Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems (Sat Nav) that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line-of-sight by radio from satellites. Receivers calculate the precise time as well as position, which can be used as a reference for scientific experiments.

As of 2010, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of being restored to full operation (21 of 24 satellites are operational). The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2014. The People's Republic of China has indicated it will expand its regional Beidou navigation system into the global Compass navigation system by 2015-2017.

The global coverage for each system is generally achieved by a constellation of 20–30 Medium Earth Orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but use orbit inclinations of >50° and orbital periods of roughly twelve hours (height 20,000 km / 12,500 miles).