Tagged: rtl-sdr

An Open Source VOR Receiver for Airspy and RTL-SDR

Thank you to Thierry Leconte (TLeconte) for writing in and submitting his new command line based open source software called vortrack. Vortrack is a simple VOR decoder which calculates the angle towards the VOR. It is compatible with both RTL-SDR and Airspy radios, and runs on Linux.

In the past we've seen several other posts about RTL-SDRs being used to decode VOR signals, but Thierry's implementation appears to be the easiest way to get a bearing straight away. You'll get the most use out of the software if you install it on a portable device like a Raspberry Pi and take it out for a drive as you'll be able to see the VOR angle changing then.

VOR stands for VHF Omnidirectional Range and is a way to help aircraft navigate by using fixed ground based beacons. The beacons are specially designed in such a way that the aircraft can use the beacon to determine a bearing towards the VOR transmitter. VOR beacons are found between 108 MHz and 117.95 MHz, and it's possible to view the raw signal in SDR#.

A DVOR Ground Station at an Airport. Source Wikipedia.
A DVOR Ground Station at an Airport. Source Wikipedia.

Motherboard Article: Creating an IMSI Catcher with an RTL-SDR

Motherboard, an online technology magazine has recently run an article titled "With $20 of Gear from Amazon, Nearly Anyone Can Make This IMSI-Catcher in 30 Minutes". The article describes how an RTL-SDR together with the IMSI-Catcher Linux software can be used to collect IMSI numbers from cellphones connected to a nearby cell tower. The IMSI is a unique number assigned to each SIM card and collecting this data could be used to identify if someone is in the area covered by the cell tower.

The IMSI-Catcher software only works with the older 2G GSM signals which are now being phased out in some countries and are relatively unused in others. Also unlike more advanced IMSI-Catchers which create a fake cell tower signal, the RTL-SDR based IMSI-Catcher can only collect IMSI numbers when the cellphone first connects to the cell tower.

One of our older posts with a YouTube tutorial video explains the RTL-SDR IMSI Catcher in more detail. 

IMSI-Catcher Python Script
IMSI-Catcher Python Script

Measuring Broadcast FM Multipath Distortion with an RTL-SDR

Over on GitHub user jj1bdx has just released a new tool called rtl_power-fm-multipath which can be used for estimating broadcast FM multipath distortion with an RTL-SDR. Broadcast FM multipath is caused when a signal is received from multiple directions due to it reflecting off and refracting through physical objects like buildings and terrain. As the reflected/refracted signals will be delayed it can cause echo like distortions in the RF signal which can cause issues like poor digital decoding, poor FM audio reception and ghosting in analogue video.

The multipath distortion estimation method works by measuring the ratio of the strength of direct/reflected radio waves which results in the desired/undesired (D/U) ratio. This measurement method was proposed by Komiya87 and JushinFM who both wrote papers in Japanese describing the method. In summary the methodology is:

  • Measure the maximum peak strength for +-50kHz spectrum of the target FM station
  • Obtain the maximum value (Lmax) and minimum value (Lmin) within the spectrum
  • Obtain the ratio of the maximum and minimum values L = Lmax / Lmin (note: Lmax and Lmin are real values (not in dB), and L must be > 1)
  • The estimated D/U ratio R = (L+1) / (L-1) (in the real value, not in dB)

The rtl_power-fm-multipath program is based on rtl_power and works by using rtl_power to record power measurements for 5 minutes, then sending the data to a peakhold function which computes the maximum power value for each frequency, and then calculations the distortion ratio.

An example of Multipath Distortion on a DAB+ Signal. From Gough Lui's post https://goughlui.com/2015/03/28/trip-to-hk-cn-2014-part-5-rtl-sdr-more-radio-ais-night-photos/
An example of Multipath Distortion on a DAB+ Signal. From Gough Lui's post at https://goughlui.com/2015/03/28/trip-to-hk-cn-2014-part-5-rtl-sdr-more-radio-ais-night-photos

Es’hail-2: First Geostationary Satellite with Amateur Radio Transponders Successfully Deployed

Today SpaceX have successfully launched and deployed the Es'hail-2 satellite which is now in geostationary orbit. This launch is special for amateur radio enthusiasts because it is the first geostationary satellite that contains an amateur radio transponder on it. The satellite is positioned at 25.5°E which is over Africa. It will cover Africa, Europe, the Middle East, India, eastern Brazil and the west half of Russia/Asia. Unfortunately, North America, Japan, most of South America, Australia and NZ miss out.

Coverage of Es'hail 2
Coverage of Es'hail 2

The satellite has a two bandwidth segments, a 250 kHz narrow band for modes like SSB, FreeDV, CW, RTTY etc, and a 8 MHz wide band for digital amateur TV (DATV) modes like DVB-S and DVB-T.

