Satellite navigation

A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows small electronic receivers to determine their location (longitude, latitude, and altitude/elevation) to high precision (within a few 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 high precision, which allows time synchronisation. 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). As of October 2018, the United States' Global Positioning System (GPS) and Russia's GLONASS are fully operational GNSSs, with China's BeiDou Navigation Satellite System (BDS) and the European Union's Galileo scheduled to be fully operational by 2020.[1][2] India, France and Japan are in the process of developing regional navigation and augmentation systems as well.

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 use orbital inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about 20,000 kilometres or 12,000 miles).

Classification

Satellite navigation systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:[3]

  • 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 provides 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, 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, G3 for GLONASS) for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection.[4][5] As of late 2018, a few consumer grade GNSS devices are being sold that leverage both, and are typically called "Dual band GNSS" or "Dual band GPS" devices.
  • Core Satellite navigation systems, currently GPS (United States), GLONASS (Russian Federation), Galileo (European Union) and Compass (China).
  • Global Satellite Based Augmentation Systems (SBAS) such as Omnistar and StarFire.
  • Regional SBAS including WAAS (US), EGNOS (EU), MSAS (Japan) and GAGAN (India).
  • Regional Satellite Navigation Systems such as China's Beidou, India's NAVIC, and Japan's proposed 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.

History and theory

Accuracy of Navigation Systems

Ground based radio navigation has long been practiced. 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.

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 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 induced by variations in the gravity field and radar refraction, among others. These were resolved by a team led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970-1973. Using real-time data assimilation and recursive estimation, the systematic and residual errors were narrowed down to a manageable level to permit accurate navigation. [6]

Part of an orbiting satellite's broadcast included its precise orbital data. In order to ensure accuracy, the US Naval Observatory (USNO) continuously observed the precise orbits of these satellites. As a satellite's orbit deviated, the USNO would send 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. 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 different satellites, thereby 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.

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). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war.

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 of geostationary, GPS, GLONASS, Galileo, Compass (MEO), International Space Station, Hubble Space Telescope, Iridium constellation and graveyard orbits, with the Van Allen radiation belts and the Earth to scale.[a] The Moon's orbit is around 9 times larger than geostationary orbit.[b] (In the SVG file, hover over an orbit or its label to highlight it; click to load its article.)
Launched GNSS 2014
launched GNSS satellites 1978 to 2014

GPS

The United States' Global Positioning System (GPS) consists of up to 32 medium Earth orbit satellites in six different orbital planes, with the exact number of satellites varying 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

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 with 24 satellites.

Galileo

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)) [7] At an estimated cost of €3 billion,[8] the system of 30 MEO satellites was originally scheduled to be operational in 2010. The original year to become operational was 2014.[9] The first experimental satellite was launched on 28 December 2005.[10] Galileo is expected to be compatible with the modernized GPS system. The receivers will be able to combine the signals from both Galileo and GPS satellites to greatly increase the accuracy. Galileo is expected to be in full service in 2020 and at a substantially higher cost.[2] The main modulation used in Galileo Open Service signal is the Composite Binary Offset Carrier (CBOC) modulation.

BeiDou-2

China has indicated their plan to complete the entire second generation Beidou Navigation Satellite System (BDS or BeiDou-2, formerly known as COMPASS), by expanding current regional (Asia-Pacific) service into global coverage by 2020.[1] The BeiDou-2 system is proposed to consist of 30 MEO satellites and five geostationary satellites. A 16-satellite regional version (covering Asia and Pacific area) was completed by December 2012.

Regional navigation satellite systems

BeiDou-1

Chinese regional (Asia-Pacific, 16 satellites) network to be expanded into the whole BeiDou-2 global system which consists of all 35 satellites by 2020.

