View source for Engineering:Global Positioning System

{{Short description|American satellite-based radio navigation service}}
{{Infobox navigation satellite system
|name = Global Positioning System (GPS)
|image = NAVSTAR GPS logo.png
|image_caption = Logo
|country = United States
|type = Military, civilian
|status = Operational
|operator = US Space Force
|coverage = Global
|precision = {{convert|30|-|500|cm|ft|sigfig=2|abbr=on}}
|satellites_nominal = 24 
|satellites_current = 38 (32 operational)
|first_launch = {{start date and age|1978|2|22}}
|last_launch = 
|launch_total = 75
|regime = 6 [[Astronomy:Medium Earth orbit|MEO]] planes
|orbit_height = {{convert|20180|km|mi|abbr=on}}
|cost = $12 billion<ref name="time-gps-cost"/><br/>(initial constellation)<br/>$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><br/>(operating cost)
|website={{URL|https://www.gps.gov|gps.gov}}
|orbit_period={{Fraction|1|2}} sd or 11 hours and 58 minutes
|revisit_period=1 sidereal day}}
{{multiple image
 | perrow = 2
 | width = 175
 | image1 = GPS-IIR.jpg | caption1 = Artist's impression of GPS Block&nbsp;IIR satellite in Earth orbit
 | image2 = Magellan GPS Blazer12.jpg | caption2 = Civilian GPS receivers ("[[Engineering:GPS navigation device|GPS navigation device]]") in a marine application
 | image3 = KyotoTaxiRide.jpg | caption3 = [[Automotive navigation system]] in a taxicab
 | image4 = 2 SOPS space systems operator 040205-F-0000C-001.jpg | caption4 = An [[Social:Air Force Space Command|Air Force Space Command]] Senior Airman runs through a checklist during Global Positioning System satellite operations.}}
{{Geodesy}}

The '''Global Positioning System''' ('''GPS'''), originally '''Navstar GPS''',<ref>{{cite web |author1=United States Department of Transportation |author2=Federal Aviation Administration |title=Global Positioning System Wide Area Augmentation System (WAAS) Performance Standard |url=https://www.gps.gov/technical/ps/2008-WAAS-performance-standard.pdf |access-date=January 3, 2012 |page=B-3 |archive-url=https://web.archive.org/web/20170427033332/http://www.gps.gov/technical/ps/2008-WAAS-performance-standard.pdf |archive-date=April 27, 2017 |date=October 31, 2008 |url-status=live}}</ref> is a [[Engineering:Radionavigation-satellite service|satellite-based radio navigation]] system owned by the United States government and operated by the United States Space Force.<ref name=":0">{{cite web |author1=United States Department of Defense |author1-link=United States Department of Defense |title=Global Positioning System Standard Positioning Service Performance Standard - 4th Edition |url=https://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf |access-date=April 21, 2017 |archive-url=https://web.archive.org/web/20170427025348/http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf |archive-date=April 27, 2017 |date=September 2008 |url-status=live}}</ref> It is one of the [[Earth:Satellite navigation|global navigation satellite systems]] (GNSS) that provide [[Geolocation|geolocation]] and [[Time transfer|time information]] to a [[Engineering:Satellite navigation device|GPS receiver]] anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.<ref>{{cite web |author1=Science Reference Section |author1-link=Library of Congress |title=What is a GPS? How does it work? |url=https://www.loc.gov/everyday-mysteries/item/what-is-gps-how-does-it-work/ |website=Everyday Mysteries |publisher=Library of Congress |access-date=April 12, 2022 |archive-url=https://web.archive.org/web/20220412090940/https://www.loc.gov/everyday-mysteries/item/what-is-gps-how-does-it-work/ |archive-date=April 12, 2022 |date=November 19, 2019 |url-status=live}}</ref> It does not require the user to transmit any data, and operates independently of any telephonic or Internet reception, though these technologies can enhance the usefulness of the GPS positioning information. It provides critical positioning capabilities to military, civil, and commercial users around the world. Although the United States government created, controls and maintains the GPS system, it is freely accessible to anyone with a GPS receiver.<ref>{{cite web|author1=((National Coordination Office for Space-Based Positioning, Navigation, and Timing)) |url=https://www.gps.gov/systems/gps/|title=What is GPS?|date=February 22, 2021|access-date=May 5, 2021|archive-url=https://web.archive.org/web/20210506000043/https://www.gps.gov/systems/gps/|archive-date=May 6, 2021 |url-status=live }}</ref>

== Overview ==
The GPS project was started by the U.S. Department of Defense in 1973. The first prototype spacecraft was launched in 1978 and the full constellation of 24 satellites became operational in 1993. Originally limited to use by the United States military, civilian use was allowed from the 1980s following an executive order from President Ronald Reagan after the Korean Air Lines Flight 007 disaster.<ref>{{cite web|url=https://www.popularmechanics.com/technology/gadgets/a26980/why-the-military-released-gps-to-the-public/|title=Why the Military Released GPS to the Public|first=Juquai|last=McDuffie|date=June 19, 2017|website=Popular Mechanics|access-date=February 1, 2020|archive-date=January 28, 2020|archive-url=https://web.archive.org/web/20200128214307/https://www.popularmechanics.com/technology/gadgets/a26980/why-the-military-released-gps-to-the-public/|url-status=live}}</ref> 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 [[Engineering:GPS Block IIIA|GPS Block IIIA]] satellites and Next Generation Operational Control System (OCX)<ref name="losangelesmil">{{cite web|url=http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=18676 |title=Factsheets: GPS Advanced Control Segment (OCX) |publisher=Losangeles.af.mil |date=October 25, 2011 |access-date=November 6, 2011 |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 U.S. Congress in 2000.

From the early 1990s, GPS positional accuracy was degraded by the United States government using a technology called [[Engineering:Error analysis for the Global Positioning System#Selective availability|Selective Availability]], which could selectively degrade or deny access to the system at any time,<ref>{{Cite news |last=Srivastava |first=Ishan |date=April 5, 2014 |title=How Kargil spurred India to design own GPS |work=The Times of India |url=https://timesofindia.indiatimes.com/home/science/How-Kargil-spurred-India-to-design-own-GPS/articleshow/33254691.cms |url-status=live |access-date=December 9, 2014 |archive-url=https://web.archive.org/web/20161215183718/https://timesofindia.indiatimes.com/home/science/How-Kargil-spurred-India-to-design-own-GPS/articleshow/33254691.cms |archive-date=December 15, 2016}}</ref> as happened to the Indian military in 1999 during the Kargil War. As a result, several non-US entities—including {{wipe|Russia}}, {{wipe|China}}, {{wipe|India}}, {{wipe|Japan}}, and the [[Social:European Union|European Union]]—have developed or are developing their own global or regional satellite navigation systems. Selective Availability was discontinued on May 1, 2000, in accordance with a bill signed into law by President Bill Clinton.<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>

When Selective Availabilty 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=22 November 2023}}</ref>

== History ==
[[File:AFSC Film, NAVSTAR GPS-Circa 1977.ogv|thumb|Air Force film introducing the Navstar Global Positioning System, circa 1977]]
[[File:GPS24goldenSMALL.gif|thumb|GPS constellation system animation]]

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 1995. Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of [[Company:The Aerospace Corporation|The Aerospace Corporation]], and [[Biography:Bradford Parkinson|Bradford Parkinson]] of the [[Organization:Applied Physics Laboratory|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 [[Biography:Gladys West|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|url=https://www.bbc.com/news/world-43812053|title=100 Women: Gladys West - the 'hidden figure' of GPS|first=Amelia|last=Butterly|work=BBC News|date=May 20, 2018|access-date=January 17, 2019|archive-url=https://web.archive.org/web/20190213112200/https://www.bbc.com/news/world-43812053|archive-date=February 13, 2019|url-status=live}}</ref><ref>{{Cite news |last=Mohdin |first=Aamna |date=2020-11-19 |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 [[Engineering:LORAN|LORAN]] and the [[Engineering:Decca Navigator System|Decca Navigator]], developed in the early 1940s.

In 1955, [[Biography:Friedwardt Winterberg|Friedwardt Winterberg]] proposed a test of [[Physics:General relativity|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 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 ([[Engineering:Sputnik 1|Sputnik 1]]) in 1957, two American physicists, William Guier and George Weiffenbach, at [[Organization:Johns Hopkins University|Johns Hopkins University]]'s [[Organization:Applied Physics Laboratory|Applied Physics Laboratory]] (APL) decided to monitor 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 [[Physics:Doppler effect|Doppler effect]], they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC to do the heavy calculations required.

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 Polaris missile, which required them to know the submarine's location.) This led them and APL to develop the [[Engineering:Transit (satellite)|TRANSIT]] system.<ref>{{citation|title=Where good ideas come from, the natural history of innovation|author=Steven Johnson|publisher=Riverhead Books|place=New York|year=2010}}</ref> In 1959, ARPA (renamed DARPA in 1972) also played a role in TRANSIT.<ref>{{cite book|title=Transit to Tomorrow. Fifty Years of Space Research at The Johns Hopkins University Applied Physics Laboratory|author1=Helen E. Worth|author2=Mame Warren|year=2009|url=http://space50.jhuapl.edu/pdfs/book.pdf|access-date=March 3, 2013|archive-date=December 26, 2020|archive-url=https://web.archive.org/web/20201226045330/http://space50.jhuapl.edu/pdfs/book.pdf|url-status=live}}</ref><ref name="Alexandrow">{{cite web|url=http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565|title=The Story of GPS|author=Catherine Alexandrow |date=April 2008|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 [[Engineering:Satellite constellation|constellation]] of five satellites and could provide a navigational fix approximately once per hour.

In 1967, the U.S. Navy developed the [[Engineering:Timation|Timation]] satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.

In the 1970s, the ground-based [[Engineering:Omega (navigation system)|OMEGA]] navigation system, based on phase comparison of signal transmission from pairs of stations,<ref>{{cite web|author=Jerry Proc |url=http://www.jproc.ca/hyperbolic/omega.html|title=Omega|publisher=Jproc.ca|access-date=December 8, 2009|archive-url=https://web.archive.org/web/20100105155410/http://www.jproc.ca/hyperbolic/omega.html |archive-date=January 5, 2010|url-status=live}}</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 [[History:Cold War|Cold War]] [[Engineering:Arms race|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. It is also the reason for the ultra-secrecy at that time. The nuclear triad consisted of the United States Navy's [[Engineering:Submarine-launched ballistic missile|submarine-launched ballistic missile]]s (SLBMs) along with {{wipe|United States Air Force}} (USAF) [[Engineering:Strategic bomber|strategic bomber]]s and [[Engineering:Intercontinental ballistic missile|intercontinental ballistic missile]]s (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier.

