Satellite constellation
Encyclopedia
A group of artificial satellites working in concert is known as a satellite constellation. Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, synchronised so that they overlap well in coverage and complement rather than interfere with other satellites' important coverage.

Overview

Low Earth orbiting satellites
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

 (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
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...

 needed to maintain its orbit
Planetary orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

. 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
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 (GPS), Galileo
Galileo positioning system
Galileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...

 and GLONASS
GLONASS
GLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...

 constellations for navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

 and geodesy
Geodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...

, the Iridium
Iridium (satellite)
Iridium Communications Inc. is a company, based in McLean, VA, United States which operates the Iridium satellite constellation, a system of 66 active satellites used for worldwide voice and data communication from hand-held satellite phones and other transceiver units...

 and Globalstar
Globalstar
Globalstar is a low Earth orbit satellite constellation for satellite phone and low-speed data communications, somewhat similar to the Iridium satellite constellation and Orbcomm satellite systems.-History:...

 satellite telephony services, the Disaster Monitoring Constellation
Disaster Monitoring Constellation
The Disaster Monitoring Constellation consists of a number of remote sensing satellites constructed by Surrey Satellite Technology Ltd and operated for the Algerian, Nigerian, Turkish, British and Chinese governments by DMC International Imaging...

 and RapidEye
RapidEye
RapidEye AG is a German geospatial information provider focused on assisting in management decision-making through services based on their own Earth observation imagery. The company owns a five satellite constellation producing 5 meter resolution imagery that was designed and implemented by ...

 for remote sensing, the Orbcomm
Orbcomm
ORBCOMM is a company that offers M2M global asset monitoring and messaging services from its constellation of 29 LEO communications satellites orbiting at 775 km. Like its voice-centric competitors Iridium and Globalstar, it filed for Chapter 11 protection, in September, 2000. ORBCOMM issued...

 messaging service, Russian elliptic orbit
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...

 Molniya
Molniya (satellite)
Molniya was a military communications satellite system used by the Soviet Union. The satellites were placed into highly eccentric elliptical orbits known as Molniya orbits, characterised by an inclination of +63.4 degrees and a period of around 12 hours...

 and Tundra constellations, the large-scale Teledesic
Teledesic
Teledesic was a company founded in the 1990s to build a commercial broadband satellite constellation for Internet services. Using low-earth orbiting satellites small antennas could be used to provide uplinks of as much as 100 Mbit/second and downlinks of up to 720 Mbit/second...

 and Skybridge broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...

 constellation proposals of the 1990s, and the proposed LEO global backhaul constellation named COMMStellation™
COMMStellation
COMMStellation is a network of 78 microsatellites with an additional six microsatellites to be used as spares. The constellation will be situated in a low Earth orbit and the satellites will travel on 6 orbital planes separated by 30 degrees of longitude. Each plane will contain 14 satellites which...

.

Broadband applications
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...

 benefit from low-latency communications
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

, so LEO satellite constellations provide an advantage over a geostationary satellite, where minimum theoretical latency 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
Spot beam
A spot beam, in telecommunications parlance, is a satellite signal that is specially concentrated in power A spot beam, in telecommunications parlance, is a satellite signal that is specially concentrated in power A spot beam, in telecommunications parlance, is a satellite signal that is specially...

 frequency use being analogous to the frequency reuse of cellular radio towers.

Satellite constellation coverage and geometry
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....

—determining 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
Satellite formation flying
Satellite formation flying is the concept that multiple satellites can work together in a group to accomplish the objective of one larger, usually more expensive, satellite...

.

Walker Constellation

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
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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

. 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
Iridium (satellite)
Iridium Communications Inc. is a company, based in McLean, VA, United States which operates the Iridium satellite constellation, a system of 66 active satellites used for worldwide voice and data communication from hand-held satellite phones and other transceiver units...

. 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.

In use

  • A-train (satellite constellation)
    A-train (satellite constellation)
    The A-train is a satellite constellation scheduled to have seven Franco-American Earth observation satellites in sun-synchronous orbit at an altitude of 690 kilometers above the Earth....

  • BGAN
    BGAN
    The Broadband Global Area Network is a global Satellite Internet Network with telephony using portable terminals. The terminals are normally used to connect a laptop computer to broadband Internet in remote locations, although as long as line-of-sight to the satellite exists, the terminal can be...

  • Compass navigation system
    Compass navigation system
    The COMPASS system is a project by China to develop an independent global satellite navigation system.COMPASS is not an extension to the previously deployed Beidou-1, but a new GNSS similar in principle to GPS, GLONASS, and Galileo.-General:The new system will be a constellation of 35 satellites,...

  • Disaster Monitoring Constellation
    Disaster Monitoring Constellation
    The Disaster Monitoring Constellation consists of a number of remote sensing satellites constructed by Surrey Satellite Technology Ltd and operated for the Algerian, Nigerian, Turkish, British and Chinese governments by DMC International Imaging...

  • Globalstar
    Globalstar
    Globalstar is a low Earth orbit satellite constellation for satellite phone and low-speed data communications, somewhat similar to the Iridium satellite constellation and Orbcomm satellite systems.-History:...

  • GLONASS
    GLONASS
    GLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...

  • Global Positioning System
    Global Positioning System
    The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

  • Iridium satellite constellation
    Iridium satellite constellation
    The Iridium satellite constellation is a large group of satellites providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface. Iridium Communications Inc...

  • Molniya
    Molniya (satellite)
    Molniya was a military communications satellite system used by the Soviet Union. The satellites were placed into highly eccentric elliptical orbits known as Molniya orbits, characterised by an inclination of +63.4 degrees and a period of around 12 hours...

  • Orbcomm
    Orbcomm
    ORBCOMM is a company that offers M2M global asset monitoring and messaging services from its constellation of 29 LEO communications satellites orbiting at 775 km. Like its voice-centric competitors Iridium and Globalstar, it filed for Chapter 11 protection, in September, 2000. ORBCOMM issued...

  • RapidEye
    RapidEye
    RapidEye AG is a German geospatial information provider focused on assisting in management decision-making through services based on their own Earth observation imagery. The company owns a five satellite constellation producing 5 meter resolution imagery that was designed and implemented by ...

  • Sirius Satellite Radio
    Sirius Satellite Radio
    Sirius Satellite Radio is a satellite radio service operating in North America, owned by Sirius XM Radio.Headquartered in New York City, with smaller studios in Los Angeles and Memphis, Sirius was officially launched on July 1, 2002 and currently provides 69 streams of music and 65 streams of...

  • TDRSS
  • XM Satellite Radio
    XM Satellite Radio
    XM Satellite Radio is one of two satellite radio services in the United States and Canada, operated by Sirius XM Radio. It provides pay-for-service radio, analogous to cable television. Its service includes 73 different music channels, 39 news, sports, talk and entertainment channels, 21 regional...


External links

Satellite constellation simulation tools:

More information:
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