KerberosSDR Preview: A 4x Coherent RTL-SDR for Direction Finding, Passive Radar and more

Over the last few months we've been working on a 4-input coherent RTL-SDR called 'KerberosSDR' (formerly known as HydraSDR) that is designed to be a low cost way to get into applications such as RF direction finding, passive radar, beam forming and more. It can also be used as a standard 4-channel SDR for monitoring multiple frequencies as well.

Phase coherent RTL-SDRs have been worked on and demonstrated several times over the past few years, but we've been disappointed to find that so far there hasn't been any easy way to replicate these experiments. The required hardware has been difficult to build and access, and the software has been kept as unreleased closed source or has been too complicated to install and use. With KerberosSDR we aim to change that by making phase coherent applications easier to access and run by providing ready to use hardware and software.

Thanks to our developer Tamás Peto, a PhD student at Budapest University of Technology and Economics whom we hired via the ad in our previous post, and the Othernet (formerly Outernet) engineering team who are our partners on this project, we've been able to build a working system, and demonstrate coherent direction finding and passive radar working as expected (demo videos below). We plan to eventually release Tamás' code as open source so that the entire community can benefit and build on it. Also if KerberosSDR turns a profit, we plan to reinvest some of the profits into continually improving the software and expanding the list of use cases.

KerberosSDR will be usable for coherent applications from ~80-100 MHz up to 1.7 GHz (as a standard receiver it will work down to 24 MHz like a regular RTL-SDR). The lower coherent limitation is due to the phase calibration board, and could be improved by custom creating a larger calibration PCB.

At the moment we are finalizing our prototype, and plan to begin final production within the next 2-3 months.

If you have any interest in KerberosSDR, please sign up to our Kerberos mailing list. This will help us gauge how many units to produce and will affect the final pricing. If you've already signed up to our weekly posts list, please sign up to this list too as it's a different list. Subscribers to this list will be the first to know when Kerberos goes on preorder, and the first 100 sales will receive a discounted price. We expect to begin taking preorders in within a month and to ship 2-3 months after preorders begin.

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Direction Finding

KerberosSDR can be used to find the bearing towards a signal using it's coherent direction finding capabilities. The software by Tamás currently implements several direction finding algorithms such as Bartlett, Capon, Maximum Entropy (MEM) and MUSIC. In the video below we show a quick test of the direction finding system working with a HackRF being used as a signal source, and four dipole antennas connected to KerberosSDR in a linear array. The MUSIC algorithm is used.

KerberosSDR Direction Finding Test

In the image below we also attempted to find the direction towards a known TETRA transmitter. We were able to confirm the direction with an Android compass app that points towards the known transmitter location. As the two angles match, we can be confident that Kerberos is finding the correct direction to the transmitter.

Finding the direction of a TETRA Transmitter
Finding the direction of a TETRA Transmitter

Passive Radar

KerberosSDR can also be used for passive radar. Normal radar systems work by transmitting a pulse of RF energy, and listening to the reflections from objects like planes, cars and ships. Passive radar works by using already existing transmitters such as those for FM/TV and listening for reflections that bounce of objects.

With a simple passive radar system you need two directional antennas and two coherent receivers. One antenna points at the transmitting 'reference' tower, and the other at the 'surveillance' area where you want to listen for reflections. It's important to try and keep as much of the reference signal out of the surveillance antenna as possible, which is why directional antennas like Yagi's are used.

The result is a doppler vs time delay graph, where the reflection of aircraft, cars, ships and other objects can be seen. The doppler gives you the speed of the object relative to your antenna and the transmitting tower, and the time delay gives you the distance relative to your antenna and the transmitter tower.

Below is an example time lapse video of KerberosSDR being used for passive radar. The reference antenna points towards a DVB-T transmitter at 588 MHz, and the surveillance antenna overlooks a small neighborhood, with aircraft sometimes flying over. The antennas we used were two very cheap TV Yagis.

You can constantly see the reflections from vehicles at small doppler values (low speeds), and every now and then you see an aircraft reflection which shows up at much higher doppler (speed) and further time delay (distance) points. 

KerberosSDR Passive Radar Timelapse Test 1

More information about KerberosSDR

KerberosSDR includes:

  • 4x Coherent R820T2 based RTL-SDR dongles with standard 24 MHz - 1.7 GHz frequency range
  • On board GPIO switched wide band noise source for sample sync and phase calibration
  • Special phase calibration PCB for 4x inputs. Required to make the Kerberos phase coherent.
  • On board USB Hub, so only one USB port is required on the PC
  • Shielded metal enclosure

KerberosSDR can also be extended to 8x receivers by daisy chaining two boards together, so that their clocks and noise sources are connected. We've also taken into account undesirable effects such as heat related PLL drift which can be an issue for phase coherence.

