Observing the 21cm Hydrogen Line with Linrad and an RTL-SDR

Over on YouTube user S53RM has uploaded a video showing his and S53MM’s observation of the 1420 MHz galactic hydrogen line with an RTL-SDR. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.

In the video they rotate their 3.6m parabolic mesh antenna dish along the Milky Way. As the dish rotates doppler shifted hydrogen line peaks can be observed on Linrad, each peak representing a different arm of the galaxy. The galaxy consists of several spinning arms, some spinning faster than others which causes the hydrogen line peaks produced by the arms to be doppler shifted by different amounts.

They used Linrad to plot the RF spectrum as they were able to use it together with a pulse generator to calibrate the RTL-SDR for a flatter frequency response.

More information about their project can be found at http://lea.hamradio.si/~s53rm/Radio%20Astronomy.htm.

Linrad showing Galactic Arm Hydrogen Line Peaks
Linrad showing Galactic Arm Hydrogen Line Peaks
Hydrogen 21cm lines with DVB-T dongle

Monitoring Military Aircraft with an RTL-SDR Part 2

Last month we posted about monitoring and logging military ADS-B data on milaircomms.com. It turns out that there is another service at www.live-military-mode-s.eu that also does military ADS-B logging. One user of live-military-mode-s.eu has recently uploaded a tutorial showing how to use a RTL-SDR to contribute to their logs. By contributing to their service you get a username and password to access members only sections of their site.

Contribution involves running an ADS-B decoder like RTL1090, sending the decoded data to Virtual Radar Server (VRS) and then using VRS to rebroadcast the data to their Mode-S Logger software.

Some Military ADS-B Logs
Some Military ADS-B Logs

Testing a FM Broadcast Bandstop Filter

Over on YouTube user Cameron Conover has been testing a simple FM broadcast bandstop filter with his HackRF. The same filter can just as easily be used with the RTL-SDR to remove broadcast FM interference and images. Cameron uses a MCM Electronics 88 – 108 MHz FM Trap which can be found very cheaply on Amazon or Ebay for around $15 USD. His video shows that the FM trap works very well and significantly reduces out of band FM interference.

HackRF One with an FM BCB filter

Analyzing a Car Security Active RFID Token with a HackRF

Some car security systems from around 2001 – 2003 use an embedded RFID tag inside the car key as an added security measure against key copying. Using his HackRF, ChiefTinker was able to analyse and decode the data from an active RFID token used in a car key. He notes that the same analysis could also be performed with an RTL-SDR dongle.

Upon powering the RFID tag with a power supply, ChiefTinker noticed that the tag emitted a short transmission every 5 seconds in the ISM band at 433.920 MHz. On closer inspection he determined that the transmitted data was encoded with a simple AM on-off keying (OOK) scheme. After importing the audio into Audacity and cleaning up the signal a little, he was able to clearly see the OOK square wave showing the transmitted binary data.

Next he analysed the data and compared the binary output against two different RFID keys. From the comparison he was able to determine that the tag simply beacons a unique serial number, which is susceptible to capture and replay attacks. After further processing he was able to convert the transmitted binary serial number into hexadecimal, then ASCII to find the unique serial number being broadcast in decimal.

RFID Car Key Tokens
RFID Car Key Tokens

RTL-SDR Tutorial: Receiving Meteor-M N2 LRPT Weather Satellite Images with an RTL-SDR

The Meteor-M N2 is a polar orbiting Russian weather satellite that was launched on July 8, 2014. Its main missions are weather forecasting, climate change monitoring, sea water monitoring/forecasting and space weather analysis/prediction.

The satellite is currently active with a Low Resolution Picture Transmission (LRPT) signal which broadcasts live weather satellite images, similar to the APT images produced by the NOAA satellites. LRPT images are however much better as they are transmitted as a digital signal with an image resolution 12 times greater than the aging analog NOAA APT signals. Some example Meteor weather images can be found on this page and the satellite can be tracked in Orbitron or online.

The RTL-SDR and other SDRs like the Funcube along with some free software can be used to receive and decode these images. LRPT images from the Meteor-M N2 are transmitted at around 137.1 MHz, so any satellite antenna like those commonly used with the NOAA weather satellites can be used.

Happysat, a satellite monitoring enthusiast has emailed us with a comprehensive tutorial showing how the RTL-SDR can be used to receive and decode these LRPT images (pdf warning) (txt file). The procedure is not quite as simple as with the NOAA satellites as it involves first pre-recording the transmission as a baseband I/Q file in SDR#, changing the sample rate in Audacity, processing the file with the Lrptrx.exe software, and then using Oleg’s LRPToffLineDecoder to finally produce the image.

