Conceptual
sketches of: (a) impact transmitter; and (b) electromechanical
pull-type actuator. Credit: arXiv:1507.01479 [physics.ins-det]
New
technology has been developed to address the illegal movement across
borders of people hidden in containers. SPIE, the international society
for optics and photonics, in comments about today's technology efforts,
said the ability to see inside sealed cargo was important. "With 200
million shipping containers being moved around the world annually, fast
and effective ways of scanning the cargo are needed." They said that
"Sensing through the walls of metal cargo containers can be done with
gamma rays, however this is not safe when considering the possibility of
people being stowed away inside these containers as a means of travel."
They said that "The only current technology that can safely sense
people behind the metal walls is acoustic."
Franklin
Felber of California-based Starmark, a company that does research and
development for the defense and health-science industries, has written a
paper that indicates success in that direction. The paper, posted on
the arXiv server, is about a high-power acoustic through-the-wall sensor
that can detect people through steel walls of cargo containers, trailer
truck bodies and train cars. Translation: it can spot stowaways. He
said the sensor has been developed and demonstrated.
The
paper is titled, "Demonstration of novel high-power acoustic
through-the-wall sensor." The technology presented, said Felber, who is
co-founder of Starmark, offers the potential for rapid nonintrusive
detection of stowaways inside closed steel cargo containers, truck
bodies, and train cars.
Felber's approach involves
a mechanical transmitter that is compact, lightweight, low-cost, can
operate on battery power and produces acoustic pulses in dense media at
one or more frequencies "orders of magnitude more powerful" than those
produced by other transmitters of comparable scale; a resonant receiver
matched to the transmitter; and methods of signal processing.
MIT
Technology Review said passive millimeter wave sensors require a source
of illumination to see though walls. Microwave radar systems do not
pass through metal walls; systems based on the detection gamma rays,
designed primarily to detect nuclear materials, are not suitable for use
on humans; acoustic sensors can work in sending signals through metal
walls but a drawback has been their lack of power to detect humans.
The
acoustic sensor that Felber developed, said MIT Technology Review, "is
both powerful and sensitive enough to detect the breathing motion of an
otherwise stationary human on the other side of a cargo container wall."
Felber
said the device is capable of nonintrusively scanning steel cargo
containers for stowaways at a rate of two containers per minute.
MIT
Technology Review discussed how his system works. Felber's acoustic
transducer operates with a nine-volt battery. "His new machine is
essentially a hammer or, as he calls it, a mechanical impact
transmitter," said the review.
"This produces a
powerful acoustic signal by repeatedly banging on a metal disc, which
then resonates at a specific frequency. When attached to a container
wall, the signal passes through into the air on the other side. An
acoustic receiver picks up any reflections from each pulse and a signal
processor then subtracts these from the reflections from the previous
pulse. Reflections that haven't changed, ones from stationary objects,
cancel out. That leaves only the reflections from moving objects, such
as people."
Device tests showed it could detect a
person on the other side of a wall either moving or stationary, from
breathing action. The author said that "Lives could even be saved of
those trying to escape detection by remaining motionless, like stowaways
inside cargo containers, who are at risk of death by dehydration on
long voyages, if undiscovered beforehand."
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