Using Raspberry Pi, RTL-SDR, and Direwolf for cheap APRS receiver

Install RTL-SDR and Direwolf

sudo apt-get update -y && sudo apt-get upgrade -y

sudo apt-get install -y rtl-sdr direwolf

sudo reboot now

Test RTL-SDR

rtl_test

Test Receiving APRS with Direwolf

rtl_fm -f 144.39M - | direwolf -r 24000 -D 1 -

Starting Direwolf on boot

We need to create a systemd service configuration so the services start on boot. We should also redirect the stdout/stderr outputs to syslog for debugging in the future.

/usr/local/bin/direwolf-aprs.sh:

#!/bin/bash

/usr/bin/rtl_fm -f 144.39M - | /usr/bin/direwolf -r 24000 -D 1 -

Make direwolf-aprs.sh executable

sudo chmod +x /usr/local/bin/direwolf-aprs.sh

Allow direwolf user to use rtl-sdr USB device

sudo wget -O /etc/udev/rules.d/rtl-sdr.rules "https://raw.githubusercontent.com/osmocom/rtl-sdr/master/rtl-sdr.rules"

sudo systemctl restart udev.service

sudo usermod -a -G plugdev direwolf

/lib/systemd/system/direwolf.service:

[Unit]
Description=DireWolf is a software "soundcard" modem/TNC and APRS decoder
Documentation=man:direwolf

[Service]
User=direwolf
SupplementaryGroups=dialout
ExecStart=/usr/local/bin/direwolf-aprs.sh
StandardOutput=syslog
StandardError=syslog
SyslogIdentifier=direwolf

[Install]
WantedBy=multi-user.target

Reload systemctl:

sudo systemctl daemon-reload

Enable the direwolf service:

sudo systemctl enable direwolf

Start the direwolf service:

sudo systemctl start direwolf

Validate direwolf service is running:

sudo systemctl status direwolf

For additional reading, a more in-depth guide for this process is the Raspberry Pi SDR IGate by WB20SZ

Optional: Adjust audio levels

To be able to decode packets received by your station efficiently, you will need to adjust the volume of the incoming audio so it’s around 50%. This is done by adjusting the volume of the Speaker in alsamixer. I typically set it to 50 on Raspberry Pi’s default device.

Check audio levels are good using direwolf logs

sudo tail -f /var/log/syslog | grep direwolf

# Example output:
# Digipeater WIDE2 (probably K6TZ-9) audio level = 47(12/17)   [NONE]   ||||||___`

audio level = 47 means 47% (which is good)

Adjust the audio levels with alsamixer

alsamixer

Esc to Exit.

Save alsa settings

sudo alsactl store

Optional: Enable logrotate

Because we are saving the output from direwolf to syslog [typically /var/log/syslog], this can lead to the root volume disk becoming full. We can enable logrotate to help mitigate and prevent any issues related to disk usage from the common log files.

sudo systemctl enable logrotate
sudo systemctl start logrotate

Connect to direwolf with APRS Client (using AGW protocol)

I use APRSIS32 on Windows, and YAAC which works on everything that has Java instdalled – but any client supporting AGW network Interface (TCP/IP) protocol can talk to direwolf.

Here’s an example of APRSIS32 connected to direwolf AGW endpoint listening on port 8080