Precise navigation would enable United States [[Engineering:Ballistic missile submarine|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 SS-24 and 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. A follow-on study, Project 57, was performed in 1963 and it was "in this study that the GPS concept was born". That same year, the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS"<ref>{{cite web |url=http://www.aero.org/publications/crosslink/summer2002/01.html|title=Charting a Course Toward Global Navigation|publisher=The Aerospace Corporation|access-date=October 14, 2013|archive-url=https://web.archive.org/web/20021101215923/http://www.aero.org/publications/crosslink/summer2002/01.html|archive-date=November 1, 2002<!--, 01:01:18-->}}</ref> and promised increased accuracy for Air Force bombers as well as ICBMs.

Updates from the Navy TRANSIT system were too slow for the high speeds of Air Force operation. The Naval Research Laboratory (NRL) continued making advances with their [[Engineering:Timation|Timation]] (Time Navigation) satellites, first launched in 1967, second launched in 1969, with the third in 1974 carrying the first [[Engineering:Atomic clock|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 ([[Engineering:SECOR|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 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, 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|url= {{google books|plainurl=y|id=mB9W3H90KDUC}}|title=The Precision Revolution: GPS and the Future of Aerial Warfare|author1=Michael Russell Rip |author2=James M. Hasik |publisher=Naval Institute Press|page=65|year=2002|isbn=978-1-55750-973-4|access-date=January 14, 2010}}</ref> Ten "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 }}</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 [[Astronomy:Ionosphere|ionospheric]] corrections to GPS location.<ref>{{cite web|url=https://www.ion.org/awards/2003-ionfellow-Klobuchar.cfm|title=ION Fellow - Mr. John A. Klobuchar|website=www.ion.org|access-date=June 17, 2017|archive-url=https://web.archive.org/web/20171004140058/https://www.ion.org/awards/2003-ionfellow-Klobuchar.cfm|archive-date=October 4, 2017|url-status=live}}</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 ([[Astronomy:Atmospheric refraction|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 [[Engineering:Boeing 747|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 Islands, 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&nbsp;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&nbsp;billion (equivalent to ${{Inflation|US|5|1994|fmt=c}} billion in {{Inflation/year|US}}).<ref>The Global Positioning System
Assessing National Policies, by Scott Pace, Gerald P. Frost, Irving Lachow, David R. Frelinger, Donna Fossum, Don Wassem, Monica M. Pinto, 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 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 Perry, in view of the widespread growth of [[Engineering:Differential GPS|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.

Since its deployment, the U.S. has implemented several improvements to the GPS service, including new signals for civil use and increased accuracy and integrity for all users, all the while maintaining compatibility with existing GPS equipment. Modernization of the satellite system has been an ongoing initiative by the U.S. Department of Defense through a series of [[Engineering:GPS Block IIIA|satellite acquisitions]] to meet the growing needs of the military, civilians, and the commercial market.

As of early 2015, high-quality Standard Positioning Service (SPS) GPS receivers provided horizontal accuracy of better than {{convert|3.5|m|sp=us||}},<ref name="gpsgovaccurary" /> although many factors such as receiver and antenna quality and atmospheric issues can affect this accuracy.

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=Steitz|first=David E.|title=National Positioning, Navigation and Timing Advisory Board Named|url=http://www.nasa.gov/home/hqnews/2007/mar/HQ_07071_National_PNT_Advisory_Board.txt|access-date=March 22, 2007|archive-url=https://web.archive.org/web/20100113234255/http://www.nasa.gov/home/hqnews/2007/mar/HQ_07071_National_PNT_Advisory_Board.txt|archive-date=January 13, 2010|url-status=live}}</ref> The executive committee is chaired jointly by the Deputy Secretaries of Defense and Transportation. Its membership includes equivalent-level officials from the Departments of State, Commerce, and Homeland Security, the Joint Chiefs of Staff and [[Organization:NASA|NASA]]. Components of the executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison.

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" and "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses".

=== Timeline and modernization ===
{{prose|section|date=July 2023}}
{|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 ''[[Earth:Inside GNSS|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
|-
! Success || Failure || In<br>preparation || Planned
|-
! I
| 1978–1985 || 10 || 1 || 0 ||  0 || 0
|-
! II
| 1989–1990 || 9 || 0 || 0 || 0 || 0
|-
! IIA
| 1990–1997 || 19 || 0 || 0 || 0 || 0
|-
! IIR
| 1997–2004 ||12|| 1 || 0 || 0 || 7
|-
! IIR-M
| 2005–2009 || 8 || 0 || 0 || 0 || 7
|-
! [[Engineering:GPS Block IIF|IIF]]
| 2010–2016  || 12 || 0 || 0 || 0 || 12
|-
! [[Engineering:GPS Block IIIA|IIIA]]
| 2018–  || 6 || 0 || 4 || 0 || 6
|-
! [[Engineering:GPS Block IIIF|IIIF]]
| —          || 0 || 0 || 0 || 22 || 0
|-
! colspan="2" | Total
| 76 || 2 || 4 || 22 || 32
|-
| colspan="7" style="font-size: smaller;" | (Last update: September 3, 2023)<br />
[[Engineering:USA-203|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 ''[[Engineering:List of GPS satellites|List of GPS satellites]]''
|}
* In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of four prototype GPS receivers in a Y configuration over White Sands Missile Range, using ground-based pseudo-satellites.<ref>{{cite web|url=https://www.rewiresecurity.co.uk/blog/gps-global-positioning-system-satellites|title=Origin of Global Positioning System (GPS)|website=Rewire Security|access-date=February 9, 2017|archive-url=https://web.archive.org/web/20170211080457/https://www.rewiresecurity.co.uk/blog/gps-global-positioning-system-satellites|archive-date=February 11, 2017|url-status=live}}</ref>
* In 1978, the first experimental Block-I GPS satellite was launched.<ref name="ieee2008" />
* In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269&nbsp;people on board, U.S. President Ronald Reagan announced that GPS would be made available for civilian uses once it was completed,<ref>{{cite book|url={{google books|plainurl=y|id=I7JRAAAAMAAJ}}|title=Technology Transfer|author1=Dietrich Schroeer |author2=Mirco Elena |publisher=Ashgate|isbn=978-0-7546-2045-7|year=2000|access-date=May 25, 2008|page=80}}</ref><ref>{{cite book|url={{google books|plainurl=y|id=_wpUAAAAMAAJ}}|title=The Precision Revolution: GPS and the Future of Aerial Warfare|author1=Michael Russell Rip |author2=James M. Hasik |publisher=Naval Institute Press|year=2002|isbn=978-1-55750-973-4|access-date=May 25, 2008}}</ref> although it had been publicly known as early as 1979, that the CA code (Coarse/Acquisition code) would be available to civilian users.<ref name="breeze-19790916-91">{{cite news |last1=Dore |first1=Richard |title=Navstar - Global system will provide accurate data for navigation |url=https://www.newspapers.com/article/the-daily-breeze-navstar-global-system/125171841/ |access-date=May 23, 2023 |newspaper=The Daily Breeze |date=September 16, 1979 |archive-url=https://web.archive.org/web/20230523113102/https://www.newspapers.com/article/the-daily-breeze-navstar-global-system/125171841/ |archive-date=May 23, 2023 |url-status=live |page=91 |location=Torrance, California |via=Newspapers.com }}</ref><ref name="breeze-19790916-97">{{cite news |last1=Dore |first1=Richard |title=Satellite technology key to GPS |url=https://www.newspapers.com/article/the-daily-breeze-satellite-technology-ke/125171984/ |access-date=May 23, 2023 |newspaper=The Daily Breeze |date=September 16, 1979 |archive-url=https://web.archive.org/web/20230523113718/https://www.newspapers.com/article/the-daily-breeze-satellite-technology-ke/125171984/ |archive-date=May 23, 2023 |url-status=live |page=97 |location=Torrance, California |via=Newspapers.com }}</ref>
* 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 [[Organization:2nd Space Operations Squadron|2nd Satellite Control Squadron]] (2SCS) located at {{wipe|Falcon Air Force Station}} in Colorado Springs, Colorado.<ref>{{cite web|title=AF Space Command Chronology |url=http://www.afspc.af.mil/heritage/chronology.asp |publisher=USAF Space Command |access-date=June 20, 2011 |archive-url=https://web.archive.org/web/20110817001221/http://www.afspc.af.mil/heritage/chronology.asp |archive-date=August 17, 2011 }}</ref><ref>{{cite web|title=FactSheet: 2nd Space Operations Squadron |url=http://www.schriever.af.mil/library/factsheets/factsheet.asp?id=4045 |publisher=USAF Space Command |access-date=June 20, 2011 |archive-url=https://web.archive.org/web/20110611205433/http://www.schriever.af.mil/library/factsheets/factsheet.asp?id=4045 |archive-date=June 11, 2011  }}</ref>
* 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, a project to create a miniature GPS receiver successfully ended, replacing the previous {{cvt|16|kg|||}} military receivers with a {{cvt|1.25|kg|||}} handheld receiver.<ref name=Alexandrow />
* In 1991, [[Company:TomTom|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.[[File:50th Space Wing.png|thumb|200px|Emblem of 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 [[Social:Air Force Space Command|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 [[Engineering:Interagency GPS Executive Board|Interagency GPS Executive Board]] to manage it as a national asset.
* In 1998, United States Vice President [[Biography:Al Gore|Al Gore]] announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety, and in 2000 the United States Congress authorized the effort, referring to it as ''GPS III''.
* 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 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 |archive-url=https://web.archive.org/web/20100528124826/http://pnt.gov/ |archive-date=May 28, 2010 }}</ref><!-- National Space-Based Positioning, Navigation, and Timing Executive Committee -->
* November 2004, [[Company:Qualcomm|Qualcomm]] announced successful tests of [[Engineering:Assisted GPS|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|archive-url=https://web.archive.org/web/20101127041459/http://www.3g.co.uk/PR/November2004/8641.htm|archive-date=November 27, 2010}}</ref>
* 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 [[Engineering:Ground segment|Ground Segment]] Control System was transferred to the new Architecture Evolution Plan.<ref>{{cite web|url=http://www.losangeles.af.mil/news/story.asp?id=123068412 |title=losangeles.af.mil |publisher=losangeles.af.mil |date=September 17, 2007 |access-date=October 15, 2010 |archive-url=https://web.archive.org/web/20110511192610/http://www.losangeles.af.mil/news/story.asp?id=123068412 |archive-date=May 11, 2011 }}</ref>
* On May 19, 2009, the United States [[Engineering:Government Accountability Office|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 [[Social:Air Force Space Command|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, Calif.{{clarify|date=March 2022|reason=What was the outcome?}}<ref>{{cite news|url=https://www.huffingtonpost.com/2010/06/01/air-force-gps-problem-gli_n_595727.html|title=Air Force GPS Problem: Glitch Shows How Much U.S. Military Relies On GPS|work=The Huffington Post|date=June 1, 2010 |first1=Dan |last1=Elliott |access-date=October 15, 2010|archive-url=https://web.archive.org/web/20110511200835/https://www.huffingtonpost.com/2010/06/01/air-force-gps-problem-gli_n_595727.html |archive-date=May 11, 2011 }}</ref>
* On February 25, 2010,<ref>{{cite web|url=http://www.losangeles.af.mil/news/story_print.asp?id=123192234 |title=Contract Award for Next Generation GPS Control Segment Announced |website=Los Angeles Air Force Base |date= February 25, 2010 |access-date=December 14, 2012 |archive-url=https://web.archive.org/web/20130723134812/http://www.losangeles.af.mil/news/story_print.asp?id=123192234 |archive-date=July 23, 2013 }}</ref> the U.S. Air Force awarded the contract to [[Company:Raytheon Company|Raytheon Company]] to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.

