At one time the FAA was talking about using ground based broadcast
stations, called LAAS (Local Area Augmentation System). (Which, by the
way, used different frequencies than the GPS constellation.) As far as I
know, none have been deployed yet or they dropped the idea.

I think there were two deployed. The manufacturer was a Canadian company
that was bought out by Honeywell.

http://gps.faa.gov/programs/laas/currentnews-text.htm