APRS is actually pretty simple protocol. Data is encoded by modulating tones in a human's hearing spectrum, so if you tuned into the APRS frequency with any-pur

APRS is actually pretty simple protocol. Data is encoded by modulating tones in a human's hearing spectrum, so if you tuned into the APRS frequency with any-purpose scanner (like most Baofengs), you would hear your childhood if you are old enough to remember 52k modems. The sound wave is easy to decode and encode with software. There are plenty of programs (Linux even has dedicated kernel modules) which turn your sound card into a modem, and the missing piece is the radio. Now, there are different approaches. If your goal is to receive only, you can plug the headphone output of the cheapest Baofeng scanner into the microphone input of your PC, run the software, and you will start seeing messages soon. If you want to transmit, you do the opposite, but you must somehow automatically enable transmission on the radio when the program wants to transmit; there are different methods of doing so, depending on the radio. Basically, you can use anything what can receive and/or transmit audio on 2m bands. Such devices are cheap and easy to buy, so people probably don't bother with assembling a dedicated hardware. Just plug it to the computer.