KrakenSDR is a Software-Defined, Coherently Operated, Five-RX-Channel Radio Based on RTL-SDR

A coherent radio allows for very interesting applications, such as radio direction finding, passive radar, and beamforming. Some use cases include:

• Physically locating an unknown transmitter of interest (e.g. illegal or interfering broadcasts, noise transmissions, or just as a curiosity)
  
• HAM radio experiments such as radio fox hunts or monitoring repeater abuse
  
• Tracking assets, wildlife, or domestic animals outside of network coverage through the use of low power beacons
  
• Locating emergency beacons for search-and-rescue teams
  
• Locating lost ships via VHF radio
  
• Passive radar detection of aircraft, boats, and drones
  
• Traffic-density monitoring via passive radar
  
• Beamforming
  
• Interferometry for radio astronomy
  

Early Access & Special Pricing for Campaign Backers!

We’ve already obtained all the long-lead-time parts for the first batch of 1,000 KrakenSDRs, so the first batch will ship about six months before any others. If you back the project during the campaign (even if you are not in the first 1,000), you’ll save $100 off the eventual retail price of $399.

KrakenSDR is KerberosSDR++

The previous version of KrakenSDR was known as KerberosSDR, which we successfully crowdfunded on Indiegogo. All backers of KerberosSDR received their orders and the relevant source code for direction finding and passive radar. KrakenSDR improves upon KerberosSDR in several important respects:

• Automatic calibration hardware. It is no longer necessary to manually calibrate and disconnect antennas during calibration. It all happens automatically when you change frequency. This will allow for KrakenSDR stations to be remotely operated.
  
• Five channels. KrakenSDR has five channels instead of four, which greatly improves-direction finding accuracy.
  
• Low-noise design. KrakenSDR has a cleaner spectrum with much less internal noise than KerberosSDR.
  
• USB Type-C ports and rugged, CNC-milled enclosure. KrakenSDR is built for high reliability in the field.
  
• Interface with external devices. Bias tees on all ports allow for LNAs and other devices to be powered easily.
  
• Improved DAQ, DSP, and GUI software. Built on the foundation of the KerberosSDR software, the KrakenSDR software adds autocalibration, tracking of intermittent signals, greater stability, arbitrary processing-block sizes, and a new web-based GUI.
  
• Software upgrades. Improvements to existing companion software and plans for new companion software.
  
• Custom Android app Custom Android app that can automatically determine the location of a transmitter and provide automatic turn-by-turn navigation to the transmitter location.
  

Features & Specifications

• Five-channel, coherent-capable RTL-SDR, all clocked to a single local oscillator
  
• Built-in automatic coherence synchronization hardware
  
• Automatic coherence synchronization and management via provided Linux software
  
• 24 MHz to 1766 MHz tuning Range (standard R820T2 RTL-SDR range, and possibly higher with hacked drivers)
  
• 4.5 V bias tee on each port
  
• Core DAQ and DSP software is open source and designed to run on a Raspberry Pi 4 (see links below)
  
• Direction-finding software for Android (free for non-commercial use)
  
• Custom antenna set available
  

What Do You Need to Get Started?

You will need the KrakenSDR, a USB-C cable, a 5V 2.4A+ USB-C power supply, and appropriate antennas for your application, such as our magnetic whip antenna set.

For computing we recommend a Raspberry Pi 4, which we will be providing ready-to-use SD card images for. Optionally for direction, finding you will want an Android phone or tablet with mobile hotspot capabilities, GPS, and compass, ideally produced within the last 3-4 years.

How KrakenSDR Works

KrakenSDR makes use of five custom RTL-SDR circuits, consisting of R820T2 and RTL2832U chips. The RTL-SDR is a well-known low-cost software defined radio, but if you throw five units together and use them on the same PC, they are not "phase coherent": each one will receive signals at slightly different phase offsets from one another. This makes it difficult or impossible to precisely measure relationships between signals that arrive at different antennas.

To achieve phase coherence, KrakenSDR drives all five RTL-SDR radios with a single clock source, and contains internal calibration hardware to allow the phase relationship between channels to be precisely measured and corrected for. Additionally, the overall design of KrakenSDR was considered to ensure phase stability, with care taken in the heat management, driver configuration, power supply, and external interference mitigation.

