Four transceivers are attached to respective wheels of a vehicle. A pair of triggering devices transmit respectively different types of trigger signal, respectively directed toward the front wheels and toward the rear wheels. One triggering device is located closer to one rear wheel than to the other,...http://www.google.de/patents/US7817026?utm_source=gb-gplus-sharePatent US7817026 - Apparatus for detecting wheel positions of vehicle based on strengths of trigger signals received by wheel-mounted transceivers
