GG-Tracker tracks your mobile using GSM Cell-location in combination with GPS. It uses both GPS and GSM networks to get a position. If only GSM is available it relies on location previously stored.This product is in a developmental phase and is only here to show you how it can be done.
The operation is very simular to Google Maps 'My location', only this time you are in control. If a GPS receiver is connected it does two things, it stores the current position and the GSM cell the phone is connected to. This position is relayed from your phone to your server using internet. The packages are pretty small to keep costs as low as possible. Personally I use MyLiveTracker to relay those positions back to me. They are then captured by GG-Tracker.
If the GPS is not connected anymore it checks the database for previously known coordinates of the GSM cells. If it was connected to GPS the program stored the GSM cells, so all it does, is look back and see if there is a proper fit, meaning that GPS is not necessary anymore for positioning.