Audiovox AS2ARSS is a vehicle reverse scanning system that uses ultrasonic sensors to detect obstacles behind your vehicle. It has three distinct "Zones" of detection, each with its own audible warning tone. "Zone 1" is 6-4 feet away from the vehicle and triggers slow beeps. "Zone 2" is 4-2 feet away and triggers rapid beeps. "Zone 3" is 2 feet or less away and triggers a solid tone. The system also has an optional visual indicator.