Detecting Pulsars (Rotating Neutron Stars) with an RTL-SDR

The RTL-SDR has been used for some time now as an amateur radio astronomy tool. Radio astronomers Peter W East and GM Gancio have recently uploaded a paper that details their experiments with detecting Pulsars with an RTL-SDR (doc file).

A pulsar is a rotating neutron star that emits a beam of electromagnetic radiation. If this beam points towards the earth, it can then be observed with a large dish antenna and a radio, like the RTL-SDR. The abstract of the paper reads: 

This project sought to determine the minimum useful antenna aperture for amateur radio astronomers to successfully detect pulsars around the Hydrogen line frequency of 1420MHz. The technique relied on the collaboration with GM Gancio, who provided RTL SDR data of the Vela pulsar (B0833-45, J0835-4510) and others, collected with a 30m radio telescope. This data was processed to determine the achievable signal-to-noise ratio from which, the minimum useful dish size necessary for some effective amateur work, could be calculated. Two software packages were developed to do synchronous integration, a third to provide a power detection function and a fourth for spectrum analysis to recover pulsar rotation rate.

With their system the authors were able to detect and measure the rotation period of the Vela pulsar. Also, from their data they were able to estimate that the minimum dish aperture required to observe the Vela pulsar would be 6m, noting that the Vela pulsar is probably the strongest pulsar ever detected. They also write that by utilizing 5 RTL-SDRs to gather 10 MHz of bandwidth together with some processing that the minimum required dish aperture could be reduced to 3.5m.

The Vela pulsar pulse power integrated over a 50 second 100MB file, combining some 560 pulsar pulses
The Vela pulsar pulse power integrated over a 50 second 100MB file, combining some 560 pulsar pulses.

In addition to these Pulsar experiments, Peter has also uploaded new papers about improving his Hydrogen Line RTL-SDR Telescope (pdf), and has updated his paper on improving the frequency stability of RTL-SDR’s with air cooling (doc file). Peter found that the frequency stability of the RTL-SDR (with standard oscillator) could be significantly improved by adding heat sinks and aircooling them. The graph from his paper below summarizes his results.

Results from air cooling the RTL-SDR.
Results from air cooling the RTL-SDR.
The air cooled and heatsinked RTL-SDRs
The air cooled and heat sinked RTL-SDRs

All of Peters papers can be found on his website at y1pwe.co.uk/RAProgs/index.html. He has many RTL-SDR radio astronomy related resources there, so check it out if you are interested.

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CubicSDR v0.1.4 Beta Released

CubicSDR is a new and upcoming multi platform open source SDR software package that is compatible with the RTL-SDR. It is similar to programs like SDR#, HDSDR and SDR-Radio. Recently the programmers have released version 0.0.4-beta which adds several new features which we have listed below:

  • Audio Spectrum visuals, drag the A/V visuals area to toggle between audio Scope and Spectrum
  • Waterfall speed can now be controlled between 1 and 1024 lines per second
  • Waterfall now continues to render while minimized or in background
  • Waterfall/Spectrum can now be zoomed to 30khz window with improved resolution
  • Spectrum averaging speed can now be controlled between 1% and 99%
  • I/Q mode for piping decimated I/Q to other applications at audio sample rate
  • Spectrum peak and floor decibels now displayed (can toggle off/on with ‘B’)
  • Can now mute demodulator with ‘M’ button or pressing ‘M” while hovering
  • Save and recall device Offset, I/Q swap, Direct sampling, Waterfall/Spectrum speed, Window state
  • Performance and UI responsiveness improvements
  • Can now use direct input for demod bandwidth
  • Direct input < 3000 now assumes Mhz
  • Additional device input sample rates
  • Improved waterfall keyboard controls via arrow keys
  • Can now specify alternate configuration name via -c (name) or -config (name) at command line
  • Automatically reduce unused buffer memory over time
  • Several crash fixes

CubicSDR is compatible with Windows, Linux and MacOS. It can be downloaded from www.cubicsdr.com.

CubicSDR v0.1.4 Demonstration

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GNU Radio Conference 2015: Presentations

The GNU Radio conference (GRCon15) is a yearly conference discussing all matters related to GNU Radio, an open source graphical block based DSP programming application that is compatible with most SDR’s, including the RTL-SDR. The conference started on August 24 and is due to close this Friday August 28, however many of the presentation slides are now available for viewing on their website.

This year there are many interesting talks, including a speech by Balint about radio direction finding, RF sniffing and digital FPV on drones. There are also several tutorial presentations that show how to install GNU Radio, how DSP sampling works, an intro to analog RF concepts and how to build a software radio from scratch.

gnuradio

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Showing how the R820T stops receiving at 1.4 GHz+ with increasing temperature

Over on YouTube RTL-SDR experimenter Adam 9A4QV has uploaded a video showing how the R820T dongle can fail to receive properly at frequencies above about 1.4 GHz as the temperature in the dongle rises. This is a known problem that may cause issues when trying to receive satellite signals like Inmarsat at 1.541450 GHz. In our own tests, the R820T2 chip appears to be much less prone to this behaviour when compared with the R820T, but still fails if the ambient temperature gets too hot, for example if left in direct sunlight. We’ve had several R820T2 RTL-SDR’s running at 1.5 GHz+ for over 48 hours when left in the shade, but not one R820T ran for more than a few minutes at those frequencies. Of course the E4000 tuner is the best RTL-SDR tuner for these GHz level frequencies, but that tuner is now rare and expensive.

