We've already got something like that. It's called Loran.

Best,

Steve Robertson

"William W. Plummer" wrote:

Satellites are great for coverage but they are expensive to maintain (
$1B per year). Would it be possible to use all those cell phone towers
as GPS extensions, providing wide area coverage and increased accuracy?
The satellite constellation would serve as the "backbone" which
provides accurate time and position information to all those cell phone
towers, but within range, the cell phone towers will provide much more
accurate location info.