At the moment we are also investigating whether singleboard computers like the Raspberry Pi 3 or Tinkerboard can be used, and there will be a header available for powering them via the Kerberos PCB. In the future we also plan to work on optimizing the code and potentially using CUDA/OpenCL GPU optimizations for passive radar so everything runs smoothly.

Once released we plan to have extensive tutorials and documentation that show exactly how to set up and replicate direction finding and passive radar experiments with low cost antennas.

Screenshots of KerberosSDR software:

Screenshots of each KerberosSDR software screen
Screenshots of each KerberosSDR software screen

Remember, if you're interested please sign up to the KerberosSDR mailing list for announcements and the chance to get in early with the cheaper first 100 units.

Be on the look out for more interesting demos that will be posted in the coming weeks!

Update: Please note that due to a Trademark complaint, we have changed the name of this unit from HydraSDR to KerberosSDR.

KerberosSDR Updates: 27 August 18

This week we've managed to get the KerberosSDR demo software made by Tamás Peto functioning on a TinkerBoard. The TinkerBoard is a US$60 single board computer. It's similar to a Raspberry Pi 3, but more powerful. We've also tested the app running on the Raspberry Pi 3 and Odroid XU4. The Pi 3 is capable of running the software but it is a little slow, and the Odroid XU4 is a little faster than the TinkerBoard. In the future we hope to further optimize the code so even Raspberry Pi 3's will be smooth.

In the video below we used a circular array of four whip antennas connected to KerberosSDR. The TinkerBoard is connected to KerberosSDR and is set up to generate a WiFi hotspot, which we connect to with an Android phone and a Windows laptop. The Windows laptop connects to the TinkerBoard's desktop via VNC, and the Android phone receives an HTML/JavaScript based compass display via an Apache server running on the Tinkerboard. With this setup we can wirelessly control and view information from KerberosSDR and the TinkerBoard.

We've also tested the KerberosSDR system on a real signal, and have found it to work as expected. More demo's of that coming later.

For more info on KerberosSDR please see our previous announcement post.

KerberosSDR Direction Finding Test 2: Tinkerboard + Circular Array

KerberosSDR Prototype
KerberosSDR Prototype with TinkerBoard Running Computations

KerberosSDR Updates: 4 September 2018

In this post we'll show an experiment that we performed which was to pinpoint the location of a transmitter using KerberosSDR's coherent direction finding capabilities. RF direction finding is the art of using equipment to determine the location of a transmitting signal. The simplest way is by using a directional antenna like a Yagi to try and determine the bearing based on signal strength. Another method is using a pseudo-doppler or coherent array of antennas to determine a bearing based on phase information.

For the test we tuned the KerberosSDR RTL-SDRs to listen to a signal at 858 MHz and then drove to multiple locations to take direction readings. The antennas were set up as a linear array of four dipole antennas mounted on the windshield of a car. To save space, the dipoles were spaced at approximately a 1/3 the frequency wavelength, but we note that optimal spacing is at half a wavelength. The four dipole antennas were connected to KerberosSDR, with a laptop running the direction finding demo software. 

Low cost direction finding array mounted to vehicle windshield.
Low cost direction finding array mounted to vehicle windshield.

Our open source demo software (to be released later when KerberosSDR ships) developed by Tamás Peto gives us a graph and compass display that shows the measured bearing towards the transmitter location. The measured bearing is relative to the antenna array, so we simply convert it by taking the difference between the car's bearing (determined approximately via road direction and landmarks in Google Earth) and the measured bearing. This hopefully results in a line crossing near to the transmitter. Multiple readings taken at different locations will end up intersecting, and where the intersection occurs is near to where the transmitter should be. 

KerberoSDR SDR Directing Finding DOA Reading
KerberoSDR SDR Directing Finding DOA Reading

In the image below you can see the five bearing measurements that we made with KerberosSDR. Four lines converge to the vicinity of the transmitter, and one diverges. The divergent reading can be explained by multipath. In that location the direct path to the transmitter was blocked by a large house and trees, so it probably detected the signal as coming in from the direction of a reflection. But regardless with four good readings it was possible to pinpoint the transmitting tower to within 400 meters.

In the future we hope to be able to automate this process by using GPS and/or e-compass data to automatically draw bearings on a map as the car moves around. The readings could also be combined with signal strength heatmap data for improved accuracy.

