Abstract: A population of locatable personal detection units (PDUs) (20) are worn by users. Any number of locators (14) are placed at known locations within an area (10) where the users tend to be. When an alarm event for an individual user occurs, a request signal is transmitted from the user's PDU (20). The request signal is received at several of the locators (14), each of which measure the power level of the request signal. A central computer (16) selects some of these locators (14) in response to the power level measurements. In sequence, the selected locators (14) transmit an interrogation signal to the PDU (20), the PDU (20) replies to the interrogation signal, and the locators (14) measure the duration transpiring between the interrogation and the reply. Based on the durations measured for at least three of the locators, the central computer (16) uses a multilateration process to localize the PDU (20).

Abstract: A high capacity time-division switching system is disclosed which includes a digital data transmission system for transmitting both digital data words and synchronizing information. The predetermined bit positions of each data word, which bit positions are to convey data only, are encoded and transmitted as a combination of the true and complement representation of those digits. The remaining bit positions, which are used to convey both data and synchronizing information, are encoded and transmitted as true representations only. A digital data receiver responds to the data so encoded by extracting synchronizing information based on the violation of the predominately true and complement coding format.