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:
- i: t/p/f
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.
List of satellite constellations
|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)
Communications satellite constellations
- Dish Network
- Sirius Satellite Radio
- XM Satellite Radio
- Molniya (discontinued)
|Broadband Global Area Network (BGAN)||Inmarsat||3 geostationary satellites||82°S to 82°N||Internet access|
|Global Xpress (GX||Inmarsat||Ka-band geostationary satellites||Internet access|
|European Aviation Network (EAN)||Inmarsat||One S-band geostationary satellite||Aeronautical Internet access|
|Globalstar||Globalstar||48 (8x6) at 1400 km, 52°||70°S to 70°N||Internet access, satellite telephony|
|Iridium NEXT||Iridium||66 (6x11) at 780 km, 86.4°||Global||Internet access, satellite telephony|
|O3b||O3b Networks (part of SES S.A.)||20 in circular equatorial orbit at 8,062 km||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 by Motorola: 63 (7 x 9) satellites at 1400 km, 48°, for Global, low-latency broadband Internet services
- Teledesic constellation: 840 satellites at 700 km  or 288 (12 x 24) satellites at 1400 km, 98.2°  for 100 Mbit/s up, 720 Mbit/s down global internet access
|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||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||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|
- first two prototypes
- Boeing Satellite is transferring the application to OneWeb
- LeoSat shut down completely in 2019 
- The OneWeb constellation had 6 pilot satellites in February 2019, 74 satellites launched as of 21 March 2020 but filed for bankruptcy on 27 March 2020
- Starlink: 540 [a] satellites launched as of 13 June 2020[update]; first mission (Starlink 0) launched on 24 May 2019; 15 satellites were retired and 5 of those retired satellites were purposely and successfully deorbited by SpaceX as of 4 June 2020[update] ; additionally 9 satellites had become non-operational  as of 4 June 2020[update]
- O3b mPower: under development
- Telesat LEO: two prototypes: 2018 launch
- CASIC Hongyun: prototype launched in December 2018
- CASC Hongyan prototype launched in December 2018, might be merged with Hongyun
- Project Kuiper: FCC filing in July 2019
Earth observation satellite constellations
- RADARSAT Constellation
- Planet Labs
- Pléiades 1A and 1B
- Disaster Monitoring Constellation
- SPOT 6 and SPOT 7
- first two prototypes
- "On the increasing number of satellite constellations". www.eso.org. Retrieved 10 June 2019.
- J. G. Walker, Satellite constellations, Journal of the British Interplanetary Society, vol. 37, pp. 559-571, 1984
- A. H. Ballard, Rosette Constellations of Earth Satellites, IEEE Transactions on Aerospace and Electronic Systems, Vol 16 No. 5, Sep. 1980.
- J. G. Walker, Comments on "Rosette constellations of earth satellites", IEEE Transactions on Aerospace and Electronic Systems, vol. 18 no. 4, pp. 723-724, November 1982.
- Khan, Farooq (9 August 2015). "Mobile Internet from the Heavens". arXiv:1508.02383 [cs.NI].
- "Globalstar satellites". www.n2yo.com. Retrieved 2019-11-22.
- Thierry Dubois (Dec 19, 2017). "Eight Satellite Constellations Promising Internet Service From Space". Aviation Week & Space Technology.
- The Boeing Company (June 22, 2016). "SAT-LOA-20160622-00058". FCC Space Station Applications. Retrieved February 23, 2018.
- The Boeing Company (June 22, 2016). "SAT-LOA-20161115-00109". FCC Space Station Applications. Retrieved February 23, 2018.
- LeoSat Enterprises. "A NEW TYPE OF SATELLITE CONSTELLATION". Retrieved February 23, 2018.
- "OneWeb asks FCC to authorize 1,200 more satellites". SpaceNews. 2018-03-20. Retrieved 2018-03-23.
- "OneWeb hardware finally coming together". SpaceNews. 3 October 2017. Retrieved 21 October 2018.
- WorldVu Satellites Limited (April 28, 2016). "ONEWEB NON-GEOSTATIONARY SATELLITE SYSTEM - ATTACHMENT A". FCC Space Station Applications. Retrieved February 23, 2018.
- WorldVu Satellites Limited (April 28, 2016). "SAT-LOI-20160428-00041". FCC Space Station Applications. Retrieved February 23, 2018.
- "Musk shakes up SpaceX in race to make satellite launch window: sources". Reuters. 30 October 2018. Retrieved 10 January 2019.
- "SpaceX Set to Launch 2 Starlink Satellites to Test Gigabit Broadband". ISPreview. 14 February 2018. Retrieved 10 January 2019.
- "This is how Elon Musk plans to use SpaceX to give internet to everyone". CNET. 21 February 2018.
- "Telesat says ideal LEO constellation is 292 satellites, but could be 512". SpaceNews. 11 September 2018. Retrieved 10 January 2019.
- Telesat Canada (August 24, 2017). "Telesat Technical Narrative". FCC Space Station Applications. Retrieved February 23, 2018.
- Telesat Canada (August 24, 2017). "SAT-PDR-20170301-00023". FCC Space Station Applications. Retrieved February 23, 2018.
- Zhao, Lei (5 March 2018). "Satellite will test plan for communications network". China Daily. Retrieved 20 December 2018.
- Jones, Andrew (13 November 2018). "China to launch first Hongyan LEO communications constellation satellite soon". GBTimes. Retrieved 20 December 2018.
- @EL2squirrel (12 December 2019). "Chinese version of OneWeb: The Hongyan system consists of 864 satellites, with 8Tbps of bandwidth, Orbital altitude 1175km" (Tweet). Retrieved 16 December 2019 – via Twitter.
- Porter, Jon (2019-04-04). "Amazon will launch thousands of satellites to provide internet around the world". The Verge. Retrieved 2019-11-17.
- "Boeing wants to help OneWeb satellite plans". Advanced Television. 2017-12-17. Retrieved 2018-10-21.
- "LeoSat, absent investors, shuts down". Cite magazine requires
- "OneWeb increases mega-constellation to 74 satellites". 2020-03-21. Retrieved 2020-04-07.
- "Coronavirus: OneWeb blames pandemic for collapse". 2020-03-30. Retrieved 2020-04-07.
- "Voluntary Petition for Non-Individuals Filing for Bankruptcy" (PDF). Omni Agent Solutions. 2020-03-27. Retrieved 2020-04-07.
- Contact lost with three Starlink satellites, other 57 healthy, SpaceNews, 1 July 2019, accessed 1 July 2019.
- Barbosa, Rui C. (21 December 2018). "Chinese Long March 11 launches with the first Hongyun satellite". NASASpaceFlight.com. Retrieved 24 December 2018.
- Barbosa, Rui (29 December 2018). "Long March 2D concludes 2018 campaign with Hongyan-1 launch". NASASpaceFlight.com. Retrieved 29 December 2018.
- @Cosmic_Penguin (14 December 2019). "Notice that these satellites from CASC are mentioned as part of a "national satellite Internet system". There are rumors that several of the planned Chinese private LEO comsat constellations have been recently absorbed into one big nationalized one" (Tweet). Retrieved 16 December 2019 – via Twitter.
|Wikimedia Commons has media related to Satellite constellations.|
Satellite constellation simulation tools:
- AVM Dynamics Satellite Constellation Modeler
- SaVi Satellite Constellation Visualization
- Transfinite Visualyse Professional