| 摘要 |
A system for tracking articles or persons comprising a base unit for transmitting a series of ultrasonic signals and an alarm signal, a remote unit, removably attached to the article or person, for receiving the series of ultrasonic signals from the base unit and for transmitting an electromagnetic signal upon receipt of the ultrasonic signal from the base unit, and a monitoring unit for receiving the alarm signal. The base unit further includes an ultrasonic signal generator, an ultrasonic signal transmitter, an electromagnetic signal receiver for receiving the signal from the remote unit, a timer for determining the amount of time from the transmission of the ultrasonic signals to the receipt of the electromagnetic signal from the remote unit, and a transmitter for transmitting an alarm signal to the remote unit and the monitoring unit when the amount of time from the transmission of the ultrasonic signals to the receipt of the electromagnetic signal from the remote unit is greater than a predetermined amount of time.
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