An apparatus for detecting obstructions behind a vehicle, employing ultrasonic waves is disclosed. The apparatus comprises an ultrasonic transmitting and receiving device disposed on the rear portion of the vehicle, an electric motor for rotating the transmitting and receiving device at a constant velocity,...http://www.google.co.uk/patents/US4618948?utm_source=gb-gplus-sharePatent US4618948 - Apparatus for detecting obstructions behind vehicle
