Manapság nagy divat a wikipedia: http://en.wikipedia.org/wiki/Global_Positioning_System
Position calculation introduction
To provide an introductory description of how a GPS receiver works, errors will be ignored in this section. Using messages received from a minimum of four visible satellites , a GPS receiver is able to determine the times sent and then the satellite positions corresponding to these times sent. The x, y, and z components of position, and the time sent, are designated as \scriptstyle\left[x_i,\, y_i,\, z_i,\, t_i\right] where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received \scriptstyle\ tr , the GPS receiver can compute the transit time of the message as \scriptstyle\left (tr-t_i\right ) . Assuming the message traveled at the speed of light, c , the distance traveled or pseudorange, \scriptstyle p_i can be computed as \scriptstyle\left (tr-t_i\right )c .
A satellite's position and pseudorange define a sphere, centered on the satellite with radius equal to the pseudorange. The position of the receiver is somewhere on the surface of this sphere. Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces.