The term GNSS is given to a worldwide position, velocity, and time determination system, that includes one or more satellite constellations, receivers, and system integrity monitoring, augmented as necessary to support the required navigation performance for the actual phase of operation. (EUROCONTROL)
Four implementations of GNSS are currently in existence or under development. These are:
Currently, the USA-operated GPS and the Russian GLONASS systems are operational. The Galileo system is currently expected to be operational in 2015. The BeiDou system is partially operational and is anticipated to be fully functional by 2020.