KrakenSDR Software

Our coherent SDR software is based on three important factors:

• Open source We provide open source code for the Data Acquisition (DAQ) software which is used to ingest RF data from all five antenna inputs, automatically calibrate and achieve phase coherence via the switches and noise source, and provide coherent samples for the next layer. This DAQ code typically runs on a Pi 4, or similar single board computer, but could also run on a PC.
  
• DSP code for specific use cases Our open source DSP code supports direction finding and passive radar. In the case of direction finding, our code implements direction finding algorithms such as MUSIC, which can also run on the same Pi 4, or PC as the DAQ code. We also provide open source DSP code for our passive radar. (As passive radar is more computationally intensive, this particular DSP code may run best on a more powerful machine.)
  
• Application layer We make use of the data coming out of the DSP layer by plotting and logging it. Generally, programs in this layer run on a separate machine. For direction finding, we are providing a free license to an Android App for mapping, logging data, and automatically estimating the transmitter location.
  

KrakenSDR Web Interface

The new KrakenSDR software comes with an easy-to-use web interface for setting up a direction finding system. With this interface it is possible to set the frequency, gains, and other advanced settings related to the DAQ code. You can also monitor the live spectrum view and graphs of the direction finding algorithm output.

Custom KrakenSDR Android App

In addition to the web interface, we have developed a companion radio direction finding Android App that can automatically determine the location of a transmitter. Since a typical Android phone has capabilities that already include necessary sensors and software like GPS, compass, mobile data, and mapping, we have been able to make use of these features to create an affordable radio direction finding system.

An example scenario might see the antenna array mounted on the roof of a car, and the KrakenSDR, Raspberry Pi 4 and Android phone inside the vehicle cabin. As the operator drives, the KrakenSDR software will be constantly providing bearings relative to the antenna array. The Android app reads in these bearings via WiFi, and adjusts them for the direction of movement determined via the Android phone’s GPS sensor, resulting in an automatic and accurate calculation of the map bearing towards the transmitter for that particular location. The app then logs this data and plots it on a map grid, which is used to automatically determine where the bearings intersect. Generally it will only take a few minutes of driving to accurately locate a transmitter with a strong continuous signal.

The app then goes a step further and provides automatic turn-by-turn navigation that will lead you to the transmitter without needing to take your eyes off the road! These are features that we’ve only seen before in high end direction finders that are prohibitively expensive to the average user.

We will be releasing our new app as a paid app on the Google Play store, but all KrakenSDR backers will receive a license for free!

Automatic Phase Calibration

Then new onboard switched noise source hardware in the KrakenSDR means that phase calibration with the noise source is now entirely automatic. So unlike the KerberosSDR, every time the frequency or gain is changed in the GUI there is no need to disconnect antennas to manually recalibrate - it just works.

Radio Direction Finding

Radio Direction Finding (RDF) refers to any technique used to determine the directional bearing towards an RF transmitter.

The simplest method is to use a directional antenna that only receives signals in the pointed direction, and manually sweep through 360 degrees, searching for which bearing angle gives the strongest signal. You then could do this in multiple locations, making note of where your bearings intersect. This simple method is considered a low accuracy manual process, and requires a tuned directional antenna.

There are more techniques such as pseudo-doppler and Watson-Watt. However, as KrakenSDR is a coherent SDR, we are able to use one of the more advanced techniques known as correlative interferometry which makes use of phase information found in an antenna array spaced out in some known pattern.

When this information is computed through an algorithm such as MUSIC, it results in a bearing towards the transmitter direction. We also receive information about the signal from the full 360 degrees around the antenna array, allowing us to better understand the multi-path environment.

Multi-path occurs when a radio signal bounces off various objects, such as buildings and hills. It can make it seem like the signal originates from the object that it bounced off. By taking readings from multiple locations we can mitigate the multi-path problem.

Click to Expand

Passive Radar

Passive Radar makes use of existing FM, TV, mobile phone or other strong broadcast transmitters. The signal from these transmitters reflects off objects such as road vehicles, ships, and aircrafts. By using two antennas on two receive channels, and an algorithm to compare the reflected signal against a clean reference copy of the actual signal, we can achieve a radar-like display of bi-static range vs doppler speed.

