories/Article/Article/974228/dod-decision-breathes-new-life-into-critical-ocx-satellite-program/https://www.defense.gov/News/News-Stories/Article/Article/974228/dod-decision-breathes-new-life-into-critical-ocx-satellite-program/ |access-date=2023-11-26 |website=U.S. Department of Defense |language=en-US}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</ref><ref>{{Cite web |title=GPS.gov: Next Generation Operational Control System (OCX) |url=https://www.gps.gov/systems/gps/control/OCX/ |access-date=2023-11-26 |website=www.gps.gov}}</ref> The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50%<ref>{{cite web|url=http://www.defenseindustrydaily.com/The-USAs-GPS-III-Satellites-04900/|title=The USA's GPS-III Satellites|date=October 13, 2011|publisher=Defense Industry Daily|access-date=October 27, 2011|archive-url=https://web.archive.org/web/20111018184806/http://www.defenseindustrydaily.com/The-USAs-GPS-III-Satellites-04900/|archive-date=October 18, 2011|url-status=live}}</ref> sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX is expected to cost millions of dollars less than the cost to upgrade OCS while providing four times the capability.
The GPS OCX program represents a critical part of GPS modernization and provides information assurance improvements over the current GPS OCS program.
* OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
* Built on a flexible architecture that can rapidly adapt to
* Provides the warfighter with more secure, actionable and predictive information to enhance situational awareness.
* Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
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* Supports higher volume near real-time command and control capabilities and abilities.
On September 14, 2011,<ref>{{cite web|url=http://www.comspacewatch.com/news/viewpr.html?pid=34625|title=GPS Completes Next Generation Operational Control System PDR|date=September 14, 2011|publisher=Air Force Space Command News Service
The project remained delayed in 2023, and was (as of June 2023) 73% over its original estimated budget.<ref>{{Cite news |date=June 21, 2023 |title=Raytheon's $7 Billion GPS Stations Are Running 73% Over Estimates |language=en |work=Bloomberg.com |url=https://www.bloomberg.com/news/articles/2023-06-21/raytheon-s-7-billion-ocx-gps-ground-stations-draw-the-ire-of-house-panel |access-date=2023-11-26}}</ref><ref>{{Cite web |last=Albon |first=Courtney |date=June 9, 2023 |title=Space Force sees further delays to 'troubled' GPS ground segment |url=https://www.c4isrnet.com/battlefield-tech/space/2023/06/09/space-force-sees-further-delays-to-troubled-gps-ground-segment/ |access-date=2023-11-26 |website=C4ISRNet |language=en}}</ref> In late 2023, Frank Calvelli, the assistant secretary of the Air Force for space acquisitions and integration, stated that the project was estimated to go live some time during the summer of 2024.<ref>{{Cite web |last=Hitchens |first=Theresa |date=November 7, 2023 |title=Next-gen GPS ground system expected to come online this summer: Calvelli |url=https://breakingdefense.com/2023/11/next-gen-gps-ground-system-expected-to-come-online-this-summer-calvelli/ |access-date=2025-03-15 |website=Breaking Defense |language=en-US }}</ref>
The US Space Force accepted delivery of OCX Blocks I and II from contractor RTX on July 1, 2025, over 8 years behind schedule and roughly $4 billion over budget due to its monolithic development and constant feature creep while in process. If current Government Accountability Office estimates hold, the new system will enter service in December 2025.
OCX Block 3F is currently in development to enable command and control of GPS IIIF satellites, currently slated to begin launching in 2027.<ref>{{Cite web |last=Hadley |first=Greg |date=July 16, 2025 |title=Space Force Accepts New GPS Control System After Years of Delays |url=https://www.airandspaceforces.com/space-force-new-operating-system-gps-accepts/ |access-date=2025-07-16 |website=Air & Space Forces Magazine|language=en-US }}</ref>
=== User segment ===
{{further|GPS navigation device}}
[[File:GPS Receivers.jpg|thumb|right|upright=0.8|GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as
[[File:Leica WM 101 at the National Science Museum at Maynooth.JPG|thumb|right|The first portable GPS survey unit, a Leica WM 101, displayed at the [[Irish National Science Museum]] at [[Maynooth]]
The user segment (US) is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a [[crystal oscillator]]). They may also include a display for providing ___location and speed information to the user.
GPS receivers may include an input for differential corrections, using the [[RTCM]] SC-104 format. This is typically in the form of an [[RS-232]] port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.{{Citation needed|date=August 2011}} Receivers with internal DGPS receivers can outperform those using external RTCM data.{{Citation needed|date=August 2011}} {{As of |2006}}, even low-cost units commonly include [[Wide Area Augmentation System]] (WAAS) receivers.
[[File:SiRF Star III основанный на GPS приёмнике с интегрированной антенной.jpg|thumb|right|upright=0.8|A typical GPS receiver with integrated antenna
Many GPS receivers can relay position data to a PC or other device using the [[NMEA 0183]] protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA),<ref>{{cite web|url=http://www.nmea.org/content/nmea_standards/nmea_standards.asp|title=Publications and Standards from the National Marine Electronics Association (NMEA)|publisher=National Marine Electronics Association|access-date=June 27, 2008|archive-url=https://web.archive.org/web/20090804071335/http://www.nmea.org/content/nmea_standards/nmea_standards.asp|archive-date=August 4, 2009
== Applications ==
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=== Civilian ===
[[File:GPS roof antenna dsc06160.jpg|thumb|right|upright=0.8|This [[antenna (radio)|antenna]] is mounted on the roof of a hut containing a scientific experiment needing precise timing.]]
[[File:GPSTest screenshot (2025).webp|thumb|Screenshot of GPSTest application showing GPS and other GNSS satellites usage in [[South Tangerang]], Indonesia (2025)]]
Many civilian applications use one or more of GPS's three basic components: absolute ___location, relative movement, and time transfer.
* [[Amateur radio]]: clock synchronization required for several digital modes such as [[FT8]], FT4 and JS8; also used with [[Automatic Packet Reporting System|APRS]] for position reporting; is often critical during emergency and disaster communications support.
* [[Atmosphere]]: studying the [[troposphere]] delays (recovery of the water vapor content) and [[ionosphere]] delays (recovery of the number of free electrons).<ref>{{cite journal |last1=Hadas |first1=T. |last2=Krypiak-Gregorczyk |first2=A. |last3=Hernández-Pajares |first3=M. |last4=Kaplon |first4=J. |last5=Paziewski |first5=J. |last6=Wielgosz |first6=P. |last7=Garcia-Rigo |first7=A. |last8=Kazmierski |first8=K. |last9=Sosnica |first9=K. |last10=Kwasniak |first10=D. |last11=Sierny |first11=J. |last12=Bosy |first12=J. |last13=Pucilowski |first13=M. |last14=Szyszko |first14=R. |last15=Portasiak |first15=K. |last16=Olivares-Pulido |first16=G. |last17=Gulyaeva |first17=T. |last18=Orus-Perez |first18=R. |title=Impact and Implementation of Higher-Order Ionospheric Effects on Precise GNSS Applications: Higher-Order Ionospheric Effects in GNSS |journal=Journal of Geophysical Research: Solid Earth |date=November 2017 |volume=122 |issue=11 |pages=9420–9436 |doi=10.1002/2017JB014750|hdl=2117/114538 |s2cid=54069697 |hdl-access=free }}</ref> Recovery of Earth surface displacements due to the atmospheric pressure loading.<ref>{{cite journal |last1=Sośnica |first1=Krzysztof |last2=Thaller |first2=Daniela |last3=Dach |first3=Rolf |last4=Jäggi |first4=Adrian |last5=Beutler |first5=Gerhard |title=Impact of loading displacements on SLR-derived parameters and on the consistency between GNSS and SLR results |journal=Journal of Geodesy |date=August 2013 |volume=87 |issue=8 |pages=751–769 |doi=10.1007/s00190-013-0644-1 |bibcode=2013JGeod..87..751S |s2cid=56017067 |url=https://boris.unibe.ch/45844/8/190_2013_Article_644.pdf |access-date=March 2, 2021 |archive-date=March 15, 2021 |archive-url=https://web.archive.org/web/20210315203121/https://boris.unibe.ch/45844/8/190_2013_Article_644.pdf |url-status=live }}</ref>
* [[Astronomy]]: both positional and [[clock synchronization]] data is used in [[astrometry]] and [[celestial mechanics]] and precise orbit determination.<ref>{{cite journal |last1=Bury |first1=Grzegorz |last2=Sośnica |first2=Krzysztof |last3=Zajdel |first3=Radosław |title=Multi-GNSS orbit determination using satellite laser ranging |journal=Journal of Geodesy |date=December 2019 |volume=93 |issue=12 |pages=2447–2463 |doi=10.1007/s00190-018-1143-1|bibcode=2019JGeod..93.2447B |doi-access=free }}</ref> GPS is also used in both [[amateur astronomy]] with [[GoTo (telescopes)|small telescopes]] as well as by professional observatories for finding [[extrasolar planet]]s.
