BMW Park Distance Control (or PDC) is a system that warns drivers of nearby objects when parking. The system works only on the rear of some older vehicles but works on both the front and rear of newer BMW models. The system uses ultrasonic sensors embedded in the vehicle to detect objects. Once the vehicle is within approximately 3 feet of the object a warning tone sounds to notify the driver. As you get closer the frequency of the warning tone's beeping increases, until it becomes a solid tone when the vehicle is approximately 12 inches away from the nearest obstruction. Depending on the vehicle model and equipment the system may also include a visual representation of the car's situation on the iDrive screen.