Category: Applications

GammaRF: Distributed Radio Signal Collection and Analysis with RTL-SDR and HackRF

Thank you to Josh for submitting news about his project called GammaRF. GammaRF is an client-server program that is used to aggregate signal information via the internet from distributed SDRs. Currently the RTL-SDR and HackRF SDRs are supported.

ΓRF (“GammaRF”, or “GRF”) is a radio signal collection, storage, and analysis system based on inexpensive distributed nodes and a central server. Put another way, it is a distributed system for aggregating information about signals, and a back-end infrastructure for processing this collected information into coherent “products”.

Nodes utilize inexpensive hardware such as RTL-SDR and HackRF radios, and computers as small and inexpensive as Intel NUCs. Each node runs modules which provide various radio monitoring functionality, such as monitoring frequencies for “hits”, watching power levels, keeping track of aircraft (through ADS-B), and more. Nodes are distributed geographically and their data is combined on the server for hybrid analysis.

A web-based system allows users to view information from and about each station in its area. Below shows the server landing page. Markers are placed at each station’s last known location (stations can be mobile or stationary.)

GammaRF Server Landing Page
GammaRF Server Landing Page

From the currently implemented modules it appears that you can monitor ADS-B, scan and monitor the power of a set of frequencies, forward the output from trunk-recorder (a P25 call recorder), scan the spectrum and monitor power levels, monitor a single frequency for activity, take a picture of a swath of RF spectrum, and collect 433 MHz ISM data. Some example applications might include:

  • Monitoring ham radio activity on repeaters in a city
  • Creating timelines of emergency services activity in an area
  • Distributed tracking of satellites and other mobile emitters
  • Monitoring power at a frequency, for example as a mobile node traverses an area (e.g. signal source location)
  • Building direction finding networks (e.g. for fox hunts)
  • Spectrum enumeration (finding channels and guessing modulation) [under development]
Monitoring Activity of an Amateur Radio Repeater
Monitoring Activity of an Amateur Radio Repeater via the 'scanner' Module

Listening to the Sound of Molecules via Nuclear Magnetic Resonance and an RTL-SDR

Over on YouTube user aonomus has uploaded a video showing how he's used an RTL-SDR to observe and listen to the radio signal generated via a chemistry lab's nuclear magnetic resonance machine. To do this he simply taps the RF output of the NMR machine which allows the RTL-SDR to listen to the signal and play it as audio. In the video he shows the sound of a sample of chloroform in acetone-d6. The demo has no real scientific purpose other than to hear the sound of the molecule. Normally the RF output goes straight into a spectrum analyzer for visual analysis.

Nuclear magnetic resonance is a technique used in chemistry for the analysis of chemicals, as well as in MRI medical imaging machines. Very basically, it works by applying a chemical sample to a strong magnetic field, exciting it with a strong pulse of RF, and listening to the echo. An echo will only occur when the radio waves are transmitted at the chemicals resonant frequency. The frequencies used are typically between 60 to 800 MHz.

A few years ago I came up with a demonstration for some high school students interested in chemistry. This demo is a modern take on a classic NMR experiment, using a low cost software defined radio to observe the FID signal as audio. In short, this demo allows you to hear the proton FID echo from the liquid sample inside the NMR magnet.

Nuclear Magnetic Resonance Demonstration Using Software Defined Radio

Measuring the SWR of FPV Antennas with an RTL-SDR

FPV stands for 'First Person View', and is a term used to describe the hobby of flying remote controlled aircraft entirely via the view from a wireless camera that transmits live video to the pilots screen or video goggles.

Part of the FPV hobby is to not only enjoy flying, but also to tweak the wireless video equipment for maximum range and reliability. This involves measuring the SWR characteristics of FPV antennas. SWR is a metric that describes how well the impedance of an antenna is matched with the receiver at a certain frequency. Poor SWR results in additional signal loss on top of cable and connector loss. We note that SWR is only one antenna metric, and doesn't take into account radiation pattern and antenna gain which is often more important, but it is the easiest metric to measure and control, and should give you some idea as to if an antenna was designed and tuned properly.

