Editing Satellite navigation

{{short description|Use of satellite signals for geo-spatial positioning}}
{{For|maneuvering satellites to maintain orbit and station|Orbital station-keeping}}
[[File:GPS Block IIIA.jpg|thumb|The [[U.S. Space Force]]'s [[Global Positioning System]] was the first global satellite navigation system and the first to be provided as a free global service.]]
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A '''satellite navigation''' or '''satnav''' system is a system that uses [[satellite]]s to provide autonomous [[geopositioning]]. A satellite navigation system with global coverage is termed '''global navigation satellite system''' ('''GNSS'''). {{As of|2024}}, four global systems are operational: the [[United States]]'s [[Global Positioning System]] (GPS), [[Russia]]'s Global Navigation Satellite System ([[GLONASS]]), [[China]]'s [[BeiDou]] Navigation Satellite System (BDS),<ref name=autogenerated1>{{cite web|url=https://www.cnn.com/2020/06/24/tech/china-beidou-satellite-gps-intl-hnk/index.html |title=China's GPS rival Beidou is now fully operational after final satellite launched |date=24 June 2020 |publisher=cnn.com |access-date=2020-06-26}}</ref> and the [[European Union|European Union's]] [[Galileo (satellite navigation)|Galileo]].<ref>{{cite web|url=https://www.euspa.europa.eu/european-space/galileo/What-Galileo |title=Galileo is the European global satellite-based navigation system |website=www.euspa.europa.eu|date=26 January 2024 |access-date=26 January 2024}}</ref> Two regional systems are operational: India's [[Indian Regional Navigation Satellite System|NavIC]]<ref name=":0" /> and Japan's [[Quasi-Zenith Satellite System|QZSS]].<ref name=":2" />

''[[Satellite-based augmentation system]]s'' (SBAS), designed to enhance the accuracy of GNSS,<ref name=":1">{{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=10 August 2019 |date=23 January 2019}}</ref> include Japan's [[Quasi-Zenith Satellite System]] (QZSS),<ref name=":1">{{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=10 August 2019 |date=23 January 2019}}</ref> India's [[GAGAN]] and the European [[EGNOS]], all of them based on GPS. Previous iterations of the BeiDou navigation system and the present [[Indian Regional Navigation Satellite System]] (IRNSS), operationally known as NavIC, are examples of stand-alone operating '''regional navigation satellite systems''' ('''RNSS''').<ref>{{Cite book |url=https://www.isro.gov.in/sites/default/files/satnav_policy-29.pdf |title=Indian Satellite Navigation Policy - 2021 (Draft) |publisher=Department of Space |year=2021 |location=Bengaluru, India |pages=7 |quote="ISRO/DOS shall work towards expanding the coverage from regional to global to ensure availability of NavIC standalone signal in any part of the world without relying on other GNSS and aid in wide utilisation of Indian navigation system across the globe." |access-date=27 July 2022 |archive-date=30 July 2021 |archive-url=https://web.archive.org/web/20210730141223/https://www.isro.gov.in/sites/default/files/satnav_policy-29.pdf |url-status=dead }}</ref>

[[Satellite navigation device]]s determine their location ([[longitude]], [[latitude]], and [[altitude]]/[[elevation]]) to high precision (within a few centimeters to meters) using [[time signal]]s transmitted along a [[Line-of-sight propagation|line of sight]] by [[radio]] from satellites. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver (satellite tracking). The signals also allow the electronic receiver to calculate the current local time to a high precision, which allows time synchronisation. These uses are collectively known as '''Positioning, Navigation and Timing''' (PNT). Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated.

Global coverage for each system is generally achieved by a [[satellite constellation]] of 18–30 [[medium Earth orbit]] (MEO) satellites spread between several [[orbital planes]]. The actual systems vary, but all use [[orbital inclination]]s of >50° and [[orbital period]]s of roughly twelve hours<!-- no, only some do, as can be seen in the table comparing systems that exists in this article --> (at an altitude of about {{convert|20000|km|mi|disp=or}}).{{citation needed (lead)|date=May 2025}}

==Classification==
{{further|GNSS augmentation}}
GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:<ref name=generations>{{cite web |url=http://www.ifatca.org/system/files/public_docs/gnss.pdf |publisher=IFATCA |title=A Beginner's Guide to GNSS in Europe |access-date=20 May 2015|archive-url=https://web.archive.org/web/20170627221451/http://www.ifatca.org/system/files/public_docs/gnss.pdf|archive-date=27 June 2017|url-status=dead }}</ref>

* '''{{visible anchor|GNSS-1}}''' is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), with [[Satellite Based Augmentation System]]s (SBAS) or [[Ground Based Augmentation System]]s (GBAS).<ref name=generations/> In the United States, the satellite-based component is the [[Wide Area Augmentation System]] (WAAS); in Europe, it is the [[European Geostationary Navigation Overlay Service]] (EGNOS); in Japan, it is the [[Multi-Functional Transport Satellite|Multi-Functional Satellite Augmentation System]] (MSAS); and in India, it is the [[GPS-aided GEO augmented navigation]] (GAGAN). Ground-based augmentation is provided by systems like the [[Local Area Augmentation System]] (LAAS).<ref name=generations/>
* '''{{visible anchor|GNSS-2}}''' is the second generation of systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system.<ref name=generations/> These systems will provide the accuracy and integrity monitoring necessary for civil navigation; including aircraft. Initially, this system consisted of only Upper [[L Band]] frequency sets (L1 for GPS, E1 for Galileo, [[L Band|and]] G1 for GLONASS). In recent years, GNSS systems have begun activating Lower [[L Band]] frequency sets (L2 and L5 for GPS, E5a and E5b for Galileo, [[L Band|and]] G3 for GLONASS) for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection.<ref>{{cite web|url=https://gssc.esa.int/navipedia/index.php/Galileo_General_Introduction|title=Galileo General Introduction - Navipedia|website=gssc.esa.int|language=en|access-date=2018-11-17}}</ref><ref name="autogenerated3">{{cite web|url=https://gssc.esa.int/navipedia/index.php/GNSS_signal|title=GNSS signal - Navipedia|website=gssc.esa.int|language=en|access-date=2018-11-17}}</ref> As of late 2018, a few consumer-grade GNSS devices are being sold that leverage both. They are typically called "Dual band GNSS" or "Dual band GPS" devices.

