If you're transmitting your own GPS signal in an Urban area someone is going to notice pretty quickly and angry pretty fast. You will interfere with both law enforcement and emergency services, and likely cause a bunch of trouble.
Thinking about how much trouble you could cause, I'm actually surprised that GPS transmissions aren't signed. I guess if you were implementing it today, that would be a logical feature to add....
After a little googling, it looks like the DoD did think of it, it's just not for the like of us:
"The Precision (P) code, sometimes called the Precise Positioning Service (PPS), is modulated onto the L1 and L2 carriers allowing for the removal of the first order effects of the ionosphere. The P code is referred to as the Y code if encrypted. Y code is actually the combination of the P code and a W encryption code and requires a DoD authorized receiver to use it. Originally the encryption was intended as a means to safe-guard the signal from being corrupted by interference, jamming, or falsified signals with the GPS signature. Because of the intent to protect against "spoofing," the encryption is referred to as "Anti-spoofing" (A-S). A-S is either "on" or it's "off;" there is no variable effect of A-S as there is with SA."
GPS degradation was turned off in the late 90s by Clinton. I'm afraid that page is woefully out of date. Everybody has access to the full precision data now.
Thinking about how much trouble you could cause, I'm actually surprised that GPS transmissions aren't signed. I guess if you were implementing it today, that would be a logical feature to add....
After a little googling, it looks like the DoD did think of it, it's just not for the like of us:
http://www.csr.utexas.edu/texas_pwv/midterm/gabor/gps.html