NAVIC

The NAVIC or NAVigation with Indian Constellation is an autonomous regional satellite navigation system developed by Indian Space Research Organisation (ISRO) which would be under the total control of Indian government. The government approved the project in May 2006, with the intention of the system completed and implemented on 28 April 2016. It will consist of a constellation of 7 navigational satellites.[11] 3 of the satellites will be 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 7.6 meters throughout India and within a region extending approximately 1,500 km around it.[12] A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India.[13] All seven satellites, IRNSS-1A, IRNSS-1B, IRNSS-1C, IRNSS-1D, IRNSS-1E, IRNSS-1F, and IRNSS-1G, of the proposed constellation were precisely launched on 1 July 2013, 4 April 2014, 16 October 2014, 28 March 2015, 20 January 2016, 10 March 2016 and 28 April 2016 respectively from Satish Dhawan Space Centre.[14][15] The system is expected to be fully operational by August 2016.[16]

QZSS

The Quasi-Zenith Satellite System (QZSS) is a proposed four-satellite regional time transfer system and enhancement for GPS covering Japan and the Asia-Oceania regions. QZSS services are available on a trial basis as of January 12, 2018, and are scheduled to be launched in November 2018. The first satellite was launched in September 2010.[17]

Comparison of systems

System BeiDou Galileo GLONASS GPS NAVIC QZSS
Owner China European Union Russia United States India Japan
Coverage Regional,
global by 2020
Global by 2020 Global Global Regional Regional
Coding CDMA CDMA FDMA CDMA CDMA CDMA
Altitude 21,150 km (13,140 mi) 23,222 km (14,429 mi) 19,130 km (11,890 mi) 20,180 km (12,540 mi) 36,000 km (22,000 mi) 32,000 km (20,000 mi)
Period 12.63 h (12 h 38 min) 14.08 h (14 h 5 min) 11.26 h (11 h 16 min) 11.97 h (11 h 58 min) 23.93-23.94 h ?
Rev./S. day 17/9 (1.888...) 17/10 (1.7) 17/8 (2.125) 2 ? ?
Satellites 23 in orbit (Oct 2018)
35 by 2020[18]
26 in orbit
6 to be launched[19]
24 by design
24 operational
1 commissioning
1 in flight tests[20]
31,[21]
24 by design
3 GEO,
5 GSO MEO
4 by the late 2010s,
7 final goal
Frequency 1.561098 GHz (B1)
1.589742 GHz (B1-2)
1.20714 GHz (B2)
1.26852 GHz (B3)
1.559–1.592 GHz (E1)

1.164–1.215 GHz (E5a/b)
1.260–1.300 GHz (E6)

1.593–1.610 GHz (G1)
1.237–1.254 GHz (G2)

1.189–1.214 GHz (G3)

1.563–1.587 GHz (L1)
1.215–1.2396 GHz (L2)

1.164–1.189 GHz (L5)

1176.45 MHz(L5)
2492.028 MHz (S)
?
Status basic nav. service by 2018 end
to be completed by H1 2020[19]
operating since 2016
2020 completion[19]
Operational Operational 7 operational ?
Precision 10m (Public)
0.1m (Encrypted)
1m (Public)
0.01m (Encrypted)
4.5m – 7.4m 15m (no DGPS or WAAS) 10m (Public)
0.1m (Encrypted)
1m (Public)
0.1m (Encrypted)
System BeiDou Galileo GLONASS GPS NAVIC QZSS

Sources: [5]

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.

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.[22]

Low Earth orbit satellite phone networks

The two current operational low Earth orbit satellite phone networks are able to track transceiver units with accuracy of a few kilometers using doppler shift calculations from the satellite. The coordinates are sent back to the transceiver unit where they can be read using AT commands or a graphical user interface.[23][24] This can also be used by the gateway to enforce restrictions on geographically bound calling plans.

See also

Notes

  1. ^ Orbital periods and speeds are calculated using the relations 4π²R³ = T²GM and V²R = GM, where R = radius of orbit in metres, T = orbital period in seconds, V = orbital speed in m/s, G = gravitational constant ≈ 6.673×1011 Nm²/kg², M = mass of Earth ≈ 5.98×1024 kg.
  2. ^ Approximately 8.6 times (in radius and length) when the moon is nearest (363 104 km ÷ 42 164 km) to 9.6 times when the moon is farthest (405 696 km ÷ 42 164 km).

