[Scan-DC] Something different -- ACARS (131.55 MHz)

[Andrew Clegg]

For a change of pace, this weekend I experimented with decoding "Aircraft 
Communications Addressing and Reporting System" (ACARS) messages from 
airplanes. I know a lot of people on this list know all about ACARS 
decoding, but this was one of my first forays, and it was quite interesting. 
It's also very simple -- all you need is an analog scanner that can tune the 
regular air band, an audio cable, and a computer. The software I used, 
called acarsd, is free (http://www.acarsd.org/).

acarsd decodes the digital bursts, and displays the decoded data on your 
screen. The data can include simple check-ins, maintenance messages, 
operations requests, weather, and a bunch of other things. The data include 
the tail number of the aircraft, and the airline and flight number if it's a 
commercial flight. acarsd displays this info, and keeps a persistent 
database of all the aircraft you've heard (by tail number). It will even 
display a photo of the specific plane you've caught, if it has a picture on 
file. (I put a screen grab at http://www.w4je.com/acarsd.png)

The fun I had was tracking how far away some of the planes were. Once acarsd 
decodes the airline and flight number, I could look up the flight on 
flightaware and see exactly where it was. I was surprised that I was hearing 
flights as far away as southwestern NC, southern NY, and over the Eastern 
Shore. The planes were at cruising altitude, so their radio line-of-sight 
distance is hundreds of km, but still, it's cool to be able to decode 
signals from that far away. I was out at my house in Shenandoah County in a 
fairly benign RF environment, using an outdoor discone with a pre-amp at the 
feed point. With that setup, I was catching an ACARS burst every 15 seconds 
or so during heavy flight times like Sunday evening. I was tuned to the 
national primary ACARS frequency, which is 131.55 MHz.

For those of you who haven't tried ACARS, it's worth a listen, especially 
since it might give a renewed purpose to a dusty old analog scanner in your 
closet. Having an outdoor antenna will help, but here in the DC area, 
there's probably enough close-by flights to be able to get at least some 
ACARS hits with an indoor antenna near a window. You can quickly judge how 
successful your setup might be just by tuning to 131.55 and seeing how many 
bursts you hear.

I'm still experimenting, and have it running now at my apartment in 
Arlington, getting bursts about every ten seconds or so with my rooftop 
discone (no pre-amp, since there's too much RF in the area). I need to learn 
more about deciphering some of the ACARS information.

Highly simplified tips and tricks from ACARS experts would be appreciated --  
for example, are there "local" ACARS frequencies for DCA and/or IAD?

Cheers,
Andy