RDS (Radio Data System) is a communication protocol standard used for embedding small amounts of digital information in traditional FM radio broadcasts. It enables radio stations to transmit data such as station identification, program information, and traffic updates.
To capture and decode RDS data, one method involves using a Software Defined Radio (SDR) along with GNU Radio and RFtap. GNU Radio provides a framework for creating software radios, while RFtap acts as a bridge between GNU Radio and conventional network monitoring and packet analysis tools like Wireshark.
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| Decoding RDS data using Wireshark. |
Unfortunately, as of the time of writing, RFtap is no longer being maintained and does not work with the latest version of GNU Radio (version 3.10.10). This post offers guidelines for rebuild and using RFtap with the new GNU Radio release.
This post assumes that the reader has access to DVB-T dongles based on the Realtek RTL2832U and a PC running Ubuntu or Debian Linux. For this, I used an RTL dongle with Rafael Micro R820T tuner and Ubuntu 24.04 LTS release.
As the first step install the following GNU Radio build dependencies into the OS:
sudo apt-get install cmake libboost-all-dev \
liblog4cpp5-dev qtcreator qtbase5-dev \
qt5-qmake python3-cheetah python3-numpy \
python3-pygtk python3-gi python3-gi-cairo \
gir1.2-gtk-4.0
sudo apt install git g++ libgmp-dev swig \
python3-mako python3-sphinx python3-lxml \
doxygen libfftw3-dev libsdl1.2-dev \
libgsl-dev libqwt-qt5-dev libqt5opengl5-dev \
python3-pyqt5 liblog4cpp5-dev libzmq3-dev \
python3-yaml python3-click \
python3-click-plugins python3-zmq python3-scipy \
libcodec2-dev libgsm1-dev libusb-1.0-0 \
libusb-1.0-0-dev libudev-dev \
python3-setuptools
sudo apt install pybind11-dev python3-matplotlib \
libsndfile1-dev libsoapysdr-dev soapysdr-tools \
python3-pygccxml python3-pyqtgraph
sudo apt install libiio-dev libad9361-dev \
libspdlog-dev python3-packaging python3-jsonschema \
python3-qtpy
sudo apt remove swig
Next, clone and build Volk (Vector-Optimized Library of Kernels)
mkdir ~/rf
cd rf
git clone --recursive https://github.com/gnuradio/volk.git
cd volk
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release -DPYTHON_EXECUTABLE=/usr/bin/python3 ../
make
sudo make install
sudo ldconfig
After installing the Volk library, we can proceed to build GNU Radio.
cd ~/rf
wget https://github.com/gnuradio/gnuradio/archive/refs/tags/v3.10.10.0.tar.gz
tar -xvf ./v3.10.10.0.tar.gz
cd gnuradio-3.10.10.0
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release -DPYTHON_EXECUTABLE=/usr/bin/python3 ../
make -j8
make test
sudo make install
sudo ldconfig
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| RFtap Encapsulation block with Socket PDU. |
Now GNU Radio is installed with all necessary components. To enable RTL SDR support, we must build and install Osmocom RTL SDR libraries and SDR components.
cd ~/rf
git clone https://gitea.osmocom.org/sdr/rtl-sdr.git
cd rtl-sdr
mkdir build
cd build
cmake ../ -DINSTALL_UDEV_RULES=ON
make
sudo make install
sudo ldconfig
cd ~/rf
git clone https://gitea.osmocom.org/sdr/gr-osmosdr
cd gr-osmosdr
mkdir build
cd build
cmake ../
make
sudo make install
sudo ldconfig
Before plugging in the RTL-SDR dongle, we need to prevent the kernel modules for the RTL-SDR USB device from being loaded into the kernel and taking ownership of the device. To do this, simply navigate to the /etc/modprobe.d directory and create a file called rtl-sdr-blacklist.conf with the following content:
# This system has librtlsdr0 installed in order to
# use digital video broadcast receivers as generic
# software defined radios.
blacklist dvb_usb_rtl28xxu
blacklist e4000
blacklist rtl2832
blacklist rtl2830
blacklist rtl2838
Next, you should clone and build the FM RDS/TMC transceiver module for GNU Radio.
cd ~/rf
wget https://github.com/bastibl/gr-rds/archive/refs/tags/v3.10.tar.gz
tar -xvf ./gr-rds\ -v3.10.tar.gz
cd gr-rds-3.10
mkdir build
cd build
cmake ..
make
sudo make install
sudo ldconfig
For the next steps, we need to have Wireshark and RFTap. Wireshark can be installed using a package manager.
sudo apt-get install wireshark
To run Wireshark without requiring root user permissions, use the following set of commands:
sudo dpkg-reconfigure wireshark-common
sudo usermod -a -G wireshark $USER
newgrp wireshark
A message may be prompted in the first step above and proceed by selecting the "Yes" option.
Now restart the OS and continue with the RFTap installation.
The official RFTap repository is no longer being maintained and is not compatible with newer versions of GNU Radio. For this step, please use the RFTap fork available in my GitHub repository. This version has been successfully tested with GNU Radio 3.10.10 and Wireshark 4.2.2.
cd ~/rf
git clone https://github.com/dilshan/gr-rftap.git
cd gr-rftap
mkdir build
cd build
cmake ..
make
sudo make install
sudo ldconfig
Now get the modifier version of rds_rx_rftap.grc from the above repository.
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| Station name received as Sirasa on 106.5MHz. |
The Wireshark Dissector file for RDS data is also available in the repository. Copy it to the ~/.config/wireshark/plugins directory. Create the directories if they do not exist.
Launch Wireshark and monitor the loopback (lo) adapter. Start GNU Radio and execute the rds_rx.grc file, which was downloaded in the above step.
If all the steps are performed correctly, the RDS data should appear in the packet list pane as UDP messages. The dissected messages can be observed through the packet bytes pane.
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