Audiovox PSD100 Digital Front or Rear 2 Sensor Upgradable System with Learning Capability 2 sensors detect obstacles over 6 feet away from the bumper System can be installed as a front or rear sensor system Learning capability Waterproof connectors allow replacement of individual sensors at the bumper System is upgradable to a 4 sensor system using PSD200/PSD200C 2 sensor add-on kits