Wikipedia

Satellite navigation

For maneuvering satellites to maintain orbit and station, seeOrbital station-keeping.

Asatellite navigationorsatnavsystem is a system that usessatellitesto provide autonomousgeopositioning.A satellite navigation system with global coverage is termedglobal navigation satellite system(GNSS). As of 2024,four global systems are operational: theUnited States'sGlobal Positioning System(GPS),Russia's Global Navigation Satellite System (GLONASS),China'sBeiDouNavigation Satellite System (BDS),[1]and theEuropean Union'sGalileo.[2]

TheU.S. Space Force'sGlobal Positioning Systemwas the first global satellite navigation system and the first to be provided as a free global service.

Satellite-based augmentation systems(SBAS), designed to enhance the accuracy of GNSS,[3]include Japan'sQuasi-Zenith Satellite System(QZSS),[3]India'sGAGANand the EuropeanEGNOS,all of them based on GPS. Previous iterations of the BeiDou navigation system and the presentIndian Regional Navigation Satellite System(IRNSS), operationally known as NavIC, are examples of stand-alone operatingregional navigation satellite systems(RNSS).[4]

Satellite navigation devicesdetermine their location (longitude,latitude,andaltitude/elevation) to high precision (within a few centimeters to meters) usingtime signalstransmitted along aline of sightbyradiofrom 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 asatellite constellationof 18–30medium Earth orbit(MEO) satellites spread between severalorbital planes.The actual systems vary, but all useorbital inclinationsof >50° andorbital periodsof roughly twelve hours (at an altitude of about 20,000 kilometres or 12,000 miles).

Classification

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Further information:GNSS augmentation

GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:[5]

  • GNSS-1is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), withSatellite Based Augmentation Systems(SBAS) orGround Based Augmentation Systems(GBAS).[5]In the United States, the satellite-based component is theWide Area Augmentation System(WAAS); in Europe, it is theEuropean Geostationary Navigation Overlay Service(EGNOS); in Japan, it is theMulti-Functional Satellite Augmentation System(MSAS); and in India, it is theGPS-aided GEO augmented navigation(GAGAN). Ground-based augmentation is provided by systems like theLocal Area Augmentation System(LAAS).[5]
  • GNSS-2is the second generation of systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system.[5]These systems will provide the accuracy and integrity monitoring necessary for civil navigation; including aircraft. Initially, this system consisted of only UpperL Bandfrequency sets (L1 for GPS, E1 for Galileo,andG1 for GLONASS). In recent years, GNSS systems have begun activating LowerL Bandfrequency sets (L2 and L5 for GPS, E5a and E5b for Galileo,andG3 for GLONASS) for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection.[6][7]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, currentlyGPS(United States),GLONASS(Russian Federation),Beidou(China) andGalileo(European Union).
  • Global Satellite-Based Augmentation Systems (SBAS) such asOmniSTARandStarFire.
  • Regional SBAS including WAAS (US), EGNOS (EU), MSAS (Japan),GAGAN(India) and SDCM (Russia).
  • Regional Satellite Navigation Systems such as India'sNAVIC,and Japan'sQZSS.
  • 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 NationalDifferential GPS(DGPS) service.
  • Regional scale GBAS such as CORS networks.
  • Local GBAS typified by a single GPS reference station operatingReal 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.[8]

History

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Further information:GPS § History,GLONASS § History,GALILEO#History,andBeiDou § History

Ground-basedradio navigationis decades old. TheDECCA,LORAN,GEEandOmegasystems used terrestriallongwaveradiotransmitterswhich 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 afix.

The first satellite navigation system wasTransit,a system deployed by the US military in the 1960s. Transit's operation was based on theDoppler effect:the satellites travelled on well-known paths and broadcast their signals on a well-knownradio 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-waverefraction,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]Using real-time data and recursive estimation, the systematic and residual errors were narrowed down to accuracy sufficient for navigation.[9]

Principles

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Further information:GPS § Principles,andGPS § Navigation equations

Part of an orbiting satellite's broadcast includes its precise orbital data. Originally, theUS 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 recentephemeris.

