Global Positioning System (GPS) is developed by U.S. Defense department and is called Navigational System with Time and Ranging Global Positioning System (NAVSTAR GPS) or simply GPS.
   For this purpose U.S. Air Force has stationed 24 satellites at an altitude of 20200km above the earth’s surface. The satellites have been positioned in such a way, at least four satellites will be visible from any point on earth.
   The user needs a GPS receiver to locate the position of any point on ground. The receive processes the signals received from the satellite and compute the position and elevation of a point with reference to datum.