For passive radar you will need to determine the location of a useful broadcast tower in your vicinity, and an appropriate direction towards your targets of interest. The geometry cannot be such that the broadcast tower and targets are in the same direction. The further apart they are in terms of angles, the better. Then you point one directional Yagi antenna towards the broadcast tower, and one towards the targets of interest. A diagram illustrates this below.

KrakenSDR Reflected Signal Diagram

Example Setup of a Simple Passive Radar

Passive Radar Range Doppler Graph after 24 hours

We are working towards releasing software that will actually be able to plot on a map where a detected object is. This system will work by making use of the full 5-channels on the KrakenSDR, by using several channels for direction finding with an array of directional Yagi antennas. We will have the bearing and range which will allow the object to be plotted on a map.

Antennas

To work as a radio direction finder, KrakenSDR needs five antennas. In order to detect signals from 360 degrees, you will need a circular array of omnidirectional antennas such as whips or dipoles. So to go along with the release of the KrakenSDR, we are offering an optional set of five magnetic whip antennas which can be mounted on, for example, the roof of a car. (Please note the magnetic whip antennas shown in the photo may differ slightly from the style shipped, but they will be equivalent value and performance.)

We have also been working with Arrow Antennas in the USA, who are producing a KrakenSDR 5-element dipole array antenna which is great for use in fixed sites (for example on the roof of a house). The antenna will be sold by Arrow antennas and we will be issuing an update when they are available for sale. This antenna has been used in all our fixed site experiments as you can see in some of our YouTube videos, and works very well. (The image below show a prototype, we’re told the final version may look slightly different.)

Standard 5-Channel Receiver

If you are not interested in coherent applications then it is also possible to use the unit as 5 seperate RTL-SDR receivers. An example use-case might be setting up a multi-purpose airband monitor. One channel monitors the VHF airband, one channel monitors ACARS/VDL2, one channel monitors ADS-B, and another monitors satellite AERO by powering an active L-Band patch antenna via the bias tee. That still leaves one receiver left over for another usecase. As KrakenSDR is based on RTL-SDR, the installation procedure for non-coherent use cases is exactly the same as for RTL-SDR, and it can be used with the standard RTL-SDR drivers.

Comparisons

¹ RDF42 with Aluminum Housing. Also requires a hardware radio scanner at additional cost

KrakenSDR vs DIY

KrakenSDR integrates the equivalent of five RTL-SDRs plus a range of supporting hardware. You could, in theory, build a comparable system, in which case you would need:

Comparison Discussion

KrakenSDR is enabling high end radio direction finding features such as automatic mapping and localization of the transmitter. When KrakenSDR is used together with the Android app there is no need to stop and manually take readings, and the system will automatically calculate the most likely transmitter location based on the data received. As far as we’re aware such functionality has only been seen before in professional military/government/commercial level gear existing in the hundreds of thousands of dollars range. Compare these videos demonstrating the Rhode & Shwarz $150k+ solution against our Android based solution to see how similar they are.

Various DIY and amateur radio focused pseudo-doppler systems such as the PA8W doppler system have existed for many years now. Pseudo-doppler based systems require special antenna arrays with rapid switching hardware built in, in order to generate the psuedo-doppler signal. Unfortunately this rapid switching can introduce distortion, interference and limits its ability to locate noise, intermittant and wideband signals. Also, in addition to the switching and pseudo-doppler processing hardware you also need to provide your own radio hardware which adds more costs.

There are also various lab-grade multi-channel coherent SDR receivers on the market, however most of these cost at least $10k+. An example is the Epiq Sidekiq x4. These high end coherent SDRs have the advantage that they are naturally coherent, meaning that software re-calibration of the phase is not required after every change in frequency. They can also transmit. The disadvantage apart from the cost is that there is usually no ready to use coherent setup or software available out of the box (or perhaps provided via a costly API subscription). These high end products are great for high level research, but certainly not affordable for every day people.

