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Low Earth orbiting satellites (LEOs) are often deployed in satellite constellations, because the coverage area provided by a single LEO satellite covers a small area, and the satellite travels at a high angular velocity to maintain its orbit. Multiple LEO satellites are needed to maintain consistent coverage. This contrasts with geostationary satellites, where a single satellite, moving with the Earth, provides permanent coverage over a large area.
Examples of satellite constellations include the Global Positioning System (GPS) and GLONASS constellations for navigation and geodesy, the Iridium and Globalstar satellite telephony services, the Orbcomm messaging service, Russian elliptical-orbit Molnya and Tundra constellations, and the Teledesic and Skybridge broadband constellation proposals. Broadband applications benefit from low-latency communications, so LEO satellite constellations provide an advantage over a geostationary satellite, where it typically takes 125 milliseconds for a signal to get from the ground to geostationary orbit. A LEO satellite constellation can also provide more capacity by frequency reuse across its coverage.
Satellite constellation coverage and geometry – determining the minimum number of satellites needed to provide a service, and their orbits – is a field in itself.
A group of satellites very close together and moving in almost identical orbits is known as a satellite cluster.