=== Awards ===
[[File:Dr Gladys West.jpg|alt=Air Force Space Commander presents Gladys West with an award as she is inducted into the Air Force Space and Missile Pioneers Hall of Fame for her GPS work on December 6, 2018.|thumb|AFSPC Vice Commander Lt. Gen. DT Thompson presents Gladys West with an award as she is inducted into the Air Force Space and Missile Pioneers Hall of Fame.]]
On February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the US's most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the USAF, the Aerospace Corporation, [[Company:Rockwell International|Rockwell International]] Corporation, and [[Company:IBM|IBM]] Federal Systems Company. The citation honors them "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50&nbsp;years ago".

Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:
* Ivan Getting, emeritus president of [[Company:The Aerospace Corporation|The Aerospace Corporation]] and an engineer at [[Organization:Massachusetts Institute of Technology|MIT]], established the basis for GPS, improving on the [[Social:World War II|World War II]] land-based radio system called LORAN (''Lo''ng-range ''R''adio ''A''id to ''N''avigation).
* [[Biography:Bradford Parkinson|Bradford Parkinson]], professor of [[Earth:Aeronautics|aeronautics]] and [[Earth:Astronautics|astronautics]] at [[Organization:Stanford University|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 |url=http://www.eurekalert.org/pub_releases/2005-11/nrl-par112205.php |title=President announces Roger Easton recipient of National Medal of Technology |archive-url=https://web.archive.org/web/20071011075824/http://eurekalert.org/pub_releases/2005-11/nrl-par112205.php |archive-date=October 11, 2007 |date=November 22, 2005 |website=EurekAlert! }}</ref>

Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force Space and Missile Pioneers Hall of Fame at Lackland A.F.B., San Antonio, Texas, March 2, 2010, for his role in space technology development and the engineering design concept of GPS conducted as part of Project 621B.

In 1998, GPS technology was inducted into the [[Organization:Space Foundation|Space Foundation]] Space Technology Hall of Fame.<ref>{{cite web|title= Inducted Technologies / 1998: Global Positioning System (GPS) |website=Space Technology Hall of Fame |url=http://www.spacetechhalloffame.org/inductees_1998_Global_Positioning_System.html |archive-url=https://web.archive.org/web/20120612064112/http://www.spacetechhalloffame.org/inductees_1998_Global_Positioning_System.html |archive-date=June 12, 2012 }}</ref>

On October 4, 2011, the [[Organization:International Astronautical Federation|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>

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; there is no other foundation; it makes things happen. And that's exactly what today's Laureates have done{{snd}} they've made things happen. 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 ==

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=2019-06-19 |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= Apr 5, 2023 }}</ref> Since the speed of [[Radio wave|radio wave]]s ([[Physics:Speed of light|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.

Each GPS satellite carries an accurate record of its own position and time, and broadcasts that data continuously. Based on data received from multiple GPS [[Astronomy:Satellite|satellite]]s, an end user's GPS receiver can calculate its own [[Four-dimensional space|four-dimensional position]] in [[Physics:Spacetime|spacetime]]; However, at a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and the deviation of its own clock from satellite time).<ref>{{Cite web |title=JAXA {{!}} Positioning to know your location and time |url=https://global.jaxa.jp/countdown/f18/overview/gps_e.html |access-date=2023-04-04 |website=global.jaxa.jp}}</ref>

=== More detailed description ===
Each GPS satellite continually broadcasts a signal ([[Carrier wave|carrier wave]] with [[Engineering:Modulation|modulation]]) that includes:
* A [[Pseudorandom binary sequence|pseudorandom]] code (sequence of ones and zeros) that is known to the receiver. By time-aligning a receiver-generated version and the receiver-measured version of the code, the time of arrival (TOA) of a defined point in the code sequence, called an epoch, can be found in the receiver clock time scale
* 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 [[Physics:Time of flight|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 pseudo-ranges. The receiver then computes its three-dimensional position and clock deviation from the four TOFs.

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 to process the TOFs.

The receiver's Earth-centered solution location is usually converted to [[Earth:Latitude|latitude]], [[Earth:Longitude|longitude]] and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to the [[Geoid|geoid]], which is essentially mean sea level. These coordinates may be displayed, such as on a [[Software:Moving map display|moving map display]], or recorded or used by some other system, such as a vehicle guidance system.

=== User-satellite geometry ===

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|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 | last1=Abel | first1=J.S. | last2=Chaffee | first2=J.W. | title=Existence and uniqueness of GPS solutions | journal=IEEE Transactions on Aerospace and Electronic Systems | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=27 | issue=6 | year=1991 | issn=0018-9251 | doi=10.1109/7.104271 | pages=952–956| bibcode=1991ITAES..27..952A }}</ref><ref name="Fang">{{cite journal | last=Fang | first=B.T. | title=Comments on "Existence and uniqueness of GPS solutions" by J.S. Abel and J.W. Chaffee | journal=IEEE Transactions on Aerospace and Electronic Systems | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=28 | issue=4 | year=1992 | issn=0018-9251 | doi=10.1109/7.165379 | page=1163| bibcode= }}</ref>

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.

=== Receiver in continuous operation ===
The description above is representative of a receiver start-up situation. Most receivers have a [[Physics:Track algorithm|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|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=[{{google books|plainurl=y|id=6P7UNphJ1z8C|page=92 |title=Extract of pages 92–93}} 92–93] |url={{google books|plainurl=y|id=6P7UNphJ1z8C}}}}</ref> More advanced navigation systems use additional sensors like a [[Physics:Compass|compass]] or an [[Earth:Inertial navigation system|inertial navigation system]] to complement GPS.

=== Non-navigation applications ===
GPS requires four or more satellites to be visible for accurate navigation. The solution of the 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|time transfer]], traffic signal timing, and synchronization of cell phone base stations, 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 on the open ocean usually has a known elevation [[Earth:Tidal range|close to 0m]], and the elevation of an aircraft may be known.{{efn|In fact, the ship is unlikely to be at precisely 0m, because of tides and other factors which create a discrepancy between mean sea level and actual sea level. In the open ocean, high and low tide typically only differ by about 0.6m, but there are locations closer to land where they can differ by over 15m. See [[Earth:Tidal range|tidal range]] for more details and references.}} Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, [[Earth:Dead reckoning|dead reckoning]], [[Earth:Inertial navigation system|inertial navigation]], or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.<ref>{{cite web|title=Continuous Navigation Combining GPS with Sensor-Based Dead Reckoning|url=http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=154870&pageID=6|archive-url=https://web.archive.org/web/20061111202317/http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=154870&pageID=6|archive-date=November 11, 2006|date=April 1, 2005|author1=Georg zur Bonsen |author2=Daniel Ammann |author3=Michael Ammann |author4=Etienne Favey |author5=Pascal Flammant |publisher=GPS World}}</ref><ref name="NAVGPS">{{cite web|title=NAVSTAR GPS User Equipment Introduction|url=http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|publisher=United States Government|access-date=August 22, 2008|archive-url=https://web.archive.org/web/20080910184805/http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|archive-date=September 10, 2008|url-status=live}} Chapter 7</ref><ref>{{cite web|title=GPS Support Notes|url=http://www.navmanwireless.com/uploads/EK/C8/EKC8zb1ITsNwDqWcqLQxiQ/Support_Notes_GPS_OperatingParameters.pdf|date=January 19, 2007|access-date=November 10, 2008|archive-url=https://web.archive.org/web/20090327051208/http://www.navmanwireless.com/uploads/EK/C8/EKC8zb1ITsNwDqWcqLQxiQ/Support_Notes_GPS_OperatingParameters.pdf|archive-date=March 27, 2009}}</ref>

== Structure ==
The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment.<ref name="breeze-19790916-97"/> The U.S. Space Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.<ref name="gps.gov">{{cite web|url=http://www.gps.gov/systems/gps |title=Global Positioning System |publisher=Gps.gov |access-date=June 26, 2010 |archive-url=https://web.archive.org/web/20100730173245/http://www.gps.gov/systems/gps |archive-date=July 30, 2010 }}</ref>

=== Space segment ===

[[File:Global Positioning System satellite.jpg|thumb|right|upright=0.8|Unlaunched GPS block II-A satellite on display at the San Diego Air & Space Museum]]
[[File:GPS24goldenSML.gif|thumb|upright=1.35|A visual example of a 24-satellite GPS constellation in motion with the Earth rotating. Notice how the number of ''satellites in view'' from a given point on the Earth's surface changes with time. The point in this example is in Golden, Colorado, USA ({{coord|39.7469|N|105.2108|W}}).]]

