GPS satellites are not in geosynchronous orbits, they are in low Earth polar orbits, and their accuracy is not as good as triangulating from cell towers can potentially be. A few years ago a special geosynchronous WAAS satellite was added that sends out a correction signal. This information is computed by land stations that know their exact location and compare it to their GPS calculated location, which is usually in error. The gravity of the Earth is not uniform and the GPS satellite positions vary constantly from their predicted positions.
Rather than just timing signals a system could be implemented to measure the phase of a radio wave when it hits. This would improve the accuracy down to a tiny fraction of an inch. Some very precise surveying equipment uses this technique.
Thanks for correcting me on that. I was thinking of television and radio communications satellites which are in geosychronous orbits.