Tag Archives: turnstile

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!