Very interesting concept, my first thought is if one would be in range of those cell phone towers. I am ok in my current location but I also have access to some prime ground some hundred miles south of me where those towers are out of range.
The term GPS when it comes to the likes of cell phones could be misleading. Take my Samsung for example, it came with the automotive type of gps but this is actually a loran signal that is decipered through the cell phone towers. (read no actual log-on to the satellites)
Your phone provider also will want to charge you for the so called GPS usage?...$9.00 per month in my case and I let it expire after the trial period as one can easily save enough to purchase a dedicated GPS unit for the auto or a decent hand held gps unit for the field in a timely manner for the subscription cost savings that I do not pay.