Global Positioning System.
In the GPS system, sattelites are used to determine the location of points on the Earth’s surface. The satellites continuously trasmit their orbit data, from which the GPS receiver can determine the location of the satellite. The distance of the satellite depends on the punctual time measurement, that is, we are measuring, that how long does it take for the radio signal projected by the satellite to get to the receiver. (…)
In our country the American NAVSTAR GPS system can be used. This system has 24 satellites floating around in 6 orbits. The satellites are located in such a way that at least 4 of them are always visible simultaneously from any point on Earth. (Geographic information system)