Global Navigation Satellite Systems
A satellite navigation or satnav system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location (longitude,latitude, and altitude/elevation) to high precision (within a few metres) using time signals transmitted along a line of sight by radio from satellites. The signals also allow the electronic receivers to calculate the current local time to high precision, which allows time synchronisation. A satellite navigation system with global coverage may be termed a global navigation satellite system (GNSS).
As of April 2013, only the United States NAVSTAR Global Positioning System (GPS) and the Russian GLONASS are global operational GNSSs. China is in the process of expanding its regional BeiDou Navigation Satellite System into the global Compass navigation system by 2020. The European Union's Galileo is a GNSS in initial deployment phase, scheduled to be fully operational by 2020 at the earliest., India has a regionalsatellite-based augmentation system, GPS Aided GEO Augmented Navigation (GAGAN), which enhances the accuracy of NAVSTAR GPS and GLONASS positions, and is developing the Indian Regional Navigation Satellite System (IRNSS). France and Japan are in the process of developing regional navigation systems.
Global coverage for each system is generally achieved by a satellite constellation of 20–30 medium Earth orbit(MEO) satellites spread between several orbital planes. The actual systems vary, but use orbital inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about 20,000 kilometres or 12,000 miles).