* [[Automated vehicle]]: applying precise vehicle ___location, coupled with [[High-definition map|highly detailed maps]], provides the context needed for cars and trucks to function without a human driver.<ref>{{Cite web |date= February 13, 2020|title=Developments in Modern GNSS and Its Impact on Autonomous Vehicle Architectures |url=https://www.swiftnav.com/resource/white-paper/developments-in-modern-gnss-and-its-impact-on-autonomous-vehicle-0 |access-date=11 June 2025 |website=www.swiftnav.com}}</ref>
* [[Cartography]]: both civilian and military cartographers use GPS extensively.
* [[Cellular telephony]]: clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for [[E911#Wireless enhanced 911|mobile emergency calls]] and other applications. The first [[Mobile GPS navigation|handsets with integrated GPS]] launched in the late 1990s. The U.S. [[Federal Communications Commission]] (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from [[Nextel]] upon launch, followed by [[Sprint Nextel|Sprint]] in 2006, and [[Verizon]] soon thereafter.
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* [[Radio occultation]] for weather and atmospheric science applications.<ref>{{cite web|url=http://www2.ucar.edu/atmosnews/just-published/12183/using-gps-improve-tropical-cyclone-forecasts|title=Using GPS to improve tropical cyclone forecasts|work=ucar.edu|access-date=May 28, 2015|archive-url=https://web.archive.org/web/20150528222132/http://www2.ucar.edu/atmosnews/just-published/12183/using-gps-improve-tropical-cyclone-forecasts|archive-date=May 28, 2015|url-status=live}}</ref>
* [[Fleet tracking]]: used to identify, locate and maintain contact reports with one or more [[fleet vehicle|fleet]] vehicles in real-time.
* [[Geodesy]]: determination of [[Earth orientation parameters]] including the daily and sub-daily polar motion,<ref>{{cite journal |last1=Zajdel |first1=Radosław |last2=Sośnica |first2=Krzysztof |last3=Bury |first3=Grzegorz |last4=Dach |first4=Rolf |last5=Prange |first5=Lars |last6=Kazmierski |first6=Kamil |title=Sub-daily polar motion from GPS, GLONASS, and Galileo |journal=Journal of Geodesy |date=January 2021 |volume=95 |issue=1 |page=3 |doi=10.1007/s00190-020-01453-w| issn=0949-7714|bibcode=2021JGeod..95....3Z |doi-access=free }}</ref> and length-of-day variabilities,<ref>{{cite journal |last1=Zajdel |first1=Radosław |last2=Sośnica |first2=Krzysztof |last3=Bury |first3=Grzegorz |last4=Dach |first4=Rolf |last5=Prange |first5=Lars |title=System-specific systematic errors in earth rotation parameters derived from GPS, GLONASS, and Galileo |journal=GPS Solutions |date=July 2020 |volume=24 |issue=3 |page=74 |doi=10.1007/s10291-020-00989-w|bibcode=2020GPSS...24...74Z |doi-access=free }}</ref> Earth's center-of-mass
* [[Geofence|Geofencing]]: [[vehicle tracking system]]s, [[Handheld tracker|person tracking systems]], and [[Tracking collar|pet tracking]] systems use GPS to locate devices that are attached to or carried by a person, vehicle, or pet. The application can provide continuous tracking and send notifications if the target leaves a designated (or "fenced-in") area.<ref name="
* [[Geotagging]]: applies ___location coordinates to digital objects such as photographs (in [[Exif]] data) and other documents for purposes such as creating map overlays with devices like [[Nikon GP-1]].
* [[GPS aircraft tracking]]
* [[GPS for mining]]: the use of RTK GPS has significantly improved several mining operations such as drilling, shoveling, vehicle tracking, and surveying. RTK GPS provides centimeter-level positioning accuracy.<ref>{{Cite book |last=Sickle |first=Jan Van |url=https://www.taylorfrancis.com/books/mono/10.4324/9780203305225/gps-land-surveyors-jan-van-sickle |title=GPS for Land Surveyors |date=October 10, 2011 |publisher=CRC Press |isbn=978-0-429-14911-5 |edition=3 |___location=Boca Raton |doi=10.4324/9780203305225}}</ref><ref>{{Cite journal |last1=Wesche |first1=Christine |last2=Eisen |first2=Olaf |last3=Oerter |first3=Hans |last4=Schulte |first4=Daniel |last5=Steinhage |first5=Daniel |date=January 2007 |title=Surface topography and ice flow in the vicinity of the EDML deep-drilling site, Antarctica |url=https://www.cambridge.org/core/journals/journal-of-glaciology/article/surface-topography-and-ice-flow-in-the-vicinity-of-the-edml-deepdrilling-site-antarctica/B796D8428791FCC28ABCF298FAC3EABA |journal=Journal of Glaciology |language=en |volume=53 |issue=182 |pages=442–448 |doi=10.3189/002214307783258512 |bibcode=2007JGlac..53..442W |issn=0022-1430}}</ref>
* [[GPS data mining]]: It is possible to aggregate GPS data from multiple users to understand movement patterns, common trajectories and interesting locations.<ref>{{cite conference |author=Khetarpaul, S. |author2=Chauhan, R. |author3=Gupta, S. K. |author4=Subramaniam, L. V. |author5=Nambiar, U. |title=Mining GPS data to determine interesting locations|year=2011|book-title=Proceedings of the 8th International Workshop on Information Integration on the Web}}</ref> GPS data is today used in transportation and disaster engineering to forecast mobility in normal and evacuation situations (e.g., hurricanes, wildfires, earthquakes).<ref>{{Cite journal |last1=Sivalingam |first1=Prahaladhan |last2=Asirvatham |first2=David |last3=Marjani |first3=Mohsen |last4=Syed Masood |first4=Jafar Ali Ibrahim |last5=Chakravarthy |first5=N. S. Kalyan |last6=Veerisetty |first6=Gopinath |last7=Lestari |first7=Martha Tri |date=April 1, 2024 |title=A review of travel behavioural pattern using GPS dataset: A systematic literature review |journal=Measurement: Sensors |volume=32 |article-number=101031 |doi=10.1016/j.measen.2024.101031 |issn=2665-9174|doi-access=free |bibcode=2024MeasS..3201031S }}</ref><ref>{{Cite book |last1=Nakajima |first1=Yuu |last2=Shiina |first2=Hironori |last3=Yamane |first3=Shohei |last4=Ishida |first4=Toru |last5=Yamaki |first5=Hirofumi |chapter=Disaster Evacuation Guide: Using a Massively Multiagent Server and GPS Mobile Phones |date=January 2007 |title=2007 International Symposium on Applications and the Internet |pages=2 |doi=10.1109/SAINT.2007.13}}</ref><ref>{{Cite journal |last1=Zhao |first1=Xilei |last2=Xu |first2=Yiming |last3=Lovreglio |first3=Ruggiero |last4=Kuligowski |first4=Erica |last5=Nilsson |first5=Daniel |last6=Cova |first6=Thomas J. |last7=Wu |first7=Alex |last8=Yan |first8=Xiang |date=June 1, 2022 |title=Estimating wildfire evacuation decision and departure timing using large-scale GPS data |url=https://www.sciencedirect.com/science/article/pii/S136192092200102X |journal=Transportation Research Part D: Transport and Environment |volume=107 |article-number=103277 |doi=10.1016/j.trd.2022.103277 |issn=1361-9209|arxiv=2109.07745 |bibcode=2022TRPD..10703277Z }}</ref><ref>{{Cite journal |last1=Yang |first1=Zhuo |last2=Franz |first2=Mark L. |last3=Zhu |first3=Shanjiang |last4=Mahmoudi |first4=Jina |last5=Nasri |first5=Arefeh |last6=Zhang |first6=Lei |date=January 1, 2018 |title=Analysis of Washington, DC taxi demand using GPS and land-use data |url=https://www.sciencedirect.com/science/article/pii/S0966692317301102 |journal=Journal of Transport Geography |volume=66 |pages=35–44 |doi=10.1016/j.jtrangeo.2017.10.021 |bibcode=2018JTGeo..66...35Y |issn=0966-6923|url-access=subscription }}</ref>
* [[GPS tour]]s: ___location determines what content to display; for instance, information about an approaching point of interest.
