Abstract: A passive keyless entry system is specifically adapted for use with automotive vehicles. The system includes a portable beacon that is carried by a vehicle operator, a receiver/controller located in the vehicle, and an antenna connected to the receiver/controller for receiving an encoded transmission from the beacon. The system is designed to automatically unlock the vehicle as the operator approaches the vehicle. The system is further designed to automatically lock the vehicle as the operator, carrying the beacon, moves away from the vehicle. The beacon includes a motion sensor to conserve battery life when the beacon is stationary. Transmission between the beacon and the receiver/controller is characterized by a magnetically coupled radio frequency signal embodying differential phase encoded data with error correction coding of the data to enhance noise immunity and signal discrimination. In addition, the transmitted signal between the beacon and the receiver is continually changing in order to deter theft.