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What is KrakenSDR?

KrakenSDR is a coherent software defined radio.

 

Coherence allows for very interesting applications, such as radio direction finding 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

  • Beamforming

  • Interferometry for radio astronomy

Radio Direction Finding

The most common application that KrakenSDR is used for is 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 the phase information found in an antenna array that is spaced out in some known pattern.

Running that information through an algorithm like MUSIC produces a bearing towards the transmitter direction. 

By taking multiple bearing readings at different locations we are able to triangulate the location of the transmitter.

What do you need to get started with radio direction finding?

You will need:

  • A KrakenSDR

  • USB Type-C cable

  • 5 V / 2.4 A+ USB Type-C power supply

  • A five element antenna array such as our Krakentenna magnetic whip antenna set

  • A portable computing device to run the KrakenSDR software

  • Android Device (Optionally for mapping)

 

For computing we recommend a Raspberry Pi 4, for which we provide a ready-to-use SD card image that you can just burn and use. 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.

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How does KrakenSDR achieve coherence?

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 throwing five units together and using them on the same PC will not make them "phase coherent" as each one will receive signals at a slightly different phase offset from the others. This makes it impossible to measure phase relationships between signals that arrive at the 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 in our software. 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

  2. Bias Tee

  3. ESD protection

  4. Noise calibration switches

  5. R820T2 tuner

  6. RTL2832U ADC

  7. Noise source

  8. USB Hub

  9. Individual tuner on/off DIP switched

  10. USB Type-C DATA

  11. USB Type-C PWR

Hardware Specifications
  • Five-channel, coherent-capable RTL-SDR, all clocked to a single local oscillator

  • Built-in automatic coherence synchronization hardware

  • 24 MHz to 1766 MHz tuning Range (standard R820T2 RTL-SDR range, and possibly higher with hacked drivers)

  • USB-C data and power connectors

  • SMA female RF input ports

  • 4.5 V bias tee on each SMA port

  • A rugged aluminum enclosure will heatsink and fan for temperature and phase stability

Software
  • Open source core DAQ and DSP software - 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 can also run on a PC. As described below we also provide open source application specific DSP code. A GNU Radio block to interface with the DAQ software is also available.​

  • DSP code for specific use cases - Our open source DSP code supports direction finding. The direction finding code implements direction-finding algorithms such as MUSIC, which can also run on the same Pi 4 or PC as the DAQ code. ​

  • 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 mobile direction finding, we are providing a free license to an Android app for mapping, logging data, and automatically estimating the transmitter location

  • For distributed fixed and mobile sites, we provide a cloud based mapping solution (currently in alpha at map.krakenrf.com)

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.

KrakenSDR Web Interface

Our KrakenSDR core 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.

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