PySDR is a free online textbook created by Dr. Marc Lichtman which explains many digital signal processing (DSP) and software defined radio (SDR) concepts in a clear, concise and easy to understand way. The guide includes multiple images and animations, as well as Python code examples.
Direction-of-Arrival (DOA) within DSP/SDR refers to the process of using an array of antennas to estimate the DOA of one or more signals received by that array. Once we know the direction a signal of interest is arriving from, we can isolate it from other signals/interference/jamming.
It is just like isolating a signal in the frequency domain by filtering it, except we are now working in the spatial domain (you can certainly combine both!).
We typically refer to the antennas that make up an array as elements, and sometimes the array is called a “sensor” instead. These array elements are most often omnidirectional antennas, equally spaced in either a line or across two dimensions.
DOA is a subset of beamforming techniques, where as the receiver, we are trying to steer a beam (our receiver’s antenna beam) towards the direction of an emitter. We may also steer a beam blindly across a wide range (e.g., 0 to 360 degrees) to figure out what signals are being received and from what direction.
A visual example of what happens to two signals when the interelement spacing of a direction finding antenna array is reduced below half a wavelength.
Over on YouTube Jon Kraft has been uploading videos explaining some interesting beamforming experiments he's been doing with his PlutoSDR. One experiment shows how to create a DIY monopulse tracker, which is a type of radio direction finding technique.
The PlutoSDR has two RX ports and two TX ports, and in this experiment he uses two directional antennas for the RX and one monopole antenna for the TX. Part 1 of this series explains standard phased array beam forming, and part 2 moves on to explain monopulse with adaptive tracking.
If you were interested in this, check out Jon's other videos on his channel. A recent video explains how time delays work in digital beamforming.
Over on the Frugal Radio YouTube channel Rob has uploaded part two of his two part series on the KrakenSDR. The KrakenSDR is our 5-channel coherent radio based on RTL-SDRs, and it can be used for applications like radio direction finding and passive radar. We successfully crowd funded the device on Crowd Supply.
In the first video Rob unboxed the KrakenSDR and set up the software. In this second video he takes the KrakenSDR out on a drive and is able to successfully locate the transmission sources of two unknown transmitters.
In the first part of the video Rob shows how he sets up his vehicle roof antennas and how he routes his cabling into the vehicle and KrakenSDR. He then shows his drivers view as he locates the site of a DMR trunked network user which ends up to be a factory plant. In his second test Rob locates a P25 transmitter site. In both tests Rob notes how he was impressed at how quickly a location was able to be determined, taking only a few minutes each time.
KrakenSDR - WOW! Amazing Direction Finding Tests : Part 2
Over on YouTube F4IPO has posted a video of him using a KrakenSDR and the KrakenSDR Android mapping app to quickly locate the source of a TETRA transmission at 427 MHz in France.
The KrakenSDR is our 5-channel coherent radio based on RTL-SDRs, and it can be used for applications like radio direction finding and passive radar. We successfully crowd funded the device on Crowd Supply.
In the video F4IPO shows a dash cam recording of his vehicle alongside a screen recording of his Android phone screen. He makes use of the auto-navigation feature which navigates him right to the radio transmit tower. He notes that the entire process to locate the transmitter only took about 5 minutes. At the end of the video he shows the antenna setup on his roof.
Over on the Frugal Radio YouTube channel Rob has uploaded part one of his two part series on the KrakenSDR. The KrakenSDR is our 5-channel coherent radio based on RTL-SDRs, and it can be used for applications like radio direction finding and passive radar. We successfully crowd funded the device on Crowd Supply.
In the video Rob unboxes his KrakenSDR, and explains how he will use it for radio direction finding. He shows his setup including the other required parts, like the Pi 4, and power supply, and then goes on to show the software installation process which involves burning an SD card and downloading an Android app. Next he sets up his antenna array by printing the antenna spacer and using the Excel antenna array calculator sheet.
Rob notes that Part 2 is coming in one to two weeks and will show him using the KrakenSDR in his vehicle to locate the source of a transmission.
If you weren't aware of it, KrakenSDR is our RTL-SDR spinoff project and is a 5-channel coherent RTL-SDR that we have successfully crowdfunded for over on Crowd Supply. KrakenSDR is the successor to our previous 4-channel coherent product called the KerberosSDR. With a radio like KrakenSDR that is capable of coherence between channels, interesting applications like direction finding and passive radar become possible. You can also use it as five independent RTL-SDRs should you chose to.
