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A satellite navigation or satnav system is a system that uses satellites to provide autonomous
geo-spatial positioning Geopositioning, also known as geotracking, geolocalization, geolocating, geolocation, or geoposition fixing, is the process of determining or estimating the geographic position of an object. Geopositioning yields a set of Geographic coordinate s ...
. It allows satellite navigation devices to determine their location ( longitude, latitude, and altitude/ elevation) to high precision (within a few centimetres to metres) using time signals transmitted along a 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). One set of critical vulnerabilities in satellite communications are the signals that govern positioning, navigation and timing (PNT). Failure to properly secure these transmissions could not only disrupt satellite networks but wreak havoc on a host of dependent systems as well. Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated. A satellite navigation system with global coverage may be termed a global navigation satellite system (GNSS). , the United States'
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
(GPS), Russia's Global Navigation Satellite System ( GLONASS),
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
's BeiDou Navigation Satellite System, and the European Union's
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
are fully operational GNSSs. Japan's Quasi-Zenith Satellite System (QZSS) is a (US) GPS satellite-based augmentation system to enhance the accuracy of GPS, with satellite navigation independent of GPS scheduled for 2023. The Indian Regional Navigation Satellite System (IRNSS) plans to expand to a global version in the long term. 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 inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about ).


Classification

GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows: * GNSS-1 is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), with Satellite Based Augmentation Systems (SBAS) or Ground Based Augmentation Systems (GBAS). 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); and in Japan, it is the Multi-Functional Satellite Augmentation System (MSAS). Ground-based augmentation is provided by systems like the Local Area Augmentation System (LAAS). * GNSS-2 is the second generation of systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system. 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, and G1 for GLONASS). In recent years, GNSS systems have begun activating lLower L-Band frequency sets (L2 and L5 for GPS, E5a and E5b for Galileo, and G3 for GLONASS) for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection. 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 core satellite navigation systems, currently
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
(United States), GLONASS (Russian Federation), Beidou (China) and
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
(European Union). * Global Satellite-Based Augmentation Systems (SBAS) such as OmniSTAR and StarFire. * Regional SBAS including WAAS (US), EGNOS (EU), MSAS (Japan), GAGAN (India) and SDCM (Russia). * Regional Satellite Navigation Systems such as India's 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.


History

Ground based radio navigation is decades old. The DECCA, LORAN, GEE and Omega systems used terrestrial longwave radio transmitters 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 Fix or FIX may refer to: People with the name * Fix (surname) Arts, entertainment, and media Films * ''Fix'' (film), a feature film by Tao Ruspoli Music * ''Fix'' (album), 2015 album by Chris Lane * "Fix" (Blackstreet song), 1997 song by Black ...
. The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the
Doppler effect The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
: 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 1970-1973, found solutions and/or corrections for many error sources. Using real-time data and recursive estimation, the systematic and residual errors were narrowed down to accuracy sufficient for navigation.


Principles

Part of an orbiting satellite's broadcast includes its precise orbital data. Originally, the 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 In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly vel ...
. 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 In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly vel ...
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 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 as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
'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.


Applications

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 A guided bomb (also known as a smart bomb, guided bomb unit, or GBU) is a precision-guided munition designed to achieve a smaller circular error probable (CEP). The creation of precision-guided munitions resulted in the retroactive renaming of ...
). 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 Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, 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. 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

In order of first launch year:


GPS

First launch year: 1978 The United States' Global Positioning System (GPS) consists of up to 32 medium Earth orbit satellites in six different orbital 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

First launch year: 1982 The formerly Soviet, and now Russian, ''Global'naya Navigatsionnaya Sputnikovaya Sistema'', (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

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. The BeiDou-3 system is proposed to consist of 30 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. 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.


Galileo

First launch year: 2011 The European Union and
European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...
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). At an estimated cost of €10 billion, the system of 30 MEO satellites was originally scheduled to be operational in 2010. The original year to become operational was 2014. The first experimental satellite was launched on 28 December 2005. Galileo is expected to be compatible with the
modernized GPS GPS Block III (previously Block IIIA) consists of the first ten GPS III satellites, which will be used to keep the Navstar Global Positioning System operational. Lockheed Martin designed, developed and manufactured the GPS III Non-Flight Sate ...
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, the last of which was launched in December 2021. The main modulation used in Galileo Open Service signal is the
Composite Binary Offset Carrier The Composite Binary Offset Carrier (CBOC) modulation is a particular implementation of the Multiplexed Binary Offset Carrier modulation and it is nowadays used by Galileo satellite signals. It is formed by addition or subtraction of two weight ...
(CBOC) modulation.


