Category: Applications

September 7, 2018

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 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 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).

Watch this video on YouTube

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.

September 7, 2018

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

[As seen on the swling Post blog]

September 5, 2018

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

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.

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

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

September 5, 2018

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.

September 4, 2018

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.

September 3, 2018

SDR# Inmarsat Decoder Plugin Now Available

Microp11, the programmer of Scytale-C a standalone Inmarsat decoder has just released a new Inmarsat decoder SDR# plugin. The plugin is currently in the "pre-alpha" stages, so is still missing some functionality and may be buggy. However, it does appear to be functional at this point in time. It can be used with RTL-SDRs, and any other SDR# compatible SDR including units running on remote SpyServers. Microp11 writes:

I ran it with SDR# version v1.0.0.1761.

If it crashes you SDR# I apologize in advance.

The auto-tracking (default on) will alter your SDR# frequency and follow the signal’s CF. When the SNR is very low, please disable it and manually tune the SDR# to try to get the CF as close to 2000 as possible.The demodulator still has plenty ideas of its own.

Use USB mode with 4000 Hz bandwidth.

For now the interface is missing the usual scatter plots.

UDP Address and UDP Port are for sending the decoded frames to the Scytale-C UI.

Offset and CF are the difference from zero error and the CF frequency of the demodulated BPSK signal.

Tx and SYM are the transmitted over UDP frames and SYM is showing the number of demodulated symbols.

A bunch of libraries are attached as extra files. Please be gentle and accept the package as it. Will clean-up in the future.

Use in conjunction with the Scytale-C UI from the archive: “x64-UI1.6-Decoder1.4.zip” (link below)

The magic line is included in the archive: “SDRSharp.ScytaleC-1.0-alpha.zip”

The files can be downloaded from https://bitbucket.org/scytalec/scytalec/downloads.

August 31, 2018

NooElec SAWBird: An LNA + Filter for GOES Weather Satellite Reception Now Available

NooElec has just released their new "SAWbird" GOES LNA for sale. This is an LNA and filter combination designed to help receive GOES weather satellite images. On the PCB is a 1688 MHz SAW filter and a low noise amplifier. It can be powered with 3V - 5.5V connected directly or via bias tee. The SAWbird is currently available on Amazon and their store for US$34.95. They also have a version for Inmarsat and Iridium, so make sure you choose the correct one.

GOES 15/16/17 are geosynchronous weather satellites that beam high resolution weather images and data. In particular they send beautiful 'full disk' images which show one side of the entire earth. As GOES satellites are in a geosynchronous orbit, the satellite is in the same position in the sky all the time, so no tracking hardware is required and images can be constantly pulled down throughout the day without having to wait for a satellite to pass over.

However, compared to the more familiar and easier to receive low earth orbit satellites such as NOAA APT and Meteor M2 LRPT, geosynchronous satellites like GOES are quite a bit further away, and transmit at 1.7 GHz. So to receive the signal you'll need a dish antenna that you can accurately point, a good low noise figure LNA and possibly a filter. So setting up a receiver is a bit more difficult when compared to receivers for NOAA and Meteor satellites. The SAWbird should help however, by providing a ready to use LNA+Filter combination.

Over the past few months several testers have already received engineering samples of the SAWbird and have been successful at receiving GOES images. From the results of several experimenters, it appears to be possible to use a cheap 2.4 GHz WiFi grid antenna with some minor modifications as a GOES satellite antenna. Get one with at least a one meter long width and bend the feed as described here or here to tune reception for the 1.7 GHz GOES frequency. Pieter Noordhuis has also shown that it's possible to use an RTL-SDR to receive GOES images, so an entire GOES system can be built on a budget.

NooElec SAWbird LNA + Filter for GOES reception.

August 30, 2018

rtl_ultrasound: Using a Piezoelectric Transducer with an RTL-SDR to Create an Ultrasound Imager

Over on GitHub an interesting project that involves using an ultrasound transducer and RTL-SDR to create a low cost 2D ultrasound imager has been uploaded. Ultrasound imagers transmit acoustic sound waves with a transducer at frequencies between 1 - 5 MHz, and then listens for the audio reflections from objects in the audio waves path. This technique is commonly used in the medical field for imaging inside the body without using damaging ionizing radiation like with x-rays.

The project by wlmeng11 is based on the open un0rick hardware, which is an open source ultrasound imager. wlmeng11's idea is to simplify and lower the cost of the un0rick hardware by replacing some expensive components like the FPGA and ADC with a computer and RTL-SDR. The simplified hardware is called "SimpleRick" and PCB and firmware files are also available on GitHub.

Regarding his choice to use SDR and RTL-SDR he writes:

Why SDR?

The analog signal produced by a B-mode ultrasound (ie. 2D imaging) is essentially an Amplitude Modulated (AM) signal. The signal's envelope (ie. amplitude) corresponds to boundary information in the physical media, and the signal's carrier frequency is equal to the resonant frequency of the transducer.

Most ultrasound systems take one of two approaches for data acquistion:

Direct sampling of the ultrasound signal: This method preserves the original signal in the time domain, accomodates any transducer frequency, and offers the best flexibility for post-processing and analysis. Both amplitude and phase information can be extracted the signal, so it is useful for both B-mode and Doppler mode imaging. However, this method requires a high sample rate ADC, as well as high bandwidth and storage for the digital data.

Envelope detection with analog hardware: Perform Amplitude Demodulation (typically with a diode-based rectifier and low pass filter) to yield an envelope signal, then acquire the envelope signal at a lower sample rate. This method reduces the bandwidth and storage requirements for the digital data, but there are a number of drawbacks:

Unless the low pass filter is adjustable, this method cannot accommodate different transducer frequencies.

The non-linearity of the diode may produce harmonic distortion.

All phase information in the signal is lost, rendering it useless for Doppler mode imaging.

It has been demonstrated by Peyton et al that quadrature sampling can be used to reduce bandwidth requirements in an ultrasound imaging system.

It turns out that quadrature modulation is essential to Software Defined Radio (SDR) because any type of amplitude modulation, frequency modulation, phase modulation, or combination of these can be expressed as a special case of quadrature modulation. Therefore, many of the software and hardware techniques used in SDR can be applied to ultrasound imaging.

Why RTL-SDR?

The RTL2832U chip in the RTL-SDR takes a hybrid approach for data acquisition. It employs a high sample rate ADC (28.8 Msps), followed by a software-configurable Digital Down Converter (DDC) that produces IQ data at a lower sample rate (up to 2.56 Msps), thus reducing bandwidth and storage requirements. We can then perform envelope detection in software.

Plus, the RTL-SDR is really cheap (under $25 on Amazon in the United States)! As such, there is a lot of software support and a large community for the RTL-SDR.

With a few software tweaks, it should be possible to substitute the RTL-SDR with a more expensive SDR (eg. AirSpy HF+, LimeSDR) for use cases that require better ADC resolution and SNR.

Some of his test results are available in his August 21 writeup. His test involves a pseudo-anechoic chamber with some steel balls to reflect the ultrasound wave. The ultrasound transducer is swept through the chamber using a servo. The results so far have been successful in reliably and repeatedly resolving imaging on objects that are about 1 cm in size.

If you're interested in the combination of acoustic transducers and SDRs, then this previous post shows using a piezo to detect ultrasound echolocation sounds from bats.

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Broadcasting Khan Academy 24/7

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