The Global Positioning System (GPS) is a satellite-based positioning system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the exact position of them. GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. With the distance measurements from a few more satellites, the receiver can determine its position with centimeter accuracy.