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Wireless network modded to see through walls

Posted: 30 Sep 2009 09:10 PM PDT
http://feedproxy.google.com/~r/arXivblog/~3/uoY0lEt9zZw/click.phdo


The way signal strength varies in a wireless network can reveal what's  
going on behind closed doors


It's every schoolboy's dream: an easy way of looking through walls
to spy on neighbours, monitor siblings and  keep tabs on the
sweet jar. And now a dream no longer...

Researchers at the University of Utah say that the way radio
signals vary in a wireless network can reveal the movement of people
behind closed doors. Joey Wilson and Neal Patwari have developed a
technique called variance-based radio tomographic imaging which
processes the signals to reveal signs of movement. They've even
tested the idea with a 34-node wireless network using the IEEE
802.15.4 wireless protocol. This is the protocol for personal area networks  
employed by home automation services such as ZigBee.


The basic idea is straightforward. The signal  strength
at any point in a network is the sum of all the paths the radio waves can  
take to get to the receiver.  Any change in the volume of
space through which the signals pass, for example caused by the
movement of a person, makes the signal strength vary. So by
"interrogating" this volume of space with many signals,
picked up by multiple receivers, it is possible to build up a picture
of the movement within it.


In tests with a 34-node network set up outside a standard living
room, Wilson and Patwari say they were able to locate moving objects in the  
room to
within a metre or so. That's not bad and the team say there is ample
potential for improvement by increasing accuracy while reducing the
number of nodes.


The advantage of this technique over others is first its cost. The
nodes in such a network are off the shelf and therefore cheap. Other  
through-wall viewing systems cost in excess of $100,000.
The second advantage is the ease with which it can be set up. Wilson and  
Patwari
say that adding a GPS receiver to each node allows it work out its
own location which should dramatically speed up the imaging process.Other  
systmes have to be "trained" to recognise the environment.


Wilson and Patwari have even worked out how their system might be used:

"We envision a building imaging scenario similar to
the following. Emergency responders, military forces, or police
arrive at a scene where entry into a building is potentially
dangerous. They deploy radio sensors around (and potentially on top
of) the building area, either by throwing or launching them, or
dropping them while moving around the building. The nodes immediately
form a network and self-localize, perhaps using information about the
size and shape of the building from a database (eg Google maps) and
some known-location coordinates (eg using GPS). Then, nodes begin to
transmit, making signal strength measurements on links which cross
the building or area of interest. The received signal strength
measurements of each link are transmitted back to a base station and
used to estimate the positions of moving people and objects within
the building."

That's ambitious but shoudl they ge ttheir system to the point where it can  
be used like this it raises another problem: the issue of privacy.

The broader issue is how such a cheap and
easy-to-configure monitoring networks might be used if they become widely  
available. What's
to stop next door's teenage brats monitoring your every move or house
breakers choosing their targets on the basis that nobody is
inside?

Of course, in the cat and mouse game of surveillance,
it shouldn't be too hard to build a device that disables such a
monitoring network. But only if you know it's there in the first
place.


There are fun and games galore to be had with this idea.


Ref: arxiv.org/abs/0909.5417: Through-Wall Motion Tracking Using  
Variance-Based Radio Tomography Networks












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