ACARS, or Aircraft Communications Addressing & Reporting System, is a digital mode for civilian and military aircraft to report various information. The ACARS system is used to free up some of the duties of the flight crew as many functions are automated.
The VHF ACARS frequencies are as follows:
129.125 Tertiary North American freq
130.025 Secondary North American freq
131.550 Primary North American freq
131.725 Primary European freq
131.550 appears to be the most commonly used frequency in North America, however, in my location in Kansas I seem to get more traffic on 130.025. The busiest frequency may vary at your location.
There are numerous programs out there for decoding ACARS. The program I use is WACARS, a freeware program available on the internet by downloading here. The program uses the sound card to decode the ACARS signal into readable text. ACARS data is all encoded in order to keep the transmissions short. The program automatically decodes the hard-to-read ACARS text into more-or-less plain language. Another neat function of WACARS is a mapping feature that will display the position of aircraft when they send position reports. The map that comes with the program is for the London, England area. The map is a standard Windows bitmap file, so you can either download a map for your own location and save it as a bitmap or draw one. There are many websites that have maps for use with Wacars, however, most of them are maps of Europe and have none for North America. A good website to generate your own map can be found at http://woodshole.er.usgs.gov/mapit/. Just enter in the coordinates you wish for the map (the same coordinates you can enter into the Wacars program) and this site will generate a map for you.
ACARS is monitored by taking the audio output from the scanner and feeding it into the line input of a computer soundcard. This can be done with a patch cord with a 3.5 mm headphone jack at both end. It's best to use a 1:1 audio transfomer between the scanner & soundcard. This isolates the soundcard from the scanner so that static charges won't fry the soundcard.
Any scanner can be used provided it receives the above frequencies, as most scanners do. A dedicated VHF aircraft receiver can be used as well. Better results may be had by tapping the discrimator circuit of the scanner, but I've found that this isn't necessary with WACARS.
To monitor ACARS with a scanner, since the data bursts are so quick, it's a good idea to leave the scanner on one channel with the squelch wide open. By doing this, you will hear everything on that channel. However, some monitors have had success scanning all the ACARS frequencies; however, you need a scanner with a quickly-opening squelch, or else most of the data will not be decoded. You will miss some traffic but will be able to monitor each channel. You will need to experiment with your scanner to see if the squelch opens quickly enough to work in this way. If you do decide to leave the scanner on one frequency though, leave the squelch open. I monitor using this method and get a lot of traffic throughout the day.
HF ACARS, or HFDL (High Frequency Data Link) can be monitored by using a shortwave receiver and computer program the same way as monitoring VHF ACARS. As with VHF ACARS, there are several programs available that will decode it. I use PC-HFDL, which can be downloaded at http://www.chbrain.dircon.co.uk/pchfdl.html.
PC-HFDL is a fairly simple and straightforward program to run. Just install it, hook your shortwave receiver to your sound card, and you're ready to go. Be sure to read the manual file which will help you get started.
HF ACARS is transmitted on several HF frequencies including the following:
5547 6532 6535 6712 8834
8843 8912 8942 8977 10084
10087 21934 17934 15025 11184
11312 11315 11348 11384 21931
13276 13315 13321 17919 17928
17967 21949 21982
All frequencies will be in upper sideband (USB) mode.
Transmissions are in the form of data bursts as on the VHF frequencies, but the bursts will sound different as they are sent at a slower data rate than the VHF ACARS signal. As with VHF ACARS monitoring, the best results are achieved by leaving the HF receiver on one frequency and monitoring for a while. The same type of isolation using a 1:1 audio transformer between the HF receiver and soundcard input is recommended.