A few posts ago, I mentioned I’d set up acarsdec on a Raspberry Pi and a USB RTL-SDR dongle. Written by Thierry Leconte F4DWV, it’s a very nice lightweight ACARS decoder that puts a relatively small load on the RPi. A detailed writeup on installing and using acarsdec states that it can handle up to 4 receive channels and with a maximum frequency spread of 1 MHz.

I found that handling 5 channels doesn’t push the CPU load too high. Tonight I found that going beyond the 1 MHz separation barrier, at least a little bit, doesn’t cause any obvious issues either. I’ve even pushed it to 8 channels, that seems to be the absolute limit, while still getting under 80% CPU usage on my RPi 2 model B. However, the error rate becomes extremely high with many messages being lost.

There are a lot of frequencies assigned for ACARS use. While there are many sites that seem to show a number of frequencies, I’ve found that a combination of acarsd.org and radioreference.com make for what seems to be the most comprehensive list.

While this is a big list, it seems that what traffic there is is scattered over just a few channels. Here in Phoenix, I hear the vast majority of all ACARS messages on either 130.025 or 136.850 MHz.  There’s some occasional stuff on 131.550, which is supposed to be the primary worldwide frequency, but it pales in comparison to the aforementioned pair. Some of the above channels are claimed to be airline-company specific, but to date I haven’t observed any evidence of that, even with over 2000 flights a day passing through my reception range.

Those two channels are way too far apart for acarsdec to decode them both in one instance. However, with two dongles I can listen to several frequencies in the low part of the band and also in the high part. I haven’t tried that yet, and I suspect it won’t work due to the CPU load with the current RPi.

There are other things that I can do first, which includes resurrecting my homebrew VHF turnstile antenna and putting an LNA/airband filter combo up on the roof right behind the antenna. That alone should help improve my reception since I’ve got about 20 m of coaxial cable between the antenna and the RTL-SDR dongle. As well, the dongle doesn’t have the greatest front end or sensitivity, so an LNA/filter can help with that as well as negating the cable loss.