This sort of capability could be useful for finding the transmit location of a mystery signal, locating a lost beacon, locating pirate or interfering transmitters, determining a source of noise and more.

KerberosSDR pinpointing a transmitters location
KerberosSDR pinpointing a transmitters location

KerberosSDR Updates 7 September 2018

For this test we parked our car to the side of a highway and pointed a cheap DVB-T Yagi antenna towards a DVB-T transmission tower, and another cheap Yagi down the road. The video shown below displays the results captured over a 5 minute period. The blips on the top half of the display indicate vehicles closing on our location (positive doppler shift), and the blips on the bottom half indicate objects moving away (negative doppler shift). 

Highway Passive Radar Traffic Monitor with DVB-T and KerberosSDR a 4x Coherent RTL-SDR

DVB-T Antennas In Car
DVB-T Antennas In Car

The resolution of each individual vehicle is not great, but it is sufficient to see the overall speed of the highway and could be used to determine if a road is experiencing traffic slowdowns or not. When larger vehicles pass by it is also obvious on the display by the brighter blip that they show. The display also shows us that the highway direction coming towards us is much busier than the direction moving away.

In the future we'll be working on optimizing the code so that the display updates much faster and smoother. It may also be possible in the future to use the third and fourth tuners to obtain even greater object resolution.

39 comments

  1. zipfly30

    How great would it be if someone started a website where folks could network and share their KerberosSDR (w/GPS) as a resource to create a networked direction-finding system. We’ve had some guys in our area intentionally jam radio repeaters. It would be nice to ‘DF’ them instantaneously based on multiple lines of bearing (LOBs).
    In fact, couldn’t we also do it with regular ‘single’ RTL-SDRs using signal strength like the KiwiSDR does it?

    • admin

      It’s possible that you could indeed set up multiple fixed position KerberosSDRs and have them each provide bearings for automatic pinpointing.

      BTW, the KiwiSDR doesn’t use signal strength, it uses TDoA (time difference of arrival) which is different.

  2. Frederick Wahl

    I’m interested in using this for falconry – tracking the transmitters attached to birds. Currently the transmitters I use are 216MHz. I DF using a 5-element yagi and offset attenuator, which works pretty good – would like to see if this is any better.

  3. Marcus Leech

    I suddenly find myself needing to think about an RFP for an imaging riometer, which would be based on an NxN array of antennae, with beam synthesis.

    What are the possibilities for “ganging” these things. In my case, ‘N’ could be as large as 8…

  4. Marcus Leech

    My own use-case would be radio astronomy. My suspicion is that I’d need to integrate their sample-alignment algorithms into my code, which isn’t a huge deal, but it’s not as nice as having hardware that is just inherently capable of doing N-channel coherence…

    • admin

      Sounds interesting. Can you elaborate a little bit on your specific use cases?

      I know a lot of RA occurs at HF. The minimum frequency of the R820T2 is 24 MHz, but the calibration board we’ll provide is probably only going to work down to around ~80 – 100 MHz (basically it’s a directional coupler). But for lower frequencies it shouldn’t be too difficult to print a larger calibration board, or increase the noise power with an external noise source. For lower HF frequencies a 4x upconverter running from the same clock might work.

      The sample alignment code should also be able to be easily implemented into other code bases.

      • Marcus Leech

        Actually, most RA occurs at VHF and above. The long wavelengths below VHF make high-gain antenna a tad awkward, and the ionosphere is partially opaque even up to 50MHz or so.
        Both simple two-antenna interferometry, and beam-forming are things that are of interest to our little enterprise: http://www.ccera.ca

        We do have “more exotic” SDR hardware, but if we can do coherence relatively cheaply, it mans we can consider more projects where coherence is a necessity…

        -Marcus

  5. Mike Frisco

    Very neat!

    I can’t say I’d have any use for the direction finding or radar features. However having more tuners in one device should hopefully make for a smoother, more compact trunk tracking experience. Maybe even have the ability to monitor two trunked systems at once!

  6. K9CQB

    I see an Android phone showing a Line Of Bearing (LOB) display in one of your photos. Will there be an Android interface for this device? That would be so useful.

    • admin

      The Android phone is just for comparing the DoA signal bearing with a magnetic bearing of the known location. But the code is running well on a Tinkerboard and Odroid XU4. With those single board computers it’s possible to create a WiFi hotspot, and VNC into the desktop to view and control the GUI on an Android phone. It’s also possible to use web server software to make the HTML based arrow display accessible via a browser on your Android device.

  7. wergor

    can you tell us more about the trademark complaint? I couldn’t find any existing hydra sdr with a quick google and us patent office search.