As FPV hobbyists are often not hams or radio professionals, most don't have access to the equipment required to measure SWR. So over on his YouTube channel bonafidepirate shows how he's been using a cheap RTL-SDR, noise source and RF Bridge to measure the SWR of his FPV antennas. The process is similar to the one shown in our tutorial, but he uses the Spektrum software which allows you to measure SWR entirely within the software itself.

In the video bonafidepirate goes over the required hardware, software and the setup, and then demonstrates several SWR scans of different FPV antennas.

DIY VSWR Meter for FPV, Lets test some antennas!

Creating a Passive Traffic Radar with DVB-T Signals and KerberosSDR our 4-tuner Coherent RTL-SDR

KerberosSDR is our upcoming low cost 4-tuner coherent RTL-SDR. With four antenna inputs it can be used as a standard array of four individual RTL-SDRs, or in coherent applications such as direction finding, passive radar and beam forming. More information can be found on the KerberosSDR main postPlease remember to sign up to our KerberosSDR mailing list on the main post or at the end of this post, as subscribers will receive a discount coupon valid for the first 100 pre-order sales. The list also helps us determine interest levels and how many units to produce.

In this post we'll show KereberosSDR being used as a passive traffic radar. Passive radar works by using an already existing transmitter such as a FM, DAB, TV or GSM and listening to the reflections of those signals created by moving objects like aircraft, boats and cars. A simple passive radar consists of two directional antennas. One antenna points at the 'reference' transmitter (the transmitting tower), and the other towards the 'surveillance' area that you want to monitor. The result is a speed vs distance plot that shows all the moving objects.

For this test we parked our car to the side of a highway and pointed a cheap DVB-T Yagi antenna towards a DVB-T transmission tower, and another cheap Yagi down the road. The video shown below displays the results captured over a 5 minute period. The blips on the top half of the display indicate vehicles closing on our location (positive doppler shift), and the blips on the bottom half indicate objects moving away (negative doppler shift). 

Highway Passive Radar Traffic Monitor with DVB-T and KerberosSDR a 4x Coherent RTL-SDR

DVB-T Antennas In Car
DVB-T Antennas In Car

The resolution of each individual vehicle is not great, but it is sufficient to see the overall speed of the highway and could be used to determine if a road is experiencing traffic slowdowns or not. When larger vehicles pass by it is also obvious on the display by the brighter blip that they show. The display also shows us that the highway direction coming towards us is much busier than the direction moving away.

In the future we'll be working on optimizing the code so that the display updates much faster and smoother. It may also be possible in the future to use the third and fourth tuners to obtain even greater object resolution.

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Othernet (formerly Outernet) Updates Lantern Backers

Othernet (formerly known as Outernet) are a providers of a free data service broadcast from satellites. They hope to build a system and low cost satellite receiver products where people can easily stream free daily data such as news, videos, books, and live audio down to a computer or phone from anywhere in the world via a device called a Lantern. It is a one way download only service, but may be useful for those in areas with limited internet, disaster preppers, or people in countries with internet censorship. The describe their mission as:

Othernet's mission is to build a universal information service; a truly pervasive multi-media service that operates in the most remote places and functions even when nothing else does.

In the past they ran a trial service on L-band satellite frequencies and used RTL-SDR dongles as the receiver. They have since discontinued that service in favor of a new Ku-band LoRa based service which can provide much more data - up to 200MB a day. The update released today was sent to Lantern backers, which was the receiver they crowdfunded for in their Kickstarter back in 2014. The update notes that the final iteration of the Lantern is close to being ready.

Broadcasting Khan Academy 24/7

Hello Backers,

Yes, we are still here. It’s been a long while since the last update, but that does not mean we have stopped–or even slowed–working on Lantern. We have been making progress, though it has been much, much slower than what everyone wants. Fortunately, we are in the final stage of development.

