Global positioning sensor (GPS) receivers are devices used within Earth’s planetary atmosphere to determine its precise geographical location.  These receivers are used in conjunction with over 20 orbiting Earth satellites to triangulate the exact three dimensional position based off of the receiver’s position about multiple satellite signals.  It is important that the receiver be in contact with at least four of these satellites in order for an accurate location to be determined.  For this reason, the orbital strategy of the GPS systems always has at least four satellites visible from any point on earth at any time.

GPS devices can determine the location of the receiver to within a few feet of accuracy about the earth’s surface.  To do this there has to be a time sync between the satellites and the receiver.  The manner in which they sync is rather complex, but with an accurate clock on both transmitter and receiver, the delays between the transmissions allow the receiver to make a very accurate distance measurement between each satellite.  When using the distance information from four different satellites, it is possible to determine three-dimensional locational accuracy.