172259 ⎘
Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using ultrasonic, sonic or infrasonic waves; Systems for determining direction or position line using amplitude comparison of signals transmitted from transducers or transducer systems having differently-oriented characteristics the signals being transmitted sequentially