04651156 is referenced by 239 patents and cites 9 patents.
An integrated radio location and communication system comprising a plurality of radio locator-transmitter devices which may be fixed to vehicles and/or carried by persons and which are capable of transmitting the position and identity of said devices to two or more centrally located fixed receiver sites. Each of said locator-transmitters is continuously phase-locked to the RF carrier of a nearby commercial broadcast station, which station is modified slightly to provide periodic transmission of a "range-tone" which is on the order of a few kilohertz in frequency and is phase-locked to the RF carrier. An identical phase-locked range-tone is likewise generated within each locator-transmitter. The two centrally located fixed receivers are also phase-locked to the broadcast station RF carrier and likewise generate a range-tone. Thus the broadcast station, the plurality of locator-transmitters and the two fixed receiver sites all generate identical range-tones and these are continuously kept in synchronism with each other by virtue of a phase-lock loop arrangement wherein all range-tones are derived from (and locked to) the RF carrier of the AM broadcast station signal. The fixed receiver sites can compute the position of each locator-transmitter by comparing the phase of the range-tone sent to them directly by the locator-transmitter with the phase of the range-tone received directly from the broadcast station. A novel frame synchronization method is disclosed whereby the position (i.e., "time-slot") of each locator-transmitter emission and its RF subchannel uniquely identifies the emitting source and thus the location as well as identity of each of the many locator-transmitters.