By their roles in the navigation system, systems can be classified as:
* There are four global satellite navigation systems, currently [[Global Positioning System|GPS]] (United States), [[GLONASS]] (Russian Federation), [[Beidou]] (China) and [[Galileo (satellite navigation)|Galileo]] (European Union).
* Global Satellite-Based Augmentation Systems (SBAS) such as [[OmniSTAR]] and [[StarFire (navigation system)|StarFire]].
* Regional SBAS including WAAS (US), EGNOS (EU), MSAS (Japan), [[GPS-aided geo-augmented navigation|GAGAN]] (India) and SDCM (Russia).
* Regional Satellite Navigation Systems such as India's [[Indian Regional Navigation Satellite System|NAVIC]], and Japan's [[QZSS]].
* Continental scale Ground Based Augmentation Systems (GBAS) for example the Australian GRAS and the joint US Coast Guard, Canadian Coast Guard, US Army Corps of Engineers and US Department of Transportation National [[Differential GPS]] (DGPS) service.
* Regional scale GBAS such as CORS networks.
* Local GBAS typified by a single GPS reference station operating [[Real Time Kinematic]] (RTK) corrections.

As many of the global GNSS systems (and augmentation systems) use similar frequencies and signals around L1, many "Multi-GNSS" receivers capable of using multiple systems have been produced. While some systems strive to interoperate with GPS as well as possible by providing the same clock, others do not.<ref>{{cite journal |last1=Nicolini |first1=Luca |last2=Caporali |first2=Alessandro |title=Investigation on Reference Frames and Time Systems in Multi-GNSS |journal=Remote Sensing |date=9 January 2018 |volume=10 |issue=2 |pages=80 |doi=10.3390/rs10010080 |bibcode=2018RemS...10...80N |doi-access=free|hdl=11577/3269537 |hdl-access=free }}</ref>

== History ==
{{further|GPS#History|GLONASS#History|Galileo (satellite navigation)#History{{!}}GALILEO#History|BeiDou#History}}
[[File:Accuracy of Navigation Systems.svg|thumb]]

Ground-based [[radio navigation]] is decades old.  The [[Decca Navigator System|DECCA]], [[LORAN]], [[Gee (navigation)|GEE]] and [[Omega Navigation System|Omega]] systems used terrestrial [[longwave]] radio [[transmitter]]s which broadcast a radio pulse from a known "master" location, followed by a pulse repeated from a number of "slave" stations.  The delay between the reception of the master signal and the slave signals allowed the receiver to deduce the distance to each of the slaves, providing a [[fix (position)|fix]].

The first satellite navigation system was [[Transit (satellite)|Transit]], a system deployed by the US military in the 1960s. Transit's operation was based on the [[Doppler effect]]: the satellites travelled on well-known paths and broadcast their signals on a well-known [[radio frequency]].  The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position. Satellite orbital position errors are caused by radio-wave [[refraction]], gravity field changes (as the Earth's gravitational field is not uniform), and other phenomena. A team, led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970 to 1973, found solutions and/or corrections for many error sources.{{citation needed|date=May 2022}} Using real-time data and recursive estimation, the systematic and residual errors were narrowed down to accuracy sufficient for navigation.<ref>Jury, H, 1973, Application of the Kalman Filter to Real-time Navigation using Synchronous Satellites, Proceedings of the 10th International Symposium on Space Technology and Science, Tokyo, 945-952.</ref>

== Principles ==
{{further|GPS#Principles|GPS#Navigation equations}}

Part of an orbiting satellite's broadcast includes its precise orbital data. Originally, the [[US Naval Observatory|US Naval Observatory (USNO)]] continuously observed the precise orbits of these satellites. As a satellite's orbit deviated, the USNO sent the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain its most recent [[ephemeris]].

Modern systems are more direct. The satellite broadcasts a signal that contains orbital data (from which the position of the satellite can be calculated) and the precise time the signal was transmitted. Orbital data include a rough [[almanac]] for all satellites to aid in finding them, and a precise ephemeris for this satellite. The orbital [[ephemeris]] is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an [[atomic clock]] to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission of three (at sea level) or four (which allows an altitude calculation also) different satellites, measuring the time-of-flight to each satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time using an adapted version of [[trilateration]]: see [[GNSS positioning calculation]] for details.

Each distance measurement, regardless of the system being used, places the receiver on a spherical shell centred on the broadcaster, at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where the shells meet, a fix is generated. However, in the case of fast-moving receivers, the position of the receiver moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that angle corresponds to the distance which the signal travels through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centred on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as [[Kalman filter]]ing to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.

