Icom R100 receiver

    Decoding ACARS: Shortwave listening (SWL-ing) is not what it used to be.  Before satellites and the Internet, the high frequency broadcast bands 49 meters, 31 meters, 25 meters, and beyond were alive with news and commentary from every conceivable point of view, as well as music, drama and other forms of entertainment. At the time of this writing (2017), the most recent major broadcaster to abandon shortwave is Radio Australia. The majority of the world is unaware of this event, but to dedicated shortwave listeners the demise of Radio Australia was a cruel blow.

    While shortwave broadcasting has steadily declined, the challenge of shortwave listening remains strong.
The number and variety of listening targets continues to fill books such as the World Radio TV Handbook.  Furthermore, many services and modalities exist today that were not imagined during the heyday of shortwave broadcasting. Instead of voice and music one can receive weather satellite images, as I described here, or images from the International Space Station, or position reports from ships at sea, or countless other services. Ham radio has also changed. In addition to Morse code and voice, multiple digital signal modes can be found across the amateur radio bands.

ACARSDeco2 (left) and acarsd (right)

    Elsewhere I described receiving data from aircraft on 1.090 GHz (ADS-B). The FAA has mandated that all US aircraft be equipped with ADS-B by the year 2020 (AOPA). However, aircraft also transmit data via a long-established system called the Aircraft Communications Addressing and Reporting System (ACARS). This system operates on various frequencies in the
aircraft voice band—the mode is AM. ACARS frequencies range from a few megahertz below the marine VHF band, so a marine band antenna should work well enough to explore the modality. As a first step I connected a small masthead-type marine VHF antenna to the NooElec dongle, and tuned SDR# to the primary worldwide ACARS frequency 131.550 MHz. Right away I heard intermittent beeps or buzzing sounds. The next problem was to locate and install ACARS decoding software.

     Previously for ADS-B decoding I had installed
ModeSDeco2. Turns out there is a companion piece called ACARSDeco2. The ACARS decoder works similarly to the ADS-B decoder, in that the software does not rely on an external receiver or SDR, and like ModeSDeco2 it formats decoded data for display in a browser. ACARSDeco2 is able to decode signals from up to three nearby frequencies concurrently. At first I was surprised to see only textual data in the program’s output. I had imagined there might be graphs or a map in the browser view. This misconception led to an abortive attempt to integrate the decoder with a virtual radar application, a sidetrack that was not particularly instructive.  

    A subsequent Internet search revealed another decoding application named acarsd that seemed to be more widely used than ACARSDeco2. I was curious as to what differences might exist between the programs. The acarsd decoder does not have a built in SDR, rather uses a sound channel for input. Therefore I configured SDR# as its signal source (same antenna and dongle as before). However, nothing decoded. Various discussion threads suggested that if there are no decodes, it is likely that the wrong sound source has been selected. But this could not have been so, as I was able to change the indicated volume level (on the acarsd status bar), by increasing or decreasing the volume in SDR#.  Sources also gave different recommendations for the volume level; some said that background static should be less than 10 while others said more than 20 (as indicated in acarsd). I tried very soft to very loud and everything in between.


    Over the course of two half days I varied every conceivable setting, although I had read that the program should simply work without changing anything, unless necessary to specify a non-default sound source. There was not a single decode! All I ever saw was the pretty startup screen, unless of course I cleared it or selected a different screen. Tables of messages received and processed, etc. were filled with zeros.

    My wife N4EFS who is good with coercing recalcitrant software to do what it is supposed to do offered to have a go at acarsd. However,
I did not want to be embarrassed by her solving the problem in a matter of minutes after I had spent hours on it, so I explained that the configuration utility was a separate executable, not an option within the acarsd program. Back and forth configuration and testing would be a time consuming effort. Then, somehow the threat of having help triggered a thought. With a different signal source, acarsd testing could be freed from dependence on the SDR. Another obvious advantage would be that the two decoding programs could be run at the same time on the same computer, one with the dongle and the other with a separate receiver.

acarsd example

    To test this idea I connected a 2-meter JPole antenna to our
Icom R100 receiver, and ran a patch cable from the radio’s external speaker jack to the computer’s primary sound card line-in jack. Success! On tuning the R100 to an active ACARS frequency, the program immediately began decoding signals. Its volume indicator was in a range previously tried. In fact, volume doesn’t seem to matter much. In the illustration above, the volume indicator reports 15 (card 1, left channel). To the ear, signals from the R100 sound about the same as those from the RTL-dongle with SDR#, maybe a little more bass. I have not tried a different SDR.

    As previously mentioned ACARSDeco2 can monitor up to 3 frequencies. However, it decodes more messages than acarsd does
at the same frequency. The comparison is not fair, of course, different antennas, likely different receiver sensitivities, different sound bandwidth and level, etc. On the other hand the acarsd program retrieves supplementary information from the Internet to produce an enhanced display, such as for example, to include origin and destination airports for a flight. This results in a more satisfying user experience.

ACARS demo: acarsdeco2+acarsd.mp4 - In the part of the demo where both applications are shown side-by-side, sound is from the external receiver (131.725 MHz).

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