A satellite constellation is a group of artificial satellites working in concert. Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, synchronized so that they overlap well in coverage, the period in which a satellite or other spacecraft is visible above the local horizon.
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.
Broadband applications benefit from low-latency communications, so 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.
A group of formation-flying satellites very close together and moving in almost identical orbits is known as a satellite cluster or Satellite formation flying.
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.
Communications satellite constellations
- Iridium Next: cockpit safety services (not passenger Wi-Fi) on land, at sea and in the skies + Aireon aircraft tracking
- Boeing: Enhanced broadband access availability in the U.S. and globally. Application filed in June 2016. In 2017 Boeing filed an application to transfer the pending application to a company controlled by OneWeb founder Greg Wyler.
- LeoSat: Global, enterprise-grade, high-speed and secure data network, satellites interconnected through laser links for a space optical backbone 1.5 times faster than terrestrial fiber backbones
- OneWeb satellite constellation: low cost mass-produced satellites, for LTE/3G/2G.Wi-Fi rooftop terminals of mobile operators and ISPs, to bridge the digital divide by 2027
- SpaceX Starlink: worldwide for individual, commercial and government institutions; optical intersatellite link; Congress pressed for spectrum sharing
- O3b, bought by SES S.A. in 2016: Covering 45°S to 45°N, synergies with SES geosynchronous-Earth-orbit and MEO satellites, O3b mPower with 4,000 steerable beams each
- Telesat LEO: Universal connectivity for business, government and individual users in areas of concentrated demand: busy airports; military operations on land, sea and air; major shipping ports; large, remote communities; optical intersatellite links, lower Mbit/s cost than others including those in development
|Constellation||Number||Manufacturer||Weight||Unveil.||Avail.||Orbit||User speed||Band||Inter-satellite links||Status|
+ Orbital ATK
|1.4 Mbit/s||L (1 – 2 GHz)
Ka (26.5 – 40 GHz)
|K 23 GHz ||Complete|
|Boeing||1,396-2,956||Boeing Satellite||N/A||2016||N/A||1,200 km
|broadband||V (40 – 75 GHz)||none ||transferring the application to OneWeb|
|LeoSat||78-108||Thales Alenia||1,250 kg
|in increments of 100 Mbit/s||Ka (26.5 – 40 GHz)||optical ||first launches in 2021|
|up to 595 Mbit/s||Ku (12–18 GHz)
Ka (26.5 – 40 GHz)
|none ||6 pilot satellites in February 2019|
|SpaceX Starlink||4,425-11,943||SpaceX||227 kg||2015||2020||550-1,325 km
|up to 1 Gbit/s||Ku (12–18 GHz)
Ka (26.5 – 40 GHz)
|optical||First batch of 60 satellites launched in May 2019|
|O3b, bought by SES S.A. in 2016||20 O3b
|Thales Alenia (O3b)
|700 kg: O3b
|1 Gbit/s for a cruise ship||Ka (26.5 – 40 GHz)||none||O3b complete|
|Telesat LEO||117-512||Airbus SSTL
|fiber-optic cable-like||Ka (26.5 – 40 GHz)||optical ||two prototypes: 2018 launch|
|CASIC Hongyun||156||2017||2022||160–2,000 km
|prototype launched in December 2018|
|CASC Hongyan||320||2017||2023||1,100 km
|prototype launched in December 2018|
- first two prototypes
Example satellite constellations
- A-train (satellite constellation)
- Compass navigation system
- Disaster Monitoring Constellation
- Galileo (satellite navigation)
- Iridium NEXT
- Planet Labs
- Pléiades 1A and 1B
- Sirius Satellite Radio
- SPOT 6 and SPOT 7
- XM Satellite Radio
- "On the increasing number of satellite constellations". www.eso.org. Retrieved 10 June 2019.
- Khan, Farooq (9 August 2015). "Mobile Internet from the Heavens". arXiv:1508.02383 [cs.NI].
- 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.
- Thierry Dubois (Dec 19, 2017). "Eight Satellite Constellations Promising Internet Service From Space". Aviation Week & Space Technology.
- "Boeing wants to help OneWeb satellite plans". Advanced Television. 2017-12-17. Retrieved 2018-10-21.
- Muri, Paul; McNair, Janise (1 April 2012). "A Survey of Communication Sub-systems for Intersatellite Linked Systems and CubeSat Missions". Journal of Communications. 7 (4). doi:10.4304/jcm.7.4.290-308.
- 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.
- "LeoSat gains Hispasat as second investor, drops demo satellite plans". SpaceNews. 2018-07-10. Retrieved 2018-10-21.
- "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.
- Barbosa, Rui C. (21 December 2018). "Chinese Long March 11 launches with the first Hongyun satellite". NASASpaceFlight.com. Retrieved 24 December 2018.
- Jones, Andrew (13 November 2018). "China to launch first Hongyan LEO communications constellation satellite soon". GBTimes. Retrieved 20 December 2018.
- Barbosa, Rui (29 December 2018). "Long March 2D concludes 2018 campaign with Hongyan-1 launch". NASASpaceFlight.com. Retrieved 29 December 2018.
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Satellite constellation simulation tools:
- AVM Dynamics Satellite Constellation Modeler
- SaVi Satellite Constellation Visualization
- Transfinite Visualyse Professional