The Open Glider Network uses a readily available, cheap software defined radio (SDR) to feed Flarm data to a central system (live.glidernet.org) to provide flight following and entertainment watching where the gliders are.
To set up a ground station you need a computer (normally an ARM based SBC like the Raspberry Pi) and a RTL-SDR USB dongle. These are available as digital TV receivers, or for a little more money as better quality amateur radio receivers. OGN have software that sets the RTL-SDR to receive on the Flarm frequency and decodes the Flarm protocol, passing the data to the central servers.
The software uses some out of date packages and this can cause some issues installing on newer operating systems. Some libraries used by the OGN software have been depreciated, and some system setup has changed.
Only some parts of the OGN software are available as source. The ogn-decode program is only available in binary.
Following the OGN install instructions get most of the way to a functioning system but there are a few gotchas.
Many new SBCs use 64bit ARM CPUs these days. Make sure you get the correct version of the software form ARM64. This has only been supported since version 0.3.2.
Both the ogn-rf and ogn-decode programs use the libjpeg8 library. This is no longer available packaged with Debian or Ubuntu.
The easiest way to get this library is to build it from source.
wget http://www.ijg.org/files/jpegsrc.v8d.tar.gz tar -xzf jpegsrc.v8d.tar.gz cd jpeg-8d ./configure --libdir=/usr/lib/ make sudo make install
For ARM64 I needed to add to the configure step:
./configure --libdir=/usr/lib/ --build=aarch64-unknown-linux-gnu
To get access to the rtl-sdr device on newer systems I needed to tell udev about it.
I found the udev rules on github at SDR-udev-rules and installed the rtl-sdr rules in /etc/udev/rules.d.