Passive Sensor that Sees Through Walls

A new technology uses the radiation given off by wi-fi devices to sense the positions of people through a one-foot-thick brick wall.

Posted on August 20, 2012 at 1:05 PM • 19 Comments

Comments

derpAugust 20, 2012 1:12 PM

Interesting, sort of reminds me of van eck boxes used in Iraq to read laptop screens inside mud brick houses. Wonder how much radiation my device is dosing me with daily if you can pick it up through walls with this thing lol

kashmarekAugust 20, 2012 1:35 PM

I am somewhat doubtful. I can't get my cell phone to handle signals through 8 inch concrete basement walls (my lower level walkout basement), but when I move to the sliding glass door in the basement, I get signal. Further, wouldn't one have to have the signal disruption occur between the wifi transmitter (cell phone or wifi router) and the exterior receiver (the disruption determines where the motion is occurring)?

Alex WAugust 20, 2012 1:58 PM

Note to self, next time the house is up for repainting - use WiFi blocking paint.

posedge clockAugust 20, 2012 2:18 PM

@derp: it gets worse.

If your wi-fi is using the 2.4GHz band then you're being hit with the exact same frequency used in a microwave oven.

MailmanAugust 20, 2012 2:59 PM

I also saw something this weekend about "femto-photography" which promises to be able to take pictures "around corners."

pjAugust 20, 2012 7:27 PM

I think there's some ambiguity between "through" and "around".

I highly doubt this device can see through solid walls, passively, using just background wifi radiation. I think this has to be a case of seeing "around" walls - a bit like using background "light bulb" radiation to see shadows "around" corners and walls.

MattAugust 21, 2012 2:30 AM

Now that we have this tech we will be able to solve all of those problems that DON'T need solving...

Danny MoulesAugust 21, 2012 3:49 AM

" I can't get my cell phone to handle signals through 8 inch concrete basement walls (my lower level walkout basement), but when I move to the sliding glass door in the basement, I get signal"

With a sufficiently advanced implementation you would only need to maintain a hold on a very weak signal; not necessarily sufficient to handle, say, a phone call. With sufficiently intelligent heuristics you could pin-point a device during good signal and then maintain an ident-lock when it moves out of the clear and into the 'dark'.

Still, you can always just blast it with interference from other devices. A bit like turning the tap on when you're being (no pun intended) tapped.

Clive RobinsonAugust 21, 2012 5:22 AM

There is actually little new in this idea other than thee post processsing techniques.

Basically the system is an ofset/standoff (ie TX and RX widley seperated) Radar using doppler detection techniques.

Arguably the first practical demonstration of radar was done in exactly this way with an oscilliscope connected to a receiver located several miles from a commercial radio transmitter working around the top end of the HF bottom end of the VHF band.

Similar wide offset systems have been looked at for dealing with "anti-radar" missiles. One such system used a "drone carried" transmitter that sent brief bursts of direct Sequence Spread Spectrum signal the doppler receivers using "matched fillter delay line" receivers then effectivly got two time delayed pulses out of the filter one from the drone transmitter and one from the target aircraft etc using several receivers the position ot the drone could be determined and from this the position of the target.

If people are old enough to remember the Russian "woodpecker" "over the horizon radar" radar this used a similar system but with a static tranmitter and a couple of receivers.

What these researchers appear to have done is use software and two or more receivers to make the equivalent of a Very Long Baseline (VLB) radio telescope thus using DSP techniques to seperate out not just the TX signals but the doppler reflections positions. Similar techniques have been used by the astromony community for finding very dense objects causing "lensing" of multiple source (stars) EM raidiation.

Oh and such narowband systems as these are fairly easy to "spoof" using multiple antennas and electronicaly controled "delay line" lengths. Which was seen as a side effect of 1980's technology commercialy available from Datong that was used as a "doppler Direction Finding" receiver.

MuffinAugust 21, 2012 5:28 AM

"If your wi-fi is using the 2.4GHz band then you're being hit with the exact same frequency used in a microwave oven."

That's like comparing candles and lasers.

kashmarekAugust 21, 2012 7:05 AM

@Danny Moules:

Will you please tell the cell phone carriers about this so they can use this technology to give me better service out of a weak signal than I am getting out of their existing "strong" signal?

Really? It seems as though there is too much technology involved here (antennas, detectable "weak" signals, heuristics, analysis, advanced implementation) to be useful. I conclude FUD.

-BAugust 21, 2012 11:51 AM

When building my new shop, I put up "Solarboard" throughout (before Tyvek, exterior paneling, roofing, etc.). When I close the metal door, it *is* a Faraday cage.

So much for my plans to stream media via WiFi from the house while working on projects.

Northern RealistAugust 21, 2012 12:22 PM

@Clive Robinson - "if people are old enohygh to remember..." -- unfortunately that's part of the problem, no one is really checking what's come before and already been done, and seems to assume that if high / advanced technology is involved it must be a new idea / concept. Perhaps it is time to add a new domain to the field of security education - "Everything Old is New Again"....

sebastian buettrichAugust 22, 2012 2:26 AM

@Northern realist and all,

and, in the TED era, not many people are really checking the physics of it all -

the waves dont care much for the business plan or even the layer2+ above, so this is neither new nor should you expect it to work very well, IRL.
i stand to be surprised, of course :)

anonAugust 22, 2012 2:03 PM

"@derp: it gets worse.

If your wi-fi is using the 2.4GHz band then you're being hit with the exact same frequency used in a microwave oven."

Last I checked all wifi is on the 2.4GHz band. So you're either completely wrong or most of the modern world is being microwaved as we speak.

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