KerberosSDR - 4 Channel Coherent RTL-SDR
A Coherent RTL-SDR with 4x Channels. For direction finding, passive radar, beam forming, or just as four RTL-SDRs!
We have developed a low-cost, 4-tuner phase-coherent RTL-SDR software defined radio. The applications could include radio direction finding, passive radar, beam forming or to simply use as four convenient RTL-SDR dongles. We have developed custom software that shows off direction finding and passive radar capabilities. The base DSP and phase synchronization code could be adapted to any coherent application that you have in mind.
The RTL-SDR software defined radio phenomenon has changed the world by unlocking low cost access to the radio spectrum. Whilst RTL-SDR is now a relatively mature commodity item that almost everyone whose tinkered with electronics has played with, there is still so much more potential to be unlocked.
A phase coherent RTL-SDR can be made out of two or more RTL-SDR dongles that share a common clock. With a bit of help from a noise source, the RTL-SDRs can be synced together. Once synced interesting applications become available, such as direction finding, beam forming and passive radar.
Phase coherent RTL-SDRs have been worked on and demonstrated several times over the past few years, but at RTL-SDR.com and Othernet 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 far too complex to install and use.
With the Kerberos SDR we aim to change that by making phase coherent applications easier to access and run by providing ready to use hardware and good demo software with an open source DSP code base that can be extended.
Please note, although we are aiming to make KerberosSDR as easy to use as possible, at least a basic to moderate level of computer and radio technical knowledge is required, or you must be willing to learn. This is an experimental product and some willingness to experiment and explore solutions is required. The demo software that we provide is capable of doing direction finding and 2-RX passive radar. Your own custom code or applications will be required for custom applications.
- The KerberosSDR Board which has:
- 4x RTL-SDR R820T2 Receivers
- A wideband noise source that can be switched in software
- USB Hub so only one USB connection is required
- A calibration board for synchronizing samples with the noise source
- A shielded metal enclosure
- Internal cables for connecting the two boards and noise source
What you'll need to provide: You'll need to provide your own antennas for your application (e.g. four magnetic whips for direction finding, two directional antennas for passive radar), a 5V USB power supply, a microUSB USB cable, and a Linux computing device like a PC/laptop or single board computer like a Raspberry Pi 3/4 or Tinkerboard. (Must run Linux natively - VMs have too much USB lag for coherency).
KERBEROSSDR HARDWARE SPECS
Each RTL-SDR on board the KerberosSDR is based on the R820T2 and RTL2832U chips, which are the same chips used in the most common RTL-SDR dongles.
- Frequency Range: 24 MHz - 1.7 GHz
- ADC Sample Rate: 2.4 MSPS
- Bit Depth: 8 Bits
KerberosSDR connects it's RTL-SDRs to the calibration board via four u.FL cables. The calibration board then has four u.FL -> SMA cables that can be used to connect to antennas.
Please note that antennas are not included with the KerberosSDR. For direction finding applications you'll need four omni-directional antennas (e.g. magnetic whips). For passive radar you'll require two directional antennas.
The KerberosSDR takes a USB power input. Any 3A supply should be sufficient. On some modern PCs you may even be able to directly power the board without any additional power supply. ***The previous version of KerberosSDR included microUSB for data and a USB-C connector for power. The USB-C connector is superior to the microUSB, but we had not implemented the USB-C specification. This caused a great deal of confusion, which is why all current KerberosSDR include two microUSB connectors.
Some applications might include:
- Pinpointing the source of VHF/UHF noise, pirates, interference, jammers, unknown signals etc using radio direction finding (RDF)
- Using passive radar to monitor aircraft that do not transmit ADS-B
- Monitoring vehicle or marine traffic with passive radar
- Direction finding for amateur radio fox hunts
- Determining the location of rescue or stolen asset beacons
- Combining multiple small dishes to create a large dish for radio astronomy via beam forming.
- Using the four tuners as standard RTL-SDRs. e.g. two for trunking, one for ADS-B and one for weather satellites.
KERBEROSSDR DEMO SOFTWARE
For development of the open source DSP software we have hired Tamás Peto, a PhD student at Budapest University of Technology and Economics. He has developed an excellent open source Linux demo application that can be used for direction finding and passive radar. The DSP and synchronization code could easily be extended to implement other applications, or extend features.
The code can be found at https://github.com/rtlsdrblog/kerberossdr, and a guide to installing it can be found at www.rtl-sdr.com/ksdr. A support forum is available at https://www.rtl-sdr.com/forum/viewforum.php?f=9.
KERBEROSSDR DIRECTION FINDING
Our open source demo software 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.
By combining angle data with our companion Android App, it is possible to drive around with the direction finding system and very accurately pinpoint the location of an RF transmitter.
Android App Download: https://play.google.com/store/apps/details?id=com.rtlsdr.realtimebearing
Below is a demo video created by Harold of the SignalsEverywhere YouTube channel. He shows how easy it is to use the KerberosSDR and Android App to locate a P25 transmitter within 15 minutes of drive time.
Below are three demonstrations that we created ourselves.
KERBEROSSDR 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.