The space segment (SS) is composed of 24 to 32 satellites, or Space Vehicles (SV), in [[Astronomy:Medium Earth orbit|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&nbsp;SVs, eight each in three approximately circular [[Astronomy:Orbital plane|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}}</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}}</ref> The six orbit planes have approximately 55° inclination (tilt relative to the Earth's [[Astronomy:Equator|equator]]) and are separated by 60° [[Right ascension|right ascension]] of the [[Astronomy:Orbital node|ascending node]] (angle along the equator from a reference point to the orbit's intersection).<ref name="GPS overview from JPO">[http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=5325 GPS Overview from the NAVSTAR Joint Program Office] {{webarchive |url=https://web.archive.org/web/20071116230801/http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=5325 |date=November 16, 2007 }}. Retrieved December 15, 2006.</ref> The [[Astronomy:Orbital period|orbital period]] is one-half of a sidereal day, ''i.e.'', 11 hours and 58 minutes, so that [[Astronomy:Satellite revisit period|the satellites pass over the same locations]]<ref>[http://metaresearch.org/cosmology/gps-relativity.asp What the Global Positioning System Tells Us about Relativity] {{webarchive |url=https://web.archive.org/web/20070104191143/http://metaresearch.org/cosmology/gps-relativity.asp |date=January 4, 2007 }}. Retrieved January 2, 2007.</ref> or almost the same locations<ref name="The GPS Satellite Constellation">{{cite web|url=http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap2/222sats.htm |title=The GPS Satellite Constellation |website=gmat.unsw.edu.au |archive-url=https://web.archive.org/web/20111022020714/http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap2/222sats.htm |archive-date=October 22, 2011  |access-date=October 27, 2011}}</ref> every day. The orbits are arranged so that at least six satellites are always within [[Line-of-sight propagation|line of sight]] from everywhere on the Earth's surface (see animation at right).<ref>{{cite web|url=http://www.navcen.uscg.gov/?pageName=gpsFaq|title=USCG Navcen: GPS Frequently Asked Questions|access-date=January 31, 2007|archive-url=https://web.archive.org/web/20110430020428/http://www.navcen.uscg.gov/?pageName=gpsFaq|archive-date=April 30, 2011|url-status=live}}</ref> The result of this objective is that the four satellites are not evenly spaced (90°) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30°, 105°, 120°, and 105° apart, which sum to 360°.<ref name=avionicswest>{{cite web|last=Thomassen|first=Keith|title=How GPS Works|url=http://avionicswest.com/Articles/howGPSworks.html|publisher=avionicswest.com|access-date=April 22, 2014|archive-url=https://web.archive.org/web/20160330083710/http://avionicswest.com/Articles/howGPSworks.html |archive-date=March 30, 2016}}</ref>

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=[{{google books|plainurl=y|id=EyFrcnSRFFgC|page=65 |title=Extract of page 65}} 65] |url={{google books|plainurl=y|id=EyFrcnSRFFgC}}}},</ref> each SV makes two complete orbits each sidereal day, repeating the same [[Astronomy:Ground track|ground track]] each day.<ref>{{cite journal|title=Finding the repeat times of the GPS constellation|author1=Agnew, D.C.  |author2=Larson, K.M.|journal=GPS Solutions|volume=11|pages=71–76|year=2007|doi=10.1007/s10291-006-0038-4|issue=1|s2cid=59397640 }} [http://spot.colorado.edu/~kristine/gpsrep.pdf This article from author's web site] {{webarchive |url=https://web.archive.org/web/20080216041650/http://spot.colorado.edu/~kristine/gpsrep.pdf |date=February 16, 2008 }}, with minor correction.</ref> This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

{{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 [[Engineering: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 Earth with a clear horizon, ensuring considerable redundancy over the minimum four satellites needed for a position.

=== 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:
# a master control station (MCS),
# an alternative master control station,
# four dedicated ground antennas, and
# six dedicated monitor stations.

The MCS can also access Satellite Control Network (SCN) ground antennas (for additional command and control capability) and NGA ([[Organization:National Geospatial-Intelligence Agency|National Geospatial-Intelligence Agency]]) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.<ref>United States Coast Guard [https://archive.today/20120712041201/http://igs.bkg.bund.de/root_ftp/IGS/mail/igsmail/year2005/5209 General GPS News 9–9–05]</ref> The tracking information is sent to the MCS at Schriever Space Force Base {{convert|25|km|mi|abbr=on}} ESE of Colorado Springs, which is operated by the [[Organization:2nd Space Operations Squadron|2nd Space Operations Squadron]] (2&nbsp;SOPS) of the U.S. Space Force. Then 2&nbsp;SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few [[Nanosecond|nanosecond]]s of each other, and adjust the [[Astronomy:Ephemeris|ephemeris]] of each satellite's internal orbital model. The updates are created by a [[Kalman filter]] that uses inputs from the ground monitoring stations, [[Astronomy:Space weather|space weather]] information, and various other inputs.<ref>USNO [http://tycho.usno.navy.mil/gpsinfo.html NAVSTAR Global Positioning System] {{Webarchive|url=https://web.archive.org/web/20060208110241/http://tycho.usno.navy.mil/gpsinfo.html |date=February 8, 2006 }}. Retrieved May 14, 2006.</ref>

When a satellite's orbit is being adjusted, the satellite is marked ''unhealthy'', so receivers do not use it. After the maneuver, engineers track the new orbit from the ground, upload the new ephemeris, and mark the satellite healthy again.

The operation control segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports GPS users and keeps the GPS operational and performing within specification.

OCS successfully replaced the legacy 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 US Department of Defense has claimed that the new capabilities provided by OCX will be the cornerstone for revolutionizing GPS's mission capabilities, enabling U.S. Space Force to greatly enhance GPS operational services to U.S. combat forces, civil partners and myriad 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%3A%2F%2Fwww.defense.gov%2FNews%2FNews-Stories%2FArticle%2FArticle%2F974228%2Fdod-decision-breathes-new-life-into-critical-ocx-satellite-program%2F |access-date=2023-11-26 |website=U.S. Department of Defense |language=en-US}}</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 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 significant 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 the changing needs of today's and future GPS users allowing immediate access to GPS data and constellation status through secure, accurate and reliable information.
* 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.
* Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
* 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|archive-url=https://web.archive.org/web/20111002043642/http://www.comspacewatch.com/news/viewpr.html?pid=34625|archive-date=October 2, 2011}}</ref> the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development. The GPS OCX program missed major milestones and pushed its launch into 2021, 5 years past the original deadline. According to the Government Accounting Office in 2019, the 2021 deadline looked shaky.<ref>{{cite web|url=https://www.gao.gov/assets/700/699234.pdf|title=GLOBAL POSITIONING SYSTEM: Updated Schedule Assessment Could Help Decision Makers Address Likely Delays Related to New Ground Control System|date=May 2019|publisher=US Government Accounting Office|access-date=August 24, 2019|archive-date=September 10, 2019|archive-url=https://web.archive.org/web/20190910233141/https://www.gao.gov/assets/700/699234.pdf|url-status=live}}</ref> 

The project remained delayed in 2023, and was (as of June 2023) 73% over its original estimated budget.<ref>{{Cite news |date=2023-06-21 |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=2023-06-09 |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=2023-11-07 |title=Next-gen GPS ground system expected to come online this summer: Calvelli |url=https://breakingdefense.sites.breakingmedia.com/2023/11/next-gen-gps-ground-system-expected-to-come-online-this-summer-calvelli/ |access-date=2023-11-26 |website=Breaking Defense |language=en-US}}</ref>

=== User segment ===
{{further|Engineering: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 these.]]
[[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 [[Engineering:Crystal oscillator|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&nbsp;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 [[Engineering:Wide Area Augmentation System|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}}</ref> references to this protocol have been compiled from public records, allowing open source tools like [[Software:Gpsd|gpsd]] to read the protocol without violating intellectual property laws.{{Clarify|What does it mean to "compile references to a protocol"?|date=February 2013}} Other proprietary protocols exist as well, such as the [[Company:SiRF|SiRF]] and [[Company:MediaTek|MTK]] protocols. Receivers can interface with other devices using methods including a serial connection, USB, or [[Bluetooth]].

== Applications ==
{{Main|Engineering:GNSS applications}}

While originally a military project, GPS is considered a [[Engineering:Dual-use technology|dual-use technology]], meaning it has significant civilian applications as well.

GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching.<ref name="gps.gov" />

=== Civilian ===
[[File:GPS roof antenna dsc06160.jpg|thumb|right|upright=0.8|This [[Engineering:Antenna (radio)|antenna]] is mounted on the roof of a hut containing a scientific experiment needing precise timing.]]

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 APRS for position reporting; is often critical during emergency and disaster communications support. 
* [[Astronomy:Atmosphere|Atmosphere]]: studying the [[Earth:Troposphere|troposphere]] delays (recovery of the water vapor content) and [[Astronomy:Ionosphere|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:Astronomy|Astronomy]]: both positional and [[Clock synchronization|clock synchronization]] data is used in [[Astronomy:Astrometry|astrometry]] and [[Physics:Celestial mechanics|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 [[Astronomy:Amateur astronomy|amateur astronomy]] with [[Astronomy:GoTo (telescopes)|small telescopes]] as well as by professional observatories for finding extrasolar planets.
* Automated vehicle: applying location and routes for cars and trucks to function without a human driver.
* [[Earth:Cartography|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 mobile emergency calls and other applications. The first 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&nbsp;callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
* [[Clock synchronization]]: the accuracy of GPS time signals (±10 ns)<ref>{{cite web|url=http://tf.nist.gov/time/commonviewgps.htm|title=Common View GPS Time Transfer|publisher=nist.gov|access-date=July 23, 2011|archive-url=https://web.archive.org/web/20121028043917/http://tf.nist.gov/time/commonviewgps.htm|archive-date=October 28, 2012}}</ref> is second only to the atomic clocks they are based on, and is used in applications such as [[GPS disciplined oscillator]]s.
* Disaster relief/emergency services: many emergency services depend upon GPS for location and timing capabilities.
* GPS-equipped [[Earth:Radiosonde|radiosonde]]s and [[Earth:Dropsonde|dropsonde]]s: measure and calculate the atmospheric pressure, wind speed and direction up to {{cvt|27|km|ft||}} from the Earth's surface.
* [[Astronomy:Radio occultation|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 [[Engineering:Fleet vehicle|fleet]] vehicles in real-time.
* [[Geodesy]]: determination of [[Astronomy:Earth orientation parameters|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 - geocenter motion,<ref>{{cite journal |last1=Zajdel |first1=Radosław |last2=Sośnica |first2=Krzysztof |last3=Bury |first3=Grzegorz |title=Geocenter coordinates derived from multi-GNSS: a look into the role of solar radiation pressure modeling |journal=GPS Solutions |date=January 2021 |volume=25 |issue=1 |page=1 |doi=10.1007/s10291-020-01037-3|bibcode=2021GPSS...25....1Z |doi-access=free }}</ref> and low-degree gravity field parameters.<ref>{{cite journal |last1=Glaser |first1=Susanne |last2=Fritsche |first2=Mathias |last3=Sośnica |first3=Krzysztof |last4=Rodríguez-Solano |first4=Carlos Javier |last5=Wang |first5=Kan |last6=Dach |first6=Rolf |last7=Hugentobler |first7=Urs |last8=Rothacher |first8=Markus |last9=Dietrich |first9=Reinhard |title=A consistent combination of GNSS and SLR with minimum constraints |journal=Journal of Geodesy |date=December 2015 |volume=89 |issue=12 |pages=1165–1180 |doi=10.1007/s00190-015-0842-0|bibcode=2015JGeod..89.1165G |s2cid=118344484 |url=https://boris.unibe.ch/71369/ }}</ref>
* Geofencing: [[Earth:Vehicle tracking system|vehicle tracking system]]s, person tracking systems, and [[Physics: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="Rouse">{{Cite encyclopedia|url=http://whatis.techtarget.com/definition/geofencing|title=What is geo-fencing (geofencing)?|language=en-US|date=December 2016|encyclopedia=WhatIs.com|publisher=TechTarget|location=Newton, Massachusetts|access-date=26 Jan 2020|last=Rouse|first=Margaret}}</ref>
* [[Engineering:Geotagging|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
* [[Engineering:GPS aircraft tracking|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.
* 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 tours: location determines what content to display; for instance, information about an approaching point of interest.
* [[Social:Mental health|Mental health]]: tracking mental health functioning and sociability.<ref>{{Cite journal |last1=Braund |first1=Taylor A. |last2=Zin |first2=May The |last3=Boonstra |first3=Tjeerd W. |last4=Wong |first4=Quincy J. J. |last5=Larsen |first5=Mark E. |last6=Christensen |first6=Helen |last7=Tillman |first7=Gabriel |last8=O'Dea |first8=Bridianne |date=2022-05-04 |title=Smartphone Sensor Data for Identifying and Monitoring Symptoms of Mood Disorders: A Longitudinal Observational Study |journal=JMIR Mental Health |language=EN |volume=9 |issue=5 |pages=e35549 |doi=10.2196/35549 | pmid=35507385|pmc=9118091 |doi-access=free }}</ref>
* [[Earth:Navigation|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>
* [[Physics:Orbit|Orbit]] determination of low-orbiting satellites with GPS receiver installed on board, such as GOCE,<ref>{{cite journal |last1=Strugarek |first1=Dariusz |last2=Sośnica |first2=Krzysztof |last3=Jäggi |first3=Adrian |title=Characteristics of GOCE orbits based on Satellite Laser Ranging |journal=Advances in Space Research |date=January 2019 |volume=63 |issue=1 |pages=417–431 |doi=10.1016/j.asr.2018.08.033|bibcode=2019AdSpR..63..417S |s2cid=125791718 }}</ref> [[Physics:GRACE and GRACE-FO|GRACE]], [[Earth:Jason-1|Jason-1]], Jason-2, [[Engineering:TerraSAR-X|TerraSAR-X]], [[Engineering:TanDEM-X|TanDEM-X]], [[Engineering:CHAMP (satellite)|CHAMP]], [[Engineering:Sentinel-3|Sentinel-3]],<ref>{{cite journal |last1=Strugarek |first1=Dariusz |last2=Sośnica |first2=Krzysztof |last3=Arnold |first3=Daniel |last4=Jäggi |first4=Adrian |last5=Zajdel |first5=Radosław |last6=Bury |first6=Grzegorz |last7=Drożdżewski |first7=Mateusz |title=Determination of Global Geodetic Parameters Using Satellite Laser Ranging Measurements to Sentinel-3 Satellites |journal=Remote Sensing |date=September 30, 2019 |volume=11 |issue=19 |page=2282 |doi=10.3390/rs11192282|bibcode=2019RemS...11.2282S |doi-access=free }}</ref> and some cubesats, e.g., [[Engineering:CubETH|CubETH]].
* [[Engineering:Phasor measurement unit|Phasor measurements]]: GPS enables highly accurate timestamping of power system measurements, making it possible to compute [[Engineering:Phasor measurement unit|phasors]].
* Recreation: for example, [[Engineering:Geocaching|Geocaching]], Geodashing, [[Engineering:GPS drawing|GPS drawing]], waymarking, and other kinds of [[Earth:Location-based game|location based mobile games]] such as [[Software:Pokémon Go|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 Multi-GNSS Processing |journal=Journal of Geophysical Research: Solid Earth |date=June 2019 |volume=124 |issue=6 |pages=5970–5989 |doi=10.1029/2019JB017443|bibcode=2019JGRB..124.5970Z |doi-access=free }}</ref> in the framework of Global Geodetic Observing System. Co-location in space between Satellite laser ranging<ref>{{cite journal |last1=Sośnica |first1=Krzysztof |last2=Thaller |first2=Daniela |last3=Dach |first3=Rolf |last4=Steigenberger |first4=Peter |last5=Beutler |first5=Gerhard |last6=Arnold |first6=Daniel |last7=Jäggi |first7=Adrian |title=Satellite laser ranging to GPS and GLONASS |journal=Journal of Geodesy |date=July 2015 |volume=89 |issue=7 |pages=725–743 |doi=10.1007/s00190-015-0810-8|bibcode=2015JGeod..89..725S |doi-access=free }}</ref> and microwave observations<ref>{{cite journal |last1=Bury |first1=Grzegorz |last2=Sośnica |first2=Krzysztof |last3=Zajdel |first3=Radosław |last4=Strugarek |first4=Dariusz |last5=Hugentobler |first5=Urs |title=Determination of precise Galileo orbits using combined GNSS and SLR observations |journal=GPS Solutions |date=January 2021 |volume=25 |issue=1 |page=11 |doi=10.1007/s10291-020-01045-3|bibcode=2021GPSS...25...11B |doi-access=free }}</ref> for deriving global geodetic parameters.<ref>{{cite journal |last1=Sośnica |first1=K. |last2=Bury |first2=G. |last3=Zajdel |first3=R. |title=Contribution of Multi-GNSS Constellation to SLR-Derived Terrestrial Reference Frame |journal=Geophysical Research Letters |date=March 16, 2018 |volume=45 |issue=5 |pages=2339–2348 |doi=10.1002/2017GL076850|bibcode=2018GeoRL..45.2339S |s2cid=134160047 }}</ref><ref>{{cite journal |last1=Sośnica |first1=K. |last2=Bury |first2=G. |last3=Zajdel |first3=R. |last4=Strugarek |first4=D. |last5=Drożdżewski |first5=M. |last6=Kazmierski |first6=K. |title=Estimating global geodetic parameters using SLR observations to Galileo, GLONASS, BeiDou, GPS, and QZSS |journal=Earth, Planets and Space |date=December 2019 |volume=71 |issue=1 |page=20 |doi=10.1186/s40623-019-1000-3|bibcode=2019EP&S...71...20S |doi-access=free }}</ref>
* 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|website=www.asme.org|language=en|archive-date=August 3, 2021|archive-url=https://web.archive.org/web/20210803230646/https://www.asme.org/topics-resources/content/gps-helps-robots-get-job-done|url-status=live}}</ref> which calculate latitude, longitude, time, speed, and heading.
* Sport: used in football and rugby for the control and analysis of the training load.<ref name="LiveViewGPS 2012-09-06">{{cite web |url=http://www.liveviewgps.com/blog/gps-tracking-technology-australian-football/ |title=The Use of GPS Tracking Technology in Australian Football |date=September 6, 2012 |access-date=September 25, 2016 |archive-url=https://web.archive.org/web/20160927063511/http://www.liveviewgps.com/blog/gps-tracking-technology-australian-football/ |archive-date=September 27, 2016 |url-status=live }}</ref>
* [[Engineering:Surveying|Surveying]]: surveyors use absolute locations to make maps and determine property boundaries.
* [[Earth:Tectonics|Tectonics]]: GPS enables direct fault motion measurement of earthquakes. Between earthquakes GPS can be used to measure [[Earth: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 [[Earth:Seismic hazard|seismic hazard]] maps.
* [[Telematics]]: GPS technology integrated with computers and mobile communications technology in [[Automotive navigation system|automotive navigation system]]s.
<!--* GPS enables researchers to explore Earth's environment including the atmosphere, ionosphere and gravity field. how???-->
<!--

Compared to a few years ago, GPS technology for handsets has matured considerably, offering much better performance in terms of sensitivity, power consumption, size and price. What is more, the OMA SUPL A-GPS standard has enabled lower cost deployment of A-GPS services that ensure a better and more consistent user experience necessary for the mass consumer market. The SUPL A-GPS standard allows network operators or handset manufacturers to deploy assistance services that reduce the time to first fix, lowers the power consumption, and enhances the sensitivity of the GPS receiver. The SUPL standard uses User Plane communication channels such as SMS and GPRS to transport the aiding data, as opposed to the control plane channels in networks, thereby reducing the load on the networks, as well as complexity and cost of service deployment. New business models have also become possible, ranging from hosted services for operators that want to minimize capital investments, to services deployed by handset vendors for end-users that cannot get similar services from their network operator yet.

The major handset software platforms and operating systems are evolving, ensuring easier integration of GPS functionality for handset manufacturers and more powerful features for application developers. Along with the improving performance of handsets, in terms of screen size, processing power and memory size, current handsets thus provide much better platforms for location-enabled applications and services than before.

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 million units in 2007 according to independent analyst firm Berg Insight. Although the number is very small in comparison with the 150 million GPS-enabled CDMA handsets sold, the number is growing rapidly. Berg Insight estimates that shipments of GPS-enabled GSM/WCDMA handsets will grow to 370&nbsp;million units in 2012, the equivalent of more than 26 percent of all GSM/WCDMA handsets sold that year. Including CDMA handsets, GPS-enabled handsets sales are estimated to reach about 560&nbsp;million, or 35&nbsp;percent of total handset shipments in 2012.

