A communications system includes at least two communications units, each communication unit including a transmitter capable of transmitting to the other unit on one or more frequencies, under the control of a mode control unit and wherein acquisition or a locking of frequencies of the two communications...http://www.google.de/patents/US5590410?utm_source=gb-gplus-sharePatent US5590410 - System and method for frequency based acquisition acknowledgment between transmitter and receiver
