raspi-pico-aprs-tnc

A TX-only TNC (Terminal Network Controller) to generate the AFSK audio tones for APRS (Automatic Packet Reporting System) messages using a Raspberry Pi Pico microcontroller board.

An analog line-out audio signal will be produced at GPIO-pin 'GP0'. You can observe it by using a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).

Basically, this is the data/signal flow:

APRS (text msg + geo-coordinates + meta-data) -> AX.25 -> PCM -> PWM -> Low-Pass filtering -> AFSK signal

Preliminaries

Your host platform is assumed to be LINUX. If you have already installed the Pico-SDK, set the PICO_SDK_PATH environment variable accordingly to avoid installing the SDK twice.

Hardware

We just need a simple band-pass filter to extract the AFSK-signal from the PWM signal:

Build the application

(cd into the cloned dir)
cmake -S . -B build
cmake --build build

Run the application

cd build
(flash 'aprs_pico.uf2' or 'aprs_pico.elf' to the Pico board as usual)

The GPIO-pin 'GP0' is the line-out for the analog AFSK-signal. You can observe it by using a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).

Modify the application

To send an APRS message of your choice, you have to modify the main() function in src/aprs_pico.c.

TODO (Aug 2021)

• Thorough evaluation
• Send the APRS message on the console (USB or UART) rather than hard-coding
• Code documentation
• Show how to connect to a Baofeng HT
• PTT control for HTs

Acknowledgements

• For APRS => AX.25 conversion I'm using a modified version of fsphil's ax25beacon
• For PCM => PWM conversion I'm using the audio lib from pico-extras (WARNING: maturity seems to be alpha/beta)