The last update described the new network technology we had developed. Our original goal was to broadcast 20 MB of content per day, which is what we were doing with our previous network. The new system is operating at 10-times that speed, which is a little over 20kbps and 200 MB of content per day. Some of the work we’ve been doing over the past few months is related to tripling our current download speeds. Our target is 60kbps, which results in over 600 MB per day. The size of the device will be similar to a standard flashlight.

At our current download speed of 20kbps, we are broadcasting both data and a 24/7 audio stream. I know many of you were interested in the educational applications that were highlighted during the campaign, which is why I’m very pleased to share that we are currently broadcasting the entirety of Khan Academy as a 24/7 audio stream. The Khan Academy library consists of over 900 separate lectures, which we’ve turned into a giant audio playlist. Now we just need to get Lanterns into everyone’s hands.

The next update will include a picture of our final antenna design. The antenna that is currently included in our DIY kit is 2-inches/5-cm across and the shape of a cone. We are trying to flatten the cone and also increase the size to about 4-inches/10-cm, which is what allows for greater download speeds. Since we are operating at microwave frequencies (12 GHz), both the design of the antenna and the parts to convert the high frequency to a lower one are pretty tricky. Microwave engineering is widely considered black magic, which is the main reason for the long break since the last update. We are close to turning the corner and are targeting the end of the year for our initial production run.

Unrelated to our technical work is our recent name change. We had been fighting a trademark issue for the past four years. We recently decided that it made more financial sense to change our name, rather than continue spending legal fees to defend our position. We are now Othernet (http://othernet.is). This name change does not mean we are going away, nor does it mean we are not delivering Lanterns. It’s just a legal hiccup.

Thanks for your patience and support while we get through the final stage of building what you all backed several years ago. I know it’s been a long time and we are making every possible effort to deliver something that exceeds everyone’s original expectations. Although it’s taking three times longer to develop and ship the product, what we now have will be ten-times more useful.

Outernet Dreamcatcher - Precursor to the Lantern
Outernet Dreamcatcher - Precursor to the Lantern
 

Locating a Radio Transmitter with Direction Finding Techniques and KerberosSDR our 4-Tuner Coherent RTL-SDR

KerberosSDR is our upcoming low cost 4-tuner coherent RTL-SDR. With four antenna inputs it can be used as a standard array of four individual RTL-SDRs, or in coherent applications such as direction finding, passive radar and beam forming. More information can be found on the KerberosSDR main post. Please remember to sign up to our KerberosSDR mailing list on the main post or at the end of this post, as subscribers will receive a discount coupon valid for the first 100 pre-order sales. The list also helps us determine interest levels and how many units to produce.

In this post we'll show an experiment that we performed which was to pinpoint the location of a transmitter using KerberosSDR's coherent direction finding capabilities. RF direction finding is the art of using equipment to determine the location of a transmitting signal. The simplest way is by using a directional antenna like a Yagi to try and determine the bearing based on signal strength. Another method is using a pseudo-doppler or coherent array of antennas to determine a bearing based on phase information.

For the test we tuned the KerberosSDR RTL-SDRs to listen to a signal at 858 MHz and then drove to multiple locations to take direction readings. The antennas were set up as a linear array of four dipole antennas mounted on the windshield of a car. To save space, the dipoles were spaced at approximately a 1/3 the frequency wavelength, but we note that optimal spacing is at half a wavelength. The four dipole antennas were connected to KerberosSDR, with a laptop running the direction finding demo software. 

Low cost direction finding array mounted to vehicle windshield.
Low cost direction finding array mounted to vehicle windshield.

Our open source demo software (to be released later when KerberosSDR ships) 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. This hopefully results in a line crossing near to the transmitter. Multiple readings taken at different locations will end up intersecting, and where the intersection occurs is near to where the transmitter should be. 

