Seeing through walls using WiFi

Turns out you don’t need to be Superman to see through walls. Researchers at University College London have developed a way to passively use WiFi as a radar system. Unlike active radar systems (which themselves send out radio waves and listen for them to echo back), passive radar systems cannot be detected.

The system is small enough to fit in a briefcase, and has been tested through a one-foot-thick brick wall. It can detect position, speed, and direction of a person moving on the other side of that wall, but cannot detect stationary object. [Karl Woodbridge] and [Kevin Chetty], the engineers behind the prototype, think it can be refined to pick up motion as minuscule as a person’s rib cage moving with each breath. For some reason we get the picture in our mind of that body scanner from the original Total Recall.

[via Reddit]

[Image Credit]

Comments

  1. charles says:

    This hit other sites some time ago. It isn’t WiFi. It only uses the same spectrum. It is rather impractical and comes with tons of caveats.

    Having anything that runs in the same spectrum within 200 feet and it’s worthless. Have anything that creates white EMI? Worthless. Large chunks of metal, magnets, really old bricks (different makeup and structure), low-e construction, appliances, moving radio reflective objects, all make this worthless.

    It is just another thing to try and get DHS to spend a few hundred million on.

    • charles says:

      Before anyone rebuffs. This is all because during the testing and development, they used a single high energy source with a known location and filtered radio background.

      Hey I live in a house with four inch thick brick walls. Router is on one side, can’t detect a thing on the other. Goes for cellphones too.

    • skeptical says:

      What’s funny is that it isn’t that hard to get outside of the 2.4ghz band. In fact, everyone seems to be late to the party – 10Ghz portables have been around for a while, and multi-band with false color and lots of trig to build a 3D model is the cool toy lately.
      Remember P.K.Dick’s* law-
      Technology always flows downhill in this order:
      Schizophrenics, then writers, then the spooks, then the rich and/or clever, the public, their kids and then it’s just part of history.

      caveat:
      Not really, I just made it up, but I’m going to attribute this to him anyway. He saw the future, darkly.

  2. woutervddn says:

    you guys have covered this technique earlier: http://hackaday.com/2009/10/01/see-through-walls-via-wireless-network/

    Cool though..

  3. Paul Potter says:

    Very impressive.

  4. mindbleach says:

    Radio-frequency ‘cameras’ like this are going to do rude things to our concept of privacy. Eventually they won’t be stymied by unexpected interference any more than a digicam with flash is upset by sunlight.

  5. John says:

    I’m curious why it’s called a passive system when the video talks about two sets of antennas, one that transmits and the other that receives the transmitted signal.

  6. The *real* information for this post is hidden behind an IEEE paywall. Just a teaser…

  7. asdf says:
    • Ben says:

      I know that HAD isn’t for political discourse, but that site is, frankly, offensive.
      The man denies the holocaust, and believes Obama to be running a “evil Obama Zionist Empire.”
      I have nothing against honest political discourse, but conspiracy theories and fabrications leave a bad taste in my mouth.

      On a brighter note, this hack is pretty cool. It’s clever of them to realize that WiFi works as a radio source for passive RADAR.

  8. Whatnot says:

    Might not be perfect but after the X’th project like that I’m seriously starting to think about how I can cheaply shield my house.. (in a not unappealing way though, sticking aluminium foil everywhere doesn’t do it.)

  9. n0lkk says:

    I admit went to visit the link with a lot of prejudice, but I’m not buying the passive radar capabilities.

  10. Alan says:

    The multiple receivers are to triangulate on the source of the Doppler shift. If there’s no WiFi present (2.4GHz) you could conceivably use your own 3G transmitter (2.1GHz) and the only thing people inside would notice is – more “signal bars” on their mobiles.

    All in all, another use for Software Defined Radios I suppose (detecting Doppler shifts, at least).

  11. Incunabulum says:

    Its not a passive radar – the correct term would be “semi-active” as it still needs an emitter, the emitter and receiver just don’t share an antenna like most radar.

    Militarily this is most commonly used to illuminate and steer missiles to targets without the missile itself having to carry an emitter and power source (it basically provides realtive bearing data to the missile and the missile steers itself towards the reflection).

    And I don’t see how it will be able to do all that is advertised if the receiver doesn’t precisely know the relative locations and timing of the emitters.

  12. Incunabulum says:

    Of course, thinking about it some more, I can see how you can detect motion and relative bearing – I can’t see how you get a decent location fix in 3 dimensions without knowing where your emitters are in 3d.

    • skeptical says:

      Your receiver fixes the point, thus with two emitters you have three points. It’s not accurate enough to pop a fly with a laser, but works fine for locating the odd human and his pop gun.

  13. Doktor Jeep says:

    Very interesting. I recall that woman who does a lot of hacks (I forget the name, sorry) making a scanner with what was either a router or a satellite receiver.

  14. Destate9 says:

    “Sonar. Like a-”
    “That’s right, like a submarine Mr. Wayne. Like a submarine.”

  15. bobjandal says:

    “passive radar systems cannot be detected” – that’s wrong – passive radar antennae can be detected due to their innate retroflective properties when excited with a strong enough impulse.

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