The downlink frequencies are at 10 GHz so a low cost TV LNB could be used as the antenna. For receiving the narrowband modes, an RTL-SDR or similar SDR could be used, and for the 8 MHz DATV modes a standard DVB-S2 set top box can be used to receive and decode the video. For uplink, the transmission frequency is at 2.4 GHz.

According to the commissioning order of the satellite, it is expected that the AMSAT transponders will be activated only after all tests have been passed, and after other higher priority commercial telecommunications systems have been activated. This is expected to take about 1-2 months.

2018: Es'hail-2 and its amateur radio payload - Graham Shirville (G3VZV) & Dave Crump (G8GKQ)

An Overview of Aircraft Communication Modes from HF to UHF

Over on YouTube icholakov has uploaded an informative video that gives an overview of the main communication modes that aircraft use from HF to UHF. In the video he also gives examples of those modes being received and decoded with an SDR.

The modes that he explains and demonstrates are VHF voice, VHF ATIS automated weather, ACARS short data messages, HF voice, HF automatic weather, HF data selective calling (SELCAL), HF data link (HFDL) and UHF ADS-B aircraft positioning.

Monitoring airplane communications

Setting up Air Traffic Control Audio Sharing with Broadcastify, RTL-Airband, RTL-SDR and a Raspberry Pi

Over on YouTube Fuzz The Pi Guy has uploaded a video tutorial showing how to set up a Broadcastify air traffic control audio feed with RTL-Airband and an RTL-SDR running on a Raspberry Pi. This allows you to publicly share your received air traffic control audio online via sites like Broadcastify.

The video is based on a comprehensive Radioreference text tutorial which takes you through the process from scratch. Setting it up involves installing the Raspbian OS, installing RTL-SDR, installing and setting up RTL-Airband, configuring ezstream and then ensuring that everything runs automatically on boot. It's a fairly involved setup process, but the video helps make things easier.

How To Setup Broadcastify On A Raspberry Pi Using RTL_AM For Aviation

RPiTX v2 Released: Easily Record and Replay with RTL-SDR and a Raspberry Pi

RPiTX is software for the Raspberry Pi which can turn it into a 5 kHz to 1500 MHz transmitter which can transmit any arbitrary signal. In order to transmit the software does not require any additional hardware apart from a wire plugged into a GPIO pin on the expansion header. It works by modulating the GPIO pin with square waves in such a way that the desired signal is generated. However, although additional hardware isn't required, if RPiTX is to be used in any actual application a band-pass filter is highly recommended in order to remove any harmonics which could interfere and jam other radio systems.

Earlier this month RPiTX was upgraded to version 2. One of the changes is a new GUI for testing the various transmission modes. Currently it is possible to transmit a chirp, FM with RDS, USB, SSTV, Opera, Pocsag, SSTV, Freedv. There is also a spectrum painter which allows you to display an image on a SDR's waterfall.

Painting an Image on a SDR Waterfall Display with RPiTX v2
Painting an Image on a SDR Waterfall Display with RPiTX v2

The RPiTX v2 update also makes recording a signal with an RTL-SDR, and replaying that signal with RPiTX significantly easier. Previously it was necessary to go through a bunch of preprocessing steps (as described in our previous tutorial) in order to get a transmittable file, but now RPiTX is capable of transmitting a recorded IQ file directly. This makes copying things like 433 MHz ISM band remotes significantly easier. One application might be to use RPiTX as an internet connected home automation tool which could control all your wireless devices.

Finally, another application of the RPiTX and RTL-SDR combination is a live RF relay. The software is able to receive a signal at one frequency from the RTL-SDR, and then re-transmit it at another frequency in real time. Additionally, it is also capable of live transmodulation, where it receives an FM radio station, demodulates and then remodulates it as SSB to transmit on another frequency.

RPiTX v2 re-transmitting a broadcast FM signal live at 434 MHz.
RPiTX v2 re-transmitting a broadcast FM signal live at 434 MHz.

Video Tutorial: Installing GNU-Radio on Windows 10

Over on YouTube user Petr Horký has uploaded a helpful tutorial video showing how to install GNU Radio on Windows 10. Petr goes through the steps from installing Python, pip and other dependencies like numpy and pyqt, to installing GNU Radio itself and then ensuring that the system PATH is set correctly.

GNU Radio is a block based programming language for building digital signal processing applications (e.g. demodulators/decoders). It is very useful for experimenting with more advanced SDR concepts, and there are also many RTL-SDR compatible applications built with GNU Radio as well. GNU Radio is typically run on Linux, but can also run on Windows now too, although perhaps not every program will be compatible.

How to install GNU Radio Companion on Windows 10 (pip, environment variables)