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What is ACARS?

ACARS stands for ‘Aircraft Communication Addressing and Reporting System’. Essentially it is a digital data link between aircraft & ground stations via radio systems. In some cases it can be used as an alternative to voice communications systems for small messages such as current frequencies. Initially it was designed to reduce the workload on Air Traffic Controll (ATC) staff and flight crew.

What do I need?

In order to receive & decode ACARS transmissions you will need a receiver capable of receiving the ACARS frequencies in use in your area. I will be dealing with 131.7250mHz which is a European secondary frequency used a lot over Britain and Ireland. You will also need a PC with a sound card that supports line in & software capable of decoding ACARS transmissions. I will be dealing with an X86 system (normal PC) running Linux (Ubuntu 7.10), however the majority of the steps in this case are platform independent (ie. they will work similarly, if not identically between Linux and Windows, sorry, no MAC’s in this one). A jack-to-jack lead (to hook the ouput of the receiver to the line in of the PC) is also required.

How do I set it up?

Firstly, the software I will be dealing with is called ACARSD and is available from http://www.acarsd.org . It is free, however, it is not open source. Depending on your platform you will download the linux or the windows version of it. The linux version comes pre-compiled.

Please see this page for instructions on how to install ACARSD in Linux & Windows.

Once ACARSD is installed, you will need to connect one end of the jack-to-jack lead to the line in port of your PC, and the other to the output of your receiver.

Tech Tip: A common term used when talking about receiving digital transmissions using consumer radio equipment is a ‘discriminator’. Generally consumer radio equipment uses a discriminator to alter the shape of audio waveforms in order to make them sound nicer to the ear, a discriminator tap, or a ‘baseband audio tap’ refers to a modification of the scanner in order to gain access to the signal BEFORE it is modified by the discriminator as this will provide better results. In this case it is not essential, but will reduce the errors encountered in decoding transmissions.

Once the receiver is connected to the PC, you can now start ACARSD, this is where the Linux and Windows versions may become different. In the linux version the command ./acarsd will produce an ncurses (text) based interface where as in windows executing acarsd.exe will produce a graphical GUI, if you desire the graphical GUI based interface you can execute ./acarsd –with-gui which will provide this.

An appropriate volume level is essential for receiving error free ACARS transmissions, through trial and error I have found a volume setting between 15 and 22 to be perfect. If you look int the bottom left of the graphical GUI in the status bar, you will see a string saying ‘VOL: x’ where x is the current volume level. If the level is too low, the signal will be too weak to be decoded reliably where if the signal is too high, it will cause distortion and render it un-usable.

Once the receiver is connected to your PC correctly and acarsd is running with a suitable input volume, you should begin to receive ACARS transmissions within a few minutes, if you dont receive any at first dont be too concerned as it does depend on the range of your receiver, and the traffic in your area.

A sample acars message:

ACARS mode: G Aircraft reg: N273AY [-]
Message label: B6 [Provide ADS report] Block id: 8 Message number: J27A
Flight number: US0700 Flight: unknown
Operator: US Airways
Message content: -
/DDLCAXA.DIS.N273AY082579

An example of the map output of ACARSD

Further Information:

For more information on how to read ACARS transmissions see this link.
For more information on ACARS as a protocol, and the history of ACARS, see this link.
For a list of standard ACARS frequencies see this link.