The basic principle behind GPS is the measurement of distance between satellites and the receiver. The distance to at least 3 satellites must be known in order to find out a position. Satellites and receivers generate duplicate radio signals at exactly the same time. As satellite signals travel at the speed of light (186,000 miles per second), they take a few hundredths of a second to reach the GPS receiver. This difference and the speed at which signal travels is used in the equation to find out the distance between the GPS receiver and the satellite.
Speed x Time = Distance
So, if it takes 0.09 of a second for a satellite's signal to reach the GPS receiver, the distance between the two must be 16,740 miles (186,000 miles per second x 0.09 seconds = 16,740 miles). The GPS receiver must be located somewhere on an imaginary sphere that has a radius of 16,740 miles.
If it takes 0.08 seconds for the signal to reach the GPS receiver from a second satellite then the receiver must be located somewhere on an imaginary sphere that has a radius of 14,880 miles, and where the two spheres intersect.
Supposing it takes 0.07 seconds for the receiver to receive a signal from a third satellite then the GPS must be located somewhere on a sphere that has a radius of 13,020 miles and where the three satellites intersect.
||Now there will be two location possibilities but one of these is located in space and is mathematically discarded by the GPS receiver as impossible.
Not only do the satellite signals contain data that the GPS receiver uses to calculate distance, but data that enables the receiver to make adjustments needed to get an accurate position. Atmospheric data is sent in the signal as the receiver has to account for delays in the time it takes for the signal to reach it. These decreases in the speed of the signal are caused by the ionosphere and the troposphere.
This information is usually used in conjunction with software on a laptop or PDA (Personal Digital Assistant) in the form of a map, to show the GPS receiver user their location.