Regional navigation satellite systems


NavIC

The NavIC or NAVigation with Indian Constellation is an autonomous regional satellite navigation system developed by
Indian Space Research Organisation The Indian Space Research Organisation (ISRO; ) is the national space agency of India, headquartered in Bengaluru. It operates under the Department of Space (DOS) which is directly overseen by the Prime Minister of India, while the Chairman ...
(ISRO). The government approved the project in May 2006, and consists of a constellation of 7 navigational satellites. 3 of the satellites are placed in the Geostationary orbit (GEO) and the remaining 4 in the 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 throughout India and within a region extending approximately around it. An Extended Service Area lies between the primary service area and a rectangle area enclosed by the
30th parallel south The 30th parallel south is a circle of latitude that is 30 degrees south of the Earth's equator. It stands one-third of the way between the equator and the South Pole and crosses Africa, the Indian Ocean, Australia, the Pacific Ocean, South Am ...
to the
50th parallel north The 50th parallel north is a circle of latitude that is 50 degrees north of the Earth's equatorial plane. It crosses Europe, Asia, the Pacific Ocean, North America, and the Atlantic Ocean. At this latitude the sun is visible for 16 hours, 22 m ...
and the
30th meridian east The meridian 30° east of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, Europe, Turkey, Africa, the Indian Ocean, the Southern Ocean, and Antarctica to the South Pole. The 30th meridian east forms a ...
to the
130th meridian east The meridian 130° east of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, Asia, Australia, the Indian Ocean, the Southern Ocean, and Antarctica to the South Pole. The 130th meridian east forms a great ...
, 1,500–6,000 km beyond borders. A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India. The constellation was in orbit as of 2018, and the system was available for public use in early 2018. NavIC provides two levels of service, the "standard positioning service", which will be open for civilian use, and a "restricted service" (an encrypted one) for authorized users (including military). There are plans to expand NavIC system by increasing constellation size from 7 to 11.


QZSS

The Quasi-Zenith Satellite System (QZSS) is a four-satellite regional
time transfer Time and frequency transfer is a scheme where multiple sites share a precise reference time or frequency. The technique is commonly used for creating and distributing standard time scales such as International Atomic Time (TAI). Time transfer sol ...
system and enhancement for
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
covering
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
and the 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. An independent satellite navigation system (from GPS) with 7 satellites is planned for 2023.


Comparison of systems

Sources: Using multiple GNSS systems for user positioning increases the number of visible satellites, improves precise point positioning (PPP) and shortens the average convergence time. 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.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.


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 systems.


Related techniques


DORIS

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.


LEO satellites

The two current operational low Earth orbit (LEO)
satellite phone A satellite telephone, satellite phone or satphone is a type of mobile phone that connects to other phones or the telephone network by radio through orbiting satellites instead of terrestrial cell sites, as cellphones do. The advantage of a sa ...
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 AT or at may refer to: Geography Austria * Austria (ISO 2-letter country code) * .at, Internet country code top-level domain United States * Atchison County, Kansas (county code) * The Appalachian Trail (A.T.), a 2,180+ mile long mountaino ...
s or a graphical user interface. 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 radionavigation. This service may also include feeder links necessary for its operation". RNSS is regarded as a safety-of-life service and an essential part of navigation which must be protected from interferences.


Classification

ITU Radio Regulations (article 1) classifies radiocommunication 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 Aeronautical radionavigation service (short: ARNS) is – according to ''Article 1.46'' of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as "''A radionavigation service intended for the benefit and for the ...
(article 1.46) ***
Aeronautical radionavigation-satellite service Aeronautical radionavigation-satellite (short: ARNSS) is – according to ''Article 1.47'' of the International Telecommunication Union's (ITU) Radio Regulations (RR)ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.47, ...
(article 1.47) ; Examples of RNSS use *Augmentation system GNSS augmentation * Automatic Dependent Surveillance–Broadcast * BeiDou Navigation Satellite System (BDS) *
GALILEO Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, European GNSS *
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
(GPS), with Differential GPS (DGPS) * GLONASS * NAVIC * Quasi-Zenith Satellite System (QZSS)


Frequency allocation

The allocation of radio frequencies is provided according to ''Article 5'' of the ITU Radio Regulations (edition 2012).''ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations'' 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.


See also

* Acronyms and abbreviations in avionics * Geoinformatics * GNSS positioning calculation * GNSS reflectometry *
GPS spoofing In the context of information security, and especially network security, a spoofing attack is a situation in which a person or program successfully identifies as another by falsifying data, to gain an illegitimate advantage. Internet Spoofing and ...
* GPS-aided geo-augmented navigation *
List of emerging technologies This is a list of emerging technologies, in-development technical innovations with significant potential in their applications. The criteria for this list is that the technology must: # Exist in some way; purely hypothetical technologies can ...
* Pseudolite * Receiver Autonomous Integrity Monitoring * Software GNSS Receiver * Space Integrated GPS/INS (SIGI) *
United Kingdom Global Navigation Satellite System The United Kingdom Global Navigation Satellite System (UK GNSS) was a UK Space Agency, United Kingdom Space Agency (UKSA) research programme which, between May 2018 and September 2020, developed outline proposals for a United Kingdom (UK) owned a ...
* UNSW School of Surveying and Geospatial Engineering


Notes


References


Further reading

* Office for Outer Space Affairs of the United Nations (2010), ''Report on Current and Planned Global and Regional Navigation Satellite Systems and Satellite-based Augmentation Systems''


External links


Information on specific GNSS systems


ESA information on EGNOS



Global Navigation Satellite System Fundamentals


Organizations related to GNSS



* ttp://www.ion.org/meetings/#gnss Institute of Navigation (ION) GNSS Meetings
The International GNSS Service (IGS)

International Global Navigation Satellite Systems Society Inc (IGNSS)

International Earth Rotation and Reference Systems Service (IERS) International GNSS Service (IGS)

US National Executive Committee for Space-Based Positioning, Navigation, and Timing

US National Geodetic Survey
Orbits for the Global Positioning System satellites in the Global Navigation Satellite System


Asia-Pacific Economic Cooperation (APEC) GNSS Implementation Team


Supportive or illustrative sites



( Java applet) Simulation and graphical depiction of the motion of space vehicles, including DOP computation.
GPS, GNSS, Geodesy and Navigation Concepts in depth
{{DEFAULTSORT:Satellite Navigation American inventions Aircraft instruments Avionics Geodesy Maritime communication Navigation