GPS

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS).

Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time.

Other similar systems are the Russian GLONASS (incomplete as of 2007), the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India.

Read more here.