Poster Presentation Sandia Helicopter Acoustic Detector*

In reviewing the safeguards plans for several proposed fuel cycle facilities, the advantages of a short-range helicopter detector became apparent. The main buildings of these new designs are generally hardened so as to provide significant delay to a helicopter-borne adversary team. Under these circumstances the sensor is only required to detect helicopters which are in their final landing phase and at close range (less than 75 meters). This shortdetection range allows the use of a fairly simple acoustic detection algorithm without making the system overly sensitive to wind noise, motor vehicles, and ventilation/heat exchange blowers. Conversely, facilities that require 3 to 5 minutes of early warning against helicopters will probably have to use a modified air traffic control radar. The Sandia helicopter acoustic detector uses a barium titanate sensor with a 2\" aluminum diaphram. This sensor, along with a small test loudspeaker, is mounted in a ported die cast plenum, and is surrounded by a rubberized horsehair windscreen. A 20 dB preamp at each sensor drives the long cable (up to 400 meters) to the central processor chassis. This chassis holds the filter/detector boards for the six sensors which are distributed around the security zones. Helicopter rotors generate impulsive pressure spikes at a basic rate of 10 to 35 hertz. Wind, at 30 mph or less, generates most of its interfering noise at frequencies below 20 Hz. Therefore, the Sandia detector is designed to only look at spectral lines between 20 and 40 Hz, and depends upon the harmonic content of the rotor pulses for detecting the lower rotor speed helicopters.