* [[
* [[Navigation]]: navigators value digitally precise velocity and orientation measurements, as well as precise positions in real-time with a support of orbit and clock corrections.<ref>{{cite journal |last1=Kazmierski |first1=Kamil |last2=Zajdel |first2=Radoslaw |last3=Sośnica |first3=Krzysztof |title=Evolution of orbit and clock quality for real-time multi-GNSS solutions |journal=GPS Solutions |date=October 2020 |volume=24 |issue=4 |page=111 |doi=10.1007/s10291-020-01026-6|bibcode=2020GPSS...24..111K |doi-access=free }}</ref>
* [[Orbit]] determination of low-orbiting satellites with GPS receiver installed * [[Phasor measurement unit|Phasor measurements]]: GPS enables highly accurate timestamping of power system measurements, making it possible to compute [[Phasor measurement unit|phasors]].
* [[Recreation]]: for example, [[Geocaching]], [[Geodashing]], [[GPS drawing]], [[waymarking]], and other kinds of [[Location-based game|___location based mobile games]] such as ''[[Pokémon Go]]''.
* [[Reference frames]]: realization and densification of the terrestrial reference frames<ref>{{cite journal |last1=Zajdel |first1=R. |last2=Sośnica |first2=K. |last3=Dach |first3=R. |last4=Bury |first4=G. |last5=Prange |first5=L. |last6=Jäggi |first6=A. |title=Network Effects and Handling of the Geocenter Motion in
* [[Robotics]]: self-navigating, autonomous robots using GPS sensors,<ref>{{Cite web|title=GPS Helps Robots Get the Job Done|url=https://www.asme.org/topics-resources/content/gps-helps-robots-get-job-done|access-date=August 3, 2021
*
* [[Surveying]]: surveyors use absolute locations to make maps and determine property boundaries.
* [[Tectonics]]: GPS enables direct fault motion measurement of [[earthquakes]]. Between earthquakes GPS can be used to measure [[Crust (geology)|crustal]] motion and deformation<ref>{{cite web|url=http://www.geodesy.cwu.edu/realtime/|title=The Pacific Northwest Geodetic Array|work=cwu.edu|access-date=October 10, 2014|archive-url=https://web.archive.org/web/20140911110131/http://www.geodesy.cwu.edu/realtime/|archive-date=September 11, 2014|url-status=live}}</ref> to estimate seismic strain buildup for creating [[seismic hazard]] maps.
* [[Telematics]]: GPS technology integrated with computers and mobile communications technology in [[automotive navigation system]]s.
<!--* GPS enables researchers to explore Earth's
<!--
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The GPS value-chain was reshaped considerably in 2007 as several specialist GPS technology developers were acquired by wireless chipset vendors. These transactions are likely to enhance the possibilities to meet handset manufacturers' demand for integrated connectivity solutions that include GPS at ever lower price points to enable true mass market deployment.
Sales of GPS-enabled GSM/WCDMA handsets grew to about 24.5
-->
==== Restrictions on civilian use ====
The U.S. government controls the export of some civilian receivers. All GPS receivers capable of functioning above {{cvt|60,000|ft|km||abbr=in|sp=us}} above sea level and {{cvt|1000|knot|m/s km/h mph|sigfig=1|abbr=in|sp=us}}, or designed or modified for use with unmanned missiles and aircraft, are classified as [[United States Munitions List|munitions]] (weapons)—which means they require [[United States Department of State|State Department]] export licenses.<ref>Arms Control Association.[http://www.armscontrol.org/documents/mtcr Missile Technology Control Regime] {{webarchive|url=https://web.archive.org/web/20080916123933/http://www.armscontrol.org/documents/mtcr |date=September 16, 2008 }}. Retrieved May 17, 2006.</ref> This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse<!-- "Coarse" is correct, as in "not finely detailed"-->/Acquisition) code.
Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule refers to operation at both the target altitude and speed, but some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches that regularly reach {{convert|30|km|ft|sigfig=1|abbr=in|sp=us}}. These limits only apply to units or components exported from the United States. A growing trade in various components exists, including GPS units from other countries. These are expressly sold as [[International Traffic in Arms Regulations|ITAR]]-free.
=== Military ===
[[File:Exelis SINCGARS RT-1523G.jpg|thumb|AN/PRC-119F SINCGARS radio, which requires accurate clock time supplied by an external GPS system to enable frequency hopping operation with other radios]] [[File:US Navy 030319-N-4142G-020 Ordnance handlers assemble Joint Direct Attack Munition (JDAM) bombs in the forward mess decks.jpg|thumb|right|Attaching a GPS guidance kit to
[[File:XM982 Excalibur inert.jpg|thumb|right
As of 2009, military GPS applications include:
* Navigation: Soldiers use GPS to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the ''Commander's Digital Assistant'' and lower ranks use the ''Soldier Digital Assistant''.<ref>{{cite web|last=Sinha|first=Vandana|url=http://gcn.com/articles/2003/07/24/soldiers-take-digital-assistants-to-war.aspx|title=Commanders and Soldiers' GPS-receivers|publisher=Gcn.com|date=July 24, 2003|access-date=October 13, 2009|archive-url=https://web.archive.org/web/20090921064048/http://gcn.com/articles/2003/07/24/soldiers-take-digital-assistants-to-war.aspx|archive-date=September 21, 2009|url-status=live}}</ref>
* Frequency-Hopping Radio Clock Coordination: Military radio systems using [[Frequency-hopping spread spectrum|frequency hopping]] modes, such as [[SINCGARS]] and [[Have Quick|HAVEQUICK]], require all radios within a network to have the same time input to their internal clocks (+/-4 seconds in the case of SINCGARS) to be on the correct frequency at a given time. Military GPS receivers, such as the [[Precision Lightweight GPS Receiver]] (PLGR) and [[Defense Advanced GPS Receiver]] (DAGR), are used by [[radio operator]]s within a radio network to properly input an accurate time to said radios internal clock. More modern military radios have internal GPS receivers that synchronize the internal clock automatically.
* Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile.{{Citation needed|date=November 2007}} These weapon systems pass target coordinates to [[precision-guided munition]]s to allow them to engage targets accurately. Military aircraft, particularly in [[air-to-ground]] roles, use GPS to find targets.
* Missile and projectile guidance: GPS allows accurate targeting of various military weapons including [[ICBM]]s, [[cruise missile]]s, [[precision-guided munition]]s and [[artillery shell]]s. Embedded GPS receivers able to withstand accelerations of 12,000 ''[[g-force|g]]''<ref name="dote-fy2003-69">{{cite report |section=Excalibur Family of Artillery Projectiles |url=https://www.dote.osd.mil/Portals/97/pub/reports/FY2003/other/2003DOTEAnnualReport.pdf |publisher=[[Director, Operational Test and Evaluation]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20211018153227/https://www.dote.osd.mil/Portals/97/pub/reports/FY2003/other/2003DOTEAnnualReport.pdf |archive-date=October 18, 2021 |year=2003 |url-status=live |page=69 |title=FY2003 Annual Report }}</ref> or about {{cvt|118|km/s2|||}} have been developed for use in {{convert|155|mm|in|sp=us|adj=on}} [[howitzer]] shells.<ref name="dote-fy2010-xm982">{{cite
* Search and rescue.
* Reconnaissance: Patrol movement can be managed more closely.
* GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor called a [[bhangmeter]], an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the [[United States Nuclear Detonation Detection System]].<ref>Sandia National Laboratory's [http://www.sandia.gov/LabNews/LN03-07-03/LA2003/la03/arms_story.htm Nonproliferation programs and arms control technology] {{Webarchive|url=https://web.archive.org/web/20060928015946/http://www.sandia.gov/LabNews/LN03-07-03/LA2003/la03/arms_story.htm |date=September 28, 2006 }}</ref><ref>{{cite
GPS type navigation was first used in war in the [[Gulf War|1991 Persian Gulf War]], before GPS was fully developed in 1995, to assist [[Coalition of the Gulf War|Coalition Forces]] to navigate and perform maneuvers in the war.