References

  1. ^ a b "Beidou satellite navigation system to cover whole world in 2020". Eng.chinamil.com.cn. Retrieved 2011-12-30.
  2. ^ a b "Galileo goes live!". europa.eu. 2016-12-14.
  3. ^ "A Beginner's Guide to GNSS in Europe" (PDF). IFATCA. Retrieved 20 May 2015.
  4. ^ "Galileo General Introduction - Navipedia". gssc.esa.int. Retrieved 2018-11-17.
  5. ^ a b "GNSS signal - Navipedia". gssc.esa.int. Retrieved 2018-11-17.
  6. ^ 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.
  7. ^ "Galileo goes live!". europa.eu. 14 December 2016.
  8. ^ "Boost to Galileo sat-nav system". BBC News. 25 August 2006. Retrieved 2008-06-10.
  9. ^ "Commission awards major contracts to make Galileo operational early 2014". 2010-01-07. Retrieved 2010-04-19.
  10. ^ "GIOVE-A launch News". 2005-12-28. Retrieved 2015-01-16.
  11. ^ "India to develop its own version of GPS". Rediff.com. Retrieved 2011-12-30.
  12. ^ S. Anandan (2010-04-10). "Launch of first satellite for Indian Regional Navigation Satellite system next year". Beta.thehindu.com. Retrieved 2011-12-30.
  13. ^ "India to build a constellation of 7 navigation satellites by 2012". Livemint.com. 2007-09-05. Retrieved 2011-12-30.
  14. ^ The first satellite IRNSS-1A of the proposed constellation, developed at a cost of 16 billion (US$280 million),[3] was[4] launched on 1 July 2013 from Satish Dhawan Space Centre
  15. ^ "ISRO: All 7 IRNSS Satellites in Orbit by March". gpsworld.com. 2015-10-08. Retrieved 2015-11-12.
  16. ^ Laiqh A. Khan (May 24, 2016). "NAVIC could be operationalised during July-August this year". The Hindu. Retrieved September 2, 2017.
  17. ^ "JAXA Quasi-Zenith Satellite System". JAXA. Archived from the original on 2009-03-14. Retrieved 2009-02-22.
  18. ^ "BeiDou Navigation Satellite System", Wikipedia, 2018-10-24, retrieved 2018-11-07
  19. ^ a b c Irene Klotz, Tony Osborne and Bradley Perrett (Sep 12, 2018). "The Rise Of New Navigation Satellites". Aviation Week & Space Technology.CS1 maint: Uses authors parameter (link)
  20. ^ "Information and Analysis Center for Positioning, Navigation and Timing".
  21. ^ "GPS Space Segment". Retrieved 2015-07-24.
  22. ^ "DORIS information page". Jason.oceanobs.com. Retrieved 2011-12-30.
  23. ^ "Globalstar GSP-1700 manual" (PDF). Retrieved 2011-12-30.
  24. ^ [1] Archived November 9, 2005, at the Wayback Machine

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. [2]

External links

Information on specific GNSS systems

Organizations related to GNSS

Supportive or illustrative sites

BeiDou Navigation Satellite System

The BeiDou Navigation Satellite System (BDS) (Chinese: 北斗卫星导航系统; pinyin: běi dǒu wèi xīng dǎo háng xì tǒng [pèi tòu wêi ɕíŋ tàu xǎŋ ɕî tʰʊ̀ŋ]) is a Chinese satellite navigation system. It consists of two separate satellite constellations. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and also known as BeiDou-1, consists of three satellites which since 2000 has offered limited coverage and navigation services, mainly for users in China and neighboring regions. Beidou-1 was decommissioned at the end of 2012.

