Radar gun teardown

[Jeri Ellsworth] is at it again, this time she takes apart a hot wheels speed gun and in the process she does a good job of  explaining how radar can be used to measure speed.  She also demonstrates a way to determine if an object is approaching or receding from the radar gun.

The Doppler shift is one way to remotely measure the speed of an object. It works by measuring the change in frequency of a wave after it strikes an object. Rather than measuring the Doppler shift of the returning wave most radar guns use the phase shift. The reason is that the frequency shift of a relativly slow object (60mph), to a relitivly high frequency signal(10GHz) is small (about 0.893Hz), where the phase shift varies based on the distance of the object.  This is all just a stepping stone in her quest to build a crude TSA body scanner.

30 thoughts on “Radar gun teardown

  1. According to a German TV expose of TSA full body scanners (watch the video after the link), they are great for peeping-toms, but not so good for detecting bomb components. After carrying bomb parts through a scanner undetected, the guy in the video assembles and ignites a thermite bomb (partially melting the frying pan he uses to hold the demonstration).


  2. The body scanners at an airport are backscatter x-ray machines. They use a very small amount of radiation to scan you, as it only has to penetrate clothing, and not flesh. Also, it’s about the same amount of radiation you’ll get in two minutes of flight at 30k feet. Being high up in the atmosphere exposes you to far more radiation than being on the ground, as there’s far less atmosphere to protect you from solar and cosmic radiation.

    Flying in the plane is what exposes you to high levels of radiation, not the backscatter x-ray machines. People really need a sense of perspective on these things. And yes, it’s enough radiation, that if you fly often, you can increase your risk of getting cancer. But, it will be from the flying, not the x-ray machines.

  3. There is a fundamental difference between different types of radiation, and how they are delivered. Radio and UV are radiation for one. Second, the backscatter xrays concentrate on the skin, instead of passing through harmlessly. Go back to the TSA blog Bob and choke on it.

  4. I worked at Raytheon this summer. Anyways they build commercial radar. The circuits are fascinating, as often the only components on the boards are transistors. The inductors and capacitors are etched on. I’d imagine Jeri would love a tour of that place :D.

  5. The scanners aren’t going anywhere; these days, unreasonable search and seizure means only the things that the folks doing the search/seizure can’t do easily. So that’s out. And even if it isn’t easy, if it can be linked with our current boogeymen (formerly communists and drug dealers, now terrorists and pedophiles) it gets a free pass.

    In addition, arguments about radiation won’t go anywhere – Look at cell phones. Basically, if it doesn’t kill you within a year, it isn’t harmful.

    It took 4 decades for warnings about smoking to get any traction, and we already knew about long term side effects in the mid-1930s. And even though the folks making cell-phones have backed down drastically on tx power for some reason, our current mantra is that “radiation must be ionizing to cause anything bad to happen”. So you can forget that approach, too.

    I will address one aspect though – look at the example images provided by manufacturers. I’m not talking the faked ones, I mean the straight from the TSA and manufacturer examples. You can clearly – CLEARLY – see bone structure in the legs and in many cases note Hip and rib structures, and sometimes shadows of fibroids and colon contents. >2cm penetration is not what I’d consider surface detection.

    The systems are clearly capable of running at variable power levels. But whatever – it’s a done deal. It won’t be long before insurance companies are onto these things – that is, after they finish correlating your legally purchased cellphone location data to the locations of drinking establishments – and start using the scanners to calculate your BMI visually using body volume/weight models.

    Btw, if you like google street-view, look up ZBV on youtube. Very cool technology, but flawed. The trucks are already wide-spread.

  6. Comparing the risk of backscatter X-rays to anything else is a false
    comparison. It’s used to allay fear when you don’t have a scientific
    basis for your argument.

    We should only consider the risks by comparing against not taking
    the risk
    . The fact that it’s small in comparison to anything is

    We get dental X-rays because tooth decay is common and life
    threatening if left untreated. Thus, the risks of not having one are
    much higher than the risks of having one and it’s a good trade off.

    Not so for backscatter at the airport. It’s a risk which is much
    smaller than the risk of not having them.

    Note that we’re in the position of completely trusting government
    and industry about the safety, and lots of really smart people who
    are experts in the field are claiming that they might be dangerous.

    Here’s my complete take on the risks analysis.


  7. 60 mph (28.8 m/s) gives me 960 Hz at 10 GHz, not sub-1Hz. Also, measuring phase shift *is* measuring the doppler shift. You’re just not measuring the absolute frequency that’s coming back (because measuring 10Ghz with a Hz accuracy isn’t fun), but you essentially de-modulate the received signal. What you get back (after a lowpass) is is the received signals shifted down by the transmitting signal – which is exactly what you’re looking for – a signal with the frequency *difference* (superposition law ftw). That also explains why you can’t measure the direction with a real signal – you have to move to a complex signal, using two transmitters and receivers (usually with a different polarization).

    You end up with the 960Hz (for 60mph) as your baseband signal, which is pretty simple to measure.

    The phase shift is *not* depending on the distance of the object (and Jeri never says that). If you want to measure the distance, one way is time of flight. Of course it’s pretty hard to send a short pulse, then measuring the sub-microsecond time. One way to solve this is to modulate the transmitting signal, for example by modulating the frequency. This can be easily done, usually by applying a saw-wave onto the supply voltage (since changing the supply voltage also changes the frequency of the oscillator). If you FM-demodulate the incoming signal, you can measure the time of flight indirectly, thus deriving the distance.

  8. Just to add to tmbinc, you don’t need 2 transmitters and 2 receivers. You only require 1, but mix the signal received both once with the raw transmitted signal and once with the transmitted signal which has a 90 degree phase shift applied. That way you basically get a quadrature output and can compare the phase of the 2 output signals to determine direction of travel.
    Most commercially available microwave modules do this mixing for you so all you need to do is measure signals <300Hz and compare phases – relatively trivial with a few opamps and a micro.


