ELECTRONIC LOCATING SYSTEM FOR PERSONS RECEIVING TELEPHONE CALLS

A method and system is disclosed for determining the location of a member of a class of individual transmitter-receiver units distributed throughout a defined facility. A central station establishes a two-way communication channel with one or more relay stations and sends a coded message identifying a particular individual unit over the communication channel. The relay stations respond to the message by sending out wake-up signals and radiant energy inquiry signals. The individual unit identified in the inquiry signal responds with a radiant energy acknowledgment signal. Embodiments of the invention disclose the use of infrared energy or ultrasonic energy for the radiant energy. Ultrasonic signals are sent on a plurality of frequencies. The relay stations send identification signals to the central station which identify the sending relay station and indicate whether an acknowledgment signal was received. The central station determines the approximate location of the individual unit from the identification signals. According to one feature of the invention, the relay stations are divided into groups in which each relay station of a group covers an area distinct from the areas covered by the remaining relay stations of the same group. In this arrangement the central station can communicate with one group of relay stations at a time.