A set of artificial satellites working together on the same mission. (Walker 1984)
A satellite constellation is a number of similar satellites, of a similar type and function, designed to be in similar, complementary, orbits for a shared purpose, under shared control. Constellations are used for navigation and geodesy (e.g. GPS, Galileo and GLONASS), satellite telephony (e.g. Iridium), or Earth Observation (e.g. DMC, PlanetLabs). More recently, companies are planning large scale constellations in low- and mid-Earth orbits to provide global satellite internet, or Internet of Things to connect machines and systems together directly. A constellation with thousands of individual units at low altitude reduces the signal latency (i.e. the time taken to signals to move from a ground station providing internet to the satellite and then on to a user), while maintaining high levels of coverage especially in remote areas without developed ground infrastructure. While there are currently around 2000 active satellites orbiting Earth, the planned satellite constellations could increase this number by tens of thousands in the coming decades (IAU, 2020).
Walker, J. G. 1984. "Satellite Constellations". Adsabs.Harvard.Edu. http://adsabs.harvard.edu/abs/1984JBIS...37..559W.
International Astronomical Union. 2020. Satellite Constellations. https://www.iau.org/public/themes/satellite-constellations/