Seeing and listening through a wall: radio waves and optical fibers


A research team from University of Utah is at work on a project that allows usage of a small network of wireless radio transceivers, in order to detect movements of people on the other side of a wall.
Thanks to this detection system, based on the RTI (Radio Tomographic Imaging) concept which allows the images to be send via airwaves, police, surveillance forces and firemen will be able to detect intruders or casualties trapped inside a building, by simply placing these small transceivers in the area.

The RTI system is basically reading the “shadows” of radio waves when they pass through a person in motion, and based on these shadows it builds an image. In an experiment that has been used as an example, scientists have created a small network with 28 transmitters, placing them in rectangular shape in a room and in the garden outside the house.

At this point, the radio signal strength was measured, first when the rectangle was empty, and then with a person inside it. By transferring the relevant data to a computer with a calculation software, a very blurred image, resembling a shadow, is created by this software.
By improving method and detection systems, it has been possible to detect human movements on the other side of a wall.
The network used is not a normal Wi-Fi network such as the one normally used at home for computers, so it can not be detected by devices using such protocol, and can be used for surveillance purposes. The Zigbee communication protocol used for this system can generally be found in automated domestic appliances.

Since it is based on radio waves, it can also work in the dark, and due to the very blurred image that can be obtained, it does not pose any kind of threat under the privacy point of view. In order to obtain images with a better quality, an optimal solution is the usage of optical fiber devices, such as for example a fiberscope, made up of a tiny cable, only 2.5 mm in diameter, which carries a drill bit camera at one end.

Just dig a hole in the wall and insert the cable, and maybe connect the fiberscope to a digital video recorder to record high quality images inside a room. Alternatively, a non invasive option, used only to listen through the wall to what happens in the room nearby, is the usage of a wall microphone.

A wall microphone is basically a technologically advanced version of our doctor’s stethoscope, equipped with high sensitivity microphones and connected to a power amplifier, which makes it capable of listening through brick walls without arising any unnecessary suspicion or attention. Once again, in this case you can use a digital recorder to store any information or evidence gathered.