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 and Skybridge 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:
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°:27/3/1 constellation. This means there are 27 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
Eight telecommunications satellite constellations are in-development in and :
- 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
- 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 residents, commerces, institutions and governments, optical intersatellite link, Congress pressed for spectrum sharing
- Samsung: worldwide affordable internet services via low-cost micro-satellites
- 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
+ Orbital ATK
|1.4 Mbit/s||L (1 – 2 GHz)
Ka (26.5 – 40 GHz)
|30 (mid-Nov 2017)|
|Boeing Satellite||N/A||2016||license +6 years||1,200 km
|broadband||V (40 – 75 GHz)|
|LeoSat||78-108||Thales Alenia||1,250 kg
|high-throughput||Ka (26.5 – 40 GHz)||2019 first launch|
|10 Tbit/s total||Ku (12–18 GHz) |
Ka (26.5 – 40 GHz)
|broadband||Ku (12–18 GHz)
Ka (26.5 – 40 GHz)
|end 2017/early 2018 prototypes launch|
|200-GB monthly for 5 billion users||V (40 – 75 GHz)|
|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)||20 O3b after the 2018-19 launches|
|Telesat LEO||117+||Airbus SSTL
|fiber-optic cable-like||Ka (26.5 – 40 GHz)||two prototypes: 2018 launch|
Example satellite constellations
- ^ 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.
Satellite constellation simulation tools:
- AVM Dynamics Satellite Constellation Modeler
- SaVi Satellite Constellation Visualization
- Transfinite Visualyse Professional