Misplaced Pages

#739260

75-471: Inmarsat-4A F4 , also known as Alphasat and Inmarsat-XL , is a large geostationary communications I-4 satellite operated by United Kingdom-based Inmarsat in partnership with the European Space Agency . Launched in 2013, it is used to provide mobile communications to Africa and parts of Europe and Asia . Inmarsat-4A F4 has been constructed by EADS Astrium and Thales Alenia Space based on

150-479: A delta-v of approximately 50 m/s per year. A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical ( equatorial eccentricity ). There are two stable equilibrium points sometimes called "gravitational wells" (at 75.3°E and 108°W) and two corresponding unstable points (at 165.3°E and 14.7°W). Any geostationary object placed between

225-427: A geostationary transfer orbit (GTO), an elliptical orbit with an apogee at GEO height and a low perigee . On-board satellite propulsion is then used to raise the perigee, circularise and reach GEO. Satellites in geostationary orbit must all occupy a single ring above the equator . The requirement to space these satellites apart, to avoid harmful radio-frequency interference during operations, means that there are

300-452: A temporary orbit , and placed in a slot above a particular point on the Earth's surface. The orbit requires some stationkeeping to keep its position, and modern retired satellites are placed in a higher graveyard orbit to avoid collisions. In 1929, Herman Potočnik described both geosynchronous orbits in general and the special case of the geostationary Earth orbit in particular as useful orbits for space stations . The first appearance of

375-496: A constellation of 7 navigational satellites. Three of the satellites are placed in geostationary orbit (GEO) and the remaining 4 in 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 metres (25 ft) throughout India and within a region extending approximately 1,500 km (930 mi) around it. An Extended Service Area lies between

450-592: A few centimeters to meters) 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). Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance

525-525: A geostationary orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith , but Smith did not go into details. British science fiction author Arthur C. Clarke popularised and expanded the concept in a 1945 paper entitled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage? , published in Wireless World magazine. Clarke acknowledged

600-415: A geostationary orbit in particular, it ensures that it holds the same longitude over time. This orbital period, T , is directly related to the semi-major axis of the orbit through the formula: where: The eccentricity is zero, which produces a circular orbit . This ensures that the satellite does not move closer or further away from the Earth, which would cause it to track backwards and forwards across

675-518: A geostationary orbit was Syncom 3 , which was launched by a Delta D rocket in 1964. With its increased bandwidth, this satellite was able to transmit live coverage of the Summer Olympics from Japan to America. Geostationary orbits have been in common use ever since, in particular for satellite television. Today there are hundreds of geostationary satellites providing remote sensing and communications. Although most populated land locations on

750-399: A known position) and providing an additional reference signal. This improves position accuracy from approximately 5m to 1m or less. Past and current navigation systems that use geostationary satellites include: Geostationary satellites are launched to the east into a prograde orbit that matches the rotation rate of the equator. The smallest inclination that a satellite can be launched into

825-408: A large area of the earth's surface, extending 81° away in latitude and 77° in longitude. They appear stationary in the sky, which eliminates the need for ground stations to have movable antennas. This means that Earth-based observers can erect small, cheap and stationary antennas that are always directed at the desired satellite. However, latency becomes significant as it takes about 240 ms for

SECTION 10

#1732863235740

900-501: A limited number of orbital slots available, and thus only a limited number of satellites can be operated in geostationary orbit. This has led to conflict between different countries wishing access to the same orbital slots (countries near the same longitude but differing latitudes ) and radio frequencies . These disputes are addressed through the International Telecommunication Union 's allocation mechanism under

975-455: 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

1050-466: 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

1125-406: A signal to pass from a ground based transmitter on the equator to the satellite and back again. This delay presents problems for latency-sensitive applications such as voice communication, so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available. Geostationary satellites are directly overhead at

1200-540: Is a stub . You can help Misplaced Pages by expanding it . This article about one or more spacecraft of the United Kingdom is a stub . You can help Misplaced Pages by expanding it . Geostationary orbit A geostationary orbit , also referred to as a geosynchronous equatorial orbit ( GEO ), is a circular geosynchronous orbit 35,786 km (22,236 mi) in altitude above Earth's equator , 42,164 km (26,199 mi) in radius from Earth's center, and following

1275-452: Is that of the launch site's latitude, so launching the satellite from close to the equator limits the amount of inclination change needed later. Additionally, launching from close to the equator allows the speed of the Earth's rotation to give the satellite a boost. A launch site should have water or deserts to the east, so any failed rockets do not fall on a populated area. Most launch vehicles place geostationary satellites directly into