Over on Reddit, some people have been discussing this issue, and have proposed that the likely cause is related to the PLL failing to lock properly at higher temperatures. A fix may be to apply a blob of solder to the vias underneath the R820T chip, and then attach a heatsink. The problem also does not occur on the Airspy, a higher performance SDR that also uses the R820T2 chip in its design. This may be due to better drivers for the Airspy, or better heat dissipation in the Airspy’s hardware design.

R820T stop receiving @ higher frequencies

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Seeing through walls with WiFi signals and USRP software defined radios

Researchers at the University College of London have found a way to use WiFi signals to see through walls, using a USRP software defined radio and software written in LabView. The researchers have shown that they are able to utilize local WiFi signals to detect and monitor moving objects such as people behind a wall in a similar fashion to how radar systems work. The advantage over traditional radar is that their system is completely passive, requiring no transmitter, other than the already ubiquitous WiFi signal.

In a demonstration the researchers showed how they were able to not only detect the presence of a person behind a wall, but also detect small hand gestures that were made.

Detecting body gestures from WiFi signals in LabView.
Detecting body gestures from WiFi signals in LabView.

It appears the researchers are patenting their work and are looking to market their technology towards military and security surveillance operators as well as towards other applications such as traffic monitoring and the monitoring of children and the elderly.

We aren’t sure what type of radio accuracy is required for a system such as this, but it may be possible that SDR’s that cost less than the USRP may also work, assuming the software technology can ever be replicated/licensed.

wifi_hostage
A proposed application of the technology: Allowing police to see through walls in a hostage situation.
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Painting on the RF Spectrum with a HackRF

Last week several people from the Chaos Communication Camp conference and others on the #hackrf IRC channel were playing around with the idea of painting pictures on the RF spectrum with the HackRF – a low cost transmit capable software defined radio. This idea works simply by modulating a signal so that it produces a desired image pattern on a frequency domain waterfall display.

To make this easier to do, GitHub user polygon has authored a Python program called Spectrum Painter which easily converts an image into an IQ file which can be transmitted with a HackRF. In addition as described in the Reddit thread linked above, a Windows program called Coagula can also be used to convert images into .wav files, which can then be transmitted on any capable radio. The RF painted images can then be received on another SDR radio like the RTL-SDR.

As always remember to only transmit at a frequency you are licensed on, or at low power in a RF controlled environment.

Below is an example image and video showing images being painted on the RF waterfall.

Spectrum painter transmitted output image
Spectrum painter transmitted output image

hackrf transmitting images in the frequency domain

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Reverse Engineering Bus Telemetry Data with an RTL-SDR

Bastian recently wrote into us at RTL-SDR.com to let us know that he’s been working on reverse engineering the bus telemetry system used in his hometown of Paderborn, Germany. Bus telemetry is often used to update live signs at bus stops that indicate based on GPS data how long a bus user needs to wait for the next bus.

Bus sign: Wireless bus telemetry updates this sign.
Bus sign: Wireless bus telemetry updates this sign.

A similar reverse engineering of bus telemetry was performed before by Oona Raissan in Helsinki, Finland. Oona found that in Helsinki bus telemetry was transmitted as a DARC subcarrier embedded in regular broadcast FM radio. In many countries bus telemetry runs through GSM or TETRA communications as well, which are encrypted and would be very difficult to decode.

However in Paderborn, Germany Bastian discovered that the bus telemetry system used a different protocol which he discovered by noticing that some very strong signals appeared on his spectrum at 150.9 MHz whenever a bus drove by his flat.

After making a recording of this signal in GQRX, bastian analysed it in Audacity and discovered that the binary data bits were encoded by the presence or absence of a half sine wave. After discovering the encoding he was then able to determine the bit rate and build a decoder in GNU Radio. His post goes into further detail about concepts he used in his GNU Radio program such as frame detection, bit stuffing and error detection.

Finally, with all his decoder program written he was able to gather lots of data from each packet such as the bus ID, line, bus stop, distance from last bus stop, delay, position and even the orientation of the bus. Bastian has also uploaded a video showing everything in action, which we have embedded below.

Bus position heatmap from data obtained via the RTL-SDR
Bus position heatmap from data obtained via the RTL-SDR

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A new AIS Decoder for the RTL-SDR on Android

A reader of our blog, EBC81, has written in to let us know about a new RTL-SDR based AIS decoder that he has written for the Android OS. AIS stands for Automatic Identification System and is used by ships to broadcast their GPS locations, to help avoid collisions and aid with rescues. An RTL-SDR with the right software can be used to receive and decode these signals, and plot ship positions on a map.

EBC81’s program is called rtl_ais_android and can be downloaded from this GitHub link. It decodes the AIS data into NMEA messages, which can then be sent via UDP to mapping programs in Android or a program like OpenCPN on your PC. To use the app you will need a USB OTG cable to connect your Android device to the RTL-SDR.

In the future EBC81 hopes to create a second app which will display the ship positions on a map.

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