The original Woodford & Nakamura TOR has a really cool hand drawn table showing all of the alternatives and their pros/cons. It was definitely for the best that

The original Woodford & Nakamura TOR has a really cool hand drawn table showing all of the alternatives and their pros/cons. It was definitely for the best that they went with the most technically challenging option (inventing atomic clocks that are small enough to go to space) since that choice is responsible for a lot of the neat aspects of GPS (users don't need to be near ground stations, they don't need their own synchronized clocks, and GPS receivers really are receive-only) Edit: just scrolled down and saw that my comment is a bit redundant with syncsynchalt's :)