The GPS frequencies are pretty much line of sight propagation only. Any multipath distortion would make the signals useless. They operate on 1.5Ghz and 1.2Ghz and you need to have uninterrupted line of sight to multiple transmitters to triangulate the position. So trying to send gps signals from land based towers would require tens of thousands of transmitters around the globe. I have used a GPS on an aircraft flying over the middle of the pacific and it works just fine. I have built and operated amateur radio gear on 1296 Mhz and with 3 meter diameter dishes worked over distances of 100 km. Knife edge diffraction and atmospheric ducting work on 1296 Mhz as well as VHF UHF frequencies, but those propagation modes won't work with gps because of the timing distortion that would introduce.
There are systems that used a land based correction system called Differential GPS, these systems use a transmitter at a known location co-ordinates to apply a correction factor to the GPS signal to get greater accuracy. The system is used in agriculture to steer machinery for sub centimeter crop sowing, they can sow between rows placed just a few centimeters apart.
Look up RTK GPS Transmitters
http://gpsworld.com/centimeter-level-rtk-accuracy-more-and-more-available-for-less-and-less/ So in summary.
1. GPS must be direct line of sight to multiple transmitters at the same time.
2. GPS works in the middle of the pacific.
3. DGPS only applies a correction factor to the basic system.
The conclusion is that GPS requires satellites in orbit. It cannot be land based.
There have been other land based positioning systems like LoranC and others, but GPS has pretty much superceded them.
More on LoranC
http://en.wikipedia.org/wiki/Loran-C