GPS was designed to be dual use for civilian and military users, and to allow completely passive navigation receivers don t emit any signals. Real time accuracy is largely of military value. One way ranging. is obtained by finding travel time of a signal from the satellite to your receiver, and multiplying by a value for the speed of light. The result is called a pseudo-range. 3 pseudo-ranges plus a clock offset value for your receiver are the minimum information, thus 4 satellites in view are the minimum.
Additional satellites strengthen the solution.
Satellite geometry can vary, hence quality of the result as well. Ranges are determined by cross correlating the code generated by your receiver with the received code each satellite has its own code. Variation in the speed of light through the atmosphere is the single largest limitation on pseudo range precision. We will look at various applications of GPS in understanding crustal motion shortly, but it is interesting to note some of the applications of the GPS signal outside of solid earth geophysics that have been developed. Because the signal delay through the atmosphere is a cause of error, a precise position is obtained by solving for the delays.