-->

==== 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 munitions (weapons)—which means they require 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]]thumb|right|Attaching a GPS guidance kit to a [[dumb bomb, March 2003]]
[[File:XM982 Excalibur inert.jpg|thumb|right|M982 Excalibur GPS-guided artillery shell]]
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 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 [[Engineering:Precision Lightweight GPS Receiver|Precison Lightweight GPS Receiver]] (PLGR) and [[Engineering:Defense Advanced GPS Receiver|Defense Advanced GPS Receiver]] (DAGR), are used by [[Social:Radio operator|radio operators]] within a radio network to properly input an accurate time to said radios internal clock. More modern military radios have internal GPS receivers that syncronize 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 munitions 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 ICBMs, [[Engineering:Cruise missile|cruise missile]]s, precision-guided munitions and artillery shells. Embedded GPS receivers able to withstand accelerations of 12,000 ''[[Physics: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 report |section=Excalibur XM982 Precision Engagement Projectiles |url=https://www.dote.osd.mil/Portals/97/pub/reports/FY2010/army/2010excalibur.pdf |publisher=Director, Operational Test and Evaluation |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20221115000445/https://www.dote.osd.mil/Portals/97/pub/reports/FY2010/army/2010excalibur.pdf |archive-date=November 15, 2022 |date=December 2010 |url-status=live |pages=65–66 |title=FY2010 Annual Report }}</ref>
* 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 [[Engineering:Bhangmeter|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 report |title=The GPS Burst Detector W-Sensor|author=Dennis D. McCrady|date=August 1994 |publisher=Sandia National Laboratories |osti=10176800 |osti-access=free}}</ref> General William Shelton has stated that future satellites may drop this feature to save money.<ref>{{cite web |url=http://www.aviationweek.com/Article.aspx?id=/article-xml/awx_01_18_2013_p0-538541.xml |title=US Air Force Eyes Changes To National Security Satellite Programs. |publisher=Aviationweek.com |date=January 18, 2013 |access-date=September 28, 2013 |archive-url=https://web.archive.org/web/20130922073035/http://www.aviationweek.com/Article.aspx?id=%2Farticle-xml%2Fawx_01_18_2013_p0-538541.xml |archive-date=September 22, 2013 |url-status=live }}</ref>
GPS type navigation was first used in war in the 1991 Persian Gulf War, before GPS was fully developed in 1995, to assist Coalition Forces to navigate and perform maneuvers in the war. The war also demonstrated the vulnerability of GPS to being [[Radio jamming|jammed]], when Iraqi forces installed jamming devices on likely targets that emitted radio noise, disrupting reception of the weak GPS signal.<ref>{{cite magazine |title = GPS and the World's First "Space War"|url = http://www.scientificamerican.com/article/gps-and-the-world-s-first-space-war/|magazine= Scientific American|access-date = February 8, 2016|first = Larry|last = Greenemeier|archive-url = https://web.archive.org/web/20160208233555/http://www.scientificamerican.com/article/gps-and-the-world-s-first-space-war/|archive-date = February 8, 2016|url-status = live}}</ref>

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>

=== Timekeeping {{anchor|GPS time|GPS time and date}} ===

==== Leap seconds ====
While most clocks derive their time from [[Physics:Coordinated Universal Time|Coordinated Universal Time]] (UTC), the atomic clocks on the satellites are set to ''GPS time''. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain new leap seconds or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with [[Physics:International Atomic Time|International Atomic Time]] (TAI) (TAI - GPS = 19&nbsp;seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks.<ref name="NAVGPS" />{{rp|at=Section 1.2.2}}

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|clock drift]] relative to [[Physics:International Atomic Time|International Atomic Time]] that the atomic clocks in GPS transmitters experience.<ref>{{cite book |author=David W. Allan |author2=Neil Ashby |author3=Clifford C. Hodge |url=https://www.hpmemoryproject.org/an/pdf/an_1289.pdf |title=The Science of Timekeeping |publisher=Hewlett Packard |year=1997 |via=HP Memory Project}}</ref> Most receivers lose some accuracy in their interpretation of the signals and are only accurate to about 100 nanoseconds.<ref>{{cite magazine |author=Peter H. Dana |author2=Bruce M Penrod|url=http://www.pdana.com/PHDWWW_files/gpsrole.pdf |title=The Role of GPS in Precise Time and Frequency Dissemination |magazine=GPS World |date=July–August 1990 |access-date=April 27, 2014|archive-url=https://web.archive.org/web/20121215031941/http://www.pdana.com/PHDWWW_files/gpsrole.pdf |archive-date=December 15, 2012|url-status=live |via=P Dana}}</ref><ref>{{cite web|url=http://www.atomic-clock.galleon.eu.com/support/gps-time-accuracy.html|title=GPS time accurate to 100 nanoseconds|publisher=Galleon|access-date=October 12, 2012|archive-url=https://web.archive.org/web/20120514001920/http://www.atomic-clock.galleon.eu.com/support/gps-time-accuracy.html|archive-date=May 14, 2012|url-status=live}}</ref>

==== Relativistic corrections ====
The GPS implements 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}}</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 |pages=1 |doi=10.12942/lrr-2003-1 |issn=1433-8351 |pmc=5253894 |pmid=28163638|bibcode=2003LRR.....6....1A }}</ref>

==== Format ====
{{see|GPS week number rollover}}
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|bit]] field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024&nbsp;weeks (19.6&nbsp;years). GPS week zero started at 00:00:00&nbsp;UTC (00:00:19&nbsp;TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47&nbsp;UTC on August 21, 1999 (00:00:19&nbsp;TAI on August 22, 1999). It happened the second time at 23:59:42&nbsp;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&nbsp;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&nbsp;weeks (157&nbsp;years), thus lasting until 2137 (157&nbsp;years after GPS week zero).

== Communication ==
{{Main|Earth: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.

=== Message format ===
:{|class="wikitable" style="float:right; margin:0 0 0.5em 1em;" border="1"
|+ {{nowrap|GPS message format}}
! Subframes !! Description
|-
| 1 || Satellite clock,<br />GPS time relationship
|-
| 2–3 || Ephemeris<br />(precise satellite orbit)
|-
| 4–5 || Almanac component<br />(satellite network synopsis,<br />error correction)
|}

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 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=Peter H. Dana|url=http://www.colorado.edu/geography/gcraft/notes/gps/gpseow.htm|title=GPS Week Number Rollover Issues|access-date=August 12, 2013|archive-url=https://web.archive.org/web/20130225182002/http://www.colorado.edu/geography/gcraft/notes/gps/gpseow.htm|archive-date=February 25, 2013}}</ref> as well as the data about the health of the satellite. The second and the third subframes contain the ''[[Astronomy:Ephemeris|ephemeris]]'' – the precise orbit for the satellite. The fourth and fifth subframes contain the ''almanac'', which contains coarse<!-- "Coarse" is correct, as in "not precision"--> orbit and status information for up to 32 satellites in the constellation as well as data related to error correction. Thus, to obtain an accurate satellite location from this transmitted message, the receiver must demodulate the message from each satellite it includes in its solution for 18 to 30 seconds. To collect all transmitted almanacs, the receiver must demodulate the message for 732 to 750 seconds or {{frac|12|1|2}} minutes.<ref>{{cite web|url=http://www.losangeles.af.mil/shared/media/document/AFD-070803-059.pdf |title=Interface Specification IS-GPS-200, Revision D: Navstar GPS Space Segment/Navigation User Interfaces |publisher=Navstar GPS Joint Program Office |page=103 |archive-url=https://web.archive.org/web/20120908003700/http://www.losangeles.af.mil/shared/media/document/AFD-070803-059.pdf |archive-date=September 8, 2012  }}</ref>

All satellites broadcast at the same frequencies, encoding signals using unique [[Code-division multiple access|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
|title=Satellite Systems for Personal Applications: Concepts and Technology
|first1=Madhavendra
|last1=Richharia
|first2=Leslie David
|last2=Westbrook
|publisher=John Wiley & Sons
|year=2011
|isbn=978-1-119-95610-5
|page=443
|url=https://books.google.com/books?id=MqPQ5CbgQ48C&pg=PT443
|access-date=February 28, 2017
|archive-url=https://web.archive.org/web/20140704134423/http://books.google.com/books?id=MqPQ5CbgQ48C&pg=PT443
|archive-date=July 4, 2014
|url-status=live
}}</ref>

The ephemeris is updated every 2 hours and is sufficiently stable for 4 hours, with provisions for updates every 6&nbsp;hours or longer in non-nominal conditions. The almanac is updated typically every 24&nbsp;hours. Additionally, data for a few weeks following is uploaded in case of transmission updates that delay data upload.{{citation needed|date=April 2021}}

=== Satellite frequencies ===
:{|class="wikitable" style="float:right; width:30em; margin:0 0 0.5em 1em;" border="1"
|+ {{nowrap|GPS frequency overview<ref name="handbook-pent">{{cite book|last1=Penttinen|first1=Jyrki T.J.|title=The Telecommunications Handbook: Engineering Guidelines for Fixed, Mobile and Satellite Systems|publisher=John Wiley & Sons|isbn=978-1-119-94488-1|url=https://books.google.com/books?id=HRQmBgAAQBAJ|language=en|date=2015}}</ref>{{rp|607}}}}
! Band !! Frequency !! Description
|-
| '''L1''' || 1575.42&nbsp;MHz || Coarse-acquisition<!-- "Coarse" is correct, as in "not precision"--> (C/A) and encrypted precision (P(Y)) codes, plus the L1 civilian ([[Earth:GPS signals#L1C|L1C]]) and military (M) codes on Block III and newer satellites.
|-
| '''L2''' || 1227.60&nbsp;MHz || P(Y) code, plus the L2C and military codes on the Block IIR-M and newer satellites.
|-
| '''L3''' || 1381.05&nbsp;MHz || Used for nuclear detonation (NUDET) detection.
|-
| '''L4''' || 1379.913&nbsp;MHz || Being studied for additional ionospheric correction.
|-
| '''L5''' || 1176.45&nbsp;MHz || Used as a civilian safety-of-life (SoL) signal on Block IIF and newer satellites.
|}

All satellites broadcast at the same two frequencies, 1.57542&nbsp;GHz (L1 signal) and 1.2276&nbsp;GHz (L2&nbsp;signal). The satellite network uses a CDMA spread-spectrum technique<ref name="handbook-pent" />{{rp|607}} where the low-bitrate message data is encoded with a high-rate [[Pseudorandom number generator|pseudo-random]] (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A&nbsp;code, for civilian use, transmits data at 1.023&nbsp;million [[Engineering:Chip (CDMA)|chips]] per second, whereas the P&nbsp;code, for U.S. military use, transmits at 10.23&nbsp;million chips per second. The actual internal reference of the satellites is 10.22999999543&nbsp;MHz to compensate for [[Physics:Theory of relativity|relativistic effects]]<ref>{{cite book|title=Global Positioning System. Signals, Measurements and Performance|edition=2nd|first1=Pratap|last1=Misra|first2=Per|last2=Enge|publisher=Ganga-Jamuna Press|year=2006|isbn=978-0-9709544-1-1|page=115|url={{google books|plainurl=y|id=pv5MAQAAIAAJ}}|access-date=August 16, 2013}}</ref><ref>{{cite book
|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&nbsp;carrier is modulated by both the C/A and P&nbsp;codes, while the L2&nbsp;carrier is only modulated by the P&nbsp;code.<ref name=avionicswest /> The P&nbsp;code can be encrypted as a so-called P(Y)&nbsp;code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y)&nbsp;codes impart the precise time-of-day to the user.