    • admin

      They haven’t registered in the US, only in their own country, but they have registered the .com domain. Probably no legal requirement to change, but if it’s going to be confusing i’d rather change now than have problems down the line.

  8. Danny Robinson VK6DVR

    Danny VK6DVR
    Tracking meteor trajectories:
    I wonder how good this HydraSDR system would be at reporting the trajectory of meteors, but more particular, the much larger bolides, some of which create a sonic boom as they enter the earth’s atmosphere, and a trajectory could help with the approximate ground impact location if they survived without a total burnout.

    VK6DVR Danny Robinson | VHF Weak-signal & Meteor-Scatter on 6-Metres | Perth, W.Australia

  9. Kevin Marriott

    very interested and the $99-$150 price tag is reasonable. I’d be willing to pay for the $150 now and to Beta test the HydraSDR and provide feedback!

      • Travis

        The screenshots look like something built in GnuRadioCompanion. Is this the case? Also, has there been any consideration to release a 2-receiver version, or at the very least to make the DoA app compatible with only 2 of the receivers active to make it easier to run on low-end PCs?
        Regardless, very, very excited to hear about this, and look forward to ordering!

        • admin

          Thanks! GNURadio isn’t involved in the code, it’s all based on Python and C.

          We’ve just confirmed that the 4-RX DF system works at a decent speed on the $60 Tinkerboard singleboard PC, so any PC with hardware roughly equivalent to that should be able to run it just fine. It also works on a Pi 3, but it’s a bit slower than we’d like at the moment. We’ll be working on optimizing the code at a later date too, so the speed may improve. A 2-RX system wouldn’t be very good as there are increased ambiguities with less receivers.

  10. JC

    So how does one know if they are actually signed up for the mailing list without any form of confirmation after hitting the button?

    • admin

      After pressing submit you should have received a ‘double opt-in’ email from Mailchimp (due to GPDR regs). You click on the confirmation in that email, and it should let you know that you’re signed up.

  11. AD5NL

    One potential application not mentioned here might be coherent averaging to reduce noise and improve SNR. With 4 SDRs the potential SNR gain is something like 6 dB, although the actual would depend on implementation.

    This might be useful for people who are listening for fairly weak signals (although ideally I suppose one would have an LNA and then a four-way splitter, or four separate antennas with 4 separate LNAs).

    Is this an application that HydraSDR might someday support?

    • admin

      Yes we’re considering applications like beamforming, averaging, noise suppression etc too. But remember it’s still an RTL-SDR, so we’re probably not going to be marketing this as a high end DX device.

    • Timmy

      The EOL(end of life) Rafael Micro R820T2 tuner is already being driven outside of specification
      The R820T/T2 official frequency range from 42 Mhz to 1002 MHz.
      But everyone who uses it for SDR has managed to get the PLL’s to lock from ~24 MHz to ~1.7 GHz.

      There is no trickery that can get that tuner chip to operate at 2.4GHz. Maybe if they used a R848 then possibly, but it would need major of testing and there would be a lot of bashing of heads, weeping and gnashing of teeth and even then it may not work, and the overall performance would be lower.

    • admin

      Unfortunately the R802T2 can’t tune that high, no amount of cooling would help. Possibly in the future some sort of cheap downconverter for WiFi could be developed, but we’d need to look into several things to maintain coherency.

  12. Timmy

    I am interested, but after reading MailChimp’s privacy policy, I am not signing up to their mailing list after reading through their privacy policy. It is basically agreeing to allow them to spy on you “Information we collect from other sources: From time to time, we may obtain information about you or your Contacts from third-party sources, such as public databases, social media platforms, third-party data providers and our joint marketing partners. We take steps to ensure that such third parties are legally or contractually permitted to disclose such information to us.” The more of it I read the less I want to use their service.

    • admin

      Mailchimp is probably the largest mailing list provider out there. I think what they’re talking about is one option in their admin panel where you can enable a paid premium option that uses the email to look up your *public* social media data, to get information like age/gender/location etc. Same as if someone put your email address into Google and tried to see what they could find. We’re not using that option.

      • Timmy

        I’m sorry but no, any marketing company that describes itself as providing “cutting-edge marketing technology” and being the “world’s largest marketing automation platform” and also describe their activity as “highly profitable”, ARE tracking people online.

        And by explicitly getting people to actually opt-in, they avoid being listed on the usual marketing opt-out websites, which assumes that everyone wants to have their online activity tracked, unless they explicitly say that they do not want that browser to be tracked:
        http://www.youronlinechoices.com/
        http://optout.aboutads.info/
        http://optout.networkadvertising.org/

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