1350-403: Is the gravitational constant , (6.674 28 ± 0.000 67 ) × 10  m kg s . The magnitude of the acceleration, a , of a body moving in a circle is given by: where v is the magnitude of the velocity (i.e. the speed) of the satellite. From Newton's second law of motion , the centripetal force F c is given by: As F c = F g , so that Replacing v with the equation for

1425-594: Is the world's most utilized satellite navigation system. First launch year: 1982 The formerly Soviet , and now Russian , 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. First launch year: 2000 BeiDou started as

1500-781: Is typically 70°, and in some cases less. Geostationary satellite imagery has been used for tracking volcanic ash , measuring cloud top temperatures and water vapour, oceanography , measuring land temperature and vegetation coverage, facilitating cyclone path prediction, and providing real time cloud coverage and other tracking data. Some information has been incorporated into meteorological prediction models , but due to their wide field of view, full-time monitoring and lower resolution, geostationary weather satellite images are primarily used for short-term and real-time forecasting. Geostationary satellites can be used to augment GNSS systems by relaying clock , ephemeris and ionospheric error corrections (calculated from ground stations of

1575-408: Is used to provide visible and infrared images of Earth's surface and atmosphere for weather observation, oceanography , and atmospheric tracking. As of 2019 there are 19 satellites in either operation or stand-by. These satellite systems include: These satellites typically capture images in the visual and infrared spectrum with a spatial resolution between 0.5 and 4 square kilometres. The coverage

SECTION 20

#1732863235740

1650-649: Is – according to Article 1.45 of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as « A radionavigation-satellite service in which earth stations are located on board ships .» ITU Radio Regulations (article 1) classifies radiocommunication services as: The allocation of radio frequencies is provided according to Article 5 of the ITU Radio Regulations (edition 2012). To improve harmonisation in spectrum utilisation, most service allocations are incorporated in national Tables of Frequency Allocations and Utilisations within

1725-662: The Alphabus satellite bus . It was the first Alphabus spacecraft to be launched, and as such it carries several experimental communications systems in addition to its commercial payload. The spacecraft had a launch mass of 6,649 kilograms (14,659 lb), and is expected to operate for at least fifteen years. Arianespace had been contracted to launch Inmarsat-4A F4, with an Ariane 5ECA rocket, flight number VA-214, delivering it and INSAT-3D into geosynchronous transfer orbit . The rocket lifted off from ELA-3 at Kourou at 19:54:07 UTC on 25 July 2013, with Inmarsat-4A F4 separating from

1800-454: 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

1875-641: The Multi-functional Satellite Augmentation System , Differential GPS , GPS-aided GEO augmented navigation (GAGAN) and inertial navigation systems . The Quasi-Zenith Satellite System (QZSS) is a four-satellite regional time transfer system and enhancement for GPS covering Japan 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

1950-473: The Radio Regulations . In the 1976 Bogota Declaration , eight countries located on the Earth's equator claimed sovereignty over the geostationary orbits above their territory, but the claims gained no international recognition. A statite is a hypothetical satellite that uses radiation pressure from the sun against a solar sail to modify its orbit. It would hold its location over the dark side of

2025-430: The centripetal force required to maintain the orbit ( F c ) is equal to the gravitational force acting on the satellite ( F g ): From Isaac Newton 's universal law of gravitation , where F g is the gravitational force acting between two objects, M E is the mass of the Earth, 5.9736 × 10  kg , m s is the mass of the satellite, r is the distance between the centers of their masses , and G

2100-451: The direction of Earth's rotation . An object in such an orbit has an orbital period equal to Earth's rotational period, one sidereal day , and so to ground observers it appears motionless, in a fixed position in the sky. The concept of a geostationary orbit was popularised by the science fiction writer Arthur C. Clarke in the 1940s as a way to revolutionise telecommunications, and the first satellite to be placed in this kind of orbit

2175-486: The speed of an object moving around a circle produces: where T is the orbital period (i.e. one sidereal day), and is equal to 86 164 .090 54  s . This gives an equation for r : The product GM E is known with much greater precision than either factor alone; it is known as the geocentric gravitational constant μ = 398 600 .4418 ± 0.0008 km s . Hence Navigation satellites Satellite-based augmentation systems (SBAS), designed to enhance

2250-424: The Earth at a latitude of approximately 30 degrees. A statite is stationary relative to the Earth and Sun system rather than compared to surface of the Earth, and could ease congestion in the geostationary ring. Geostationary satellites require some station keeping to keep their position, and once they run out of thruster fuel they are generally retired. The transponders and other onboard systems often outlive

2325-1061: 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), in Russia by the System for Differential Corrections and Monitoring (SDCM), and in Asia, by Japan's Multi-functional Satellite Augmentation System (MSAS) and India's GPS-aided GEO augmented navigation (GAGAN). 27 operational + 3 spares Currently: 26 in orbit 24 operational 2 inactive 6 to be launched Using multiple GNSS systems for user positioning increases