Of course as KrakenSDR is an RTL-SDR based system, it is possible to DIY 5x RTL-SDRs into a coherent system just like KrakenSDR, and having seen people do this in the past was exactly the inspiration for building the KrakenSDR. However, once you have obtained all the components required to build your own system we think you’d be close to, or exceeding the cost of the KrakenSDR. Not to mention the time, research and testing required to build such a system. We havved provided the DAQ+DSP as opensource code for the KrakenSDR as well as DIY users, and we do reinvest by continually improving this software and building new software to lower the barrier to new use-cases over time. However, due to ongoing costs relating to MapBox usage fees, and possible server costs for future multi-KrakenSDR networking enhancements, we do need to charge non-KrakenSDR customers for use of the Android app and possible future software.

Ongoing Work

DAQ & direction of arrival (DOA / radio direction finding)

Work on the DAQ and DSP software is coming along well. It is stable on a Raspberry Pi 4 and is nearly complete. We are continuously adding minor features and monitoring for bugs to fix. Intermittent "bursty"-signal handing is already working, and we are well on our way to improving its sensitivity to weak, bursty, narrowband CW signals, which can still be problematic to detect. The Android app is currently being field tested as well.

Passive radar software

Work on new passive radar software is also ongoing, and we expect to have a quick-start guide and examples ready for experimentation before we begin shipping. As of now, it also remains possible to use the older KerberosSDR software for passive radar, but we believe the new DAQ core software will run things much more smoothly. The goal for our new software is not only to plot a range-doppler map, but to combine it with direction-finding and to plot radar detections on a map. To do so, it might need to run on something that’s faster than a Raspberry Pi 4, such as a GPU-based device like the NVIDIA Jetson.

Beamforming & interferometry

One application at which we think KrakenSDR will excel at is amateur radio astronomy via interferometry. The ability to combine multiple small hydrogen line dishes, spread out over several meters of area, should result in a much greater radio imaging resolution without the need to deal with a single huge dish. It may also allow for a beam to be electrically steered, which would obviate the need to rotate the dishes.

Advanced direction finding & advanced log management

At the moment, networked direction finding (direction finding via multiple fixed or mobile sites spread out around a city or area) is possible via the third party RDF Mapper software, but we aim to create our own advanced platform in the near future. The goal is to have software that will automatically log the event, notify users when a signal of interest appears and automatically determine the location of the transmitter. The list of use cases for this might include:

• helping coast guards locate distressed marine pleasure-crafts that typically do not have AIS via their VHF radios
  
• locating beacons for animal, wildlife, or asset tracking
  
• monitoring for illegal or interfering transmissions
  

Our core DAQ + DSP software will also be updated to support the monitoring of multiple simultaneous channels within the available 2.56 MHz bandwidth, and with a scanning and beacon-ID detection feature.

Research into field applications

One example we hope to test is the operation of KrakenSDR on a drone. With a line of sight from up in the sky, it should take very little time to locate a transmitter!

Another interesting application might be the combination of a patch antenna array, KrakenSDR and Augmented Reality. This could essentially grant the super-power of being able to ‘see’ RF.

Support & Documentation

Our DAQ firmware + Direction Finding DSP code is available on our GitHub at https://github.com/krakenrf. Please be aware that prior to official release, everything is kept in the development branches while we are continually adding new features and fixing bugs. Upon shipping, we will have a ready-to-use .IMG file that can be burned onto an SD card for the Raspberry Pi 4, which will be the fastest way to get up and running with the KrakenSDR software.

We will also be releasing a variety of tutorials that will show you exactly how to get started with using KrakenSDR for direction finding and passive radar.

Manufacturing Plan

We have a good relationship with our manufacturer who has built previous products for us, including the KerberosSDR and the current KrakenSDR prototypes, so we are confident in their processes. At the moment, we have all the long lead time components already in our stock, and after crowdfunding, we will immediately begin ordering the other common components and proceed with manufacturing.

Our metal enclosure is still being finalized, and will be similar to the prototype images seen in this campaign. We will have updates on the final design as the campaign goes on.

Fulfillment & Logistics

We will be making use of Crowd Supply’s & Mouser’s logistics network for the distribution of the crowdfunded units.

Risks & Challenges

The pandemic has resulted in many issues in terms of delayed work and supply shortages. While we believe that most of those problems are now behind us, there is always the possibility of more lockdowns and supply chain/shipping issues. However, having most of the long lead time parts already in our stock, we believe that the main risks have been mitigated.

Funding ends on Nov 25, 2021 at 03:59 PM PST (11:59 PM UTC)