GPS's vulnerability to jamming is a threat that continues to grow as jamming equipment and experience grows.<ref>{{cite web|url=https://www.militaryaerospace.com/articles/2016/06/gps-jamming-satellite-navigation.html|title=GPS jamming is a growing threat to satellite navigation, positioning, and precision timing|website=www.militaryaerospace.com|date=June 28, 2016 |access-date=March 3, 2019|archive-url=https://web.archive.org/web/20190306044006/https://www.militaryaerospace.com/articles/2016/06/gps-jamming-satellite-navigation.html|archive-date=March 6, 2019|url-status=live}}</ref><ref>{{cite web |url=https://www.nbcnews.com/news/us-news/gps-under-attack-crooks-rogue-workers-wage-electronic-war-n618761 |title=GPS Under Attack as Crooks, Rogue Workers Wage Electronic War |work=NBC News |last=Brunker |first=Mike |date=August 8, 2016 |archive-url=https://web.archive.org/web/20190306051331/https://www.nbcnews.com/news/us-news/gps-under-attack-crooks-rogue-workers-wage-electronic-war-n618761 |archive-date=March 6, 2019 |url-status=live |access-date=December 15, 2021}}</ref> GPS signals have been reported to have been jammed many times over the years for military purposes. Russia seems to have several objectives for this approach, such as intimidating neighbors while undermining confidence in their reliance on American systems, promoting their GLONASS alternative, disrupting Western military exercises, and protecting assets from drones.<ref>{{cite web|url=https://rntfnd.org/2018/04/30/russia-undermining-worlds-confidence-in-gps/|title=Russia Undermining World's Confidence in GPS|date=April 30, 2018|access-date=March 3, 2019|archive-url=https://web.archive.org/web/20190306050610/https://rntfnd.org/2018/04/30/russia-undermining-worlds-confidence-in-gps/|archive-date=March 6, 2019|url-status=live}}</ref> China uses jamming to discourage US surveillance aircraft near the contested [[Spratly Islands]].<ref>{{cite web|url=https://rntfnd.org/2016/09/26/china-jamming-us-forces-gps/|title=China Jamming US Forces' GPS|date=September 26, 2016|access-date=March 3, 2019|archive-url=https://web.archive.org/web/20190306050548/https://rntfnd.org/2016/09/26/china-jamming-us-forces-gps/|archive-date=March 6, 2019|url-status=live}}</ref> [[North Korea]] has mounted several major jamming operations near its border with South Korea and offshore, disrupting flights, shipping and fishing operations.<ref>{{cite web|url=https://www.popularmechanics.com/military/weapons/a20289/north-korea-jamming-gps-signals/|title=North Korea Is Jamming GPS Signals|first=Kyle|last=Mizokami|date=April 5, 2016|website=Popular Mechanics|access-date=March 3, 2019|archive-url=https://web.archive.org/web/20190306043300/https://www.popularmechanics.com/military/weapons/a20289/north-korea-jamming-gps-signals/|archive-date=March 6, 2019|url-status=live}}</ref> Iranian Armed Forces disrupted the civilian airliner plane Flight [[PS752]]'s GPS when it shot down the aircraft.<ref>{{Cite web|date=December 29, 2020|title=Iran Spokesman Confirms Mysterious Disruption Of GPS Signals In Tehran|url=https://iranintl.com/en/iran/iran-spokesman-confirms-mysterious-disruption-gps-signals-tehran|access-date=July 12, 2021|website=Iran International|language=en|archive-date=July 12, 2021|archive-url=https://web.archive.org/web/20210712184125/https://iranintl.com/en/iran/iran-spokesman-confirms-mysterious-disruption-gps-signals-tehran|url-status=live}}</ref><ref>{{Cite web|date=July 12, 2021|title=Evidence shows Iran shot down Ukrainian plane 'intentionally' {{!}} AvaToday|url=https://avatoday.net/node/14295|access-date=July 12, 2021|archive-url=https://web.archive.org/web/20210712184447/https://avatoday.net/node/14295|archive-date=July 12, 2021}}</ref>
In the [[Russo-Ukrainian War]], GPS-guided munitions provided to Ukraine by NATO countries experienced significant failure rates as a result of Russian electronic warfare. Excalibur artillery shells efficiency rate hitting targets dropped from 70% to 6% as Russia adapted its electronic warfare activities.<ref>{{cite news | url=https://www.businessinsider.com/russian-cheap-electronic-warfare-keeps-beating-us-precision-weapons-2024-4 | title=Russian forces have hit on a cheap way to foil US precision weapons in Ukraine | work=[[Business Insider]] |date=April 30, 2024 |first1=Chris |last1=Panella }}</ref>
=== Timekeeping {{anchor|GPS time|GPS time and date}} ===
==== Leap seconds ====
While most clocks derive their time from [[Coordinated Universal Time]] (UTC), the atomic clocks on the satellites are set to
The GPS navigation message includes the difference between GPS time and UTC. {{As of|2017|1|post=,}} GPS time is 18 seconds ahead of UTC because of the leap second added to UTC on December 31, 2016.<ref>{{cite web|title=Notice Advisory to Navstar Users (NANU) 2016069 |access-date=June 25, 2017 |url=https://www.navcen.uscg.gov/?Do=gpsArchives&path=nanu&year=2016&file=25665&type=messageBody--nanuId--NANUS&name=2016069.txt |archive-url=https://web.archive.org/web/20170525063405/https://www.navcen.uscg.gov/?Do=gpsArchives&path=nanu&year=2016&file=25665&type=messageBody--nanuId--NANUS&name=2016069.txt |archive-date=May 25, 2017 |publisher=GPS Operations Center}}</ref> Receivers subtract this offset from GPS time to calculate UTC and specific time zone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits).
==== Accuracy ====
GPS time is theoretically accurate to about 14 nanoseconds, due to the [[clock drift]]
==== Relativistic corrections ====
The GPS implements{{Citation needed|reason=The two provided references are not citations about what is really implemented. The first one only suggest to implement it in the future and the 2nd, while from an author who worked in that field, only explains how such an implementation could work.|date=August 2025}} two major corrections to its time signals for relativistic effects: one for relative velocity of satellite and receiver, using the special theory of relativity, and one for the difference in gravitational potential between satellite and receiver, using general relativity. The acceleration of the satellite could also be computed independently as a correction, depending on purpose, but normally the effect is already dealt with in the first two corrections.<ref>{{Citation|last1=Fliegel|first1=Henry F.|last2=DiEsposti|first2=Raymond S.|date=December 1996|publisher=The Aerospace Corporation|___location=El Segundo, CA|title=GPS and relativity overview|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a516975.pdf|access-date=December 7, 2022|archive-date=March 6, 2023|archive-url=https://web.archive.org/web/20230306071351/https://apps.dtic.mil/dtic/tr/fulltext/u2/a516975.pdf|url-status=dead}}</ref><ref>{{Cite journal |last=Ashby |first=Neil |date=2003 |title=Relativity in the Global Positioning System |journal=Living Reviews in Relativity |volume=6 |issue=1 |article-number=1 |doi=10.12942/lrr-2003-1 |doi-access=free |issn=1433-8351 |pmc=5253894 |pmid=28163638|bibcode=2003LRR.....6....1A }}</ref>
==== Format ====
{{
As opposed to the year, month, and day format of the [[Gregorian calendar]], the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-[[bit]] field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). It happened the second time at 23:59:42 UTC on April 6, 2019. To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern in the future the modernized GPS civil navigation (CNAV) message will use a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until 2137 (157 years after GPS week zero).
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{{Main|GPS signals}}
The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military.<ref>{{cite web |title=GPS.gov: Performance Standards & Specifications |url=https://www.gps.gov/technical/ps/ |website=www.gps.gov |access-date=June 21, 2024}}</ref>
=== Message format ===
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Each GPS satellite continuously broadcasts a ''navigation message'' on L1 (C/A and P/Y) and L2 (P/Y) frequencies at a rate of 50 bits per second (see [[bitrate]]). Each complete message takes 750 seconds ({{frac|12|1|2}} minutes) to complete. The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long. Subframes 4 and 5 are [[commutation (telemetry)|subcommutated]] 25 times each, so that a complete data message requires the transmission of 25 full frames. Each subframe consists of ten words, each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes in a frame times 25 frames in a message, each message is 37,500 bits long. At a transmission rate of 50-bit/s, this gives 750 seconds to transmit an entire [[GPS Almanac|almanac message (GPS)]]. Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.<ref>{{cite web |url=http://gpsinformation.net/gpssignal.htm |title=Satellite message format |publisher=Gpsinformation.net |access-date=October 15, 2010 |archive-url=https://web.archive.org/web/20101101021138/http://gpsinformation.net/gpssignal.htm |archive-date=November 1, 2010 |url-status=live }}</ref>
The first subframe of each frame encodes the week number and the time within the week,<ref>{{cite web |author=Dana |first=Peter H.
All satellites broadcast at the same frequencies, encoding signals using unique [[code-division multiple access]] (CDMA) so receivers can distinguish individual satellites from each other. The system uses two distinct CDMA encoding types: the coarse<!-- "Coarse" is correct, as in "not precision"-->/acquisition (C/A) code, which is accessible by the general public, and the precise (P(Y)) code, which is encrypted so that only the U.S. military and other NATO nations who have been given access to the encryption code can access it.<ref>{{cite book
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! Band !! Frequency !! Description
|-
| '''L1''' || 1575.42 MHz || Coarse-acquisition<!-- "Coarse" is correct, as in "not precision"--> (C/A) and encrypted precision (P(Y)) codes, plus the L1 civilian ([[GPS signals#L1C|L1C]]) and military (M) codes on
|-
| '''L2''' || 1227.60 MHz || P(Y) code, plus the [[L2C]] and military codes on the Block IIR-M and newer satellites.
