Satellite constellation

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); or satellite programs (such as Landsat), which are generations of satellites launched in succession; satellite fleets, which are groups of satellites from the same manufacturer or operator that function independently from each other (not as a system). Satellites in medium Earth orbit (MEO) and low Earth orbit (LEO) are often deployed in satellite constellations, because the coverage area provided by a single satellite only covers a small area that moves as the satellite travels at the high angular velocity needed to maintain its orbit. Many MEO or LEO satellites are needed to maintain continuous coverage over an area. This contrasts with geostationary satellites, where a single satellite, at a much higher altitude and moving at the same angular velocity as the rotation of the Earth's surface, provides permanent coverage over a large area. For some applications, in particular digital connectivity, the lower altitude of MEO and LEO satellite constellations provide advantages over a geostationary satellite, with lower path losses (reducing power requirements and costs) and latency. The propagation delay for a round-trip internet protocol transmission via a geostationary satellite can be over 600 ms, but as low as 125 ms for a MEO satellite or 30 ms for a LEO system.