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A global navigation satellite system (GNSS) is a type of satellite navigation that provides global coverage. A GNSS is defined by a constellation of orbiting satellites working together with a network of ground control stations and receivers that calculate ground positions through an adapted version of trilateration.
To date, there are only two operational GNSSs, the United States’ NAVSTAR Global Positioning System (GPS) and the Russian Federation’s Global Navigation Satellite System (GLONASS). However there are two other satellites undergoing development, the European Union’s Galileo and China’s Compass or BeiDou-2.
A global navigation satellite system is a constellation of satellites that provide geo-spatial positioning to many devices autonomously, allowing electronic devices with the appropriate receivers to determine their precise location on the surface of the Earth.
The initial motivation for a satellite system was for military applications, but it has now progressed to more extensive civil applications, including the following:
Generally the global coverage can be achieved through a satellite constellation of 20 to 30 medium Earth orbit (MEO) satellites. Each satellite would be placed between several orbital planes. Current systems vary, but altogether set the orbital inclinations to > 50° and their orbital periods to roughly 12 hours at an altitude of almost 12,000 miles (20,000 km).