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| '''L4''' || 1379.913 MHz || Being studied for additional ionospheric correction.
|-
| '''L5''' || 1176.45 MHz ||
|}
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|title=A Software-Defined GPS and Galileo Receiver. A single-Frequency Approach|first1=Kai|last1=Borre|first2=Dennis|last2=M. Akos|first3=Nicolaj|last3=Bertelsen|first4=Peter|last4=Rinder|first5=Søren Holdt|last5=Jensen|publisher=Springer|year=2007|isbn=978-0-8176-4390-4|page=18|url={{google books|plainurl=y|id=x2g6XTEkb8oC}}}}</ref> that make observers on the Earth perceive a different time reference with respect to the transmitters in orbit. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.<ref name=avionicswest /> The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.
The L3 signal at a frequency of 1.38105 GHz is used to transmit data from the satellites to ground stations. This data is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and near space.<ref>{{cite web
The L4 band at 1.379913 GHz is being studied for additional ionospheric correction.<ref name="handbook-pent" />{{rp|607}}
The L5 frequency band at 1.17645 GHz was added in the process of [[GPS modernization]]. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that provides this signal was launched in May 2010.<ref name="dailytech1">{{cite news |url=http://www.dailytech.com/First+Block+2F+GPS+Satellite+Launched+Needed+to+Prevent+System+Failure/article18483.htm |title=First Block 2F GPS Satellite Launched, Needed to Prevent System Failure |work=DailyTech |access-date=May 30, 2010
{{update|section|date=May 2021}}
In 2011, a conditional waiver was granted to [[LightSquared]] to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify
=== Demodulation and decoding ===
<!-- Demodulation is done with carrier frequency; decoding is done with Gold Code. -->
[[File:gps ca gold.svg|thumb|right|upright=0.8|Demodulating and Decoding GPS Satellite Signals using the Coarse<!-- "Coarse" is correct, as in "not precision"-->/Acquisition [[Gold code]]
Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary [[sequence]] known as a [[Gold code]]. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver.<ref>{{cite web|url=http://www.navcen.uscg.gov/?pageName=gpsAlmanacs|title=GPS Almanacs, NANUS, and Ops Advisories (including archives)|publisher=United States Coast Guard|work=GPS Almanac Information|access-date=September 9, 2009|archive-url=https://web.archive.org/web/20100712223936/http://www.navcen.uscg.gov/?pageName=gpsAlmanacs|archive-date=July 12, 2010|url-status=live}}</ref><ref>"George, M., Hamid, M.
If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range 1 through 32. If the almanac information is not in memory, the receiver enters a search mode until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern. There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data.
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{{See also|Pseudorange}}
=== Problem
The receiver uses messages received from satellites to determine the satellite positions and time sent. The ''x, y,'' and ''z'' components of satellite position and the time sent (''s'') are designated as [''x<sub>i</sub>, y<sub>i</sub>, z<sub>i</sub>, s<sub>i</sub>''] where the subscript ''i'' denotes the satellite and has the value 1, 2, ..., ''n'', where ''n'' ≥ 4. When the time of message reception indicated by the on-board receiver clock is
For n satellites, the equations to satisfy are:
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:<math>d_i = \sqrt{(x-x_i)^2 + (y-y_i)^2 + (z-z_i)^2}</math>
Defining ''pseudoranges'' as <math> p_i = \left ( \tilde{t}_i - s_i \right )c</math>, we see they are biased versions of the true range:
:<math>p_i = d_i + bc, \;i=1,2,...,n</math> .<ref name=GPS_BASICS_Blewitt>section 4 beginning on page 15 [http://www.nbmg.unr.edu/staff/pdfs/Blewitt%20Basics%20of%20gps.pdf
Since the equations have four unknowns [''x, y, z, b'']—the three components of GPS receiver position and the clock bias—signals from at least four satellites are necessary to attempt solving these equations. They can be solved by algebraic or numerical methods. Existence and uniqueness of GPS solutions are discussed by Abell and Chaffee.<ref name="Abel1" /> When ''n'' is greater than four, this system is [[Overdetermined system|overdetermined]] and a [[Mean|fitting method]] must be used.
The amount of error in the results varies with the received satellites' locations in the sky, since certain configurations (when the received satellites are close together in the sky) cause larger errors.
=== Geometric interpretation ===
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==== Spheres ====
[[File:2D Trilat Scenario 2019-0116.jpg|thumb|2-D Cartesian true-range multilateration (trilateration) scenario
The measured ranges, called pseudoranges, contain clock errors. In a simplified idealization in which the ranges are synchronized, these true ranges represent the radii of spheres, each centered on one of the transmitting satellites. The solution for the position of the receiver is then at the intersection of the surfaces of these spheres; see [[trilateration]] (more generally, true-range multilateration). Signals from at minimum three satellites are required, and their three spheres would typically intersect at two points.<ref>{{cite web|url=http://www.math.nus.edu.sg/aslaksen/gem-projects/hm/0203-1-10-instruments/modern.htm|title=Modern navigation|work=math.nus.edu.sg|access-date=December 4, 2018|archive-url=https://web.archive.org/web/20171226024421/http://www.math.nus.edu.sg/aslaksen/gem-projects/hm/0203-1-10-instruments/modern.htm|archive-date=December 26, 2017
In practice, there are many sources of inaccuracy besides clock bias, including random errors as well as the potential for precision loss from subtracting numbers close to each other if the centers of the spheres are relatively close together. This means that the position calculated from three satellites alone is unlikely to be accurate enough. Data from more satellites can help because of the tendency for random errors to cancel out and also by giving a larger spread between the sphere centers. But at the same time, more spheres will not generally intersect at one point. Therefore, a near intersection gets computed, typically via least squares. The more signals available, the better the approximation is likely to be.
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[[File:Hyperbolic Navigation.svg|thumb|219x219px|Three satellites (labeled as "stations" A, B, C) have known locations. The true times it takes for a radio signal to travel from each satellite to the receiver are unknown, but the true time differences are known. Then, each time difference locates the receiver on a branch of a hyperbola focused on the satellites. The receiver is then located at one of the two intersections.]]
If the pseudorange between the receiver and satellite ''i'' and the pseudorange between the receiver and satellite ''j'' are subtracted, {{
==== Inscribed sphere ====
[[File:Descartes Circles.svg|thumb|A smaller circle ({{color|red|'''red'''}}) inscribed and tangent to other circles ({{color|black|'''black'''}}), that need not necessarily be mutually tangent
The receiver position can be interpreted as the center of an [[inscribed sphere]] (insphere) of radius ''bc'', given by the receiver clock bias ''b'' (scaled by the speed of light ''c''). The insphere ___location is such that it touches other spheres. The [[Circumscribed sphere|circumscribing spheres]] are centered at the GPS satellites, whose radii equal the measured pseudoranges ''p''<sub>i</sub>. This configuration is distinct from the one described above, in which the spheres' radii were the unbiased or geometric ranges ''d''<sub>i</sub>.<ref name=HWLW />{{rp|36–37}}<ref name="Hoshen 1996">{{cite journal
==== Hypercones ====
The clock in the receiver is usually not of the same quality as the ones in the satellites and will not be accurately synchronized to them. This produces [[pseudorange]]s with large differences compared to the true distances to the satellites. Therefore, in practice, the time difference between the receiver clock and the satellite time is defined as an unknown clock bias ''b''. The equations are then solved simultaneously for the receiver position and the clock bias.
