"A phone's GPS accurate to roughly 15 feet, although military GPS devices can be accurate to centimeters"
I don't think this is any longer true. As I understand it, GPS signals were originally designed to be available in two flavours - a civilian one, that was made deliberately less accurate, and a military one, that had the *real* data. The reason for this was - the US military didn't want to be giving foreign powers the ability to target their cruise missiles more effectively.
Now when the gulf war came, military GPS devices were in very short supply. Several soldiers received gifts of yachting units from family and friends and were using them to navigate through Iraq - as a result of this the military decided to broadcast the civilian signals at the same accuracy as the military ones.