A satellite constellation is a group of artificial satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global or near-global coverage, such that at any time everywhere on Earth at least one satellite is visible. Satellites are typically placed in sets of complementary orbital planes and connect to globally distributed ground stations. They may also use inter-satellite communication.
Satellite constellations should not be confused with:
Satellites in medium Earth orbit (MEO) and low Earth orbit (LEO) are often deployed in satellite constellations, because the coverage area provided by a single satellite only covers a small area that moves as the satellite travels at the high angular velocity needed to maintain its orbit. Many MEO or LEO satellites are needed to maintain continuous coverage over an area. This contrasts with geostationary satellites, where a single satellite, at a much higher altitude and moving at the same angular velocity as the rotation of the Earth's surface, provides permanent coverage over a large area.
For some applications, in particular digital connectivity, the lower altitude of MEO and LEO satellite constellations provide advantages over a geostationary satellite, with lower path losses (reducing power requirements and costs) and latency. [2] The propagation delay for a round-trip internet protocol transmission via a geostationary satellite can be over 600 ms, but as low as 125 ms for a MEO satellite or 30 ms for a LEO system. [3]
Examples of satellite constellations include the Global Positioning System (GPS), Galileo and GLONASS constellations for navigation and geodesy in MEO, the Iridium and Globalstar satellite telephony services and Orbcomm messaging service in LEO, the Disaster Monitoring Constellation and RapidEye for remote sensing in Sun-synchronous LEO, Russian Molniya and Tundra communications constellations in highly elliptic orbit, and satellite broadband constellations, under construction from Starlink and OneWeb in LEO, and operational from O3b in MEO.
There are a large number of constellations that may satisfy a particular mission. Usually constellations are designed so that the satellites have similar orbits, eccentricity and inclination so that any perturbations affect each satellite in approximately the same way. In this way, the geometry can be preserved without excessive station-keeping thereby reducing the fuel usage and hence increasing the life of the satellites. Another consideration is that the phasing of each satellite in an orbital plane maintains sufficient separation to avoid collisions or interference at orbit plane intersections. Circular orbits are popular, because then the satellite is at a constant altitude requiring a constant strength signal to communicate.
A class of circular orbit geometries that has become popular is the Walker Delta Pattern constellation. This has an associated notation to describe it which was proposed by John Walker. [4] His notation is:
where:
For example, the Galileo navigation system is a Walker Delta 56°: 24/3/1 constellation. This means there are 24 satellites in 3 planes inclined at 56 degrees, spanning the 360 degrees around the equator. The "1" defines the phasing between the planes, and how they are spaced. The Walker Delta is also known as the Ballard rosette, after A. H. Ballard's similar earlier work. [5] [6] Ballard's notation is (t,p,m) where m is a multiple of the fractional offset between planes.
Another popular constellation type is the near-polar Walker Star, which is used by Iridium. Here, the satellites are in near-polar circular orbits across approximately 180 degrees, travelling north on one side of the Earth, and south on the other. The active satellites in the full Iridium constellation form a Walker Star of 86.4°: 66/6/2, i.e. the phasing repeats every two planes. Walker uses similar notation for stars and deltas, which can be confusing.
These sets of circular orbits at constant altitude are sometimes referred to as orbital shells.
In spaceflight, an orbital shell is a set of artificial satellites in circular orbits at a certain fixed altitude. [7] In the design of satellite constellations, an orbital shell usually refers to a collection of circular orbits with the same altitude and, oftentimes, orbital inclination, distributed evenly in celestial longitude (and mean anomaly).[ citation needed ] For a sufficiently high inclination and altitude the orbital shell covers the entire orbited body. In other cases the coverage extends up to a certain maximum latitude.[ citation needed ]
Several existing satellite constellations typically use a single orbital shell. New large megaconstellations have been proposed that consist of multiple orbital shells. [7] [8]
| Name | Operator | Satellites and orbits (latest design, excluding spares) | Coverage | Services | Status | Years in service |
|---|---|---|---|---|---|---|
| Global Positioning System (GPS) | USSF | 24 in 6 planes at 20,180 km (55° MEO) | Global | Navigation | Operational | 1993–present |
| GLONASS | Roscosmos | 24 in 3 planes at 19,130 km (64°8' MEO) | Global | Navigation | Operational | 1995–present |
| Galileo | EUSPA, ESA | 24 in 3 planes at 23,222 km (56° MEO) | Global | Navigation | Operational | 2019–present |
| BeiDou | CNSA | Global | Navigation | Operational |
| |
| NAVIC | ISRO |
| Regional | Navigation | Operational | 2018–present |
| QZSS | JAXA |
| Regional | Navigation | Operational | 2018–present |
| Name | Operator | Constellation design | Coverage | Freq. | Services |
|---|---|---|---|---|---|
