ACARS is a digital data link system for the transmission of messages between aircraft and ground stations. The transmissions usually take place in VHF band, between 130 and 137 MHz. The wide availability of cheap software defined radios makes it easy for anyone to receive and decode ACARS transmissions. Since RTL2832U based USB sticks are the cheapest SDR hardware this is what I will use for reception. There are many tutorials on this subject and most of them involve piping an audio stream from a SDR application to an ACARS decoder application. This is not reliable and takes a lot of resources. More than that, by selecting one frequency, demodulating it and sending the audio stream to ACARS decoder software you are limited to that single frequency.
But there are a few tools that work directly with RTL-SDR hardware, are free, cross-platform and perform ACARS demodulation. More important than that, the software tools you will see here can listen to multiple frequencies at once. The only requirement is that those frequencies should fit in the bandwidth of RTL-SDR (about 2.4 MHz).
