GNSS or Global Position Navigation System is a constellation of satellites used for many purposes like navigation, positioning and time distribution applications. Currently there are two GNSS systems in operation: the US-based Global positioning system (GPS) and the Russian GLONASS. A third system, Galileo, is currently being developed in Europe. Of these GPS is the most popular and widely used navigation system. So the working of a GNSS system can be understood by looking at the working of GPS as a typical example.