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 roughalmanacfor all satellites to aid in finding them, and a precise ephemeris for this satellite. The orbitalephemerisis transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses anatomic clockto 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 oftrilateration:seeGNSS positioning calculationfor details.

Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal 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 changes the distance 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 asKalman filteringto combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.

Einstein's theory ofgeneral relativityis 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.[10]

Applications

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GNSS satellites used for navigation on a smartphone in 2021
Main article:GNSS applications
Further information: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. (SeeGuided bomb). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing thefog of war.

Now a global navigation satellite system, such asGalileo,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, insurance, energy, etc.[11][12]

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.

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Clickable image, highlightingmedium altitude orbits around Earth,[a]fromLow Earthto the lowestHigh Earth orbit(geostationary orbitand itsgraveyard orbit,at one ninth of theMoon's orbital distance),[b]with theVan Allen radiation beltsand theEarthto scale
Launched GNSS satellites 1978 to 2014

In order of first launch year:

GPS

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Main article:Global Positioning System

First launch year: 1978

The United States' Global Positioning System (GPS) consists of up to 32medium Earth orbitsatellites in six differentorbital planes.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

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Main article:GLONASS

First launch year: 1982

The formerlySoviet,and nowRussian,Global'nayaNavigatsionnayaSputnikovayaSistema,(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

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Main article: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.[13]The BeiDou-3 system is proposed to consist of 30MEOsatellites 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.[14]On 23 June 2020, the BDS-3 constellation deployment is fully completed after the last satellite was successfully launched at theXichang Satellite Launch Center.[15]

Galileo

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Main article:Galileo (satellite navigation)

First launch year: 2011

TheEuropean UnionandEuropean Space Agencyagreed in March 2002 to introduce their own alternative to GPS, called theGalileo positioning system.Galileo became operational on 15 December 2016 (global Early Operational Capability, EOC).[16]At an estimated cost of €10 billion,[17]the system of 30MEOsatellites was originally scheduled to be operational in 2010. The original year to become operational was 2014.[18]The first experimental satellite was launched on 28 December 2005.[19]Galileo is expected to be compatible with themodernized GPSsystem. 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,[20]the last of which was launched in December 2021.[21][22]The main modulation used in Galileo Open Service signal is theComposite Binary Offset Carrier(CBOC) modulation.

Regional navigation satellite systems

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Main article:NavIC

TheNavIC(acronym forNavigation with Indian Constellation) is an autonomous regional satellite navigation system developed by theIndian Space Research Organisation(ISRO). TheIndian governmentapproved the project in May 2006. It consists of a constellation of 7 navigational satellites.[23]Three of the satellites are placed ingeostationary orbit (GEO)and the remaining 4 ingeosynchronous 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 7.6 metres (25 ft) throughoutIndiaand within a region extending approximately 1,500 km (930 mi) around it.[24]An Extended Service Area lies between the primary service area and a rectangle area enclosed by the30th parallel southto the50th parallel northand the30th meridian eastto the130th meridian east,1,500–6,000 km beyond borders.[25]A goal of complete Indian control has been stated, with thespace segment,ground segmentand user receivers all being built in India.[26]

The constellation was in orbit as of 2018, and the system was available for public use in early 2018.[27]NavIC provides two levels of service, the "standard positioning service", which will be open for civilian use, and a "restricted service" (anencryptedone) for authorized users (including military). There are plans to expand NavIC system by increasing constellation size from 7 to 11.[28]

India plans to make the NavIC global by adding 24 moreMEOsatellites. The Global NavIC will be free to use for the global public.[29]

Early BeiDou

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Main articles:BeiDou-1andBeiDou-2

The first two generations of China's BeiDou navigation system were designed to provide regional coverage.

Augmentation

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GNSS augmentationis 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, theWide Area Augmentation System,theEuropean Geostationary Navigation Overlay Service,theMulti-functional Satellite Augmentation System,Differential GPS,GPS-aided GEO augmented navigation(GAGAN) andinertial navigation systems.

