Satellites are equipped with an atomic clock that allows for a very precise determination of time. At coordinated intervals, the satellites emit radio signals that contain information about the time when they are sent and the satellite's current location in orbit. The radio signals travel to a receiver at a known speed (the speed of light). A receiver can therefore determine its 3-D space position relative to the satellites, as well as time, depending on how long the signal travels for, from the satellites to the receiver.
Most satellite navigation applications in Europe today rely on the United States' Global Positioning System (GPS). The European Union and the European Space Agency are currently developing Galileo (expected to be operational in 2014), Europe's independent Global Navigation Satellite System for civilian use with a positioning accuracy varying from 10m to less than 1m.
Many operatioanal satellite navigation-based services exist and are being used today. They have revolutionised land, maritime, air transport, they offer innovative solutions in agriculture, risk management, building and many fields of activity.
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