Bike-Mounted Synthetic-Aperture Radar Makes Detailed Images

Synthetic-aperture radar, in which a moving radar is used to simulate a very large antenna and obtain high-resolution images, is typically not the stuff of hobbyists. Nobody told that to [Henrik Forstén], though, and so we’ve got this bicycle-mounted synthetic-aperture radar project to marvel over as a result.

Neither the electronics nor the math involved in making SAR work is trivial, so [Henrik]’s comprehensive write-up is invaluable to understanding what’s going on. First step: build a 6-GHz frequency modulated-continuous wave (FMCW) radar, a project that [Henrik] undertook some time back that really knocked our socks off. His FMCW set is good enough to resolve human-scale objects at about 100 meters.

Moving the radar and capturing data along a path are the next steps and are pretty simple, but figuring out what to do with the data is anything but. [Henrik] goes into great detail about the SAR algorithm he used, called Omega-K, a routine that makes use of the Fast Fourier Transform which he implemented for a GPU using Tensor Flow. We usually see that for neural net applications, but the code turned out remarkably detailed 2D scans of a parking lot he rode through with the bike-mounted radar. [Henrik] added an auto-focus routine as well, and you can clearly see each parked car, light pole, and distant building within range of the radar.

We find it pretty amazing what [Henrik] was able to accomplish with relatively low-budget equipment. Synthetic-aperture radar has a lot of applications, and we’d love to see this refined and developed further.

[via r/electronics]

19 thoughts on “Bike-Mounted Synthetic-Aperture Radar Makes Detailed Images

    1. The broader the beam, the better. Amazing fact: SAR resolution is independent of distance. The further something is, the more times it is pinged. In SONAR that call the result a “fish arc”, that funny shape caused by the distance to a target changing as you approach and pass by. The limiting factors are probably position, heading, and speed precision, hardware noise, sampling jitter, etc. This is really cool.

        1. The math is far from trivial. I would ignore the FFT. It is just used as a fast way to do de-convolution and correlation, which are long tedious processes of multiplication and addition. And there is a lot going on in a convolution integral or sum, but it is the same thing done over and over across a range – for each output point! With a handle on con/cor one can muddle through the rest.

          I think he sends a chirp, which a linear FM signal and is the optimal signal for detecting itself among noise. It is also the optimal filter to use in detecting the refelcted copy of itself. A long chirp goes out and is reflected back. When you correlate it with itself, there is a sharp peak when everything lines up. They call it pulse compression because that long pulse can be used to precisely measure distances and the output is very short.

  1. how much radiation is this putting out?
    microwaves are not good for you
    it there is enough power output to map through buildings I would be very concerned about anyone nearby

    1. shoo fear mongerer, shoo

      read about the radar itself – https://hackaday.com/2014/12/03/extremely-detailed-fmcw-radar-build/
      23dBm output power (200mW), harmless

      Microwaves are no more dangerous then UHF or VHF, if you stand too close to a working antenna fed by a 100W amp for long enough, you can get burns from those as well. Something like your cell phone however, would never be able to burn you with RF, same goes for almost any hand-held radio. It’s all about the power. The lights in your room probably dump more energy onto you then this small radar can.

      1. @mack_10 Little understanding, causes some people to have big fears. Sometimes putting things in perspective can help, or just cause them to have even more to fear.

        A typical microwave oven is legally permitted by the FDA (21 CFR 1030.10) to leak about 1dB less energy, than this device intentionally outputs. Or to summarise from the document the attenuation used in the construction of a microwave oven must, after it is sold, reduce the energy emitted to a maximum of 5mW per square centimetre at a distance of 5 cm from the external surface of the oven (which would be a power density from an isotropic antenna of about 22 dBm).

        The tiny amount of energy used is a bit more focused with this device using a horn, but after you factor in free-space path loss (inverse square law), and the attenuation caused by buildings, your fears sound a lot like “If someone gently whispers into a speaking-trumpet (bullhorn) a mile away, could that cause me to go deaf.”

        There is just not enough power.

        1. Another factor that points to it being unlikely to cause harm: microwave ovens operate at 2.4ghz, the resonate frequency of water, to make water molecules vibrate thus heating your food. This radar operates at 6ghz, which does not effect water in the same way.

          1. ” 2.4ghz, the resonate frequency of water,”
            Please stop propagating this grossly incorrect meme. 2.4 GHz is nowhere near any sort of “resonant frequency” of water.
            The absorption peak of water is around 10-20 GHz, depending on temperature. If a microwave oven were to operate at that frequency, the penetration depth would be just a millimeter or so, rather than the 20 mm or so it is at the usual microwave oven frequency of 2.45 GHz.

          2. @Paul Are you SURE that the absorption peak of water is around 10-20 GHz
            https://i.imgur.com/R4yaH0M.jpg
            My guess would be that the peak absorption would be in the PHz range for each of the three possible Molecular vibration modes of water molecules (Symmetrical stretching, Scissoring (Bending), Asymmetrical stretching). The O-H bond has a bond length of about 96 pm, and that is why I would predict it would be in the petahertz range.

            And I know that the above plot is for water vapour, but the overall shape would is similar
            (red) liquid water
            (green) atmospheric water vapor
            (blue line) ice
            https://upload.wikimedia.org/wikipedia/commons/9/97/Water_infrared_absorption_coefficient_large.gif

          3. @Truth: not having directly measured it myself above 1.3 GHz (the limit of my instruments at the time), I have to put some faith in the literature. See for example, Meissner & Wentz “The Complex Dielectric Constant of Pure and Sea Water…” (doi: 10.1109/TGRS.2004.831888) , especially Fig 3. More accessible is http://www1.lsbu.ac.uk/water/microwave_water.html#microw — see (e.g.) Fig 1 and the following figure. Gabriel & Gabriel also did very thorough study for the US Air Force of measurements in body tissues, some of which are predominantly water, and agree well with the the above. A calculator using their data is available at http://niremf.ifac.cnr.it/tissprop/

            Short answer: yes, absorption in liquid water peaks around 10-20 GHz, and is much greater than at 2.45 GHz. Of course, there are other (and higher) absorption peaks, especially as you go higher into the THz, LWIR and all the way to visible.

            In the PHz, which is in the ultraviolet and soft x-ray range, you’re going to get entirely different absorption mechanisms (ionization, mostly), but no resonance effects.

          4. IIRC there are 3 ways to heat up water with RF. First is a Maxwell Induction Current in which the electric field slams polar molecules back and forth and causes colisions. 1MHz will do. Second is rotational. A frequency that makes a polar molecule rotate, which is the Microwave Oven. Third is a frequency that makes the molecule vibrate due to the chemical bonds, which is the much higher frequencies.

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