Friday, October 9, 2015

Tracking aircraft with ACARS

This morning I was using my SDRPlay and heard some AM data packets near 130 MHz. I knew this was the aircraft band and so they must be ACARS packets.

Transmissions in the aircraft band are in AM, as opposed to most other VHF services which use FM. Because they use VHF, you need line of sight to hear airplanes, but that doesn't mean they have to be passing overhead; on the contrary, planes usually fly so high that people in other states have line of sight.

You can download an ACARS decoder here.

I knew there were programs that could decode ACARS packets, but didn't expect it to work very well. Thankfully, I was wrong.

I upgraded to Windows 10 and thought I'd mention that I was skeptical when I saw how old this program looks. A lot of old decoding programs were for Windows 95/98 and expect different sound API's than what Windows Vista and up are willing to provide. Fortunately, acarsd runs just fine on Windows 10.

Here is what an ACARS packet looks like:
(click to zoom)

Download acarsd (the program above) and extract the ZIP. There will be a file called installer.exe. It's not a normal installer as it just extracts some files to a folder you choose. When it is done, open that folder and choose acarsd.exe.

You need a way to pipe the audio into the program. It will expect the sound to come from the default recording device. I prefer to use the Stereo Mix in my sound card. Be sure it is set as the default recording device.

If you sound card doesn't have Stereo Mix, make sure you have the latest official drivers from your manufacturer (not built-in Windows drivers) or use VBcable (which is free.)

After listening to some packets, the program will look like this:
(click to zoom)

You can type the aircraft registration number in a search engine to find current flight plan. Sometimes there will even be GPS coordinates and "CREW ACKNOWLEDGEMENT" messages.

The picture below shows that you really can receive packets even if the plane doesn't pass over your state. (I'm in South Carolina)

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