Inmarsat-4A F4 - Misplaced Pages Continue

2400-400: The absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station-keeping places a limitation on the lifetime of the satellite. Hall-effect thrusters , which are currently in use, have the potential to prolong the service life of a satellite by providing high-efficiency electric propulsion . For circular orbits around a body,

2475-633: The accuracy of GNSS, include Japan's Quasi-Zenith Satellite System (QZSS), 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 ). Satellite navigation devices determine their location ( longitude , latitude , and altitude / elevation ) to high precision (within

2550-453: 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. 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

2625-458: The concept was seen as impractical, so Hughes often withheld funds and support. By 1961, Rosen and his team had produced a cylindrical prototype with a diameter of 76 centimetres (30 in), height of 38 centimetres (15 in), weighing 11.3 kilograms (25 lb), light and small enough to be placed into orbit. It was spin stabilised with a dipole antenna producing a pancake shaped beam. In August 1961, they were contracted to begin building

2700-531: The connection in his introduction to The Complete Venus Equilateral . The orbit, which Clarke first described as useful for broadcast and relay communications satellites, is sometimes called the Clarke orbit. Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt. In technical terminology the orbit is referred to as either a geostationary or geosynchronous equatorial orbit, with

2775-493: 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 , 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

2850-659: The equator and appear lower in the sky to an observer nearer the poles. As the observer's latitude increases, communication becomes more difficult due to factors such as atmospheric refraction , Earth's thermal emission , line-of-sight obstructions, and signal reflections from the ground or nearby structures. At latitudes above about 81°, geostationary satellites are below the horizon and cannot be seen at all. Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes. A worldwide network of operational geostationary meteorological satellites

2925-474: The equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation. The correction of this effect requires station-keeping maneuvers with a maximal delta-v of about 2 m/s per year, depending on the desired longitude. Solar wind and radiation pressure also exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits. In

3000-542: The following properties: An inclination of zero ensures that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the Earth-centered Earth-fixed reference frame). The orbital period is equal to exactly one sidereal day. This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties. For

3075-596: 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

Inmarsat-4A F4 - Misplaced Pages Continue

3150-774: The future is enormous, including both the public and private sectors across numerous market segments such as science, transport, agriculture, insurance, energy, 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. In order of first launch year: 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

3225-509: 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 3 geostationary satellites . 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

3300-533: The global public. The first two generations of China's BeiDou navigation system were designed to provide regional coverage. 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 ,

3375-523: The ground. All geostationary satellites have to be located on this ring. A combination of lunar gravity, solar gravity, and the flattening of the Earth at its poles causes a precession motion of the orbital plane of any geostationary object, with an orbital period of about 53 years and an initial inclination gradient of about 0.85° per year, achieving a maximal inclination of 15° after 26.5 years. To correct for this perturbation , regular orbital stationkeeping maneuvers are necessary, amounting to

3450-473: 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

3525-731: The last of which was launched in December 2021. The main modulation used in Galileo Open Service signal is the Composite Binary Offset Carrier (CBOC) modulation. The NavIC (acronym for Navigation with Indian Constellation ) is an autonomous regional satellite navigation system developed by the Indian Space Research Organisation (ISRO). The Indian government approved the project in May 2006. It consists of

3600-525: The navigation system, systems can be classified as: 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. Ground-based radio navigation is decades old. The DECCA , LORAN , GEE and Omega systems used terrestrial longwave radio transmitters which broadcast

3675-440: 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. The original motivation for satellite navigation was for military applications. Satellite navigation allows precision in

3750-451: 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

3825-466: 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

SECTION 50

#1732863235740

3900-476: The planet now have terrestrial communications facilities ( microwave , fiber-optic ), with telephone access covering 96% of the population and internet access 90%, some rural and remote areas in developed countries are still reliant on satellite communications. Most commercial communications satellites , broadcast satellites and SBAS satellites operate in geostationary orbits. Geostationary communication satellites are useful because they are visible from

3975-456: 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. 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

4050-572: 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 . Aeronautical radionavigation-satellite ( ARNSS ) is – according to Article 1.47 of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as « A radionavigation service in which earth stations are located on board aircraft .» Maritime radionavigation-satellite service ( MRNSS )

4125-623: The real satellite. They lost Syncom 1 to electronics failure, but Syncom 2 was successfully placed into a geosynchronous orbit in 1963. Although its inclined orbit still required moving antennas, it was able to relay TV transmissions, and allowed for US President John F. Kennedy in Washington D.C., to phone Nigerian prime minister Abubakar Tafawa Balewa aboard the USNS Kingsport docked in Lagos on August 23, 1963. The first satellite placed in