The second generation of the system, officially called the BeiDou Navigation Satellite System (BDS) and also known as COMPASS or BeiDou-2, became operational in China in December 2011 with a partial constellation of 10 satellites in orbit. Since December 2012, it has been offering services to customers in the Asia-Pacific region. On December 27, 2018, Beidou-3 officially began to provide global services.In 2015, China started the build-up of the third generation BeiDou system (BeiDou-3) in the global coverage constellation. The first BDS-3 satellite was launched on 30 March 2015. As of January 2018, nine BeiDou-3 satellites have been launched. BeiDou-3 will eventually consist of 35 satellites and is expected to provide global services upon completion in 2020. When fully completed, BeiDou will provide an alternative global navigation satellite system to the United States owned Global Positioning System (GPS), the Russian GLONASS or European Galileo systems and is expected to be more accurate than these. It was claimed in 2016 that BeiDou-3 will reach millimeter-level accuracy (with post-processing).According to China Daily, in 2015, fifteen years after the satellite system was launched, it was generating a turnover of $31.5 billion per annum for major companies such as China Aerospace Science and Industry Corp, AutoNavi Holdings Ltd, and China North Industries Group Corp.On 27 December 2018, BeiDou Navigation Satellite System started to provide global services.

Commercial use of space

Commercial use of space is the provision of goods or services of commercial value by using equipment sent into Earth orbit or outer space. Examples of the commercial use of space include satellite navigation, satellite television and commercial satellite imagery. Operators of such services typically contract the manufacturing of satellites and their launch to private or public companies, which form an integral part of the space economy. Some commercial ventures have long-terms plans to exploit natural resources originating outside Earth, for example asteroid mining. Space tourism, currently an exceptional activity, could also be an area of future growth, as new businesses strive to reduce the costs and risks of human spaceflight.

The first commercial use of outer space occurred in 1962, when the Telstar 1 satellite was launched to transmit television signals over the Atlantic Ocean. By 2004, global investment in all space sectors was estimated to be $50.8 billion.

As of 2010, 31% of all space launches were commercial.

Comparison of satellite navigation software

This is a list of notable commercial satellite navigation software (also known as GPS software) for various device, with a specific focus on mobile phones, tablets, tablet PC's, (Android, iOS, Windows).

DORIS (geodesy)

Doppler Orbitography and Radiopositioning Integrated by Satellite or, in French, Détermination d'Orbite et Radiopositionnement Intégré par Satellite (in both case yielding the acronym DORIS) is a French satellite system used for the determination of satellite orbits (e.g. TOPEX/Poseidon) and for positioning.

Differential GPS

Differential Global Positioning Systems (DGPS) are enhancements to the Global Positioning System (GPS) which provide improved location accuracy, in the range of operations of each system, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations.

Each DGPS uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellite system and known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range.

The United States Coast Guard (USCG) and the Canadian Coast Guard (CCG) each run DGPS systems in the United States and Canada on longwave radio frequencies between 285 kHz and 325 kHz near major waterways and harbors. The USCG's DGPS system was named NDGPS (Nationwide DGPS) and was jointly administered by the Coast Guard and the U.S. Department of Defense's Army Corps of Engineers (USACE). It consisted of broadcast sites located throughout the inland and coastal portions of the United States including Alaska, Hawaii and Puerto Rico. Other countries have their own DGPS system.

A similar system which transmits corrections from orbiting satellites instead of ground-based transmitters is called a Wide-Area DGPS (WADGPS) or Satellite Based Augmentation System.

European GNSS Agency

The European Global Navigation Satellite Systems Agency (European GNSS Agency; GSA; formerly European GNSS Supervisory Authority) is the agency of the European Union (EU) that aims to ensure that essential public interests are properly defended and represented in connection with satellite navigation programmes of the union: Galileo and European Geostationary Navigation Overlay Service (EGNOS). The aim of the former is provide a modern European alternative to the established American system, GPS. Established in 2004 and based in Prague, Czech Republic since 1st September 2012, the agency is responsible for managing and monitoring the use of the program funds. It will help the European Commission deal with any matters relating to satellite radio-navigation.

In June 2018, the European Commission proposed that the agency be transformed into an EU Agency for the Space Programme, with oversight of space situational awareness.

European Geostationary Navigation Overlay Service

The European Geostationary Navigation Overlay Service (EGNOS) is a satellite based augmentation system (SBAS) developed by the European Space Agency and EUROCONTROL on behalf of the European Commission. It supplements the GPS, GLONASS and Galileo by reporting on the reliability and accuracy of their positioning data and sending out corrections.