=== Solution methods ===
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Bancroft's method is algebraic, as opposed to numerical, and can be used for four or more satellites. When four satellites are used, the key steps are inversion of a 4x4 matrix and solution of a single-variable quadratic equation. Bancroft's method provides one or two solutions for the unknown quantities. When there are two (usually the case), only one is a near-Earth sensible solution.<ref name=Bancroft />
When a receiver uses more than four satellites for a solution, Bancroft uses the [[generalized inverse]] (i.e., the pseudoinverse) to find a solution. A case has been made that iterative methods, such as the Gauss–Newton algorithm approach for solving over-determined [[non-linear least squares]]
Leick et al. (2015) states that "Bancroft's (1985) solution is a very early, if not the first, closed-form solution."<ref>{{cite book|title=GNSS Positioning Approaches – GPS Satellite Surveying, Fourth Edition – Leick |publisher= Wiley Online Library|doi=10.1002/9781119018612.ch6|pages=257–399|chapter = GNSS Positioning Approaches|year = 2015|isbn = 9781119018612}}</ref>
Other closed-form solutions were published afterwards,<ref name="Kleus">Alfred Kleusberg, "Analytical GPS Navigation Solution", ''University of Stuttgart Research Compendium'', 1994.</ref><ref name="Oszczak">Oszczak, B., "New Algorithm for GNSS Positioning Using System of Linear Equations
=== 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. (
| url = http://www.gps.gov/systems/gps/modernization/sa/faq/
| title = Frequently Asked Questions About Selective Availability
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| 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 =
| archive-date = June 16, 2015
}}</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
== Accuracy enhancement and surveying ==
{{
== Regulatory spectrum issues concerning GPS receivers ==
In the United States, GPS receivers are regulated under the [[Federal Communications Commission]]'s (FCC) [[Title 47 CFR Part 15|Part 15]] rules. As indicated in the manuals of GPS-enabled devices sold in the United States, as a Part 15 device, it "must accept any interference received, including interference that may cause undesired operation".<ref>{{cite web|url=http://stellarsupport.deere.com/en_US/support/pdf/om/en/ompfp11008_sf3000.pdf |title=2011 John Deere StarFire 3000 Operator Manual |publisher=John Deere |access-date=November 13, 2011 |archive-url=https://web.archive.org/web/20120105123842/http://stellarsupport.deere.com/en_US/support/pdf/om/en/ompfp11008_sf3000.pdf |archive-date=January 5, 2012 }}</ref> With respect to GPS devices in particular, the FCC states that GPS receiver manufacturers "must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum".<ref name="FCC.gov">{{cite web|url=http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-11-57A1.pdf|title=Federal Communications Commission Report and Order In the Matter of Fixed and Mobile Services in the Mobile Satellite Service Bands at 1525–1559 MHz and 1626.5–1660.5 MHz|publisher=Federal Communications Commission|date=April 6, 2011|access-date=December 13, 2011|archive-url=https://web.archive.org/web/20111216043702/http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-11-57A1.pdf|archive-date=December 16, 2011|url-status=dead }}</ref> For the last 30 years, GPS receivers have operated next to the Mobile Satellite Service band, and have discriminated against reception of mobile satellite services, such as Inmarsat, without any issue.
The spectrum allocated for GPS L1 use by the FCC is 1559 to 1610 MHz, while the spectrum allocated for satellite-to-ground use owned by Lightsquared is the Mobile Satellite Service band.<ref>{{cite web|url=http://transition.fcc.gov/oet/spectrum/table/fcctable.pdf|title=Federal Communications Commission Table of Frequency Allocations|publisher=Federal Communications Commission|date=November 18, 2011|access-date=December 13, 2011|archive-url=https://web.archive.org/web/20111216043702/http://transition.fcc.gov/oet/spectrum/table/fcctable.pdf|archive-date=December 16, 2011|url-status=live}}</ref> Since 1996, the FCC has authorized licensed use of the spectrum neighboring the GPS band of 1525 to 1559 MHz to the [[Virginia]] company [[LightSquared]]. On March 1, 2001, the FCC received an application from LightSquared's predecessor, [[Motient]] Services, to use their allocated frequencies for an integrated satellite-terrestrial service.<ref name=FCC>{{cite web|url=http://licensing.fcc.gov/cgi-bin/ws.exe/prod/ib/forms/reports/related_filing.hts?f_key=200647&f_number=SATASG2001030200017|title=FCC Docket File Number: SATASG2001030200017, "Mobile Satellite Ventures LLC Application for Assignment and Modification of Licenses and for Authority to Launch and Operate a Next-Generation Mobile Satellite System"|page=9|publisher=Federal Communications Commission|date=March 1, 2001|access-date=December 14, 2011|archive-url=https://web.archive.org/web/20120114225139/http://licensing.fcc.gov/cgi-bin/ws.exe/prod/ib/forms/reports/related_filing.hts?f_key=200647&f_number=SATASG2001030200017|archive-date=January 14, 2012|url-status=live}}</ref> In 2002, the U.S. GPS Industry Council came to an out-of-band-emissions (OOBE) agreement with LightSquared to prevent transmissions from LightSquared's ground-based stations from emitting transmissions into the neighboring GPS band of 1559 to 1610 MHz.<ref>{{cite web|url=http://fjallfoss.fcc.gov/ecfs/document/view?id=6515082621|title=U.S. GPS Industry Council Petition to the FCC to adopt OOBE limits jointly proposed by MSV and the Industry Council|publisher=Federal Communications Commission|date=September 4, 2003|access-date=December 13, 2011}}{{dead link|date=August 2023|bot=medic}}{{cbignore|bot=medic}}</ref> In 2004, the FCC adopted the OOBE agreement in its authorization for LightSquared to deploy a ground-based network ancillary to their satellite system – known as the Ancillary Tower Components (ATCs) – "We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service."<ref name="hraunfoss.fcc.gov">{{cite web |title=Order on Reconsideration |url=http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-162A1.pdf |date=July 3, 2003 |access-date=October 20, 2015 |archive-url=https://web.archive.org/web/20111020215425/http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-162A1.pdf |archive-date=October 20, 2011 |url-status=live }}</ref> This authorization was reviewed and approved by the U.S. Interdepartment Radio Advisory Committee, which includes the [[U.S. Department of Agriculture]], U.S. Space Force, U.S. Army, [[U.S. Coast Guard]], [[Federal Aviation Administration]], [[National Aeronautics and Space Administration]] (NASA), [[United States Department of the Interior|U.S. Department of the Interior]], and [[U.S. Department of Transportation]].<ref>{{cite web|url=http://www.gps.gov/congress/hearings/2011-09-HASC/knapp.pdf|title=Statement of Julius P. Knapp, Chief, Office of Engineering and Technology, Federal Communications Commission|publisher=gps.gov|date=September 15, 2011|page=3|access-date=December 13, 2011|archive-url=https://web.archive.org/web/20111216043738/http://www.gps.gov/congress/hearings/2011-09-HASC/knapp.pdf|archive-date=December 16, 2011|url-status=live}}</ref>
In January 2011, the FCC conditionally authorized LightSquared's wholesale customers—such as [[Best Buy]], [[Sharp Corporation|Sharp]], and [[C Spire]]—to only purchase an integrated satellite-ground-based service from LightSquared and re-sell that integrated service on devices that are equipped to only use the ground-based signal using LightSquared's allocated frequencies of 1525 to 1559 MHz.<ref>{{cite web|url=http://hraunfoss.fcc.gov/edocs_public/attachmatch/DA-11-133A1.pdf|title=FCC Order, Granted LightSquared Subsidiary LLC, a Mobile Satellite Service licensee in the L-Band, a conditional waiver of the Ancillary Terrestrial Component "integrated service" rule|work=Federal Communications Commission|publisher=FCC.Gov|date=January 26, 2011|access-date=December 13, 2011|archive-url=https://web.archive.org/web/20111216043715/http://hraunfoss.fcc.gov/edocs_public/attachmatch/DA-11-133A1.pdf|archive-date=December 16, 2011|url-status=live}}</ref> In December 2010, GPS receiver manufacturers expressed concerns to the FCC that LightSquared's signal would interfere with GPS receiver devices<ref name="gpsworld-20110301" /> although the FCC's policy considerations leading up to the January 2011 order did not pertain to any proposed changes to the maximum number of ground-based LightSquared stations or the maximum power at which these stations could operate. The January 2011 order makes final authorization contingent upon studies of GPS interference issues carried out by a LightSquared led working group along with GPS industry and Federal agency participation. On February 14, 2012, the FCC initiated proceedings to vacate LightSquared's Conditional Waiver Order based on the NTIA's conclusion that there was currently no practical way to mitigate potential GPS interference.
GPS receiver manufacturers design GPS receivers to use spectrum beyond the GPS-allocated band. In some cases, GPS receivers are designed to use up to 400 MHz of spectrum in either direction of the L1 frequency of 1575.42 MHz, because mobile satellite services in those regions are broadcasting from space to ground, and at power levels commensurate with mobile satellite services.<ref>{{cite web|url=http://www.gpsworld.com/gnss-system/news/javad-ashjaee-discuss-javad-gnss-lightsquared-tech-december-8-webinar-12337 |title=Javad Ashjaee GPS World webinar |date=December 8, 2011 |publisher=gpsworld.com |access-date=December 13, 2011 |archive-url=https://web.archive.org/web/20111126033508/http://www.gpsworld.com/gnss-system/news/javad-ashjaee-discuss-javad-gnss-lightsquared-tech-december-8-webinar-12337 |archive-date=November 26, 2011 }}</ref> As regulated under the FCC's Part 15 rules, GPS receivers are not warranted protection from signals outside GPS-allocated spectrum.<ref name="FCC.gov" /> This is why GPS operates next to the Mobile Satellite Service band, and also why the Mobile Satellite Service band operates next to GPS. The symbiotic relationship of spectrum allocation ensures that users of both bands are able to operate cooperatively and freely.
