Relay attack prevention for passive entry passive start (peps) vehicle security systems

A passive entry passive start (PEPS) vehicle security system (10) configured to thwart a relay attack on the system (10). The system (10) includes one or more ultra wideband transceivers (UWBX) installed on a vehicle (14) and configured to transmit a request pulse (24) at a request time. A mobile UWBX (30), possibly installed in a nomadic device (16) such as a smart phone, is configured to transmit a reply pulse (32) in response to the request pulse (24). A controller (18) is configured to determine a distance (12) between each UWBX and the mobile UWBX (30) based on a time interval between the request time and a time that corresponds to when the reply pulse (32) is received by the each UWBX. The controller (18) may also be configured to unlock doors of the vehicle (14) only if the distance (12) is less than an unlock threshold.