We wanted to note that all units preordered through the Crowd Supply crowd funding campaign are now at the Crowd Supply / Mouser warehouse, and the majority have already been shipped out to customers!
The KrakenSDR with 5x Krakentenna's for Radio Direction Finding
Additional units for new purchasers are in a mixture of production and freighting and will be available for fulfillment as soon as we can. We are constrained by supply and production time, so if you're interested in a KrakenSDR, please get your order in so that you have an earlier place in the queue.
KrakenSDR ProductionKrakenSDR Boxes
Other Recent KrakenSDR Updates
Wiki Manual: Our Wiki manual and guide is up at https://github.com/krakenrf/krakensdr_docs/wiki. It covers topics from what you need to get started, radio direction finding theory and background, antenna array setup, KrakenSDR Web-GUI software guide, Android App guide and a Passive Radar guide.
Install Scripts, VirtualBox Images, Docker: For general vehicle based direction finding, which is the most popular application, we recommend using our premade Raspberry Pi 4 image for easy almost plug and play setup. But to ease installation on other computing devices (especially as the Pi 4 stock is non-existent at the moment due to the supply chain crisis) we've now created an automatic Linux install script and a Virtual Box image which can be run on Windows or Linux host machines. Third parties have also released a Docker container. See this page on our Wiki for more information.
Customer Feedback: We've also had some great customer feedback so far with one user submitting examples of his success in locating transmitters like a 162 MHz NOAA weather station, and various fox hunt beacons.
KrakenSDR Direction Finding with Android App
Arrow Antennas: For fixed site direction finding installs, Arrow Antennas is now shipping their 5-element dipole antenna array.
If you're interested in the latest KrakenSDR updates, please follow us on the Crowd Supply mailing list, and keep an eye out for our official krakenrf.com website releasing soon.
KrakenSDR is our 5-channel coherent RTL-SDR product that we have successfully crowdfunded for over on CrowdSupply. KrakenSDR is the successor to our previous 4-channel product called the KerberosSDR and will begin shipping to initial supporters within the next few months. Along with the new hardware developments, KrakenSDR comes with a new opensource codebase that is also compatible with the KerberosSDR.
With a coherent SDR like KrakenSDR or KerberosSDR, interesting applications such as radio direction finding and passive radar become possible.
Unlike the newer KrakenSDR, the KerberosSDR is unable to automatically calibrate without manual intervention on the hardware. However, it is possible to upgrade the KerberosSDR with some third party hardware switches provided by Corey Koval of Lakeshore Labs. With these switches and the new software the KerberosSDR can be made to automatically calibrate like the KrakenSDR.
Corey also has created DF-Aggregator, which is open source software that allows users to plot bearing data from one or more KerberosSDR, KrakenSDR or other radio direction finding devices on a map.
Recently, Corey has demonstrated some changes to our codebase that allow the new KrakenSDR software to directly upload to DF-Aggregator. Over on his YouTube channel, Aaron who runs the DragonOS channel has uploaded a video that shows exactly how to set this up. In the future we plan on integrating support for DF-Aggregator directly into our core code.
With a 5-channel phase coherent RTL-SDR interesting applications like radio direction finding (RDF), passive radar and beam forming become possible. It can also be used as five separate RTL-SDRs for multichannel monitoring.
KrakenSDR Updates
Like many other projects we have been severely delayed by COVID work restrictions and the effects it's having on the supply chain, and I'd like to thank everyone who is keen to get a hold of a KrakenSDR for their patience. But the ball is rolling faster now and we have finally received our latest KrakenSDR prototypes! Testing has been ongoing for the last few days, and apart from a few minor issues everything is working brilliantly. At this stage we are confident in the design and are making plans to begin the crowdfunding campaign soon.
The latest KrakenSDR Prototype PCB running on a Pi 4.
Supply Chain Constraints
The first batch will unfortunately be limited to 1000 units maximum due to supply constraints and we expect this first batch to be ready 2-3 months after the campaign finishes. So if you are after a unit ASAP, please ensure you are on the CrowdSupply mailing list as we fully expect demand for the first batch to outstrip the supply.