The L3 signal at a frequency of 1.38105&nbsp;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 |url=https://fas.org/spp/military/program/nssrm/initiatives/usnds.htm |title=United States Nuclear Detonation Detection System (USNDS) |website=Fas.org |access-date=November 6, 2011 |archive-url=https://web.archive.org/web/20111010123718/http://www.fas.org/spp/military/program/nssrm/initiatives/usnds.htm |archive-date=October 10, 2011 |url-status=dead }}</ref> One usage is the enforcement of nuclear test ban treaties.

The L4 band at 1.379913&nbsp;GHz is being studied for additional ionospheric correction.<ref name="handbook-pent" />{{rp|607}}

The L5 frequency band at 1.17645&nbsp;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 |archive-url=https://web.archive.org/web/20100530023659/http://www.dailytech.com/First+Block+2F+GPS+Satellite+Launched+Needed+to+Prevent+System+Failure/article18483.htm |archive-date=May 30, 2010  }}</ref> On February 5, 2016, the 12th and final Block IIF satellite was launched.<ref>{{cite web|url=https://www.ulalaunch.com/about/news-detail/2016/02/05/united-launch-alliance-successfully-launches-gps-iif-12-satellite-for-u.s.-air-force|title=United Launch Alliance Successfully Launches GPS IIF-12 Satellite for U.S. Air Force|website=www.ulalaunch.com|access-date=February 27, 2018|archive-url=https://web.archive.org/web/20180228161519/https://www.ulalaunch.com/about/news-detail/2016/02/05/united-launch-alliance-successfully-launches-gps-iif-12-satellite-for-u.s.-air-force|archive-date=February 28, 2018|url-status=live}}</ref> The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. "L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy."<ref>{{cite web|title=Air Force Successfully Transmits an L5 Signal From GPS IIR-20(M) Satellite |url=http://www.losangeles.af.mil/news/story.asp?storyID=123144001 |publisher=LA AFB News Release |access-date=June 20, 2011 |archive-url=https://web.archive.org/web/20110521025953/http://www.losangeles.af.mil/news/story.asp?storyID=123144001 |archive-date=May 21, 2011 }}</ref>


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 issues that might arise due to the larger signal power from the LightSquared terrestrial network. The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, essentially repurposing what was supposed to be a "quiet neighborhood" for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the impact of the lower 10&nbsp;MHz of spectrum is minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10&nbsp;MHz intended for use by LightSquared may have some impact on GPS devices. There is some concern that this may seriously degrade the GPS signal for many consumer uses.<ref name="gpsworld-20110301">{{cite web|url=https://www.gpsworld.com/the-system-test-data-predicts-disastrous-gps-jamming-by-fcc-authorized-broadcaster/ |title=The System: Test Data Predicts Disastrous GPS Jamming by FCC-Authorized Broadcaster |date=March 1, 2011 |publisher=GPS World |access-date=November 6, 2011 |archive-url=https://web.archive.org/web/20111011082258/http://www.gpsworld.com/gnss-system/news/data-shows-disastrous-gps-jamming-fcc-approved-broadcaster-11029 |archive-date=October 11, 2011 |url-status=live }}</ref><ref>{{cite web|url=http://www.saveourgps.org/studies-reports.aspx|title=Coalition to Save Our GPS|publisher=Saveourgps.org|access-date=November 6, 2011|archive-url=https://web.archive.org/web/20111030072958/http://saveourgps.org/studies-reports.aspx|archive-date=October 30, 2011}}</ref> ''Aviation Week'' magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system.<ref name="aviationweek1">{{cite magazine|title=LightSquared Tests Confirm GPS Jamming |url=http://www.aviationweek.com/aw/generic/story.jsp?id=news/awx/2011/06/09/awx_06_09_2011_p0-334122.xml&headline=LightSquared%20Tests%20Confirm%20GPS%20Jamming&channel=busav |magazine=Aviation Week |access-date=June 20, 2011 |archive-url=https://web.archive.org/web/20110812045607/http://www.aviationweek.com/aw/generic/story.jsp?id=news%2Fawx%2F2011%2F06%2F09%2Fawx_06_09_2011_p0-334122.xml&headline=LightSquared%20Tests%20Confirm%20GPS%20Jamming&channel=busav |archive-date=August 12, 2011 }}</ref>

=== 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|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., and Miller A. {{PDFWayback |date=20071122063244 |url=http://www.xilinx.com/support/documentation/application_notes/xapp217.pdf |archive-url=https://web.archive.org/web/20071122063244/http://www.xilinx.com/support/documentation/application_notes/xapp217.pdf |archive-date=2007-11-22 |url-status=live |title=Gold Code Generators in Virtex Devices |126&nbsp;KB}}</ref>

If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range&nbsp;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&nbsp;code pattern. There can be a delay of up to 30&nbsp;seconds before the first estimate of position because of the need to read the ephemeris data.

Processing of the navigation message enables the determination of the time of transmission and the satellite position at this time. For more information see [[Earth:GPS signals#Demodulation and decoding|Demodulation and Decoding, Advanced]].

== Navigation equations ==
{{Further|Engineering:GNSS positioning calculation}}

=== Problem statement ===
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''&nbsp;≥&nbsp;4. When the time of message reception indicated by the on-board receiver clock is <math>\tilde{t}_i</math>, the true reception time is <math>t_i = \tilde{t}_i - b</math>, where ''b'' is the receiver's clock bias from the much more accurate GPS clocks employed by the satellites. The receiver clock bias is the same for all received satellite signals (assuming the satellite clocks are all perfectly synchronized). The message's transit time is <math>\tilde{t}_i - b - s_i</math>, where ''s<sub>i</sub>'' is the satellite time. Assuming the message traveled at [[Physics:Speed of light|the speed of light]], ''c'', the distance traveled is <math>\left(\tilde{t}_i - b - s_i\right) c</math>.

For n satellites, the equations to satisfy are:
:<math>d_i = \left( \tilde{t}_i - b - s_i \right)c, \; i=1,2,\dots,n</math>
where ''d<sub>i</sub>'' is the geometric distance or range between receiver and satellite ''i'' (the values without subscripts are the ''x, y,'' and ''z'' components of receiver position):
:<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 Geoffery Blewitt: Basics of the GPS Technique] {{Webarchive|url=https://web.archive.org/web/20130922064413/http://www.nbmg.unr.edu/staff/pdfs/Blewitt%20Basics%20of%20gps.pdf |date=September 22, 2013 }}</ref><ref name=Bancroft>{{cite web|url=http://www.macalester.edu/~halverson/math36/GPS.pdf|archive-url=https://web.archive.org/web/20110719232148/http://www.macalester.edu/~halverson/math36/GPS.pdf|archive-date=July 19, 2011|title=Global Positioning Systems|access-date=October 15, 2010}}</ref>
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.  Receivers usually calculate a running estimate of the error in the calculated position. This is done by multiplying the basic resolution of the receiver by quantities called the [[Engineering:Dilution of precision (navigation)|geometric dilution of position]] (GDOP) factors, calculated from the relative sky directions of the satellites used.<ref>{{cite web|url=http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#Gdop|title=Geometric Dilution of Precision (GDOP) and Visibility|first=Peter H.|last=Dana|publisher=University of Colorado at Boulder|access-date=July 7, 2008|archive-url=https://web.archive.org/web/20050823013233/http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#Gdop|archive-date=August 23, 2005}}</ref> The receiver location is expressed in a specific coordinate system, such as latitude and longitude using the WGS 84 geodetic datum or a country-specific system.<ref>{{cite web|url=http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#PosVelTime|title=Receiver Position, Velocity, and Time|author=Peter H. Dana|publisher=University of Colorado at Boulder|access-date=July 7, 2008|archive-url=https://web.archive.org/web/20050823013233/http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#PosVelTime|archive-date=August 23, 2005}}</ref>

=== Geometric interpretation ===
The GPS equations can be solved by numerical and analytical methods. Geometrical interpretations can enhance the understanding of these solution methods.

==== 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|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}}</ref> One of the points is the location of the receiver, and the other moves rapidly in successive measurements and would not usually be on Earth's surface.

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.

==== Hyperboloids ====
[[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, {{nobreak|1=''p<sub>i</sub>'' − ''p<sub>j</sub>''}}, the common receiver clock bias (''b'') cancels out, resulting in a difference of distances {{nobreak|1=''d<sub>i</sub>'' − ''d<sub>j</sub>''}}. The locus of points having a constant difference in distance to two points (here, two satellites) is a [[Hyperbola|hyperbola]] on a plane and a hyperboloid of revolution (more specifically, a two-sheeted hyperboloid) in 3D space (see [[Multilateration]]). Thus, from four pseudorange measurements, the receiver can be placed at the intersection of the surfaces of three hyperboloids each with [[Focus (geometry)|foci]] at a pair of satellites. With additional satellites, the multiple intersections are not necessarily unique, and a best-fitting solution is sought instead.<ref name="Abel1" /><ref name="Fang" /><ref>{{cite book|author1=Gilbert Strang|author2=Kai Borre|title=Linear Algebra, Geodesy, and GPS|url=https://books.google.com/books?id=MjNwWUY8jx4C&pg=PA449|year=1997|publisher=SIAM|isbn=978-0-9614088-6-2|pages=448–449|access-date=May 22, 2018|archive-date=October 10, 2021|archive-url=https://web.archive.org/web/20211010021202/https://books.google.com/books?id=MjNwWUY8jx4C&pg=PA449|url-status=live}}</ref><ref>{{cite book|author=Audun Holme|title=Geometry: Our Cultural Heritage|url=https://books.google.com/books?id=zXwQGo8jyHUC&pg=PA338|year=2010|publisher=Springer Science & Business Media|isbn=978-3-642-14441-7|page=338|access-date=May 22, 2018|archive-date=October 10, 2021|archive-url=https://web.archive.org/web/20211010021203/https://books.google.com/books?id=zXwQGo8jyHUC&pg=PA338|url-status=live}}</ref><ref name="HWLW">{{cite book|author1=B. Hofmann-Wellenhof|author2=K. Legat|author3=M. Wieser|title=Navigation|url=https://books.google.com/books?id=losWr9UDRasC&pg=PA36|year=2003|publisher=Springer Science & Business Media|isbn=978-3-211-00828-7|page=36|access-date=May 22, 2018|archive-date=October 10, 2021|archive-url=https://web.archive.org/web/20211010021203/https://books.google.com/books?id=losWr9UDRasC&pg=PA36|url-status=live}}</ref><ref name="Groves2013">{{cite book | last=Groves | first=P.D. | title=Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems, Second Edition | publisher=Artech House | series=GNSS/GPS | year=2013 | isbn=978-1-60807-005-3 | url=https://books.google.com/books?id=t94fAgAAQBAJ | access-date=February 19, 2021 | page= | archive-date=March 15, 2021 | archive-url=https://web.archive.org/web/20210315202930/https://books.google.com/books?id=t94fAgAAQBAJ | url-status=live }}</ref>

==== 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|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| author = Hoshen J| year = 1996| title = The GPS Equations and the Problem of Apollonius| journal = IEEE Transactions on Aerospace and Electronic Systems| volume = 32| pages = 1116–1124| doi = 10.1109/7.532270| issue = 3| bibcode = 1996ITAES..32.1116H| s2cid = 30190437}}</ref>

==== 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 [[Earth:Pseudorange|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.  The solution space [''x, y, z, b''] can be seen as a four-dimensional [[Physics:Spacetime|spacetime]], and signals from at minimum four satellites are needed. In that case each of the equations describes a [[Hypercone|hypercone]] (or spherical cone),<ref>{{cite journal|title=GPS Solutions: Closed Forms, Critical and Special Configurations of P4P | doi=10.1007/PL00012897 | volume=5|issue=3 |journal=GPS Solutions|pages=29–41 | last1 = Grafarend | first1 = Erik W.|year=2002 | bibcode=2002GPSS....5...29G | s2cid=121336108 }}</ref> with the cusp located at the satellite, and the base a sphere around the satellite. The receiver is at the intersection of four or more of such hypercones.