KerberoSDR SDR Directing Finding DOA Reading
KerberoSDR SDR Directing Finding DOA Reading

In the image below you can see the five bearing measurements that we made with KerberosSDR. Four lines converge to the vicinity of the transmitter, and one diverges. The divergent reading can be explained by multipath. In that location the direct path to the transmitter was blocked by a large house and trees, so it probably detected the signal as coming in from the direction of a reflection. But regardless with four good readings it was possible to pinpoint the transmitting tower to within 400 meters.

In the future we hope to be able to automate this process by using GPS and/or e-compass data to automatically draw bearings on a map as the car moves around. The readings could also be combined with signal strength heatmap data for improved accuracy.

This sort of capability could be useful for finding the transmit location of a mystery signal, locating a lost beacon, locating pirate or interfering transmitters, determining a source of noise, for use during fox hunts and more.

KerberosSDR pinpointing a transmitters location
KerberosSDR pinpointing a transmitters location
KerberosSDR Prototype
KerberosSDR Prototype

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Grid-2-Audio: Analyzing the Mains Electrical Grid Waveform with a PC Soundcard

Over on Hackaday and Hackaday.io we've seen an interesting project by David Scholten called "Grid-2-Audio". The project's goal is to build a safe device for monitoring the mains electrical grid waveform via a power jack and PC soundcard. This is essentially an SDR, with the soundcard acting as the ADC for the 50Hz grid signal, and Grid-2-Audio acting as a safety isolator and signal preconditioner. About why you might want to monitor the mains power signal, David writes:

There is a lot to be observed from the waveform of the electrical mains. Harmonics, transient changes, periodic fluctuations, frequency shifts, impedance, power line communications - These all give clues as to the state of the country's electrical transmission system (or what loads your neighbour has connected). Platforms like MATLAB allow for the easy analysis of waveforms through powerful software tools, but only once the signal has been acquired. 

The final product will be a black box with mains plug and a 3.5mm audio jack ready to plug into your soundcard. In order to make the device safe, mains isolation transformers are used as well as good PCB design practices that isolate live and safe areas on the PCB. In the design care is also taken to maintain signal integrity and to not introduce noise by ensuring that the power supply draws minimal sinusoidal current, and is in phase with the voltage.

Grid-2-Audio PCB Rendering
Grid-2-Audio PCB Rendering

SegDSP: Distributed Cloud Based SDR with SpyServer

Over the last few months Lucas Teske (author of the Open Satellite Project) has been working on a piece of software called "SegDSP". The idea appears to create a web GUI based SDR receiver for SpyServer streams which can be used to create a cloud of channel demodulators, essentially segmenting the DSP computation burden over multiple computers.

SpyServer is a SDR server application that is compatible with Airspy products and RTL-SDRs. It allows you to connect to these SDRs remotely over a network or internet connection. The SDR server computer sends the radio IQ data over the network allowing you to perform processing remotely. A major advantage of SpyServer compared to other SDR server applications is that it only sends the raw IQ data for the portion of the spectrum that you're interested in which can save a lot of bandwidth.

One key application that Lucas envisions for SegDSP is using it with cloud clusters of single board computers (SBC) like the Raspberry Pi 3. The philosophy is that there will be specific roles for each SBC machine. For example you might have some SDR machines running SpyServers, some processing machines for demodulating and decoding multiple channels, and a storage machine for recording data. Then you can dynamically spawn / despawn workers when needed (for example only spawning a machine when a LEO satellite with data to decode passes over).

SegDSP development is still in the early stages, and appears to only have the web GUI set up at the moment with a few demodulators. But keep an eye on his Twitter @lucasteske for updates too. Lucas also did a talk at the last CyberSpectrum meetup. His talk can be found at 1:30:00 in the recording.

SegDSP Screenshot Demodulating WBFM
SegDSP Screenshot Demodulating WBFM