[[Einstein]]'s theory of [[general relativity]] is applied to GPS time correction, the net result is that time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds per day.<ref name=einstein>{{cite web |url=https://www.e-education.psu.edu/geog862/node/1714|publisher=The Pennsylvania State University|title=Relativistic Effects on the Satellite Clock}}</ref>

==Applications==
[[File:GPSTest screenshot (2025).webp|thumb|GNSS satellites (GPS, GLONASS, Galileo, and BeiDou) and a GNSS augmentation system (Quasi-Zenith Satellite System) used on a smartphone in [[South Tangerang]], [[Indonesia]] (2025)]]

{{Main|GNSS applications}}
{{further|Automotive navigation system}}
The original motivation for satellite navigation was for military applications. Satellite navigation allows precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from mis-directed weapons. (See [[Guided bomb]]). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the [[fog of war]].

Now a global navigation satellite system, such as [[Galileo (satellite navigation)|Galileo]], is used to determine users location and the location of other people or objects at any given moment. The range of application of satellite navigation in the future is enormous, including both the public and private sectors across numerous market segments such as science, transport, agriculture, etc.<ref>{{Cite web|url=https://www.gsa.europa.eu/galileo/applications|title=Applications|date=2011-08-18|website=www.gsa.europa.eu|language=en|access-date=2019-10-08}}</ref>

The ability to supply satellite navigation signals is also the ability to deny their availability. The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires.

==Global navigation satellite systems==
{{Comparison satellite navigation orbits}}
[[File:Launched GNSS 2014.jpg|thumb|Launched GNSS satellites 1978 to 2014]]

In order of first launch year:

===GPS===
{{Main|Global Positioning System}}
First launch year: 1978

The United States' Global Positioning System (GPS) consists of up to 32 [[medium Earth orbit]] satellites in six different [[orbital plane (astronomy)|orbital plane]]s. The exact number of satellites varies as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is the world's most utilized satellite navigation system.

===GLONASS===
{{Main|GLONASS}}
First launch year: 1982

The formerly [[Soviet Union|Soviet]], and now [[Russia]]n, '''''Glo'''bal'naya '''Na'''vigatsionnaya '''S'''putnikovaya '''S'''istema'', (GLObal NAvigation Satellite System or GLONASS), is a space-based satellite navigation system that provides a civilian radionavigation-satellite service and is also used by the Russian Aerospace Defence Forces. GLONASS has full global coverage since 1995 and with 24 active satellites.

===BeiDou===
{{Main|BeiDou Navigation Satellite System}}
First launch year: 2000

BeiDou started as the now-decommissioned Beidou-1, an Asia-Pacific local network on the geostationary orbits. The second generation of the system BeiDou-2 became operational in China in December 2011.<ref>{{Cite news|date=2012-03-08|title=China's GPS rival is switched on|language=en-GB|work=BBC News|url=https://www.bbc.com/news/technology-16337648|access-date=2020-06-23}}</ref> The BeiDou-3 system is proposed to consist of 30 [[Medium Earth orbit|MEO]] satellites and five geostationary satellites (IGSO). A 16-satellite regional version (covering Asia and Pacific area) was completed by December 2012. Global service was completed by December 2018.<ref>{{Cite web|url=http://en.beidou.gov.cn/WHATSNEWS/201812/t20181227_16837.html|title=The BDS-3 Preliminary System Is Completed to Provide Global Services|website=news.dwnews.com|access-date=2018-12-27}}</ref> On 23 June 2020, the BDS-3 constellation deployment is fully completed after the last satellite was successfully launched at the [[Xichang Satellite Launch Center]].<ref>{{Cite web|title=APPLICATIONS-Transport|url=http://en.beidou.gov.cn/WHATSNEWS/202006/t20200623_20692.html|access-date=2020-06-23|website=en.beidou.gov.cn}}</ref>

===Galileo===
{{Main|Galileo (satellite navigation)}}
First launch year: 2011

The [[European Union]] and [[European Space Agency]] agreed in March 2002 to introduce their own alternative to GPS, called the [[Galileo positioning system]]. Galileo became operational on 15 December 2016 (global Early Operational Capability, EOC).<ref>{{cite news |publisher=europa.eu |url=http://europa.eu/rapid/press-release_IP-16-4366_en.htm |title=Galileo goes live!|date=14 December 2016}}</ref> At an estimated cost of €10 billion,<ref>{{cite news |publisher=BBC News |url=http://news.bbc.co.uk/1/hi/sci/tech/5286200.stm |title=Boost to Galileo sat-nav system |date=25 August 2006 |access-date=2008-06-10}}</ref> the system of 30 [[Medium Earth orbit|MEO]] satellites was originally scheduled to be operational in 2010. The original year to become operational was 2014.<ref>{{cite web|url= http://europa.eu/rapid/pressReleasesAction.do?reference=IP/10/7&language=en |title= Commission awards major contracts to make Galileo operational early 2014 |date=2010-01-07 |access-date=2010-04-19}}</ref> The first experimental satellite was launched on 28 December 2005.<ref>{{cite web|url=http://www.esa.int/Our_Activities/Navigation/The_future_-_Galileo/First_Galileo_Launch/GIOVE-A_launch_news|title=GIOVE-A launch News|date=2005-12-28|access-date=2015-01-16}}</ref> Galileo is expected to be compatible with the [[GPS modernization|modernized GPS]] system. The receivers will be able to combine the signals from both Galileo and GPS satellites to greatly increase the accuracy. The full Galileo constellation consists of 24 active satellites,<ref>{{cite news |title=Galileo begins serving the globe |url=https://www.internationales-verkehrswesen.de/galileo-begins-serving-the-globe/ |work=INTERNATIONALES VERKEHRSWESEN |date=23 December 2016 |language=de-DE}}</ref> the last of which was launched in December 2021.<ref>{{cite news |title=Soyuz launch from Kourou postponed until 2021, 2 others to proceed |url=https://www.spacedaily.com/reports/Soyuz_launch_from_Kourou_postponed_until_2021_2_others_to_proceed_999.html |work=Space Daily |date=19 May 2020}}</ref><ref name=autogenerated2>{{cite web|url=https://www.gsa.europa.eu/galileo/services/initial-services |title=Galileo Initial Services |website=gsa.europa.eu|date=9 December 2016 |access-date=25 September 2020}}</ref> The main modulation used in Galileo Open Service signal is the [[Composite Binary Offset Carrier]] (CBOC) modulation.

