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 satellite clusters, which are groups of satellites moving very close together in almost identical orbits (see satellite formation flying), satellite programs (such as Landsat), which are generations of satellites launched in succession, and satellite fleets, which are groups of satellites from the same manufacturer or operator that function independently from each other (not as a system).
Low Earth orbiting satellites (LEOs) are often deployed in satellite constellations, because the coverage area provided by a single LEO satellite only covers a small area that moves as the satellite travels at the high angular velocity needed to maintain its orbit. Many LEO satellites are needed to maintain continuous coverage over an area. This contrasts with geostationary satellites, where a single satellite, moving at the same angular velocity as the rotation of the Earth's surface, provides permanent coverage over a large area.
Examples of satellite constellations include the Global Positioning System (GPS), Galileo and GLONASS constellations for navigation and geodesy, the Iridium and Globalstar satellite telephony services, the Disaster Monitoring Constellation and RapidEye for remote sensing, the Orbcomm messaging service, Russian elliptic orbit Molniya and Tundra constellations, the large-scale Teledesic, Skybridge, and Celestri broadband constellation proposals of the 1990s, and more recent systems such as O3b or the OneWeb proposal.
For applications which benefit from low-latency communications, LEO satellite constellations provide an advantage over a geostationary satellite, where minimum theoretical latency from ground to satellite is about 125 milliseconds, compared to 1–4 milliseconds for a LEO satellite. A LEO satellite constellation can also provide more system capacity by frequency reuse across its coverage, with spot beam frequency use being analogous to the minimum number of satellites needed to provide a service, and their orbits—is a field in itself.
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. His notation is:
where: i is the inclination; t is the total number of satellites; p is the number of equally spaced planes; and f is the relative spacing between satellites in adjacent planes. The change in true anomaly (in degrees) for equivalent satellites in neighbouring planes is equal to f*360/t.
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. 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.
|Name||Operator||Satellites and orbits
(latest design, excluding spares)
|Coverage||Service(s)||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||GSA, ESA||24 in 3 planes at 23,222 km (56° MEO)||Global||Navigation||Operational||2019-present|
|BeiDou||CNSA||3 geostationary at 35,786 km (GEO)
3 in 3 planes at 35,786 km (55° GSO)
24 in 3 planes at 21,150 km (55° MEO)
|NAVIC||ISRO||3 geostationary at 35,786 km (GEO)
4 in 2 planes at 250-24,000 km (29° GSO)
|QZSS||JAXA||1 geostationary at 35,786 km (GEO)
3 in 3 planes at 32,600-39,000 (43° GSO)
|Broadband Global Area Network (BGAN)||Inmarsat||3 geostationary satellites||82°S to 82°N||Internet access|
|Global Xpress (GX)||Inmarsat||Geostationary satellites||Ka-band||Internet access|
|European Aviation Network (EAN)||Inmarsat||1 geostationary satellite||Regional||S-band||Aeronautical Internet access|
|Globalstar||Globalstar||48 at 1400 km, 52° (8 planes)||70°S to 70°N||Internet access, satellite telephony|
|Iridium NEXT||Iridium||66 at 780 km, 86.4° (6 planes)||Global||Internet access, satellite telephony|
|O3b||O3b Networks (SES S.A.)||20 at 8,062 km, 0° (circular equatorial orbit)||45°S to 45°N||Internet access|
|Orbcomm||ORBCOMM||17 at 750 km, 52° (OG2)||65°S to 65°N||"IoT and M2M communication", 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||Internet access, satellite telephony|
Some systems were proposed but never realised:
|Celestri||Motorola||63 satellites at 1400 km, 48° (7 planes)||Ka-band (20/30 Ghz)||Global, low-latency broadband Internet services||Abandoned in May 1998|
|Teledesic||Teledesic||840 satellites at 700 km, 98.2° (21 planes) [1994 design]
288 satellites at 1400 km, 98.2° (12 planes) [1997 design]
|Ka-band (20/30 Ghz)||100 Mbit/s up, 720 Mbit/s down global internet access||Abandoned in October 2002|
Other systems are proposed or currently being developed:
|Boeing||Boeing Satellite||1,396-2,956||N/A||2016||N/A||1,200 km
|broadband||V (40 – 75 GHz)||none |
|LeoSat||Thales Alenia||78-108||1,250 kg
|100 Mbit/s increments||Ka (26.5 – 40 GHz)||optical |
|up to 595 Mbit/s with 32ms latency||Ku (12–18 GHz)
Ka (26.5 – 40 GHz)
|Starlink||SpaceX||4,425-11,943||260 kg||2015||2020||550-1,325 km
|up to 1 Gbit/s with 20ms latency||Ku (12–18 GHz)
Ka (26.5 – 40 GHz)
|1 Gbit/s for a cruise ship
45°S to 45°N
|Ka (26.5 – 40 GHz)||none|
|Telesat LEO||Airbus SSTL
|fiber-optic cable-like||Ka (26.5 – 40 GHz)||optical |
|Project Kuiper||Amazon||3236||2019||590–630 km
|56°S to 56°N|
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