Tag Archives: aviation

Update on acarsdec reception

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.

Freq (MHz) acarsd RR
129.125
130.025
130.425
130.450
130.825
131.125
131.425
131.450
131.475
131.525
131.550
131.725
131.850
136.575
136.650
136.675
136.700
136.725
136.750
136.775
136.800
136.850
136.900

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.

Decoding not one, not two, not three, but four ACARS channels on one USB SDR Stick!

ACARS (Aircraft communications and reporting system) monitoring has never been easier. Used to be one had to set up an AM aviation receiver or scanner and feed the audio output into a computer soundcard to receive and decode the 2400 baud ACARS packets. As well, since there are well over a dozen possible AM VHF aviation/airband channels where ACARS might be found, it was a real nuisance to scan. Even fast scanners listening to only a few radio channels could capture only a portion of the messages transmitted. Not any more!

Now acarsdec is available, and works incredibly well. An extremely neat feature is its ability to listen simultaneously (not sequentially, but really in parallel) to up to four channels in any 1 MHz piece of radio band, decode all packets heard and UDP them to an Ethernet address of your choice.

I used to have two Radio Shack PRO2052 scanners, each monitoring 4 ACARS frequencies, a sound card for each, as shown on the right in the following diagram from 2006.

N7UV_Main Station_ACARS-APRS

Back then, the antenna was a pair of dipoles, one horizontal, the other vertical, with a 90° phasing line in-between, providing an antenna with circular polarization for improved reception. I tested acarsdec against a scanner monitoring the same 4 channels, and acarsdec won hands-down.

Since acarsdec can monitor only 1 MHz continuous spectrum, I’ve set one up to listen to 136.700, 136.750, 136.800, and 136.850 MHz ACARS channels. The vast majority of traffic is on 136.850 MHz, but every once in a while I hear something on the other ones. I wish I could monitor a set of channels spaced more widely, but this is pretty darned good as it is!