1.1 - TRIANGULATION

First consider how distance measurements from three satellites can pinpoint you in space (we will see later how our receiver measures this distances). Suppose we measure our distance from a satellite and find it to be 20,000 Km. Knowing that we're 20,000 Km from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 20,000 Km.