The FCC adopted rules in February 2003 that allowed Mobile Satellite Service (MSS) licensees such as LightSquared to construct a small number of ancillary ground-based towers in their licensed spectrum to "promote more efficient use of terrestrial wireless spectrum".<ref>{{cite web|url=http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-15A1.pdf|title=FCC Order permitting mobile satellite services providers to provide an ancillary terrestrial component (ATC) to their satellite systems|work=Federal Communications Commission|date=February 10, 2003|access-date=December 13, 2011|archive-url=https://web.archive.org/web/20111216043720/http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-15A1.pdf|archive-date=December 16, 2011|url-status=live}}</ref> In those 2003 rules, the FCC stated: "As a preliminary matter, terrestrial [Commercial Mobile Radio Service ('CMRS')] and MSS ATC are expected to have different prices, coverage, product acceptance and distribution; therefore, the two services appear, at best, to be imperfect substitutes for one another that would be operating in predominantly different market segments ... MSS ATC is unlikely to compete directly with terrestrial CMRS for the same customer base...". In 2004, the FCC clarified that the ground-based towers would be ancillary, noting: "We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service."<ref name="hraunfoss.fcc.gov" /> In July 2010, the FCC stated that it expected LightSquared to use its authority to offer an integrated satellite-terrestrial service to "provide mobile broadband services similar to those provided by terrestrial mobile providers and enhance competition in the mobile broadband sector".<ref>{{cite web|url=http://www.federalregister.gov/articles/2010/08/16/2010-19824/fixed-and-mobile-services-in-the-mobile-satellite-service#p-31|title=Federal Communications Commission Fixed and Mobile Services in the Mobile Satellite Service|work=Federal Communications Commission|date=July 15, 2010|access-date=December 13, 2011|archive-url=https://web.archive.org/web/20120527223503/https://www.federalregister.gov/articles/2010/08/16/2010-19824/fixed-and-mobile-services-in-the-mobile-satellite-service#p-31|archive-date=May 27, 2012|url-status=live}}</ref> GPS receiver manufacturers have argued that LightSquared's licensed spectrum of 1525 to 1559 MHz was never envisioned as being used for high-speed wireless broadband based on the 2003 and 2004 FCC ATC rulings making clear that the Ancillary Tower Component (ATC) would be, in fact, ancillary to the primary satellite component.<ref name="LightSquared DOD GPS Spec">{{cite web|url=http://saveourgps.org/pdf/SIS_DOD_Response_Statement_08122011.pdf|title=SIS DOD Response Statement|archive-url=https://web.archive.org/web/20121213185643/http://saveourgps.org/pdf/SIS_DOD_Response_Statement_08122011.pdf|archive-date=December 13, 2012}}</ref> To build public support of efforts to continue the 2004 FCC authorization of LightSquared's ancillary terrestrial component vs. a simple ground-based LTE service in the Mobile Satellite Service band, GPS receiver manufacturer [[Trimble Navigation]] Ltd. formed the "Coalition To Save Our GPS".<ref name="Coalition To Save Our GPS">{{cite web|url=http://saveourgps.org/|title=Coalition to Save Our GPS|publisher=Saveourgps.org|access-date=November 6, 2011|archive-url=https://web.archive.org/web/20111024192351/http://saveourgps.org/|archive-date=October 24, 2011}}</ref>
The FCC and LightSquared have each made public commitments to solve the GPS interference issue before the network is allowed to operate.<ref>{{cite web |author=Carlisle |first=Jeff |date=June 23, 2011 |title=Testimony of Jeff Carlisle, LightSquared Executive Vice President of Regulatory Affairs and Public Policy to U.S. House Subcommittee on Aviation and Subcommittee on Coast Guard and Maritime Transportation |url=http://ssv.cachefly.net/lightsquared/wp-content/uploads/2011/06/LSQ-Testimony-Package.pdf |archive-url=https://web.archive.org/web/20110929064959/http://ssv.cachefly.net/lightsquared/wp-content/uploads/2011/06/LSQ-Testimony-Package.pdf |archive-date=September 29, 2011 |access-date=December 13, 2011}}</ref><ref>{{cite web |author=Genachowski |first=Julius |date=May 31, 2011 |title=FCC Chairman Genachowski Letter to Senator Charles Grassley |url=http://www.lightsquared.com/documents/FCC%20Julius%20Genachowski%20letter%20to%20Senator%20Grassley%20-%20May%2031,%202011.pdf |archive-url=https://web.archive.org/web/20120113093239/http://www.lightsquared.com/documents/FCC%20Julius%20Genachowski%20letter%20to%20Senator%20Grassley%20-%20May%2031%2C%202011.pdf |archive-date=January 13, 2012 |access-date=December 13, 2011}}</ref> According to Chris Dancy of the [[Aircraft Owners and Pilots Association]], airline pilots with the type of systems that would be affected "may go off course and not even realize it".<ref name=Tessler /> The problems could also affect the Federal Aviation Administration upgrade to the [[air traffic control]] system, [[United States Defense Department]] guidance, and local [[emergency service]]s including [[9-1-1|911]].<ref name=Tessler>{{cite news|url=http://www.thesunnews.com/2011/04/07/2085752/internet-network-may-jam-gps-in.html |title=Internet network may jam GPS in cars, jets |last=Tessler |first=Joelle |work=The Sun News |date=April 7, 2011 |access-date=April 7, 2011 |archive-url=https://web.archive.org/web/20110501134549/http://www.thesunnews.com/2011/04/07/2085752/internet-network-may-jam-gps-in.html |archive-date=May 1, 2011 }}</ref>
On February 14, 2012, the FCC moved to bar LightSquared's planned national broadband network after being informed by the [[National Telecommunications and Information Administration]] (NTIA), the federal agency that coordinates spectrum uses for the military and other federal government entities, that "there is no practical way to mitigate potential interference at this time".<ref name=FCC20120214>FCC press release [http://www.fcc.gov/document/spokesperson-statement-ntia-letter-lightsquared-and-gps "Spokesperson Statement on NTIA Letter – LightSquared and GPS"] {{Webarchive|url=https://web.archive.org/web/20120423172022/http://www.fcc.gov/document/spokesperson-statement-ntia-letter-lightsquared-and-gps |date=April 23, 2012 }}. February 14, 2012. Accessed March 3, 2013.</ref><ref>Paul Riegler, FBT. [http://www.frequentbusinesstraveler.com/2012/02/fcc-bars-lightsquared-broadband-network-plan/ "FCC Bars LightSquared Broadband Network Plan"]. {{Webarchive|url=https://web.archive.org/web/20130922055621/http://www.frequentbusinesstraveler.com/2012/02/fcc-bars-lightsquared-broadband-network-plan/|date=September 22, 2013}}. February 14, 2012. Retrieved February 14, 2012.</ref> LightSquared is challenging the FCC's action.{{update inline|date=March 2021}}
== Similar systems ==
{{Main|Satellite navigation}}
{{Comparison satellite navigation orbits}}
<!--"Comparison satellite navigation orbits" creates the info-graphic to the right.-->
Following the United States's deployment of GPS, other countries have also developed their own satellite navigation systems. These systems include:
* The Russian Global Navigation Satellite System ([[GLONASS]]) was developed at the same time as GPS, but suffered from incomplete coverage of the globe until the mid-2000s.<ref>{{cite magazine|title=Russia Launches Three More GLONASS-M Space Vehicles|url=http://www.insidegnss.com/node/982|magazine=[[Inside GNSS]]|access-date=December 26, 2008|archive-url=https://web.archive.org/web/20090206081945/http://insidegnss.com/node/982|archive-date=February 6, 2009}}</ref> GLONASS reception in addition to GPS can be combined in a receiver thereby allowing for additional satellites available to enable faster position fixes and improved accuracy, to within {{convert|2|m|sp=us|spell=in|ft}}.<ref>{{cite web|url=http://blog.clove.co.uk/2012/01/10/glonass-the-future-for-all-smartphones/|title=GLONASS the future for all smartphones? |author1=Jon |date=January 10, 2012|website=Clove Blog|access-date=October 29, 2016|archive-url=https://web.archive.org/web/20160310151239/http://blog.clove.co.uk/2012/01/10/glonass-the-future-for-all-smartphones/|archive-date=March 10, 2016}}</ref><ref>{{cite journal | doi=10.15292/geodetski-vestnik.2022.01.49-59 | title=Challenges related to the determination of altitudes of mountain peaks presented on cartographic sources | year=2022 | last1=Chwedczuk | first1=Katarzyna | last2=Cienkosz | first2=Daniel | last3=Apollo | first3=Michal | last4=Borowski | first4=Lukasz | last5=Lewinska | first5=Paulina | last6=Guimarães Santos | first6=Celso Augusto | last7=Eborka | first7=Kennedy | last8=Kulshreshtha | first8=Sandeep | last9=Romero-Andrade | first9=Rosendo | last10=Sedeek | first10=Ahmed | last11=Liibusk | first11=Aive | last12=MacIuk | first12=Kamil | journal=Geodetski Vestnik | volume=66 | pages=49–59 | s2cid=247985456 | doi-access=free }}</ref> In October 2011, the full orbital constellation of 24 satellites enabled full global coverage. The GLONASS satellites' designs have undergone several upgrades, with the latest version, [[GLONASS-K2]], launched in 2023.<ref name="tsr-20221219">{{cite web |last=Hendrickx |first=Bart |url=https://www.thespacereview.com/article/4502/1 |title=The secret payloads of Russia's Glonass navigation satellites |work=[[The Space Review]] |date=19 December 2022 |access-date=20 December 2022 |archive-date=20 December 2022 |archive-url=https://web.archive.org/web/20221220033222/https://www.thespacereview.com/article/4502/1 |url-status=live }}</ref>
