Installing and Using the Direwolf
Software Modem on Raspberry Pi

Direwolf is a software "soundcard" AX.25 packet modem/TNC and APRS encoder/decoder. It can be used stand-alone to observe APRS traffic, as a tracker, digipeater, APRStt gateway, or Internet Gateway (IGate). For the windows or Linux version please visite the following site.
https://github.com/wb2osz/direwolf/releases

For those interested in experimenting with this DIY approach to Packet Radio, Chris Hood (WB4ULK) has produced a series of four (short) YouTube videos you may find helpful:

In this series, Chris makes reference to two additional items you may need or want to complete your project:

(1) The RA-35 Radio Adapter Interface Board
(NOTE: This should work fine for most needs, but there is a newer product line you might also review: The DRA Series Digital Radio Adapter Interface)

(2) EasyTerm Cross-Platform Serial Port Emulator software

Many Raspberry Pi users also install 'Xastir', an APRS Linux application that allows users to send and receive position reports, messages, weather data, and other information over packet radio. For more information, please visit The Xastir APRS Program site