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Microwave interactions with motion sensors

  1. Jun 14, 2012 #1
    Hello everyone here at the physics forum. Forgive me if I am not posting this question in the correct forum. I am not a physicist, but I do know my way around a little bit. Now on to my question. I am a security consultant who conducts security audits and specializes more specifically in physical penetration testing. For those of you who do not know what this is, PPT is a security audit in which one is given permission to try to circumvent a client's security system through physical means. I.E. breaking in, safecracking, covert infiltration, etc. Now that you have my background, a client recently asked me a question I couldn't answer because it had never come up before and I hadn't thought about it. He asked if his microwave based motion sensors could be circumvented by either flooding the space in front of the sensor with microwaves or maybe more specifically using a device (like a Tunnel Diode) that can match the frequency the sensor is using to give it a false "map" of the room. Is something like this possible?

    Thank you very much for assistance!

  2. jcsd
  3. Jun 15, 2012 #2


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    Securityman, Welcome to Physics Forums!

    A continuous wave (CW) microwave motion sensor uses the Doppler principle to detect motion of reflective targets (usually people). If the target is moving towards the unit the received frequency is higher; if moving away the frequency is lower. The receiver detects this difference and trips the alarm. Even if the area was flooded with radiation this higher/lower frequency still gets to the detector.

    Sophisticated active jamming of CW radar by enemy forces is possible: a modern fighter aircraft’s fire control radar and air-to-air missiles must be made immune to this kind of jamming. I estimate that no burglar would have this kind of equipment mounted on his body to confuse the microwave motion detectors. My advice is: Possible, but extremely unlikely. Assure your customers there is no need to worry about jammers.

    Some would-be intruders attempt to avoid detection by deliberately moving very slowly to deceive the sensors and also use the location of the dead zones to evade detection. This is why for a really secure area microwave sensors are augmented with passive infrared sensors. Lastly, if the intruder has access to “stealth clothing” he or she may not reflect enough microwave signal to be detected.
  4. Jun 15, 2012 #3
    Thank you very much for your detailed and very helpful response Bobbywhy. I figured that the two scenarios I provided wouldn't be feasible. Some of my clients are high profile tech and weapons research companies so they tend to attract some of the more sophisticated hostiles. What exactly is the "stealth clothing" you are referring to, and could you explain the CW jamming in further detail? Is the CW device separate from the missiles and aircraft, and more importantly do you know how these devices are counteracted?

    Thank you again!

  5. Jun 15, 2012 #4


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    Sorry, but much of electronic countermeasures (ECM) technology is classified.

    If you want to assure your customers that their intruder detection system is reliable (and does not generate too many false alarms) then you may consider trying to infiltrate the space yourself. Challenge that customer to a test of his suggestion. You might, for example, arm yourself with a portable microwave motion detector transmitting the identical frequency of the installed one. It may saturate the fixed receiver and allow your ingress undetected. If you could find “stealth” radar-absorbing clothing that would seem to be the easiest way to pass undetected. (if I tell you any more I'd have to kill you...JOKE!)

    “Basic radar ECM strategies are (1) radar interference, (2) target modifications, and (3) changing the electrical properties of air.[1] Interference techniques include jamming and deception. Jamming is accomplished by a friendly platform transmitting signals on the radar frequency to produce a noise level sufficient to hide echoes.[1] The jammer's continuous transmissions will provide a clear direction to the enemy radar, but no range information.[1] Deception may use a transponder to mimic the (pulse) radar echo with a delay to indicate incorrect range.[1] Transponders may alternatively increase return echo strength to make a small decoy appear to be a larger target.[1] Target modifications include radar absorbing coatings and modifications of the surface shape to either "stealth" a high-value target or enhance reflections from a decoy.[1] Dispersal of small aluminum strips called chaff is a common method of changing the electromagnetic properties of air to provide confusing radar echoes.”

    Two additional sites you may want to study:
  6. Jun 16, 2012 #5


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    I should imagine that the manufacturers would have some idea about the performance of their products in this respect. They may not want to tell you how they actually work, of course, but there should be some information available about their product testing.
  7. Jun 16, 2012 #6
    Securityman, is there any way we can know your intentions aren't nefarious?

    I know how to walk through these detectors but I don't want to tell you because I don't know you.
  8. Jun 16, 2012 #7


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    There is no way to know. If he is legit the information can be found elsewhere.

    << Link deleted by Moderator >>
    Last edited by a moderator: Jun 16, 2012
  9. Jun 16, 2012 #8


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    This thread is now closed.
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