The received signal includes both the direct transmitted signal and multipath signals which reflect from the ground and nearby structures and objects. Moving objects e.g. human and vehicles, produce changes in the net vector sum of the received signal.
Detection occurs when the resulting received signal crosses a predetermined threshold.
The transmitted signal is tone modulated to eliminate mutual interference when multiple sensors are operated in close proximity. The primary detection mode is the beam break mode where the target passes directly between the transmitter and receiver. A second and equally important mode is the multipath reflection mode in which the reflected wave from an off-axis target destructively interferes with the direct wave at the receiver. Alarms are produced when the transmitted signal or its modulation is disturbed, when the equipment is jammed, or when the enclosure tamper switches are activated. The transmitter and receiver are equipped with 12Volt Batteries and charging circuits to provide a four hour battery back-up in the event the fixed power source is unavailable.