QZSS

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Main article:Quasi-Zenith Satellite System

The Quasi-Zenith Satellite System (QZSS) is a four-satellite regionaltime transfersystem and enhancement forGPScoveringJapanand theAsia-Oceaniaregions. 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.[30]An independent satellite navigation system (from GPS) with 7 satellites is planned for 2023.[31]

EGNOS

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This section is an excerpt fromEuropean Geostationary Navigation Overlay Service.[edit]
Map of the EGNOS ground network

TheEuropean Geostationary Navigation Overlay Service(EGNOS) is asatellite-based augmentation system(SBAS) developed by theEuropean Space AgencyandEUROCONTROLon behalf of theEuropean Commission.Currently, it supplementsGPSby reporting on the reliability and accuracy of their positioning data and sending out corrections. The system will supplementGalileoin the future version 3.0.

EGNOS consists of 40 Ranging Integrity Monitoring Stations, 2 Mission Control Centres, 6 Navigation Land Earth Stations, the EGNOS Wide Area Network (EWAN), and 3geostationary satellites.[32]Ground stations determine the accuracy of the satellite navigation systems data and transfer it to the geostationary satellites; users may freely obtain this data from those satellites using an EGNOS-enabled receiver, or over the Internet. One main use of the system is inaviation.

According to specifications, horizontal position accuracy when using EGNOS-provided corrections should be better than seven metres. In practice, the horizontal position accuracy is at the metre level.

Similar service is provided in North America by theWide Area Augmentation System(WAAS), in Russia by theSystem for Differential Corrections and Monitoring(SDCM), and in Asia, by Japan'sMulti-functional Satellite Augmentation System(MSAS) and India'sGPS-aided GEO augmented navigation(GAGAN).

Galileoand EGNOS received a budget of €14.6 billion for its six-year, 2021–2027, research and development period.[33]

Comparison of systems

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System BeiDou Galileo GLONASS GPS NavIC QZSS
Owner China European Union Russia United States India Japan
Coverage Global Global Global Global Regional Regional
Coding CDMA CDMA FDMA&CDMA CDMA CDMA CDMA
Altitude
km (mi) 		21,150
(13,140) 		23,222
(14,429) 		19,130
(11,890) 		20,180
(12,540) 		36,000
(22,000) 		32,600–39,000
(20,300–24,200)[34]
Period 12.88 h
(12 h 53 min) 		14.08 h
(14 h 5 min) 		11.26 h
(11 h 16 min) 		11.97 h
(11 h 58 min) 		23.93 h
(23 h 56 min) 		23.93 h
(23 h 56 min)
Rev./S. day 13/7(1.86) 17/10(1.7) 17/8(2.125) 2 1 1
Satellites BeiDou-3:
28 operational
(24 MEO, 3 IGSO, 1 GSO)
5 in orbit validation
2 GSO planned 20H1
BeiDou-2:
15 operational
1 in commissioning 		By design:

27 operational + 3 spares

Currently:

26 in orbit
24 operational

2 inactive
6 to be launched[35]

24 by design
24 operational
1 commissioning
1 in flight tests[36] 		24 by design
30 operational[37] 		8 operational
(3 GEO, 5GSOMEO) 		4 operational (3 GSO, 1 GEO)
7 in the future
Frequency
GHz 		1.561098 (B1)
1.589742 (B1-2)
1.20714 (B2)
1.26852 (B3) 		1.559–1.592 (E1)
1.164–1.215 (E5a/b)
1.260–1.300 (E6) 		1.593–1.610 (G1)
1.237–1.254 (G2)
1.189–1.214 (G3) 		1.563–1.587 (L1)
1.215–1.2396 (L2)
1.164–1.189 (L5) 		1.57542 (L1)
1.17645 (L5)
2.49202 (S) 		1.57542 (L1C/A, L1C, L1S)
1.22760 (L2C)
1.17645 (L5, L5S)
1.27875 (L6)[38]
Status Operational[39] Operating since 2016
2020 completion[35] 		Operational Operational Operational Operational
Accuracy
m (ft) 		3.6 (12) (public)
0.1 (0.33) (encrypted) 		0.2 (0.66) (public)
0.01 (0.033) (encrypted) 		2–4 (6.6–13.1) 0.3–5 (0.98–16.40)
(no DGPS or WAAS) 		1 (3.3) (public)
0.1 (0.33) (encrypted) 		1 (3.3) (public)
0.1 (0.33) (encrypted)
System BeiDou Galileo GLONASS GPS NavIC QZSS
Sources:[7][40][41]

