Lighthouses and Ter'rists and Bombs, oh my!
The problem with ground-based stations is one of line-of-sight. As well as the "probability it'll get hit by an RPG or car bomb" factor. Not to mention if there was a power-cut. You'd have half of the UK's GPS-navigated drivers ploughing headlong through pedestrianised areas and walls because their GPS hadn't told them to turn...
Orbital global positioning is the way to go. It's resiliant to attack, reliable (as they don't want to have to send another satellite up there!), and relatively simple to decode. Plus it's free to use and has all sorts of extra uses (New Scientist not too long ago had a short article about someone using the transmissions from the GPS satellites to find water)
Also, DGPS receivers (Which would be similar to what you're proposing) are a little too big compared to the tiny chewing-gum-packet sized GPS devices available today. Check out ARQuake from the Uni of South Australia Wearables Lab- that's DGPS based.
Finally, signals coming down the way are far better in urban environments than signals going sideways through buildings or relying on signal diffraction around buildings to provide signals to areas behind large buildings. Either that or the power rating would be high enough to cook birds and the occasional light aircraft...
Lighthouses would be great unless the terrorists were armed with easily concealable spray paint. You can just imagine the headlines- "spray paint banned!" "Giving your car a new paint job- OF DEATH" "Paintball guns: The silent killer. Also they give out CO2 with every shot hence are evil."
eLORAN sounds like it's a good investment. Though it does need a better name- perhaps LORANe (as in Lorraine)? Nerds like techs with womens names so that should give it a better chance of succeeding!