    You are not entitled to you own facts. If you would like to make such a claim than please post source documentation. Who performed the test to make such a finding? What equipment was used?

  10. To all the bodyscanner whiners: there’s a difference between backscatter x-ray and tetrawave scanners. Both are in use.

    Try to specify which you are complaining about when complaining.

  11. I think it might be interesting to try and get doppler from a laser pointer and a IR sensitive transistor.
    Use a CW signal and then use the same reflected/transmitted wave interaction to measure doppler.

  12. @draeath doesn’t it depend on the altitude? I’m pretty sure that a hole through the airplane would heavily alter the atmosphere inside the vessel :) I’m not saying this is enough, just that it could be a big problem (depending on size since i don’t think a small amount of thermite is going to melt a very large hole through a plane).

  13. @Eirinn I wrote the software that goes into [some] aircraft cabin pressurization systems.

    The short answer is it’s a serious event, but no one is expected to die.

    At high altitudes, passengers will go unconscious in 15 seconds or so (and the length of time you can hold your breath doesn’t affect this). The masks will fall and everyone is expected to put them on immediately.

    The same is true for the pilot and crew, but their “masks” are fully pressurized atmosphere units and not the trickle of oxygen that the passengers get.

    The emergency procedure is to dive the aircraft as fast as possible to a safe altitude. Once below 12,000 feet or so, the passengers will wake up.

    It’s the “emergency procedure” that’s important. Pilots are trained to take that action in the event of a sudden decompression, and bring the aircraft to an altitude where having a hole in the fuselage isn’t life threatening.

    (I’ve glossed over some details for simplicity)

  14. @alan turing’s dog

    Well I can think of a few reasons to reduce Tx power of cell phones without the ‘OOOOOO it’s gonna gimme cancer in my brainz’ reason. Higher Tx power -> lower battery life, components \emph{may} be larger, and it \emph{may} force reduce the number of users able to connect (just a guess as there is a finite amount of codes, time slots, and frequency bins with increases powers, comes increased range, maybe enough for more towers to receive the signal and thus that code/time slot/frequency slot is in use)

    I also call BS on this ‘the X-Rays are concentrated in the skin’. So what tells the X-Rays to stop right within the skin layer? I’m more for the millimeter wave as it doesn’t use X-Rays and provides a much more ‘intimate’ picture for our wonderful friends in the TSA. The millimeter wave is a much more interesting method as it uses Frequency Modulated Continuous Wave RADAR (which I suspected first over a pulsed UWB RADAR). FMCW radar, in principle, is much closer to hobbyists (with lots large chunk, about $1000??? + prototype costs, of change and RF circuit know how) to get small range resolutions. I think I remember seeing a DDS or other digital oscillator that can sweep several hundred megahertz, maybe close to 1 GHz, on digikey. FMCW RADAR should be able to tell the distance and the velocity (speed + is it coming closer or moving further away) of an object.

  15. Very interesting video, and Jeri sounds hot. But what I want to figure out is how to connect a camera to the Hot Wheels Radar Gun so I can nail those SOBs who go 50MPH on my dead-end, school-zoned (20MPH only) street with lots of kids running around.

  16. You can also get psuedo-quadrature by splitting the IF output into two and using two sample-and-holds fed with a clock 90 degrees out of phase or you can perform the quadrature split at a lower IF frequency if your 10 GHz Gunn/DRO source doesn’t provide that output.

  17. For version 2.0 would be a nice feature to add also the target’s acceleration calculated as the derivative of the speed, and the position delta integrating the speed signal output.

    Aren’t like that how the intelligent car anticollision warning radar systems works? ;)

  18. @draeath and Eirinn: The point isn’t the /type/ of bomb that got through, but that one got through. Besides, all you need is some mercury (http://en.wikipedia.org/wiki/Mercury_(element)#Mercury_and_aluminium).


    1) I can’t wait for that LNB article. I have a big pile of the suckers kicking around (which I’ll put on the HaD Classifieds if anyone wants some).

    2) Seriously, what’s up with the dark videos?

    3) Umm, yeah, the cop thing was quite freaky. Not your best idea :P

  19. Yeah, me too – Jeri’s videos all seem to need better lighting and more contrast. Don’t know what tools she’s using, but contrast and balance is easy to come by.

    And that cop thing… Well, as the safety SS has been claiming, some derangement was inevitable for her, but that was kind of sudden. $5 says she’ll start using hand puppets any day now.

    @cornelius785 – you asked:
    “So what tells the X-Rays to stop right within the skin layer?”
    Basically, absorption and backscatter, the heart of most imaging technologies. The radars are called back scatter because they bounce off harder atoms in the body – mostly carbon, I’d guess. That’s why old school carbon paper (and not the fake purple ink capsule kind) blocks these scans.

    These cannot penetrate very far without being attenuated to the point where the returned signal can be detected.

  20. oops, I meant cannot be detected.

    @draeath – I wouldn’t go so far as to claim that thermite would be harmless to heavy aircraft. I don’t know what airbus/boeing fuel systems look like, but I’ll bet the tanks are under the cargo deck. I doubt any one guy could get a significant amount of thermite mixed up, and that the odds of bringing a plane down are pretty damn slim, but it’s not really a non-event, you know?

    Actually, I’m surprised they haven’t been trotting out the scary anthrax thing, which always plays to the “momma, I’m scared!” crowd.

    Releasing any of the thousands of “killer” bio agents on a plane would be waaaaaaay worse than blowing it up – but I guess that until someone in the administration has a fiduciary interest in selling a solution, it won’t make the evening news.

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