But if you are willing to wait, batch 2 will be still be available at the campaign special price. we will have a second batch available for early preorder at a discount (sorry due to higher than expected shipping and skyrocketing component prices we can't discount the second batch at the moment). Please keep in mind that the second batch will be at least 6 months away due to the long supply chain resulting from the pandemic.
Next Steps
The next stages in hardware development will involve finalizing our custom milled aluminum enclosure, testing one last prototype, and beginning mass manufacturing when the crowd funding campaign is over.
Work on the software is ongoing, but the beta version of our new DAQ firmware and direction finding DSP software layer is stable and already available on the krakensdr GitHub at https://github.com/krakenrf. Everything resides in the development branches and there is full documentation on the code structure available in the Documentation folder. This code can also be used on the KerberosSDR by editing the configuration files to specify 4 receivers instead of 5.
By the time the units ship out we will have a ready to use SD card image for the Raspberry Pi 4 and a quickstart guide available.
KrakenSDR DAQ and DOA DSP Web Interface
Android App
We have also been working at improving the Android direction finding companion app. This app was made during the KerberosSDR release a couple of years ago, and is used to plot and log the direction finding bearings being generated by the Kerberos/KrakenSDR unit, combining it against the GPS and movement data generated by the Android phone. This Android phone + KrakenSDR combination results in a powerful multipath resistant radio direction finding tool, and once enough data has been collected (usually after a few minutes of driving) it is able to determine where the most likely transmitter location is.
The upgraded app makes use of the full 360 degrees of direction of arrival and multipath data that is generated by the KrakenSDR, resulting in a more accurate determination of the transmitter location, and a better understanding of the uncertainties. It also allows users to visualize multipath. There are also various bug fixes and improvements made overall. We are planning to transition this app into a paid app, but all KrakenSDR backers will receive a license for free and the older KerberosSDR app will remain free.
KrakenSDR Android App Improvements
KrakenSDR Antennas
To work as a radio direction finder, KrakenSDR needs five antennas. If you plan to use them in a circular array, they need to be omnidirectional antennas such as whips or dipoles. So to go along with the KrakenSDR we will be selling an optional set of five magnetic whip antennas which can be mounted on for example, the roof of a car. (Please note the magwhips shown in the photo may differ slightly from the final ones sold).
KrakenSDR Magnetic Whips on a Car Roof
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 (not by us), and the future link (not active yet) will be http://www.arrowantennas.com/arrowii/kraken.html. We expect them to generate this page within the next few days. This antenna has been used in all our fixed site experiments as you can see in some of the YouTube videos, and works very well. (The image below show a prototype, we're told the final version may look slightly different.)
Arrow Antenna 5-element antenna array for the KrakenSDR
Future Work
DAQ & Direction of Arrival (DOA / Radio Direction Finding) :
Work on the DAQ and DSP software is coming along well and this is mostly complete and runs stable on a Raspberry Pi 4. There are just now bug fixes and minor features being added. Intermittent 'bursty' signal handing is already working, but we are working on improving it's sensitivity to weak bursty narrowband CW signals which can still be problematic to detect. The Android app is also currently being field tested.
Passive Radar:
Work on new passive radar software is also ongoing and we expect to have something ready for experimentation and with quickstart guides before shipping. At the moment it is also still possible to use the older KerberosSDR software for passive radar, but we believe the new DAQ core software will run things much smoother. The goal for the new software is to not only plot a range-doppler map, but to combine it with direction finding and be able to plot radar detections on a map. This feature may require operation on a device faster than the Raspberry Pi 4, such as GPU based device like a NVIDIA Jetson.
Beam Forming, Interferometry:
One application we think the KrakenSDR would be great with 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 much greater radio imaging resolution, without needing to deal with a single huge dish. It may also allow for electrically steering a beam without needing 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 and alert when a signal of interest appears. For some examples we can see this being used to help coastguard locate distressed marine pleasurecraft that typically do not have AIS via their VHF radios, locate emergency beacons, for animal/wildlife/asset tracking, and monitoring for illegal/interference transmissions.
At this stage the core DAQ+DSP software will also be updated to support monitoring 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 great line of sight from up in the sky, localizing a transmitter should be fast. Another example could be actually visualizing signals like light via augmented reality.
Some of our previous KerberosSDR and KrakenSDR posts might also be of interest.
