Method and apparatus to secure a system for passive unlocking of a vehicle system against relay station attacks

A method to secure a system for passive unlocking of a vehicle system against relay station attacks, comprising:

• transmitting signals having first and second radio frequencies between a vehicle-based device configured to send on the first frequency and to receive on the second frequency, and a driver-based device configured to receive on the first frequency and send on the second frequency;
• receiving, via the vehicle-based and driver-based devices, at least one ambient noise signal sequence on the first and second radio frequencies;
• sending the at least one ambient noise signal sequence from either the vehicle-based device to the driver-based device or from the driver-based device to the vehicle-based device
• comparing the ambient noise signal sequences received by the vehicle-based device and the driver-based device via cross-correlation;
• and
• unlocking the vehicle if the cross-correlation exceeds an associated correlation threshold a phase shift of the ambient noise signal sequences is less than an associated phase shift threshold.