1.3 - TRIANGULATION

Next, say we measure our distance to a third satellite and find out that it's 21,000 Km away. That narrows our position down even further, to the two points where the 21,000 Km sphere cuts through the circle that's the intersection of the first two spheres. So by ranging from three satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected.