GPS 
Global Positioning System 
The global positioning system is a satellite navigation system used to precisely determine terrestrial position, velocity, and time. Once exclusively used by the U.S. military, GPS is now available to the worldwide general public. The GPS system is made up of 24 satellites that continually orbit the earth and relay signals to corresponding GPS receivers on earth. Because the speeds and distances of the satellites are precisely calibrated, you can use them to determine the latitude, longitude, and even altitude of a fixed point on earth. The satellites also contain atomic clocks that are accurate to within 1-billionth of a second.