| Broadband Global Area Network (BGAN) | Inmarsat | 3 geostationary satellites | 82°S to 82°N | Internet access | |
| Global Xpress (GX) | Inmarsat | 5 Geostationary satellites [9] | Ka band | Internet access | |
| Globalstar | Globalstar | 48 at 1400 km, 52° (8 planes) [10] | 70°S to 70°N [10] | Internet access, satellite telephony | |
| Iridium | Iridium Communications | 66 at 780 km, 86.4° (6 planes) | Global |
| Internet access, satellite telephony |
| O3b | SES S.A. | 20 at 8,062 km, 0° (circular equatorial orbit) | 45°S to 45°N | Ka band | Internet access |
| Orbcomm | ORBCOMM | 17 at 750 km, 52° (OG2) | 65°S to 65°N | IoT and M2M, AIS | |
| Defense Satellite Communications System (DSCS) | 4th Space Operations Squadron | Military communications | |||
| Wideband Global SATCOM (WGS) | 4th Space Operations Squadron | 10 geostationary satellites | Military communications | ||
| ViaSat | Viasat, Inc. | 4 geostationary satellites | Varying | Internet access | |
| Eutelsat | Eutelsat | 20 geostationary satellites | Commercial | ||
| Thuraya | Thuraya | 2 geostationary satellites | EMEA and Asia | L band | Internet access, satellite telephony |
| Starlink | SpaceX | LEO in several orbital shells
|
| Internet access [11] [12] [13] | |
| OneWeb constellation | Eutelsat (completed merger in Sep 2023) | 882–1980 [14] (planned) Total number of operational satellites: 634 as of 20 May 2023 | Global | Internet access |
Other Internet access systems are proposed or currently being developed:
| Constellation | Manufacturer | Number | Weight | Unveil. | Avail. | Altitude | Offer | Band | Inter-sat. links |
|---|---|---|---|---|---|---|---|---|---|
| IRIS² | European Space Agency | TBD | TBD | ||||||
| O3b mPOWER, (SES S.A.) | Boeing | 13 | 1700 kg | 2017 | Q2 2024 [16] |
| Ka (26.5–40 GHz) | None | |
| Telesat LEO | 117–512 [19] | — | 2016 | 2027 | 1,000–1,248 km 621–775 mi | Fiber-optic cable-like | Ka (26.5–40 GHz) | Optical [20] [21] | |
| Hongyun [22] | CASIC | 156 | 2017 | 2022 | 160–2,000 km 99–1,243 mi | ||||
| Hongyan [23] | CASC | 320-864 [24] | 2017 | 2023 | 1,100–1,175 km 684–730 mi | ||||
| Hanwha Systems [25] | 2000 | 2022 | 2025 | ||||||
| Project Kuiper | Amazon | 3236 | 2019 | 2024 | 590–630 km 370–390 mi | 56°S to 56°N [26] |
Some systems were proposed but never realized:
| Name | Operator | Constellation design | Freq. | Services | Abandoned date |
|---|---|---|---|---|---|
| Celestri | Motorola | 63 satellites at 1400 km, 48° (7 planes) | Ka band (20/30 GHz) | Global, low-latency broadband Internet services | 1998 May |
| Teledesic | Teledesic |
| Ka band (20/30 GHz) | 100 Mbit/s up, 720 Mbit/s down global internet access | 2002 October |
| LeoSat | Thales Alenia | 78–108 satellites at 1400 km | Ka (26.5–40 GHz) | High-speed broadband internet | 2019 |
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. Many communications satellites are in geostationary orbit 22,300 miles (35,900 km) above the equator, so that the satellite appears stationary at the same point in the sky; therefore the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite. Others form satellite constellations in low Earth orbit, where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never more than about one-third of the radius of Earth.
The Ka band is a portion of the microwave part of the electromagnetic spectrum defined as frequencies in the range 26.5–40 gigahertz (GHz), i.e. wavelengths from slightly over one centimeter down to 7.5 millimeters. The band is called Ka, short for "K-above" because it is the upper part of the original NATO K band, which was split into three bands because of the presence of the atmospheric water vapor resonance peak at 22.24 GHz (1.35 cm), which made the center unusable for long range transmission. The 30/20 GHz band is used in communications satellite uplinks in either the 27.5 GHz or 31 GHz bands, and in high-resolution, close-range targeting radars aboard military airplanes. Some frequencies in this radio band are used for vehicle speed detection by law enforcement. The Kepler Mission used this frequency range to downlink the scientific data collected by the space telescope.
A satellite telephone, satellite phone or satphone is a type of mobile phone that connects to other phones or the telephone network by radio link through satellites orbiting the Earth instead of terrestrial cell sites, as cellphones do. Therefore, they can work in most geographic locations on the Earth's surface, as long as open sky and the line-of-sight between the phone and the satellite are provided. Depending on the architecture of a particular system, coverage may include the entire Earth or only specific regions. Satellite phones provide similar functionality to terrestrial mobile telephones; voice calling, text messaging, and low-bandwidth Internet access are supported through most systems. The advantage of a satellite phone is that it can be used in such regions where local terrestrial communication infrastructures, such as landline and cellular networks, are not available.
Satellite Internet access is Internet access provided through communication satellites; if it can sustain high speeds, it is termed satellite broadband. Modern consumer grade satellite Internet service is typically provided to individual users through geostationary satellites that can offer relatively high data speeds, with newer satellites using Ku band to achieve downstream data speeds up to 506 Mbit/s. In addition, new satellite internet constellations are being developed in low-earth orbit to enable low-latency internet access from space.
