Abstract: A security system including tracking units or readers carried or mounted on mobile units to operatively communicate status signals indicating identity, location, direction of travel, and a number of alarm conditions to a local monitoring station. The tracking units or readers are equipped with an infrared receiver to receive infrared location signals transmitted by fixed infrared transmitters located throughout the area to be monitored. Each infrared transmitter transmits a unique location signal. The tracking unit is operative to transmit a radio frequency watchdog signal that includes the last two location signals received, indicating the time lapsed since receipt of those location signals, a unique identification signal indicative of the and any applicable alarm signals triggered by predetermined conditions to which the tracking unit is also operative to detect.