Science Copperfield, Second Life asked:
Why do we use satellites for positioning instead of ground based methods?
We put this to Professor Todd Humphreys:
Todd - Well in fact, the early systems were ground based. There was the LORAN (LOng RAnge Navigation) system developed around the second World War that worked faithfully for many years and is still in use in some parts of the world. The trouble is that GPS enjoys great advantages over these ground based systems. Itís got global coverage and its signals arenít distorted by the local topography. So itís just been a question of success. GPS has line of sight global signals.