* China's [[BeiDou Navigation Satellite System]] began global services in 2018 and finished its full deployment in 2020. It consists of satellites in three different orbits, including 24 satellites in medium-circle orbits (covering the world), 3 satellites in inclined geosynchronous orbits (covering the Asia-Pacific region), and 3 satellites in geostationary orbits (covering China).<ref>{{cite news |title=China launches final satellite in GPS-like Beidou system |url=https://phys.org/news/2020-06-china-satellite-gps-like-beidou.html |access-date=June 24, 2020 |work=phys.org |agency=The Associated Press |date=June 23, 2020 |archive-url=https://web.archive.org/web/20200624080233/https://phys.org/news/2020-06-china-satellite-gps-like-beidou.html |archive-date=June 24, 2020 |url-status=live}}</ref>
* The [[Galileo (satellite navigation)|Galileo navigation satellite system]], a global system being developed by the European Union and other partner countries, began operation in 2016,<ref>{{cite web|url=http://www.dw.com/en/galileo-navigation-satellite-system-goes-live/a-36422029|title=Galileo navigation satellite system goes live|publisher=dw.com|access-date=December 17, 2016|archive-url=https://web.archive.org/web/20171018202016/http://www.dw.com/en/galileo-navigation-satellite-system-goes-live/a-36422029|archive-date=October 18, 2017|url-status=live}}</ref> and has been fully deployed by 2020. In November 2018, the [[Federal Communications Commission|FCC]] approved use of Galileo in the [[United States|US]].<ref>{{cite news |title=FCC approves use of Galileo in the US |url=https://galileognss.eu/fcc-approves-use-of-galileo-in-the-us/ |publisher=Galileo |date=19 November 2018}}</ref> As of September 2024, there are 25 [[List of Galileo satellites#Satellites|launched satellites]] that operate in the constellation.<ref>{{cite web|url=https://www.gsc-europa.eu/system-service-status/constellation-information|title=Constellation Information {{!}} European GNSS Service Centre|website=www.gsc-europa.eu|access-date=17 October 2019}}</ref><ref name="First Pair">{{cite web|url=https://insidegnss.com/galileo-elliptical-auxiliary-satellites-removed-from-service/|title=Galileo Elliptical Auxiliary Satellites Removed from Service|publisher=Inside GNSS|date=23 February 2021|access-date=17 December 2021}}</ref><ref>{{cite journal|last1=Hadas|first1=Tomasz|last2=Kazmierski |first2=Kamil|last3=Sośnica|first3=Krzysztof|title=Performance of Galileo-only dual-frequency absolute positioning using the fully serviceable Galileo constellation|journal=GPS Solutions|date=7 August 2019 |volume=23|issue=4|page=108 |doi=10.1007/s10291-019-0900-9|bibcode=2019GPSS...23..108H |doi-access=free}}</ref> It is expected that the next generation of satellites will begin to become operational after 2026 to replace the first generation, which can then be used for backup capabilities.
* Japan's [[Quasi-Zenith Satellite System]] (QZSS) is a GPS [[satellite-based augmentation system]] to enhance GPS's accuracy in Asia-Oceania, with satellite navigation independent of GPS scheduled for 2023.<ref>{{cite web |last1=Kriening |first1=Torsten |title=Japan Prepares for GPS Failure with Quasi-Zenith Satellites |url=https://spacewatch.global/2019/01/japan-prepares-for-gps-failure-with-quasi-zenith-satellites/ |website=SpaceWatch.Global |access-date=August 10, 2019 |date=January 23, 2019 |archive-date=April 19, 2019 |archive-url=https://web.archive.org/web/20190419093030/https://spacewatch.global/2019/01/japan-prepares-for-gps-failure-with-quasi-zenith-satellites/ |url-status=live }}</ref>
* The [[Indian Regional Navigation Satellite System]] (Operational name 'NavIC', Navigation with Indian Constellation), deployed by India.
==Backup system==
In the event of adverse [[space weather]] or the deployment of an anti-satellite weapon against GPS, the United States has no terrestrial backup system. The potential cost of such an event to the U.S. economy is estimated at $1 billion per day. The [[LORAN-C]] system was turned off in North America in 2010 and Europe in 2015. [[eLoran]] is proposed as an American terrestrial backup system, but as of 2024 has not received approval or funding.<ref>{{cite news |last1=Hegyi |first1=Nate |last2=Wong |first2=Wailin |date=September 27, 2024 |title=Losing GPS would cost the U.S. $1 billion a day. So why is there no backup? |url=https://www.npr.org/2024/09/27/nx-s1-5127737/losing-gps-would-cost-the-u-s-1-billion-a-day-so-why-is-there-no-backup |publisher=[[NPR]]}}</ref>
China continues to operate LORAN-C transmitters,<ref>{{cite web |author=Goward |first=Dana |date=November 15, 2019 |title=China leads world with plan for 'comprehensive' PNT |url=https://www.gpsworld.com/china-leads-world-with-plan-for-comprehensive-pnt/ |publisher=GPS World}}</ref> and Russia has a similar system called [[CHAYKA]] ("Seagull").
== See also ==
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* [[GPS satellite blocks]]
* [[GPS signals]]
* [[
* [[GPS/INS]]
* [[GPS spoofing]]
* [[Indoor positioning system]]
* [[Local-area
* [[Local positioning system]]
* [[Military invention]]
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* [[Notice Advisory to Navstar Users]]
* [[S-GPS]]
* [[High-altitude balloon#Geostationary balloon satellite|Geostationary balloon satellite]]
{{div col end}}
== Notes ==
{{
== References ==
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{{Library resources box}}
* {{cite web|title=NAVSTAR GPS User Equipment Introduction|url=http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|date=September 1996|publisher=United States Coast Guard|access-date=August 22, 2008|archive-date=October 21, 2013|archive-url=https://web.archive.org/web/20131021060507/http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|url-status=dead}}
* {{cite book|url={{google books|plainurl=y|id=lvI1a5J_4ewC}}|title=The global positioning system|author1=Parkinson |author2=Spilker |publisher=American Institute of Aeronautics and Astronautics|isbn=978-1-56347-106-3|year=1996}}
* {{cite book |url={{google books|plainurl=y|id=t1lBTH42mOcC}} |title=GPS and Galileo |
* {{cite book |title=The American Practical Navigator – Chapter 11 ''Satellite Navigation'' |author=Bowditch |first=Nathaniel
* [http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-540-principles-of-the-global-positioning-system-spring-2012/ Global Positioning System]. Open Courseware from [[MIT|Massachusetts Institute of Technology]], 2012.
* {{cite book |title=Pinpoint: How GPS is Changing Technology, Culture, and Our Minds |author=Milner |first=Greg
== External links ==
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-->
{{Commons}}
* [https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss
* [https://www.gps.gov/ GPS.gov] – General public education website created by the U.S. Government
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{{USAF system codes}}
{{Orienteering}}
{{
[[Category:Global Positioning System| ]]
[[Category:20th-century inventions]]
[[Category:Equipment of the United States Space Force]]
[[Category:Programs of the United States Space Force]]
[[Category:Military equipment introduced in the 1970s]]
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