What is ACARS?
ACARS is an acronym for Aircraft Communications Addressing and Reporting System which is a digital communications system that aircraft use to send and receive short messages to and from ground stations.
Standard ACARS transmits at a frequency of 131.550 MHz, which is in the receivable range of the RTL-SDR. The RTL-SDR software radio can be used as a radio scanner for listening to these digital messages, and with the help of some decoding software, can be used to decode and display the messages. The messages you can receive will be from nearby aircraft and ground stations. Most messages will be unreadable data intended for computers, but you can find out what is flying near you by decoding the flight number and aircraft registration details sent with every message.
There is also HF ACARS, which is used for long distance communications. In this article the focus will be on VHF ACARS, as receiving HF ACARS is a little different.
Examples of the RTL-SDR being used to decode ACARS
YouTube user Superphish shows a timelapse over 5 hours of ACARS traffic and decoding using SDR# and decoding program acarsd. He used a J-Pole antenna. (2021 Update: please see note regarding acarsd no longer working as expected below in the tutorial)