The first satellite gives you a sphere (a shell really) The second satellite gives you a circle of intersection between the spheres. The third satellite gives

The first satellite gives you a sphere (a shell really) The second satellite gives you a circle of intersection between the spheres. The third satellite gives you two points where three spheres intersect. One of these is usually in space so it's discarded. The fourth one lets you correct for timing errors in the reciever. A microsecond uncertainty corresponds to around 300m (i.e. c * 1µs). You could get coarse information with three satellites, for instance if you already know your altitude, but four+ is preferable.