ACARSDEC

Aircraft Communication Addressing and Reporting System (or ACARS) is a digital datalink system for transmission of small messages between aircraft and ground stations via VHF radio.

Acarsdec is an open source, realtime ACARS demodulator and position decoder for Linux.

HOW DOES IT WORK ?

To receive ACARS you need at least an AM VHF air band receiver tuned to one of these frequencies:

• 131.725 Europe primary
• 131.525 European secondary
• 131.550 USA primary
• 130.025 USA secondary
• 131.450 Japan primary

(These frequencies are the most common, Google is your friend for other frequencies)

Audio output from this receiver is send to the soundcard input of your PC under Linux. Then, acarsdec will demodulate the signals sent by aircraft and print the received messages on its standard output in airnav log text format.

BUILDING IT

On a Linux system, you will need libsnd library, alsa audio system and gcc/make installed.

Then just type: make

USING IT

acarsdec could be called with the following options:

acarsdec [-LR][-s noport] -d alsapcmdevice | -f sndfile
 -f sndfile :           decode from file sndfile (ie: a .wav file)
 -d alsapcmdevice :     decode from soundcard input alsapcmdevice (ie: hw:0,0)
 [-LR] :                disable left or right channel decoding of stereo signal (save cpu)
 [-s noport ] :         "xastir" mode : act as an APRS local server, on port : noport (see below)

Input could be mono or stereo but with 48Khz sampling frequency. If stereo, acarsdec will demod the 2 channels independently (if no L or R options specified)

Typical usage for realtime decoding is : acarsdec -d hw:0

Be sure that correct record level is set for the used soundcard input. For testing, you could try to record your receiver output at 48khz sampling frequency with any audio recording tool.

Save as wav file, then decode it by: acarsdec -f audiofile.wav

USING IT WITH XASTIR

acarsdec have a special output mode to use it with APRS position reporting plotting program: xastir.

In this mode, acarsdec acts as a very basic local aprsd server. ACARS messages, and in particular, position report messages are converted to APRS format, so you can plot aircraft positions on a map.

PS: position decoding is in experimental stage. Mail me if you find errors or lack of position reporting.

Start acarsdec with the following option: acarsdec -d hw:0 -s 14000

Then in xastir, choose:

• Interface → Interface Control → Add
• Select: Internet Server, then Add
• Set Host at 127.0.0.1, Port 14000, Don't allow transmitting, then OK.

This will add an interface in the Interface Control dialog.

Then select this interface and press start. To check that acarsdec send messages to xastir, select View→Incoming traffic

ACARS messages look like that in xastir:

F-XXYZ>ACARS:>Fid:AFXXXX Lbl:Q0

Lots of ACARS messages are messages without position report, so be patient before seeing aircraft plotted on the map.