Design for an automated door locks(smart key less entry)

A situation occurs when we lock our car doors with key inside the car.For this a transmitter is attached to the keychain and a transponder inside the car.When we lock with key inside the car, the transponder activates the transmitter, which inturn will make a hand-shake signal back to the transponder.Thus the transpon knows that the key is with in the range of the car. There will be a mechanism which will automatically open the door