Tomographic motion detection

Typical alarms use PIR (passive infrared), microwave or ultrasound detectors for motion detection. PIR are by far the most common type of detector – they work by detecting changes in infrared emitted by warm bodies. They are cheap, very reliable, and actually quite hard to beat.

Laser break beams are only really seen in films, though simple active infra-red break beams are often used on scaffolding alarms.

The problem with all of these is that they cannot see through objects. A common method of circumventing PIR detectors is to “mask” them – you either cover them  using paint (or another infrared opaque coating) or simply put something like a box in front of them. Higher security systems have “anti-masking” detectors which use an active element to check that their view has not been masked.

It can mean that complex, cluttered, or continually changing spaces need a lot of PIRs to be adequately covered.

Step in a new type of motion detection – tomographic motion detection. This sounds really clever and innovative. You might have heard of tomography from the medical world – CT scan stands for computerised tomography. It means “imaging by cross section”. Xandem have come to the market with a new detector that uses 2.4GHz radio signals to detector motion in a space.

A group of wireless nodes form a mesh of connections, as shown in this image from the patent:

Mesh network

Mesh network

Each one of those lines represents a radio path. The system uses 2.4GHz signals, the same as with WiFi or Bluetooth. These are heavily attenuated by anything containing water – such as the human body. A human body placed in the radio path of any two nodes will reduce the received signal strength (RSS).

By carefully measuring the RSS from each node to each other and doing some clever processing, you should be able to build up an image of what the area usually looks like. Any significant disturbance would signal an alarm. Hence, motion of a human body can be detected.

This would work through walls, shelves, furniture and so on – as long as the signal strength is attenuated too much.

This is clever stuff. Very easy to fit (though you do need power to each node), and probably very hard to beat. It is expensive though.

For those interested, here is a link to the patent:

And I have pulled a picture of the PCB from the FCC report on it:



The markings on the main IC are not visible, but based on the frequency, size of the package, crystal frequency, crystal connections and antenna connections, this is a TI CC2540 RF SoC – a brother to the CC1110 RF SoC, using an 8051 core connected to a RF transceiver.

Interestingly there is a micro-USB and debugging connector on the board as well!

5 thoughts on “Tomographic motion detection

  1. Permalink  ⋅ Reply

    Myke Scott

    September 9, 2013 at 9:44am

    Many thanks for your clear and in-depth description of the tomographic motion detection principle. However, I have two questions:

    I live in an old farm house with thick rammed-earth walls. I want to put a perimeter detection ring in my yard around the house to warn me of intruders (thugs) before they get close to the house so I have time to barricade myself and my family in a strong room from where we can make rescue cell phone calls (we live in a part of the world where murder, rape and mahem are rampant). My concern is that the radio paths of the detectors in the mesh will not be able to properly communicate with each other because of so many thick walls they have to go through. On Xandem’s website they only speak of using this system in warehouses or inside of houses where I assume the walls are not very thick and the detectors are close to each other. Do you have any opinion on this?

    2. Will the radio signals created by the 2.4GHz transmitter interfere negatively with my existing wifi modem/router?

    Many thanks in advance for any assistance you can render.

  2. Permalink  ⋅ Reply

    Ronald Henderson

    May 25, 2014 at 10:30pm

    Came upon a nice little article about TMD here.

    • Permalink  ⋅ Reply


      May 18, 2016 at 11:11am

      2.4GHz outdoor based detection will work until the first rain or fog, since 2.4GHz is affected with water…

  3. Permalink  ⋅ Reply


    August 24, 2015 at 9:00pm

    Hi. Question please. I know this is microwave,will this be able to work outside,because of trees and so on. Please let me know. Thanks

  4. Permalink  ⋅ Reply


    May 18, 2016 at 11:09am

    how many sensors you need to cover an area of ~30m2?
    what would happen if someone moves around (not inside) an area covered with wireless nodes?
    Can you detect motionless human presence with tomography? Since radio waves penetrate through walls, how can you focus the detection to a specific room area?

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