The tech is getting good. Bargain basement warfare for those willing to put in the work. -NCS

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 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 auto-calibration, 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 KrakenSDR, a USB Type-C cable, a 5 V / 2.4 A+ USB Type-C power supply, and antennas—such as our magnetic whip antenna set—that are appropriate for your application.

For computing we recommend a Raspberry Pi 4, for which we will be providing ready-to-use SD card images. Optionally, for direction finding, you will want an Android phone or tablet with mobile-hotspot capabilities, GPS, and a compass, ideally produced within the last three to four 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 (SDR), but throw five units together and using them on the same PC will not make them “phase coherent;” each one will receive signals at a slightly different phase offset from the others. This makes it difficult or impossible to achieve a high degree of precision when measuring 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 measured precisely and corrected for. Additionally, the overall design of KrakenSDR works to ensure phase stability, with care taken in the areas of heat management, driver configuration, power supply, and external-interference mitigation.

1. SMA Antenna inputs 5. R820T2 tuner 9. Individual tuner on/off DIP switched
2. Bias Tee 6. RTL2832U ADC 10. USB Type-C DATA
3. ESD protection 7. Noise source 11. USB Type-C PWR
4. Noise calibration switches 8. USB Hub

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 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 (SBC), but could also run on a PC.
  • DSP code for specific use cases Our open source DSP code supports direction finding and passive radar. That 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 output from the direction-finding algorithm.

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 include necessary sensors and software like GPS, compass, mobile data, and mapping, we have made use of those features to create an affordable radio direction finding system.

An example scenario might see the antenna array mounted on the roof of a car, with KrakenSDR, a Raspberry Pi 4, and an Android phone inside the vehicle cabin. As the operator drives, the KrakenSDR software will constantly provide bearings relative to the antenna array. The Android app receives these bearings via Wi-Fi 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 most potential users would find prohibitively expensive.

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 KrakenSDR means that phase calibration with the noise source is now entirely automatic. So, unlike with KerberosSDR, changing the frequency or gain in the GUI does not require disconnecting antennas to manually recalibrate – it just works.

Radio Direction Finding

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

The simplest method is to use an antenna that only receives signals from the direction in which it is pointed, then manually sweeping through 360 degrees to identify the bearing angle that receives the strongest signal. You could then do this from multiple locations and make note of where your bearings intersect. Unfortunately, this “simple” method requires a tuned directional antenna and a manual, error-prone process.

There are other techniques as well, 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.

Running that information through an algorithm like MUSIC produces a bearing toward the transmitter direction. KrakenSDR also receives signal data from the full 360-degrees around its antenna array, which gives it a better “picture” of multi-path environments that occur when a radio signal bounces off objects like buildings and hills. Multi-path environments can make it seem like a signal originated from an object that merely reflected it. 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, and other strong broadcast transmitters. The signal from these transmitters reflects off objects such as road vehicles, ships, and aircraft. 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 toward 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 toward the broadcast tower and the other toward the targets of interest. The diagram, photograph, and plot below illustrate this configuration:

KrakenSDR Reflected Signal Diagram

Example Setup of a Simple Passive Radar

Passive Radar Range Doppler Graph after 24 hours

We are working toward a release of software that will actually be able to plot the location of a detected object on a map. It will leverage all five channels on KrakenSDR, using several of them for direction finding with an array of directional Yagi antennas. By obtaining the bearing and range, we will be able to plot the object on a map.


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 KrakenSDR, we are offering an optional set of five magnetic whip antennas that you can mount, for example, on the roof of your car. (Please note the magnetic whip antennas shown below may differ slightly from the ones we end up shipping, but they will be equivalent in value and performance.)

We have also been working with the US-based company, Arrow Antennas, who are producing a five-element dipole array for KrakenSDR that is great for use in fixed sites (on the roof of a house, for example). That 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 of our fixed-site experiments, and you can see it in some of our YouTube videos. It works extremely well! (The image below shows a prototype. We’re told the final version may look slightly different.)

Standard Five-Channel Receiver

If you are not interested in coherent applications, it is also possible to use KrakenSDR as five separate RTL-SDR receivers. An example use-case might be setting up a multi-purpose airband monitor. One channel monitors the VHF airband, one monitors ACARS/VDL2, one monitors ADS-B, and another monitors satellite AERO by powering an active L-Band patch antenna via the bias tee. (And that still leaves one receiver left over for some other application!) 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.