==Regional navigation satellite systems==

===NavIC===
{{Main|NavIC}}
The '''NavIC''' (acronym for '''Navigation with Indian Constellation''') is an autonomous regional satellite navigation system developed by the [[Indian Space Research Organisation]] (ISRO). The [[Government of India|Indian government]] approved the project in May 2006. It consists of a constellation of 7 navigational satellites.<ref>{{cite web|url=http://www.rediff.com/news/2007/sep/27gps.htm |title=India to develop its own version of GPS |work=Rediff.com |access-date=2011-12-30}}</ref> Three of the satellites are placed in [[Geostationary orbit|geostationary orbit (GEO)]] and the remaining 4 in [[Geosynchronous orbit|geosynchronous orbit (GSO)]] to have a larger signal footprint and lower number of satellites to map the region. It is intended to provide an all-weather absolute position accuracy of better than {{convert|7.6|m}} throughout [[India]] and within a region extending approximately {{convert|1500|km|abbr=on}} around it.<ref>{{cite web|author=S. Anandan |url=http://www.thehindu.com/sci-tech/article393892.ece |title=Launch of first satellite for Indian Regional Navigation Satellite system next year |publisher=Beta.thehindu.com |date=2010-04-10 |access-date=2011-12-30}}</ref> An Extended Service Area lies between the primary service area and a rectangle area enclosed by the [[30th parallel south]] to the [[50th parallel north]] and the [[30th meridian east]] to the [[130th meridian east]], 1,500–6,000 km beyond borders.<ref>{{Cite web|title=IRNSS Programme - ISRO|url=https://www.isro.gov.in/irnss-programme|access-date=2018-07-14|website=www.isro.gov.in|language=en|archive-date=2022-03-02|archive-url=https://web.archive.org/web/20220302041206/https://www.isro.gov.in/irnss-programme|url-status=dead}}</ref> A goal of complete Indian control has been stated, with the [[Satellite space segment|space segment]], [[ground segment]] and user receivers all being built in India.<ref>{{cite web|url=http://www.livemint.com/2007/09/05002237/India-to-build-a-constellation.html |title=India to build a constellation of 7 navigation satellites by 2012 |publisher=Livemint.com |date=2007-09-05 |access-date=2011-12-30}}</ref>

The constellation was in orbit as of 2018, and the system was available for public use in early 2018.<ref>{{cite web|url=https://www.ibtimes.co.in/indias-own-gps-irnss-navic-made-by-isro-go-live-early-2018-728409|title=India's own GPS IRNSS NavIC made by ISRO to go live in early 2018|website=International Business Times|date=28 May 2017|access-date=29 April 2021|author=Rohit KVN}}</ref> NavIC provides two levels of service, the "standard positioning service", which will be open for civilian use, and a "restricted service" (an [[Encryption|encrypted]] one) for authorized users (including military). There are plans to expand NavIC system by increasing constellation size from 7 to 11.<ref name=":0">{{Cite news|url=https://economictimes.indiatimes.com/news/science/navigation-satellite-clocks-ticking-system-to-be-expanded-isro/articleshow/59082657.cms|title=Navigation satellite clocks ticking; system to be expanded: ISRO|last=IANS|date=2017-06-10|work=The Economic Times|access-date=2018-01-24}}</ref>

India plans to make the NavIC global by adding 24 more [[Medium Earth orbit|MEO]] satellites. The Global NavIC will be free to use for the global public.<ref>{{cite news |title=ISRO to boost NavIC, widen user base of location system |first=Jacob |last=Koshy |work=[[The Hindu]] |date=October 26, 2022 |url=https://www.thehindu.com/news/national/isro-to-upgrade-indian-gps-navic-to-widen-user-base/article66056505.ece |url-access=subscription |url-status=live |archive-url=https://web.archive.org/web/20230920113926/https://www.thehindu.com/news/national/isro-to-upgrade-indian-gps-navic-to-widen-user-base/article66056505.ece |archive-date= Sep 20, 2023 }}</ref>

===Early BeiDou===
{{Main|BeiDou-1|BeiDou-2}}
The first two generations of China's BeiDou navigation system were designed to provide regional coverage.

===Korea===
{{Main|Korean Positioning System}}
The Korean Positioning System (KPS) is currently in development and expected to be operational by 2035.<ref name="l316">{{cite web | last=Byung-wook | first=Kim | title=LIG Nex1 to develop Korea's own satellite navigation system | website=The Korea Herald | date=2021-09-26 | url=https://www.koreaherald.com/article/2695812 | language=ko | access-date=2025-02-24}}</ref><ref name="f292">{{cite journal | last1=Choi | first1=Byung-Kyu | last2=Roh | first2=Kyoung-Min | last3=Ge | first3=Haibo | last4=Ge | first4=Maorong | last5=Joo | first5=Jung-Min | last6=Heo | first6=Moon Beom | title=Performance Analysis of the Korean Positioning System Using Observation Simulation | journal=Remote Sensing | volume=12 | issue=20 | date=2020-10-15 | issn=2072-4292 | doi=10.3390/rs12203365 | doi-access=free | page=3365| bibcode=2020RemS...12.3365C }}</ref>

==Augmentation==
[[GNSS augmentation]] is a method of improving a navigation system's attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process, for example, the [[Wide Area Augmentation System]], the [[European Geostationary Navigation Overlay Service]], the [[Multi-functional Satellite Augmentation System]], [[Differential GPS]], [[GPS-aided GEO augmented navigation]] (GAGAN) and [[inertial navigation system]]s.

