A vehicle accessory might first transmit a signal to your iPhone that identifies the current or future location of the vehicle. Your iPhone would then generate one or more virtual geofences around the location of the vehicle.
Since your iPhone can repeatedly estimate its own location, it would determine whether it has crossed a geofence. It could even estimate its motion, detecting the direction in which it is crossing the geofence.
Once the geofence is crossed, your iPhone can generate and transmit a signal to the vehicle. The accessory would then interpret the signal and co-ordinate the control of vehicle functions in response.
For example, function control rules can identify parameters related to door locking, trunk opening, vehicle running, heater or cooling operation, defroster operation, music selection or status, accessory power states, seat warmers, navigation operations, etc. Upon detecting a particular geofence crossing (e.g., and a direction in which an ingress or egress of the geofence is made), the mobile phone can generate and transmit a second signal to the vehicle accessory identifying the function control
to be implemented.
More details at the link below...
Read More [via MacRumors]