The Global Positioning System (GPS) was developed by the U.S. military as a navigation aid. It consists of 27 satellites orbiting the Earth at an altitude of about 12,000 miles. The orbits were selected so that at least four GPS satellites will be above the horizon at all times, everywhere in the world.
In order to determine your location, a GPS receiver first establishes communication with four or more of the GPS satellites. The GPS receiver can calculate its position by determining how far away the satellites are.
Each GPS satellite broadcasts a signal at a specific time. Because the signals travel at the speed of light, they will arrive at the receiver after short delays that depend on the distance between the receiver and each of the satellites. The receiver then calculates its position relative to the satellites. It can be challenging to visualize in three dimensions how to determine your position from the knowledge of satellite positions. Here are examples of how it might work in imaginary one- and two-dimensional worlds.
Suppose you are standing in a very long tunnel. If you know that you are 7 miles from beacon A, then you must be standing either at mile marker 3 or mile marker 17. If, however, you also know that you are 3 miles from beacon B, then you must be at mile marker 17. In fact, as long as you know both your distance to A and to B, you will always be able to tell where you are in the tunnel.
Now imagine standing on a two-dimensional plane. If you know you are ten miles away from beacon A and fifteen miles from beacon B, then you could be at either point X or Y.
However, if you know that you are also 5 miles from beacon C, then you know that you are, without a doubt, at point X.
In a one-dimensional world, you need two beacons to determine your position; in a two-dimensional world, you need three beacons; and in a three-dimensional world like ours, you need four beacons (or GPS satellites) to determine your position.