EGNOS consists of a network of about 40 ground stations and 3 geostationary satellites. Ground stations determine accuracy data of the satellite navigation systems 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 in aviation.

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 the Wide Area Augmentation System (WAAS), and in Asia, by Japan's Multi-functional Satellite Augmentation System (MSAS) and India's GPS Aided GEO Augmented Navigation (GAGAN).

GIOVE

GIOVE [ˈdʒɔve], or Galileo In-Orbit Validation Element, is the name for two satellites built for the European Space Agency (ESA) to test technology in orbit for the Galileo positioning system.Giove is the Italian word for "Jupiter". The name was chosen as a tribute to Galileo Galilei, who discovered the first four natural satellites of Jupiter, and later discovered that they could be used as a universal clock to obtain the longitude of a point on the Earth's surface.

The GIOVE satellites are operated by the GIOVE Mission (GIOVE-M) segment in the frame of the risk mitigation for the In Orbit Validation (IOV) of the Galileo positioning system.

Galileo (satellite navigation)

Galileo is the global navigation satellite system (GNSS) that went live in 2016, created by the European Union (EU) through the European GNSS Agency (GSA), headquartered in Prague in the Czech Republic, with two ground operations centres, Oberpfaffenhofen near Munich in Germany and Fucino in Italy. The €10 billion project is named after the Italian astronomer Galileo Galilei. One of the aims of Galileo is to provide an independent high-precision positioning system so European nations do not have to rely on the U.S. GPS, or the Russian GLONASS systems, which could be disabled or degraded by their operators at any time.

The use of basic (lower-precision) Galileo services will be free and open to everyone. The higher-precision capabilities will be available for paying commercial users. Galileo is intended to provide horizontal and vertical position measurements within 1-metre precision, and better positioning services at higher latitudes than other positioning systems.

Galileo is also to provide a new global search and rescue (SAR) function as part of the MEOSAR system.

The first Galileo test satellite, the GIOVE-A, was launched 28 December 2005, while the first satellite to be part of the operational system was launched on 21 October 2011. As of July 2018, 26 of the planned 30 active satellites are in orbit. Galileo started offering Early Operational Capability (EOC) on 15 December 2016, providing initial services with a weak signal, and is expected to reach Full Operational Capability (FOC) in 2019. The complete 30-satellite Galileo system (24 operational and 6 active spares) is expected by 2020. It is expected that the next generation of satellites will begin to become operational by 2025 to replace older equipment. Older systems can then be used for backup capabilities.

There are 22 satellites in usable condition (satellite is operational and contributing to the service provision), 2 satellites are in "testing" and 2 more are marked as not available. .

Indian Regional Navigation Satellite System

The Indian Regional Navigation Satellite System (IRNSS), with an operational name of NAVIC ("sailor" or "navigator" in Sanskrit, Hindi and many other Indian languages and also standing for NAVigation with Indian Constellation), is an autonomous regional satellite navigation system that provides accurate real-time positioning and timing services. It covers India and a region extending 1,500 km (930 mi) around it, with plans for further extension. 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. The system at present consists of a constellation of seven satellites, with two additional satellites on ground as stand-by.The constellation is in orbit as of 2018, and the system was expected to be operational from early 2018 after a system check. NAVIC will provide 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). Due to the failures of one of the satellites and its replacement, no new date for operational status has been set.

There are plans to expand NavIC system by increasing constellation size from 7 to 11.

L band

The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz).

List of BeiDou satellites

This is a list of past and present satellites of the BeiDou/Compass navigation satellite system. As of November 2018, 33 satellites are operational: 6 in geostationary orbits (GEO), 6 in 55-degree inclined geosynchronous orbits (IGSO) and 21 in Medium Earth orbits (MEO). Furthermore, 6 satellites (3 in Medium Earth orbit, 1 in geostationary orbit and 2 in inclined geosynchronous orbit) are undergoing testing or commissioning. The full constellation will consists of 35 satellites and expected to be completed by 2020.

