US Patent 9894613 Time of flight based passive entry/passive start system

A passive entry-passive start portable device detection system. A portable device includes a transceiver. At least one vehicle-based transceiver broadcasts an interrogation signal. The transceiver of the portable device transmits a response signal in response to the interrogation signal. A controller determining at least a range of the portable device as a function of the time-of-flight of the interrogation and response signals. The controller controls a measurement duty cycle of the interrogation signal as a function of the determined range.