Satellite flare, also known as satellite glint, is a satellite pass visible to the naked eye as a brief, bright "flare". It is caused by the reflection toward the Earth below of sunlight incident on satellite surfaces such as solar panels and antennas. Streaks from satellite flare are a form of light pollution that can negatively affect ground-based astronomy, stargazing, and indigenous people.
Telesat, formerly Telesat Canada, is a Canadian satellite communications company founded on May 2, 1969. The company is headquartered in Ottawa.
SES S.A. is a Luxembourgish satellite telecommunications network provider supplying video and data connectivity worldwide to broadcasters, content and internet service providers, mobile and fixed network operators, governments and institutions.
A medium Earth orbit (MEO) is an Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between 2,000 and 35,786 km above sea level.
Spacecraft collision avoidance is the implementation and study of processes minimizing the chance of orbiting spacecraft inadvertently colliding with other orbiting objects. The most common subject of spacecraft collision avoidance research and development is for human-made satellites in geocentric orbits. The subject includes procedures designed to prevent the accumulation of space debris in orbit, analytical methods for predicting likely collisions, and avoidance procedures to maneuver offending spacecraft away from danger.
O3b Networks Ltd. was a network communications service provider building and operating a medium Earth orbit (MEO) satellite constellation primarily intended to provide voice and data communications to mobile operators and Internet service providers. O3b Networks became a wholly owned subsidiary of SES S.A. in 2016 and the operator name was subsequently dropped in favour of SES Networks, a division of SES. The satellites themselves, now part of the SES fleet, continue to use the O3b name.
The Iridium satellite constellation provides L band voice and data information coverage to satellite phones, satellite messenger communication devices and integrated transceivers. Iridium Communications owns and operates the constellation, additionally selling equipment and access to its services. It was conceived by Bary Bertiger, Raymond J. Leopold and Ken Peterson in late 1987 and then developed by Motorola on a fixed-price contract from July 29, 1993, to November 1, 1998, when the system became operational and commercially available.
O3b is a satellite constellation in Medium Earth orbit (MEO) owned and operated by SES, and designed to provide low-latency broadband connectivity to remote locations for mobile network operators and internet service providers, maritime, aviation, and government and defence. It is often referred to as O3b MEO to distinguish these satellites from SES's forthcoming O3b mPOWER constellation.
High-throughput satellite (HTS) is a communications satellite that provides more throughput than a classic FSS satellite for the same amount of allocated orbital spectrum, thus significantly reducing cost-per-bit. ViaSat-1 and EchoStar XVII do provide more than 100 Gbit/s of capacity, which is more than 100 times the capacity offered by a conventional FSS satellite. When it was launched in October 2011 ViaSat-1 had more capacity than all other commercial communications satellites over North America combined.
Starlink is a satellite internet constellation operated by American aerospace company SpaceX, providing coverage to over 70 countries. It also aims for global mobile phone service after 2023.
Eutelsat OneWeb is a subsidiary of Eutelsat Group providing broadband satellite Internet services in low Earth orbit (LEO). The company is headquartered in London, and has offices in Virginia, US and a satellite manufacturing facility in Florida – Airbus OneWeb Satellites – that is a joint venture with Airbus Defence and Space.
A satellite internet constellation is a constellation of artificial satellites providing satellite internet service. In particular, the term has come to refer to a new generation of very large constellations orbiting in low Earth orbit (LEO) to provide low-latency, high bandwidth (broadband) internet service.
Soyuz flight VS22 was a rocket launch conducted by multinational launch service provider Arianespace. It was the sixteenth launch of a Soyuz-ST-B launch vehicle, and the 22nd launch of a Soyuz-2 series launch vehicle from the Ensemble de Lancement Soyouz at the Guiana Space Centre. After two scheduling delays and a 33-minute logistical delay, the rocket lifted off on 4 April 2019, and successfully delivered to medium Earth orbit the final four satellites in the O3b broadband satellite constellation, which services Latin America, Africa, and Oceania. After four previous Soyuz flights delivered the constellation's first sixteen satellites, the launch increased the constellation's throughput by 26 per cent. The flight marked the second occasion in which two Soyuz-2 launch vehicles were launched on the same day, occurring hours after the launch of Progress MS-11 from the Baikonur Cosmodrome.
The Celestri Multimedia LEO System was a planned Low Earth orbit (LEO) satellite constellation, which was intended to offer global, low-latency broadband Internet services via Ka-band radio links. It was planned by Motorola circa 1997-1998 as one of the earliest "Internet in the sky" constellations, and as a successor to the company's Iridium satellite constellation, but never built or launched.
O3b mPOWER is a communications satellite system currently under construction and deployment. The first two satellites were launched on 16 December 2022 and commercial service is expected to begin "early Q2 2024". Owned and operated by SES, O3b mPOWER initially comprises 6 high-throughput and low-latency satellites in a medium Earth orbit (MEO), along with ground infrastructure and intelligent software, to provide multiple terabits of global broadband connectivity for applications including cellular backhaul to remote rural locations and simultaneous international IP trunking.
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