Tags: radio winlink
Identify the used transmission protocol using sigint wiki (Sounds a lot like APRS)
Decode it using multimon-ng with AFSK1200, you'll get some metadata and an encoded message.
The metadata points to WinLink. Setup Winlink and soundmodem, with the correct setup you'll get the message containing the flag.
Basically, setup old hamradio software to get the flag, done ;)