4200-532: The rocket around 27 minutes later. The spacecraft operates in a geostationary orbit at a longitude of 25 degrees east. As of 29 October 2013, it is in an orbit with a perigee of 35,771 kilometres (22,227 mi), an apogee of 35,771 kilometres (22,227 mi) and 0.14 degrees inclination to the equator. The orbit had a semimajor axis of 42,157.20 kilometres (26,195.27 mi) and eccentricity of 0.0003552, giving it an orbital period of 1,435.75 minutes, or 23.92 hours. The satellite

4275-524: The same plane, altitude and speed; however, the presence of satellites in eccentric orbits allows for collisions at up to 4 km/s. Although a collision is comparatively unlikely, GEO satellites have a limited ability to avoid any debris. At geosynchronous altitude, objects less than 10 cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence. Despite efforts to reduce risk, spacecraft collisions have occurred. The European Space Agency telecom satellite Olympus-1

4350-548: 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. Using real-time data and recursive estimation,

4425-407: The satellite. The coordinates are sent back to the transceiver unit where they can be read using AT commands or a graphical user interface . This can also be used by the gateway to enforce restrictions on geographically bound calling plans. The International Telecommunication Union (ITU) defines a radionavigation-satellite service ( RNSS ) as "a radiodetermination-satellite service used for

4500-524: 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

4575-405: The sky. A geostationary orbit can be achieved only at an altitude very close to 35,786 kilometres (22,236 miles) and directly above the equator. This equates to an orbital speed of 3.07 kilometres per second (1.91 miles per second) and an orbital period of 1,436 minutes, one sidereal day . This ensures that the satellite will match the Earth's rotational period and has a stationary footprint on

SECTION 60

#1732863235740

4650-407: 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

4725-461: 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 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,

4800-628: The systematic and residual errors were narrowed down to accuracy sufficient for navigation. 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 . Modern systems are more direct. The satellite broadcasts

4875-444: The terms used somewhat interchangeably. The first geostationary satellite was designed by Harold Rosen while he was working at Hughes Aircraft in 1959. Inspired by Sputnik 1 , he wanted to use a geostationary satellite to globalise communications. Telecommunications between the US and Europe was then possible between just 136 people at a time, and reliant on high frequency radios and an undersea cable . Conventional wisdom at

4950-485: The thruster fuel and by allowing the satellite to move naturally into an inclined geosynchronous orbit some satellites can remain in use, or else be elevated to a graveyard orbit . This process is becoming increasingly regulated and satellites must have a 90% chance of moving over 200 km above the geostationary belt at end of life. Space debris at geostationary orbits typically has a lower collision speed than at low Earth orbit (LEO) since all GEO satellites orbit in

5025-501: The time was that it would require too much rocket power to place a satellite in a geostationary orbit and it would not survive long enough to justify the expense, so early efforts were put towards constellations of satellites in low or medium Earth orbit. The first of these were the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962. Although these projects had difficulties with signal strength and tracking, issues that could be solved using geostationary orbits,

5100-556: 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 inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about 20,000 kilometres or 12,000 miles). GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows: By their roles in

5175-497: 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. India plans to make the NavIC global by adding 24 more MEO satellites. The Global NavIC will be free to use for

5250-726: 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 . 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). At an estimated cost of €10 billion,

5325-494: Was launched in 1963. Communications satellites are often placed in a geostationary orbit so that Earth-based satellite antennas do not have to rotate to track them but can be pointed permanently at the position in the sky where the satellites are located. Weather satellites are also placed in this orbit for real-time monitoring and data collection, and navigation satellites to provide a known calibration point and enhance GPS accuracy. Geostationary satellites are launched via

5400-626: Was launched in September 2010. An independent satellite navigation system (from GPS) with 7 satellites is planned for 2023. 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 . Currently, it supplements GPS by reporting on the reliability and accuracy of their positioning data and sending out corrections. The system will supplement Galileo in

5475-551: Was marginally worse than the 0.91 m of the QZSS GEO satellites. 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

5550-490: Was struck by a meteoroid on August 11, 1993, and eventually moved to a graveyard orbit , and in 2006 the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable, although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017, both AMC-9 and Telkom-1 broke apart from an unknown cause. A typical geostationary orbit has

5625-668: Was used as part of an in-orbit verification of the ESA European Data Relay System . In 2014, data from the Sentinel-1A satellite in LEO was transmitted via an optical link to the Alphasat in GEO and then relayed to a ground station using a Ka band downlink. The new system can offer speeds up to 7.2 Gbit/s in the future. This article about one or more communications satellites

#739260