===QZSS===
{{Main|Quasi-Zenith Satellite System}}
The Quasi-Zenith Satellite System (QZSS) is a four-satellite regional [[time transfer]] system and enhancement for [[Global Positioning System|GPS]] covering [[Japan]] and the [[Asia-Pacific|Asia-Oceania]] regions. QZSS services were available on a trial basis as of January 12, 2018, and were started in November 2018. The first satellite was launched in September 2010.<ref name=":2">{{cite web |url=http://qzss.jaxa.jp/is-qzss/qzss_e.html |title=About QZSS |access-date=2009-02-22 |publisher=JAXA |url-status=dead |archive-url=https://web.archive.org/web/20090314085502/http://qzss.jaxa.jp/is-qzss/qzss_e.html |archive-date=2009-03-14 }}</ref> An independent satellite navigation system (from GPS) with 7 satellites is planned for 2023.<ref>{{cite web |title=Japan mulls seven-satellite QZSS system as a GPS backup |url=https://spacenews.com/japan-mulls-seven-satellite-qzss-system-as-a-gps-backup/ |first1=Caleb |last1=Henry |website=SpaceNews.com |access-date=10 August 2019 |date=15 May 2017 |url-status=live |archive-url=https://archive.today/20231209214741/https://spacenews.com/japan-mulls-seven-satellite-qzss-system-as-a-gps-backup/ |archive-date= 9 Dec 2023 }}</ref>

===EGNOS===
{{excerpt|EGNOS}}

==Comparison of systems==
{| class="wikitable" style="text-align:center;"
|-
! System
! [[Beidou Navigation Satellite System|BeiDou]]
! [[Galileo (satellite navigation)|Galileo]]
! [[GLONASS]]
! [[Global Positioning System|GPS]]
! [[Indian Regional Navigation Satellite System|NavIC]]
! [[Quasi-Zenith Satellite System|QZSS]]
|-
! Owner
| [[China]]
| [[EU|European Union]]
| [[Russia]]
| [[United States]]
| [[India]]
| [[Japan]]
|-
! Coverage
| Global
| Global
| Global
| Global
| Regional
| Regional
|-
! [[Channel access method|Coding]]
| [[CDMA]]
| [[CDMA]]
| [[FDMA]] & [[CDMA]]
| [[CDMA]]
| [[CDMA]]
| [[CDMA]]
|-
! Altitude<br />km (mi)
| {{convert|21,150|km|abbr=values|disp=br()}}
| {{convert|23,222|km|abbr=values|disp=br()}}
| {{convert|19,130|km|abbr=values|disp=br()}}
| {{convert|20,180|km|abbr=values|disp=br()}}
| {{convert|36,000|km|abbr=values|disp=br()}}
| {{convert|32,600-39,000|km|abbr=values|disp=br()}}<ref name=h2a>{{cite web |website=NASASpaceFlight.com |url=https://www.nasaspaceflight.com/2017/10/japans-h-2a-rocket-qzss-4-launch/ |title=Japan's H-2A conducts QZSS-4 launch |url-status=live |archive-url=https://web.archive.org/web/20171010005756/https://www.nasaspaceflight.com/2017/10/japans-h-2a-rocket-qzss-4-launch/ |archive-date=2017-10-10 |first1=William |last1=Graham |date=9 October 2017}}</ref>
|-
! Period
| 12.88&nbsp;h<br /> (12&nbsp;h 53&nbsp;min)
| 14.08&nbsp;h<br /> (14&nbsp;h &nbsp;5&nbsp;min)
| 11.26&nbsp;h<br /> (11&nbsp;h 16&nbsp;min)
| 11.97&nbsp;h<br /> (11&nbsp;h 58&nbsp;min)
| 23.93&nbsp;h<br /> (23&nbsp;h 56&nbsp;min)
| 23.93&nbsp;h<br /> (23&nbsp;h 56&nbsp;min)
|-
! Rev./[[sidereal day|S. day]]
| 13/7 {{Gray|(1.86)}}
| 17/10 {{Gray|(1.7)}}
| 17/8 {{Gray|(2.125)}}
| 2
| 1
| 1
|-
! Satellites
| BeiDou-3:<br /> 28 operational<br /> (24 MEO, 3 IGSO, 1 GSO)<br /> 5 in orbit validation<br /> 2 GSO planned 20H1<br />BeiDou-2:<br /> 15 operational<br /> 1 in commissioning 
| By design:
27 operational + 3 spares

Currently:

26 in orbit<br />[[List of Galileo satellites|24 operational]]

