The Global Positioning System, usually called GPS, is a satellite navigation system. More than two dozen GPS satellites orbit the Earth and transmit radio signals which allow any GPS receiver near the planet to determine its location, speed and direction.

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude). Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

http://en.wikipedia.org/wiki/GPS

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GPS means Global positioning system.

Basically your hand device has to be GPS enabled and then your device is connected to the satellite system available exclusively for GPS customers and then device shows the position of the device at a particular location (at a particular time). For this GPS mapping software is the key thing available with the GPS service provider.
Time interval between transmission of signal( coded message) by the satellite and reception at the receiver will give you a range. the range is not true range, becos satellite clock and receiver clock may not be synchronised, and the radio waves dont travel in a straight line but are deviated by the atmosphere. the range so obtained by the receiver is called pseudo range. the satellites position is known to the satellite as tare being monitored (24 X 7) by ground stations. The signal transmitted by the satellite which is used for range measurement also has the information about the satellites exact position modulated. this information helps to determine GPS receiver position.

The satellite position is send in XYZ co-ordinate system.

In the range equation

Range Squared = (X-a)squared + (Y-b) squared + (Z-c) squared

unknown is GPS receiver position (a,b,c)

As a,b and c are 3 unknowns you have to solve 4 range equations. for 4 range equations GPS receiver has to get signals from 4 satellites.(3D position).

this is how a gps functions.

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