The tutorial also shows an alternative and faster Linux based method using some GNU Radio scripts, but with the final processing still done with Oleg’s decoder in Windows.

The tutorial can be downloaded in PDF form from this link or alternatively in a text file here.

The Meteor-M2 Satellite
The Meteor-M2 Satellite
An Example LRPT Image Received with an RTL-SDR from the Meteor-2 M2.
An Example LRPT Image Received with an RTL-SDR from the Meteor-2 M2.
Another Sample LRPT Image
Another Sample LRPT Image
What a LRPT signal looks like in SDR#
What a LRPT signal looks like in SDR#

Hak5: Installing RTL-SDR on Linux

In this episode of Hak5, a popular YouTube technology channel, Shannon shows how to use the RTL-SDR on Debian Linux. She shows how to install the RTL-SDR drivers from scratch if using a distribution without them pre-installed and also shows how to install and use rtl_fm, a command line FM demodulator.

WiFi Birdhouses and Linux RTL-SDR Setup, Hak5 1703

Two New SDR# Plugins Released: ShortWave Info and DCS Decoder

Recently two new SDR# plugins have just been released.

The first is a plugin which shows the name and language of the shortwave station that is currently tuned in using data from short-wave.info. It can be downloaded from http://sourceforge.net/projects/sdrsharpshortwaveinfoplugin/.

short-wave.info SDR# Plugin
short-wave.info SDR# Plugin

The second plugin is a Digital Code Squelch (DCS) decoder plugin. The plugin will display the DCS codes that are transmitted with the signal and will display all possible compatible codes. DCS is a squelching system similar to CTCSS which allows for radio user sharing by ensuring that radio users are not bothered by communications not intended for them. The DCS Decoder plugin can be downloaded from http://www.rtl-sdr.ru/page/novyj-plagin-dcs-decoder (note page in Russian).

Digital Code Squelch (DCS) Decoder Plugin for SDR#
Digital Code Squelch (DCS) Decoder Plugin for SDR#

SDR Touch Updated to Version 2.0

SDR Touch, the popular Android based software defined radio software for the RTL-SDR has been updated to version 2.0. This new version is a complete rewrite with many optimizations listed below.

  • 100% rewritten from scratch
  • Improved reception sensitivity and quality
  • Optimized engine
  • GUI overhaul (Landscape mode, more flexible)
  • 16 bit audio
  • FIR filtering

The author also writes that the rewrite allows for new features coming out in the future such as adjustable bandwidth, FFT size, plugins and a separate GUI for in-car use. SDR Touch is available from the Android Play store.

SDR Touch Android GUI for RTL-SDR
SDR Touch Android GUI for RTL-SDR

Experimenting with the R820T2 Tuner Chip

The R820T is the tuner chip that is used in the most popular RTL-SDR dongles. It turns out that there is also an R820T2 tuner chip available, but it does not seem to be used in any RTL-SDR dongles that we know of. According to superkuh’s RTL-SDR notes, the R820T2 has better sensitivity over the R820T with an apparent 6dB lower noise floor. It also has a wider IF bandwidth which makes no difference to the RTL-SDRs 3.2 MHz maximum bandwidth, but is why the Airspy with its 10 MHz of bandwidth is using the R820T2 in its design.

Nobu an RTL-SDR experimenter who had previously experimented with dongles retrofitted with TCXO’s has now retrofitted a standard RTL-SDR dongle with an R820T2 tuner chip (note that this post is in Japanese). The Google translation of this post is a little to difficult understand, but it seems that Nobu did notice an improvement due to the lower noise floor. If anyone can understand Japanese we’d appreciate confirmation on this in the comments.

R820T2 Tuner Chip
R820T2 Tuner Chip

SWSCAN – A Console Based Shortwave Broadcast Scanner for the RTL-SDR

Over on the Reddit discussion boards user gat3way has posted about his newly released software project called swscan. Swscan is a Linux console based program that can be used to scan and listen to shortwave broadcast stations. It has a built in database of shortwave station frequencies as well as their broadcast schedules and it will even show you the stations power level and distance you are from the transmitter. Swscan is based on GNU Radio 3.7, so you will need to have that installed first.

As shortwave stations exist at frequencies below the normal tuning range of the RTL-SDR, you will need an upconverter or be using the latest R820T experimental driver which can tune down to around 1 MHz.

Swscan can be downloaded from http://www.gat3way.eu/poc/swscan.tgz.

Console GUI for swscan.
Console GUI for swscan.