2 inactive<br />6 to be launched<ref name="AvWeek12sep2018" />
| 24 by design<br />24 operational<br />1 commissioning<br />1 in flight tests<ref>{{cite web |url= https://www.glonass-iac.ru/en/ |title= Information and Analysis Center for Positioning, Navigation and Timing |access-date= 2018-07-21 |archive-date= 2018-07-21 |archive-url= https://web.archive.org/web/20180721162605/https://www.glonass-iac.ru/en/ |url-status= dead }}</ref>
| 24 by design<br/ >30 operational<ref>{{cite web|url=http://www.gps.gov/systems/gps/space/#generations|title=GPS Space Segment|access-date=2015-07-24}}</ref>
| 8 operational<br />(3 GEO, 5 [[geosynchronous|GSO]] MEO)
| 4 operational (3 GSO, 1 GEO)<br />7 in the future
|-
! Frequency<br />GHz
| 1.561098&nbsp;(B1)<br />1.589742&nbsp;(B1-2)<br />1.20714&nbsp;(B2)<br />1.26852&nbsp;(B3)
| 1.559–1.592 (E1)<br>1.164–1.215 (E5a/b)<br />1.260–1.300 (E6)
| 1.593–1.610 (G1)<br />1.237–1.254 (G2)<br>1.189–1.214 (G3)
| 1.563–1.587 (L1)<br />1.215–1.2396 (L2)<br>1.164–1.189 (L5)
| 1.57542 (L1)<br />1.17645 (L5)<br />2.49202 (S)
| 1.57542 (L1C/A, L1C, L1S)<br />1.22760 (L2C)<br />1.17645 (L5, L5S)<br />1.27875 (L6)<ref>{{cite web |url= https://qzss.go.jp/overview/services/sv03_signals.html |title=送信信号一覧 |access-date=2019-10-25}}</ref>
|-
! Status
| Operational<ref name="bdsStatus20200623">{{Cite web|url=https://phys.org/news/2020-06-china-satellite-gps-like-beidou.html|title=China launches final satellite in GPS-like Beidou system|publisher=phys.org|access-date=24 June 2020|archive-url=https://web.archive.org/web/20200624080233/https://phys.org/news/2020-06-china-satellite-gps-like-beidou.html|archive-date=24 June 2020|url-status=live}}</ref>
| Operating since 2016<br />2020 completion<ref name=AvWeek12sep2018>{{cite news  |url= http://aviationweek.com/world-satellite-business-week/rise-new-navigation-satellites |url-access=subscription |title= The Rise Of New Navigation Satellites |date= Sep 12, 2018 |author1=Irene Klotz |author2=Tony Osborne |author3=Bradley Perrett |work= Aviation Week Network |url-status=live |archive-url=https://web.archive.org/web/20231025102726/https://aviationweek.com/defense-space/space/rise-new-navigation-satellites |archive-date=Oct 25, 2023 }}</ref>
| Operational
| Operational
| Operational
| Operational
|-
! Accuracy<br />m (ft)
| {{convert|3.6|m|ft|abbr=values}} (public)<br />{{convert|0.1|m|ft|abbr=values}} (encrypted)
| {{convert|0.2|m|ft|abbr=values}} (public)<br />{{convert|0.01|m|ft|abbr=values}} (encrypted)
| {{convert|2|-|4|m|ft|abbr=values}}
| {{convert|0.3|-|5|m|ft|abbr=values}}<br>(no DGPS or WAAS)
| {{convert|1|m|ft|abbr=values}} (public)<br />{{convert|0.1|m|ft|abbr=values}} (encrypted)
| {{convert|1|m|ft|abbr=values}} (public)<br />{{convert|0.1|m|ft|abbr=values}} (encrypted)

|-
! System
! [[Beidou Navigation Satellite System|BeiDou]]
! [[Galileo (satellite navigation)|Galileo]]
! [[GLONASS]]
! [[Global Positioning System|GPS]]
! [[Indian Regional Navigation Satellite System|NavIC]]
! [[Quasi-Zenith Satellite System|QZSS]]
|-
| colspan=7 | Sources:<ref name="autogenerated3"/><ref>{{Cite book |last1=Aswal |first1=Dinesh K. |url=https://books.google.com/books?id=I73SEAAAQBAJ&pg=PA512 |title=Handbook of Metrology and Applications |last2=Yadav |first2=Sanjay |last3=Takatsuji |first3=Toshiyuki |last4=Rachakonda |first4=Prem |last5=Kumar |first5=Harish |date=2023-08-23 |publisher=Springer Nature |isbn=978-981-99-2074-7 |pages=512 |language=en}}</ref><ref>{{Cite web |date=August 2023 |title=NAVIC SIGNAL IN SPACE ICD FOR STANDARD POSITIONING SERVICE IN L1 FREQUENCY |url=https://www.isro.gov.in/media_isro/pdf/SateliteNavigation/NavIC_SPS_ICD_L1_final.pdf |access-date=20 September 2024 |website=ISRO}}</ref>
|}

