A navigation system that makes use of a constellation of satellites in low earth orbit, each emitting carefully timed signals. A ground-based receiver can determine its location on earth to a high degree of accuracy by carefully measuring the reception times of signals from a number of satellites currently above the horizon. A system with global coverage is known as a global navigation satellite system (GNSS). The most widely used is the GPS system deployed by the USA, but the European Space Agency (ESA) is now deploying its own Galileo system, Russia is developing and deploying GLONASS, and China has the Beidou system.