=== Solution methods ===

==== Least squares ====
When more than four satellites are available, the calculation can use the four best, or more than four simultaneously (up to all visible satellites), depending on the number of receiver channels, processing capability, and geometric dilution of precision (GDOP).

Using more than four involves an over-determined system of equations with no unique solution; such a system can be solved by a least-squares or weighted least squares method.<ref name=GPS_BASICS_Blewitt />

:<math>\left( \hat{x},\hat{y},\hat{z},\hat{b} \right) = \underset{\left( x,y,z,b \right)}{\arg \min} \sum_i \left( \sqrt{(x-x_i)^2 + (y-y_i)^2 + (z-z_i)^2} + bc - p_i \right)^2</math>

==== Iterative ====
Both the equations for four satellites, or the least squares equations for more than four, are non-linear and need special solution methods. A common approach is by iteration on a linearized form of the equations, such as the [[Gauss–Newton algorithm]].

The GPS was initially developed assuming use of a numerical least-squares solution method—i.e., before closed-form solutions were found.

==== Closed-form ====
One closed-form solution to the above set of equations was developed by S. Bancroft.<ref name=Bancroft /><ref name=Bancroft1985>{{cite journal |last1=Bancroft |first1=S. |date=January 1985 |title=An Algebraic Solution of the GPS Equations |journal=IEEE Transactions on Aerospace and Electronic Systems |volume=AES-21 |issue=1 |pages=56–59 |doi=10.1109/TAES.1985.310538 |bibcode=1985ITAES..21...56B|s2cid=24431129 }}</ref> Its properties are well known;<ref name="Abel1" /><ref name="Fang" /><ref name="Chaffee">Chaffee, J. and Abel, J., "On the Exact Solutions of Pseudorange Equations", ''IEEE Transactions on Aerospace and Electronic Systems'', vol:30, no:4, pp: 1021–1030, 1994</ref> in particular, proponents claim it is superior in low-GDOP situations, compared to iterative least squares methods.<ref name=Bancroft1985 />

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|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|non-linear least squares]] problems, generally provide more accurate solutions.<ref name="Sirola2010">{{cite conference |last1=Sirola |first1=Niilo |date=March 2010 |title=Closed-form algorithms in mobile positioning: Myths and misconceptions |book-title=7th Workshop on Positioning Navigation and Communication |conference=WPNC 2010 |pages=38–44 |doi=10.1109/WPNC.2010.5653789|citeseerx=10.1.1.966.9430 }}</ref>

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", ''Proceedings of the 26th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2013)'', Nashville, TN, September 2013, pp. 3560–3563.</ref> although their adoption in practice is unclear.

=== Error sources and analysis ===
{{Main|Engineering: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), [[Astronomy:Ephemeris|ephemeris]] and clock data, multipath signals, and natural and artificial interference. Magnitude of residual errors from these sources depends on geometric dilution of precision. Artificial errors may result from jamming devices and threaten ships and aircraft<ref>Attewill, Fred. (February 13, 2013) [http://metro.co.uk/2013/02/13/vehicles-that-use-gps-jammers-are-big-threat-to-aircraft-3474922/ Vehicles that use GPS jammers are big threat to aircraft] {{Webarchive|url=https://web.archive.org/web/20130216014922/http://metro.co.uk/2013/02/13/vehicles-that-use-gps-jammers-are-big-threat-to-aircraft-3474922/ |date=February 16, 2013 }}. Metro.co.uk. Retrieved on 2013-08-02.</ref> or from intentional signal degradation through selective availability, which limited accuracy to ≈ {{cvt|6-12|m||-1|}}, but has been switched off since May 1, 2000.<ref>{{cite web
 | url = http://www.gps.gov/systems/gps/modernization/sa/faq/
 | title = Frequently Asked Questions About Selective Availability
 | publisher = National Coordination Office for Space-Based Positioning, Navigation, and Timing (PNT)
 | quote = Selective Availability ended a few minutes past midnight EDT after the end of May 1, 2000. The change occurred simultaneously across the entire satellite constellation.
 | date = October 2001
 | access-date = June 13, 2015
 | archive-url = https://web.archive.org/web/20150616044948/http://www.gps.gov/systems/gps/modernization/sa/faq/
 | archive-date = June 16, 2015
| url-status = live
 }}</ref><ref>{{Cite web|url=https://blackboard.vuw.ac.nz/bbcswebdav/pid-1444805-dt-content-rid-2193398_1/courses/2014.1.ESCI203/Esci203_2014_GPS_1.pdf|title=Blackboard}}</ref>

== Accuracy enhancement and surveying ==
{{excerpt|GNSS enhancement}}

== Regulatory spectrum issues concerning GPS receivers ==
In the United States, GPS receivers are regulated under the Federal Communications Commission's (FCC) 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&nbsp;MHz and 1626.5–1660.5&nbsp;MHz|publisher=FCC.gov|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=live}}</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&nbsp;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=FCC.gov|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&nbsp;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=FCC.gov|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&nbsp;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=FCC.gov|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, [[Organization:National Aeronautics and Space Administration|National Aeronautics and Space Administration]] (NASA), 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 [[Company:Best Buy|Best Buy]], [[Company: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&nbsp;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&nbsp;MHz of spectrum in either direction of the L1 frequency of 1575.42&nbsp;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|publisher=FCC.gov|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&nbsp;... 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|publisher=FCC.gov|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&nbsp;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">[http://saveourgps.org/pdf/SIS_DOD_Response_Statement_08122011.pdf] {{webarchive|url=https://web.archive.org/web/20121213185643/http://saveourgps.org/pdf/SIS_DOD_Response_Statement_08122011.pdf|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|url=http://ssv.cachefly.net/lightsquared/wp-content/uploads/2011/06/LSQ-Testimony-Package.pdf |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 |author=Jeff Carlisle |date=June 23, 2011 |access-date=December 13, 2011 |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 }}</ref><ref>{{cite web|url=http://www.lightsquared.com/documents/FCC%20Julius%20Genachowski%20letter%20to%20Senator%20Grassley%20-%20May%2031,%202011.pdf |title=FCC Chairman Genachowski Letter to Senator Charles Grassley |author=Julius Genachowski |date=May 31, 2011 |access-date=December 13, 2011 |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 }}</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 [[Engineering:Air traffic control|air traffic control]] system, United States Defense Department guidance, and local emergency services including 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|Earth:Satellite navigation}}
{{Comparison satellite navigation orbits}}
<!--"Comparison satellite navigation orbits" creates the info-graphic to the right.-->

Following the United States' deployment of GPS, other countries have also developed their own satellite navigation systems. These systems include:

* The Russian Global Navigation Satellite System ([[Astronomy:GLONASS|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=[[Earth:Inside GNSS|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>

* China's [[Engineering:BeiDou Navigation Satellite System|BeiDou Navigation Satellite System]] began global services in 2018 and finished its full deployment in 2020.<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 navigation satellite system, a global system being developed by the [[Social:European Union|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 is expected to be fully deployed by 2020.{{needs update|date=September 2021}}

* 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 [[Astronomy:Indian Regional Navigation Satellite System|Indian Regional Navigation Satellite System]], deployed by India.

== See also ==
{{div col|colwidth=15em}}
* [[Engineering:List of GPS satellites|List of GPS satellites]]
* [[Engineering:GPS satellite blocks|GPS satellite blocks]]
* [[Earth:GPS signals|GPS signals]]
* [[Engineering:Satellite navigation software|Satellite navigation software]]
* GPS/INS
* GPS spoofing
* [[Indoor positioning system]]
* [[Engineering:Local-area augmentation system|Local-area augmentation system]]
* [[Earth:Local positioning system|Local positioning system]]
* Military invention
* [[Earth:Mobile phone tracking|Mobile phone tracking]]
* [[Navigation paradox]]
* [[Engineering:Notice Advisory to Navstar Users|Notice Advisory to Navstar Users]]
* [[Engineering:S-GPS|S-GPS]]
* Geostationary balloon satellite
{{div col end}}

== Notes ==
{{notes}}

== References ==
{{reflist|colwidth=30em}}

== Further reading ==
{{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}}
* {{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|author1=Jaizki Mendizabal |author2=Roc Berenguer |author3=Juan Melendez |publisher=McGraw Hill|isbn=978-0-07-159869-9|year=2009}}
* {{cite book|title=The American Practical Navigator – Chapter 11 ''Satellite Navigation''|author=Nathaniel Bowditch|publisher=United States government|year=2002|title-link=s:The American Practical Navigator}}
* [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, 2012
* {{cite book|title=Pinpoint: How GPS is Changing Technology, Culture, and Our Minds|author=Greg Milner|publisher=W. W. Norton|year=2016|isbn=978-0-393-08912-7}}

== External links ==
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{{Commons}}
* [https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/gps/ FAA GPS FAQ]
* [https://www.gps.gov/ GPS.gov] – General public education website created by the U.S. Government

{{GPS satellites}}
{{Satellite navigation systems}}
{{USAF space vehicles}}

[[Category:Global Positioning System| ]]

{{Sourceattribution|Global Positioning System}}