Using multiple GNSS systems for user positioning increases the number of visible satellites, improves precise point positioning (PPP) and shortens the average convergence time.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/pii/S0273117718304745 | doi=10.1016/j.asr.2018.06.008 | title=Assessing the latest performance of Galileo-only PPP and the contribution of Galileo to Multi-GNSS PPP | year=2019 | last1=Xia | first1=Fengyu | last2=Ye | first2=Shirong | last3=Xia | first3=Pengfei | last4=Zhao | first4=Lewen | last5=Jiang | first5=Nana | last6=Chen | first6=Dezhong | last7=Hu | first7=Guangbao | journal=Advances in Space Research | volume=63 | issue=9 | pages=2784–2795 | bibcode=2019AdSpR..63.2784X | s2cid=125213815 | url-access=subscription }}</ref>
The signal-in-space ranging error (SISRE) in November 2019 were 1.6&nbsp;cm for Galileo, 2.3&nbsp;cm for GPS, 5.2&nbsp;cm for GLONASS and 5.5&nbsp;cm for BeiDou when using real-time corrections for satellite orbits and clocks.<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 |year=2020 |volume=24 |issue=111 |page=111 |doi=10.1007/s10291-020-01026-6 |bibcode=2020GPSS...24..111K |doi-access=free}}</ref> The average SISREs of the BDS-3 MEO, IGSO, and GEO satellites were 0.52 m, 0.90 m and 1.15 m, respectively. Compared to the four major global satellite navigation systems consisting of MEO satellites, the SISRE of the BDS-3 MEO satellites was slightly inferior to 0.4 m of Galileo, slightly superior to 0.59 m of GPS, and remarkably superior to 2.33 m of GLONASS. The SISRE of BDS-3 IGSO was 0.90 m, which was on par with the 0.92 m of QZSS IGSO. However, as the BDS-3 GEO satellites were newly launched and not completely functioning in orbit, their average SISRE was marginally worse than the 0.91 m of the QZSS GEO satellites.<ref name=":1" />

==Related techniques==
{{further|Satellite geodesy#Radio techniques}}

===DORIS===
{{Main|DORIS (satellite system)}}
Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a French precision navigation system. Unlike other GNSS systems, it is based on static emitting stations around the world, the receivers being on satellites, in order to precisely determine their orbital position. The system may be used also for mobile receivers on land with more limited usage and coverage. Used with traditional GNSS systems, it pushes the accuracy of positions to centimetric precision (and to millimetric precision for altimetric application and also allows monitoring very tiny seasonal changes of Earth rotation and deformations), in order to build a much more precise geodesic reference system.<ref>{{cite web|url=http://www.aviso.altimetry.fr/en/doris.html |title=DORIS information page |publisher=Jason.oceanobs.com |access-date=2011-12-30}}</ref>

===LEO satellites===
The two current operational [[low Earth orbit]] (LEO) [[satellite phone]] networks are able to track transceiver units with accuracy of a few kilometres using doppler shift calculations from the satellite. The coordinates are sent back to the transceiver unit where they can be read using [[AT command]]s or a [[graphical user interface]].<ref>{{cite web |url=http://common.globalstar.com/doc/common/en/products/gsp1700_usermanual.pdf |title=Globalstar GSP-1700 manual |access-date=2011-12-30 |archive-date=2011-07-11 |archive-url=https://web.archive.org/web/20110711101531/http://common.globalstar.com/doc/common/en/products/gsp1700_usermanual.pdf |url-status=dead }}</ref><ref>{{cite web |url=http://www.skyhelp.net/acrobat/jan_05/Iridium%20SBD-FAQ%201-05.pdf |last=Rickerson |first=Don |date=January 2005 |title=Iridium SMS and SBD |publisher=Personal Satellite Network, Inc. |archive-url=https://web.archive.org/web/20051109014357/http://www.skyhelp.net/acrobat/jan_05/Iridium%20SBD-FAQ%201-05.pdf |archive-date=9 November 2005 |url-status=dead}}</ref> This can also be used by the gateway to enforce restrictions on geographically bound calling plans.

==International regulation==
The [[International Telecommunication Union]] (ITU) defines a '''radionavigation-satellite service''' ('''RNSS''') as "a [[radiodetermination-satellite service]] used for the purpose of [[Radio navigation|radionavigation]]. This service may also include [[feeder link]]s necessary for its operation".<ref>ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.43, definition: ''radionavigation-satellite service ''</ref>

RNSS is regarded as a [[Safety service|safety-of-life service]] and an essential part of [[navigation]] which must be protected from [[Interference (communication)|interferences]].

''' Aeronautical radionavigation-satellite ''' ('''ARNSS''') is – according to ''Article 1.47'' of the [[International Telecommunication Union|International Telecommunication Union's]] (ITU) [[ITU Radio Regulations|Radio Regulations]] (RR)<ref>ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.47, definition: ''aeronautical radionavigation service ''</ref> – defined as «''A [[radionavigation service]] in which [[earth station]]s are located on board aircraft''.»

''' Maritime radionavigation-satellite service ''' ('''MRNSS''') is – according to ''Article 1.45'' of the [[International Telecommunication Union|International Telecommunication Union's]] (ITU) [[ITU Radio Regulations|Radio Regulations]] (RR)<ref>ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.45, definition: ''maritime radionavigation-satellite service''</ref> – defined as «''A [[radionavigation-satellite service]]  in which earth stations are located on board ships''.»

===Classification===
ITU Radio Regulations (article 1) classifies [[radio]]communication services as:

* [[Radiodetermination service]] (article 1.40)
*[[Radiodetermination-satellite service]] (article 1.41)
*[[Radionavigation service]] (article 1.42)
**'''Radionavigation-satellite service''' (article 1.43)
**[[Maritime radionavigation service]] (article 1.44)
***[[Maritime radionavigation-satellite service]] (article 1.45)
**[[Aeronautical radionavigation service]] (article 1.46)
***[[Aeronautical radionavigation-satellite service]] (article 1.47)

; Examples of RNSS use

*Augmentation system [[GNSS augmentation]]
*[[Automatic Dependent Surveillance–Broadcast]]
*[[BeiDou|BeiDou Navigation Satellite System]] (BDS)
*[[Galileo (satellite navigation)|GALILEO]], European GNSS
*[[Global Positioning System]] (GPS), with [[Differential GPS]] (DGPS)
*[[GLONASS]]
*[[Indian Regional Navigation Satellite System|NAVIC]]
*[[Quasi-Zenith Satellite System]] (QZSS)

===Frequency allocation===
{{further |Frequency allocation}}
The allocation of radio frequencies is provided according to ''Article 5'' of the ITU Radio Regulations (edition 2012).<ref>''ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations''</ref>

To improve harmonisation in spectrum utilisation, most service allocations are incorporated in national Tables of Frequency Allocations and Utilisations within the responsibility of the appropriate national administration. Allocations are:

* primary: indicated by writing in capital letters
* secondary: indicated by small letters
* exclusive or shared utilization: within the responsibility of administrations.

