GPS is a system which shows exact position on earth, anytime in any weather and anywhere.24 GPS satellites orbit at 11,000 nautical miles(1 nautical mile=1,852 m, it is a unit of length that is about one minute of arc of longitude at the equator) continuously monitored by ground stations located worldwide. Satellites transmits signal that are received by GPS receiver, through which we can determine exact position namely longitude, latitude, altitude with time.

    GPS has three parts-Space segment, user segment and the control segment. Space segment consists of 24 satellites. User segments consists of all receivers which one can hold. Control station(i.e. ground stations) take care of control segment that make sure satellites are working properly.

    Satellites are equipped with precise locks with accurate time of 3ns(if anyone don't know, 1ns=3X10^9s).Receiver detects the time signal(transmitted along with their orbital parameters) & calculates distance of satellite. By getting signals from 3 different satellites, receiver gets exact position. By getting information from fourth satellite, it's able to get time also. Thus, in all only 4 satellites are needed to get 4 parameters which detect the positions viz. x, y, z and t. At any time and at any location always a minimum of four satellites will be visible for receiver.