NAVSTAR satellite, 1986.

Artist's impresion. This navigational satellite, also know as GPS-12, uses the Global Positioning System with a constellation of 18 satellites giving the system an accuracy of about 6 metres. Each NAVSTAR transmits information about its position in orbit, together with precise time checks. Since the speed at which radio signals travel is constant, it is posible for a user to listen to the signals from a number of satellites and determine his own position by triangulation.