[hrbuckley]

A cell phone communicates with the network through one cell (a radio tower that interfaces the carriers network to the cellphone frequencies) at a time. A change from one cell to another, or a significant change in signal strength can indicate a change in location. You will need to develop a set of appropriate heuristics to determine when it is appropriate to start the GPS. Starting and stopping the GPS on a cycle shorter than 10 minutes actually takes more power than just letting it run.

If you are not familiar with these concepts I would suggest that you do some research into how mobile devices operate before taking on such a complex project.