List of Galileo satellites

This is a list of past and present satellites of the Galileo navigation system. As of February 2019, 22 satellites are declared operational, 2 satellites placed in non-nominal orbit are declared in testing, and 2 satellites are currently not usable. The two GIOVE prototype vehicles were retired in 2012.

MTSAT Satellite Augmentation System

Multi-functional Satellite Augmentation System (MTSAT or MSAS) is a Japanese satellite based augmentation system (SBAS), i.e. a satellite navigation system which supports differential GPS (DGPS) to supplement the GPS system by reporting (then improving) on the reliability and accuracy of those signals. MSAS is operated by Japan's Ministry of Land, Infrastructure and Transport Japan Civil Aviation Bureau (JCAB). Tests have been accomplished successfully, MSAS for aviation use was commissioned on 27 September 2007.

The use of SBASs, such as MSAS, enables an individual GPS receiver to correct its own position, offering a much greater accuracy. Typically GPS signal accuracy is improved from some 20 meters to approximately 1.5–2 meters in both the horizontal and vertical dimensions.

MSAS provides a similar service to Wide Area Augmentation System (WAAS) in North America, and European Geostationary Navigation Overlay Service (EGNOS) in Europe.

Point of interest

A point of interest, or POI, is a specific point location that someone may find useful or interesting. An example is a point on the Earth representing the location of the Space Needle, or a point on Mars representing the location of the mountain, Olympus Mons. Most consumers use the term when referring to hotels, campsites, fuel stations or any other categories used in modern (automotive) navigation systems.

Users of a mobile devices can be provided with geolocation and time aware POI service, that recommends geolocations nearby and with a temporal relevance (e.g. POI to special services in a ski resort are available only in winter).

In medical fields such as histology/pathology/histopathology, points of interest are selected from the general background in a field of view; for example, among hundreds of normal cells, the pathologist may find 3 or 4 neoplastic cells that stand out from the others upon staining.

A region of interest (ROI) and a volume of interest (VOI) are similar in concept, denoting a region or a volume (which may contain various individual POIs).

The term is widely used in cartography, especially in electronic variants including GIS, and GPS navigation software. In this context the synonym waypoint is common.

A GPS point of interest specifies, at minimum, the latitude and longitude of the POI, assuming a certain map datum. A name or description for the POI is usually included, and other information such as altitude or a telephone number may also be attached. GPS applications typically use icons to represent different categories of POI on a map graphically.

TERCOM

Terrain Contour Matching, or TERCOM, is a navigation system used primarily by cruise missiles. It uses a pre-recorded contour map of the terrain that is compared with measurements made during flight by an on-board radar altimeter. A TERCOM system considerably increases the accuracy of a missile compared with inertial navigation systems (INS). The increased accuracy allows a TERCOM-equipped missile to fly closer to obstacles and generally lower altitudes, making it harder to detect by ground radar.

Transit (satellite)

The Transit system, also known as NAVSAT or NNSS (for Navy Navigation Satellite System), was the first satellite navigation system to be used operationally. The system was primarily used by the U.S. Navy to provide accurate location information to its Polaris ballistic missile submarines, and it was also used as a navigation system by the Navy's surface ships, as well as for hydrographic survey and geodetic surveying. Transit provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use as well.

Tsikada

Tsikada (Russian: Цикада meaning cicada), is a Russian satellite navigation system including ten Low Earth Orbit (LEO) satellites. It transmits the same two carrier frequencies as the U.S. TRANSIT satellite system. The first satellite was launched in 1974.

Tsiklon (satellite)

Tsiklon (meaning cyclone, Russian: Циклон) is the first Soviet satellite navigation system, developed in the former Soviet Union and now operated by the Russian Space Forces.

From 1967 to 1978 a total of 31 Tsiklon satellites were launched onboard Kosmos-3 and Kosmos-3M rockets, from the Kapustin Yar and Plesetsk launch sites. The project was conceived in the 1950s and the draft proposal was approved in 1962, but was not made operational until 1972 due to delays.The successor satellites to Tsiklon were Parus and Sfera. Currently, Russia operates the GLONASS system.

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