
MIT researchers use Wi-Fi to recognize people through walls - thro1237
http://gizmodo.com/wifi-networks-can-now-identify-who-you-are-through-wall-1738998333
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tempodox
All of a sudden, tin foil hats get a whole new meaning.

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falcolas
The problem with tinfoil hats is that it just makes your signature more
unique. Tinfoil reflects radio waves better than skin and other clothing.

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arama471
Unless you change up the shape/location of the tinfoil regularly, in which
case the system will likely think you're a different person every time.

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lordnacho
Is there something special about the WiFi spectrum that makes this work? Does
it work with any ambient radio frequency noise?

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Jach
The benefit of wifi spectrum is that the parts are cheap and you generally
won't get in trouble for transmitting signals without a license, which is what
their system does. (It's not passive.) Unfortunately the wifi spectrum isn't
that great for through-wall imaging in general -- you're going to lose a lot
of transmission power if the wall is composed of just about anything besides
drywall, glass, or wood, on top of the massive loss of power you get by
default since the received signal's power is inversely proportional to the
distance raised to the fourth power.

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darkmighty
Is there a range of frequencies that does better? Don't frequencies that go
through walls easily also go through bodies easily, or are there frequencies
where the body is particularly reflective w.r.t. concrete walls?

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shalmanese
Scanners designed for this purpose use X-rays and are in use by counter-
terrorism and law enforcement units: [http://www.pbs.org/wnet/need-to-
know/opinion/google-street-v...](http://www.pbs.org/wnet/need-to-
know/opinion/google-street-view-angers-advocates-but-can-it-see-inside-your-
car/3400/)

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darkmighty
This is a different technology, those are not radars as far as I know, and I'm
not sure it's possible to build x-ray radars due to low
reflectance/scattering.

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JusticeJuice
Is there a reason why very little information showed up from the legs? Are
jeans just great at absorbing but not reflecting wifi signals?

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Jach
The problem is that legs make poor backwards reflectors due to their shape, so
the reflected signals aren't detected since they're sent elsewhere. Though the
material can make a huge difference too. (e.g. wool and nylon can
significantly reduce the reflectivity, as can the presence of wrinkles.) The
best reflective part of the body is the torso (especially the back) since it's
relatively flat.

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corvus_sapiens
So recognition doesn't work if it was calibrated while you were wearing
different clothing? That kills some of the use cases.

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Jach
The wall being made of reinforced concrete also kills a few use cases. But
human detection isn't hopeless in general. You can do pretty well (even gender
distinction) by having a physically based model of what you're expecting, e.g.
humans, on the ground level instead of at some angle, within a certain range,
potentially moving but you can detect heartbeats too, and there are models of
how the human body behaves at rest and while moving. (Edit: the assumption of
movement really helps a lot since you can diff samples and subtract out all
the static bits. So even if clothing attenuates the signal by x%, x < 100, if
the human's the only thing in the scene that's moving you're ok.)

(I'm actually impressed/jealous with what they accomplished since it was
something I tried and failed to do alone for a school project based on
[http://ocw.mit.edu/resources/res-ll-003-build-a-small-
radar-...](http://ocw.mit.edu/resources/res-ll-003-build-a-small-radar-system-
capable-of-sensing-range-doppler-and-synthetic-aperture-radar-imaging-january-
iap-2011/))

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aaronkrolik
We had a wifi radar at school. Was scary good. Only downside was that it
jammed wifi for half the engineering building

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dang
Url changed from [http://www.theverge.com/2015/10/28/9625636/rf-capture-mit-
wi...](http://www.theverge.com/2015/10/28/9625636/rf-capture-mit-wifi-
tracking-surveillance-technology), which points to this.

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govindpatel
what happens when two person stand behind one another. keeping their bodies
close to each other. Is there is any way to detect how many person are
actually standing there?

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TeMPOraL
I also would like to know the theoretical answer. The practical will be, I
guess, "wait for them to start moving around".

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Natanael_L
Physics is crazy:
[http://web.media.mit.edu/~raskar/cornar/](http://web.media.mit.edu/~raskar/cornar/)

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TeMPOraL
> _Physics is crazy_

This is my new favourite response.

