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

KerberosSDR is now available for pre-order over on Indiegogo!

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

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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. 

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!

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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). 

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.

KerberosSDR Updates 27 September 2018

In this post we're showing some more passive radar demos. The first video is a time lapse of aircraft coming in to land at a nearby airport. The setup consists of two DVB-T Yagi antennas, with KerberosSDR tuned to a DVB-T signal at 584 MHz. The reference antenna points towards a TV tower to the west, and the surveillance antenna points south. Two highlighted lines indicate roughly where reflections can be seen from within the beam width (not taking into account blockages from mountains, trees etc).

The second video shows a short time lapse of a circling helicopter captured by the passive radar. The helicopter did not show up on ADS-B. On the left are reflections from cars and in the middle you can see the helicopter's reflection moving around.

We are expecting to receive the final prototype of KerberosSDR within the next few weeks. If all is well we may begin taking pre-orders shortly after confirming the prototype.

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Bob Thornton

Can we network multiple units together for more accurate Direction Finding?

I would like to have one set up on out tower site, base station and mobile vehicle for good triangulation.


I now own 4 KerberosSDR devices, 2 of them are connected to Tinkerboard-S and 2 of them are running on RasPi3-B+
I also want to network them so we can geo-locate a ‘jammer’ more quickly and accurately. We think we may have an intentional repeater jammer (for our ham radio repeaters) and we’re worried he may be mobile. The only way to geo-locate a mobile jammer in a timely manner is to use a few stationary RDF sites that are networked together – and of course networked to a mobile KerberosSDR just for ‘last mile’ geolocation.


I love you guys!!!
I’ll go there and check it out.
You realize this is going to make everybody go out and buy more KerberosSDRs just to tinker with this?


très interessant, mais est-ce que cela fonction sous windows10?
bonne continuation.


I bought 2 KerberosSDR’s and I bought a Tinkerboard S and a Raspberry PI 3+ to use each with.
I was going to connect them with a short USB to MicroUSB cable, but then I noticed that the Tinkerboard and Pi will ‘fit’ directly onto the KerberosSDR’s board via the 40-pin header bar. When you remove the enclosure bottom there’s even instructions on the board next to the 40-pin header bar that say:
“Please remove JP2 jumper for Raspberry Pi operation”
“Power booth KerberosSDR and RPi fromUSB PWR.”

Will this work with both the Pi and the Tinkerboard?
What are the drawbacks?


What is the production and shipping status at this time?


I was looking at your setup for passive radar. I use an RTL-SDR dongle for AIS on my sailboat. It was nice to see traffic and the circling airplane on your tests. But in both scenarios you and the reference signal was stationary. In a sailboat the reference signal will be stationary but the boat will be moving. Albeit under 9 MPH but course changes would be an issue. If using Yagi antennas one would have to change the aim of the antenna if the boat tacked onto a different course. Has it been tested at all in this type of scenario? With Omni directional antennae?


Is it possible to use Downconvertor to get it find directions for higher freqvencies above 1.7 ghz?


As long as the received phase difference is maintained. So multiple down converters with a common lower jitter reference clock from all of them and using equal cable lengths in each to maintain the phase difference.

christo nel

I am also interested in this. Just to be clear on what your answer means.
Is it possible to give a better reference to the board as it is?
What is the tuning range of the receivers?



The first question I do not know for sure, but there appears to be a U-FL connector marked “CLK_I/O” on the top right of the board.
That is in the article above search for “1.7 GHz” which is only mentioned twice.

James Finch

Wondering if there is an external clock capability to refine into a GPSDO system?

Would be interesting to couple the position and clock information from a GPS for both functions.


Thank you!


It’s stated in the write-up that the RPi3 is a little slow for this application. Was a 32 bit OS being used on the RPi3 at the time? If so, would it be worth trying a 64 bit OS on the RPi3 for a performance improvement? This one here seems quite stable and versatile..

.. and offers both CLI and GUI versions.
In my experience it out-performed the 64 bit RPi3 openSUSE image that was released around the same time.


I appreciate you taking the time to try this, thanks! I incorrectly assumed the older pi3b was being used in testing, rather than the newer pi3b+, sorry to send you down the garden path.
Perhaps this 64bit OS here would be more useful:
I have no experience with this particular distro but it appears to be suitable for the pi3b+ and may work better than the previously suggested distro for this application.
This particular 64bit OS has shown considerable improvements (~30-40% in some use cases) over the 32bit alternatives – see paper here:
Regardless, great to hear that the pi3b+ now produces good results using the 32bit OS!

Bruce P. Smith

Very interested in building a unit with Raspberry Pi if proving out. Presently have Byonics DF and working on Rashawk . Bruce P, Smith, LtCol., Civil Air Patrol, Florida Wing SAR Officer


Hi. Do you ship to military intallations such as APOs? Thanks.


I’m not ready to pay by my CC. Is it possible to pay by Paypal ?


There are devices used in vessels called sattelite compass, those devices have a set of gps antenas (2 or 3) mounted in a line or triangle set, usually mounted permanently on a mast. They compute the signals from the three antenas and are able to find the heading of the vesel like a magnectic compass, but instead of using earth’s magnectic field it uses GPS signals. I read some time ago that they do that by calculating the fase diference on the received signals.
Could such a task be accomplished using kerberos as a receiver?


You would need external filters and externally powered LNA’s (no Bias-T) to be able to receive GPS. But if you have the required hardware to be able to see signals from the GPS satellites then, on paper at least, I don’t see why not with a large enough spacing between the antennas.


Greetings. I am very interested in the KerberosSDR for DF and remote sensing applications like passive radar as well as other non-coherent applications like monitoring and recording several radio channels simultaneously. I plan on using Linux based software for these purposes and I plan on making my setup mobile, that is to say laptop based. I notice that in the KerberosSDR Direction Finding Test youtube video that the laptop being used is a Dell and it appears to be running an Ubuntu based Linux distro. For compatibility’s sake I’d like to know what model Dell laptop was used in these tests, I’d like to try to acquire that same model for use in my setup or if you have a different make/model you would recommend that plays well with Linux, I’d love to hear your thoughts. Thanks.


I´m planning to buy five, any discount?


Is the source code for the recent experiments currently available? It may perhaps accelerate development if it’s open to the community for contributions.


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?

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.

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…

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…

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:

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…



Have you tested the range of the calibration now? Will it work up to 1.7 GHz?

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!


what about the Rock64?


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.

Andres M.

Cuando el soft de utilizacion, sea en Windows,seria interesante para mi.


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.

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

John Scherer

This would be a fun application! Reminded me of a paper I saw some time back about passive radar imaging of meteor trails. I had to go dig it up!

John Scherer

This would be a fun application! Reminded me of a paper I saw some time back about passive radar imaging of meteor trails. I had to go dig it up!

And This one on Hackaday:

Dan Finch

Perhaps you can apply direction finding to meteor skip or broadcast stations.


Damn this looks cool. Eagerly awaiting more news!

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!


is the application for windows ?


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!


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


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?

Sdr funhouse

Would be great if you could use it for 2.4 ghz wifi. Would the chip be able to do that with some cooling?


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.


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.


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:




interesting, will keep an eye on it.
can you give us a price estimate?

Doc Oct

That’s pretty reasonable. Definitely interested if it works out.