Decoding APRS with a Raspberry Pi 4 and an RTL-SDR dongle
Continuing its experiments with the RTL-SDR V4 dongle, framboise314 has published a guide to decoding APRS on a Raspberry Pi 4. The idea is to turn the small board into an amateur radio receiving station, listening on the European VHF frequency of 144.800 MHz.
The article starts by explaining what APRS (Automatic Packet Reporting System) is. Invented in the late 1980s, it is packet radio: very short bursts of audio in 1200-baud AFSK, wrapped in AX.25 frames. They are used for position tracking, telemetry (weather stations, the battery status of an isolated relay) and short messages. The network works without the Internet thanks to digipeaters, and iGates bridge it to the web. The author also compares it with Meshtastic, whose main difference lies in regulations and frequencies.
On the practical side, the setup goes through GQRX, tuned carefully, and then through Dire Wolf, a purely software TNC that decodes the frames. The tutorial covers the installation, routing audio over UDP, and a very real pitfall: GQRX sends over IPv6 by default, so the stream has to be forced to IPv4 for Dire Wolf to receive anything. Finally, it deals with a hardware problem, thermal throttling of the processor under the load of signal processing, which must be addressed if you want a station that runs reliably around the clock.
What I found pleasant is the tone of someone who practised packet radio in the 1990s and is rediscovering the technique with hardware that costs a few tens of euros. A good starting point for anyone who wants to combine Raspberry Pi and radio.
Source: Framboise 314
The article starts by explaining what APRS (Automatic Packet Reporting System) is. Invented in the late 1980s, it is packet radio: very short bursts of audio in 1200-baud AFSK, wrapped in AX.25 frames. They are used for position tracking, telemetry (weather stations, the battery status of an isolated relay) and short messages. The network works without the Internet thanks to digipeaters, and iGates bridge it to the web. The author also compares it with Meshtastic, whose main difference lies in regulations and frequencies.
On the practical side, the setup goes through GQRX, tuned carefully, and then through Dire Wolf, a purely software TNC that decodes the frames. The tutorial covers the installation, routing audio over UDP, and a very real pitfall: GQRX sends over IPv6 by default, so the stream has to be forced to IPv4 for Dire Wolf to receive anything. Finally, it deals with a hardware problem, thermal throttling of the processor under the load of signal processing, which must be addressed if you want a station that runs reliably around the clock.
What I found pleasant is the tone of someone who practised packet radio in the 1990s and is rediscovering the technique with hardware that costs a few tens of euros. A good starting point for anyone who wants to combine Raspberry Pi and radio.
Source: Framboise 314
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