Using multiple GNSS systems for user positioning increases the number of visible satellites, improves precise point positioning (PPP) and shortens the average convergence time.[42] The signal-in-space ranging error (SISRE) in November 2019 were 1.6 cm for Galileo, 2.3 cm for GPS, 5.2 cm for GLONASS and 5.5 cm for BeiDou when using real-time corrections for satellite orbits and clocks.[43]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.[3]

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Further information:Satellite geodesy § Radio techniques

DORIS

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Main article: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.[44]

LEO satellites

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The two current operationallow Earth orbit(LEO)satellite phonenetworks 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 usingAT commandsor agraphical user interface.[45][46]This can also be used by the gateway to enforce restrictions on geographically bound calling plans.

International regulation

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TheInternational Telecommunication Union(ITU) defines aradionavigation-satellite service(RNSS) as "aradiodetermination-satellite serviceused for the purpose ofradionavigation.This service may also includefeeder linksnecessary for its operation ".[47]

RNSS is regarded as asafety-of-life serviceand an essential part ofnavigationwhich must be protected frominterferences.

Aeronautical radionavigation-satellite(ARNSS) is – according toArticle 1.47of theInternational Telecommunication Union's(ITU)Radio Regulations(RR)[48]– defined as «Aradionavigation servicein whichearth stationsare located on board aircraft.»

Maritime radionavigation-satellite service(MRNSS) is – according toArticle 1.45of theInternational Telecommunication Union's(ITU)Radio Regulations(RR)[49]– defined as «Aradionavigation-satellite servicein which earth stations are located on board ships.»

Classification

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ITU Radio Regulations (article 1) classifiesradiocommunicationservices as:

Examples of RNSS use

Frequency allocation

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Further information:Frequency allocation

The allocation of radio frequencies is provided according toArticle 5of the ITU Radio Regulations (edition 2012).[50]

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.
Allocation to services
Region 1 Region 2 Region 3
5 000–5 010 MHz
AERONAUTICAL MOBILE-SATELLITE (R)
AERONAUTICAL RADIONAVIGATION
RADIONAVIGATION-SATELLITE (Earth-to-space)

See also

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Notes

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  1. ^Orbital periods and speeds are calculated using the relations 4π2R3=T2GMandV2R=GM,whereRis the radius of orbit in metres;Tis the orbital period in seconds;Vis the orbital speed in m/s;Gis the gravitational constant, approximately6.673×10−11Nm2/kg2;Mis the mass of Earth, approximately 5.98×1024kg (1.318×1025lb).
  2. ^Approximately 8.6 times when the Moon is nearest(that is,⁠363,104 km/42,164 km⁠),to 9.6 times when the Moon is farthest(that is,⁠405,696 km/42,164 km⁠)

References

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  45. ^"Globalstar GSP-1700 manual"(PDF).Archived fromthe original(PDF)on 2011-07-11.Retrieved2011-12-30.
  46. ^Rickerson, Don (January 2005)."Iridium SMS and SBD"(PDF).Personal Satellite Network, Inc. Archived fromthe original(PDF)on 9 November 2005.
  47. ^ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.43, definition:radionavigation-satellite service
  48. ^ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.47, definition:aeronautical radionavigation service
  49. ^ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.45, definition:maritime radionavigation-satellite service
  50. ^ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations

Further reading

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Information on specific GNSS systems

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Supportive or illustrative sites

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