GPS is a system which shows the exact position on the earth, anytime in any weather and anywhere. There are 24 GPS satellites orbiting at 11,000 nautical miles above the earth. They are continuously monitored by ground station located worldwide. The satellites transmit signals that can be detected by anyone with a GPS receiver. Using the receiver one can determine exact position namely longitude, latitude altitude and the time, with greater precision. GPS has 3 parts: the space segment, user segment and the central segment. Space segment consist of 24 satellites as stated before. User segment consist of receivers which we can hold in hand or mount in a car and this gives the exact location on the earth. The control segment consist of ground stations that make sure that the satellites are working properly.
The satellite are equipped with precise clocks that keep accurate time to within 3 nano seconds (3x 10-9 secs). The time signals are transmitted along with their orbital parameters. The receiver detects the time signal and calculate the distance of the satellite. By getting the signals from 3 different satellites and by doing mathematical calculations, the receiver is able to give the exact position where the receiver's located. By getting the information from 4th satellite, it is able to give the time also.
Thus 4 satellites are required to compute the 4 dimensions x, y, z and time. At any time and at any location always a minimum of 4 satellites will be visible for the receiver.
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