{| class=wikitable
|- bgcolor="#CCCCCC" align="center"
|align="center" colspan="3"| '''Allocation to services'''
|- align="center"
|  align="center" | [[International Telecommunication Union region|Region 1]]
| &nbsp;&nbsp;&nbsp;&nbsp; Region 2 &nbsp;&nbsp;&nbsp;&nbsp;
| &nbsp;&nbsp;&nbsp;&nbsp; Region 3 &nbsp;&nbsp;&nbsp;&nbsp;
|-
|colspan="3"|5 000–5 010&nbsp;MHz<br />
::::: AERONAUTICAL MOBILE-SATELLITE (R) <br />AERONAUTICAL RADIONAVIGATION <br />'''RADIONAVIGATION-SATELLITE (Earth-to-space)'''
|-
|}

==Alternatives==
'''Alternative Positioning, Navigation and Timing''' ('''AltPNT''') refers to the concept of  as an alternative to GNSS. Such alternatives include:<ref>{{Cite web|url=https://spacenews.com/can-altpnt-really-replace-gps/|title=Can "AltPNT" Really Replace GPS?|first=Sean|last=Gorman|date=July 2, 2024}}</ref>
*[[Inertial navigation system]]s (INS)
*[[eLORAN]]
*[[Terrain-based navigation]] (TBN)
*[[Visual Positioning Systems]] (VPS) 
*[[LiDAR]]

==See also==
{{Portal|Spaceflight}}
{{div col|colwidth=20em}}
<!-- Please respect alphabetical order -->
*[[Acronyms and abbreviations in avionics]]
*[[Geoinformatics]]
*[[GNSS positioning calculation]]
*[[GNSS reflectometry]]
*[[GPS spoofing]]
*[[GPS-aided geo-augmented navigation]]
*[[List of emerging technologies]]
*[[Moving map display]]
*[[Pseudolite]]
*[[Receiver Autonomous Integrity Monitoring]]
*[[Software GNSS Receiver]]
*[[Space Integrated GPS/INS]] (SIGI)
*[[United Kingdom Global Navigation Satellite System]]
*[[UNSW School of Surveying and Geospatial Engineering]]
{{div col end}}

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist}}

==Further reading==
* Office for Outer Space Affairs of the United Nations (2010), ''[http://www.oosa.unvienna.org/pdf/publications/icg_ebook.pdf Report on Current and Planned Global and Regional Navigation Satellite Systems and Satellite-based Augmentation Systems]''.

==External links==
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===Information on specific GNSS systems===
* [http://www.esa.int/esaNA/GGG63950NDC_egnos_0.html ESA information on EGNOS]
* [https://web.archive.org/web/20020621064652/http://www.astronautix.com/craft/beidou.htm Information on the Beidou system]
* [https://www.ieee.li/pdf/viewgraphs/gnss_fundamentals.pdf Global Navigation Satellite System Fundamentals]

===Organizations related to GNSS===
* [http://www.unoosa.org/oosa/en/SAP/gnss/icg.html United Nations International Committee on Global Navigation Satellite Systems (ICG)]
* [http://www.ion.org/meetings/#gnss Institute of Navigation (ION) GNSS Meetings]
* [http://www.igs.org/ The International GNSS Service (IGS)]
* [http://www.ignss.org/ International Global Navigation Satellite Systems Society Inc (IGNSS)]
* [http://www.iers.org/MainDisp.csl?pid=84-63 International Earth Rotation and Reference Systems Service (IERS) International GNSS Service (IGS)]
* [http://www.pnt.gov/ US National Executive Committee for Space-Based Positioning, Navigation, and Timing]
* [http://www.ngs.noaa.gov/orbits/ US National Geodetic Survey] Orbits for the Global Positioning System satellites in the Global Navigation Satellite System
* [http://facility.unavco.org/science_tech/gnss_modernization.html UNAVCO GNSS Modernization]
* [http://www.apecgit.org/ Asia-Pacific Economic Cooperation (APEC) GNSS Implementation Team]

===Supportive or illustrative sites===
* [http://rhp.detmich.com/gps.html GPS and GLONASS Simulation] ([[Java applet]]) Simulation and graphical depiction of the motion of space vehicles, including [[Dilution of precision (GPS)|DOP]] computation.
* [http://northsurveying.com/index.php/soporte/gnss-and-geodesy-concepts GPS, GNSS, Geodesy and Navigation Concepts in depth]

=== Alternatives to GNSS ===
* [https://afwerxchallenge.com/altpntchallenge USSF Alternative Positioning, Navigation, & Timing Challenge Definition Workshop]
* [https://spacenews.com/startup-pnt-services/ Startups map out strategies to augment or backup GPS]
* [https://spacenews.com/competing-with-uncle-sams-free-space-offerings/ Competing with Uncle Sam’s free space offerings]

{{TimeSig}}
{{Satellite navigation}}
{{Satellite navigation systems}}
{{Spaceflight}}

{{Authority control}}

{{DEFAULTSORT:Satellite Navigation}}
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