Scanner Setup

There are websites which relay ATC communications (LiveATC.net), but there are none that I am aware of that relay CTAF communications at uncontrolled airports. So I did this as a fun project. Within a short time, there was a fatal mid-air collision at Blue Ash (Cincinnati) airport, and my audio archive was the only record of the radio communication betwee the two airplanes. It was featured on several local tv stations and newspapers, and was also given to the NTSB.

Since many people wanted to know the details of this setup, here is a brief description.

At first I was using a Uniden Bearcat base station scanner, which is available for under $100. Then I switched to the ICOM IC-A6, which is an aviation transceiver. There was a huge difference in sensitivity between the two models. The ICOM was able to pick up distant aircraft that would not even register as a blip on the Bearcat model. Click here for a comparison.

The receiver scans through several pre-programmed frequencies, pausing whenever an active signal above the squelch level is received. The audio output is fed directly to the sound card line-input on the computer, and all tasks such as live streaming, recording, encoding and website uploads are performed continuously in the background with no manual intervention.

The most important component is the antenna. Comm radios are vertically polarized, so the regular TV antenna with horizontal elements will not work. I am using the Scantenna available from Grove.

Since VHF transmissions are line-of-sight, the antenna should ideally be higher than any nearby structure. In my case the antenna is actually below many trees in the area. But the receptiion is still pretty decent and I can pick up transmissions from airports 50 miles away.




The antenna is mounted on a Radio Shack mast.

Computer Setup

The operating system is Linux, which is also my generic web server and file server. All new Linux kernels now come with ALSA (Advanced Linux Sound Architecture), which is a powerful module for handling audio.

Live Streaming

Streaming is done with Ices2/Icecast. Ices2 processes the soundcard input and sends it to the Icecast server for streaming. Both processes are started during system boot from /etc/rc.local:

/usr/local/bin/icecast -c /usr/local/etc/icecast.xml -b
sleep 5
su nonroot_user -c "/usr/local/bin/ices /usr/local/share/ices/ices-alsa.xml"

Ices2 can only stream Ogg Vorbis format, which I am told is better (and free) compared to mp3. Winamp player already contains the ogg codec, but Windows Media Player user would need to install it separately.

Recording

Concurrent with live streaming, the audio can also be saved to disk for archiving. It is important to use dsnoop as the audio device instead of hwplug to prevent one program from locking the audio device. Recording is quite simple using arecord, which saves it as a wav file. After every hour of recording, the output file is fed to sox which trims out the silent periods. Finally, lame is used to encode the audio into mp3. A perl script is used to catalog all files into a web format.

Since aviation communication is very sporadic, there is no point in continuously recording everything. By cutting out the silent space between the individual transmissions the file size and listening time can be significantly reduced. As you can see from the examples, one clock hour may only contain about 10 minutes of audio. Archiving one full month of audio for half a dozen airports only takes up about 200MB of disk space, so the resource demands are not too huge. The only drawback with cutting out the dead space is that the timestamp of each transmission is lost in the process.

Here is the batch file called start_record that does the audio caputure.

#!/bin/bash
RND=$(date +%s)
DD=$(date +%Y%m%d%H)
AUD_PATH=/path/scan_audio
cd $AUD_PATH
perl metar.pl $(date +%Y%m%d%H).txt
arecord -q -D plug:dsnoop -d 3600 -r 11025 -f S16_LE raw/$RND.wav
sox -q raw/$RND.wav raw/$RND-1.wav silence 1 0.1 3% -10 0.1 3%
/usr/local/bin/lame --quiet raw/$RND-1.wav $DD.mp3
perl create.pl
rm -f raw/$RND.wav
rm -f raw/$RND-1.wav

Here is the crontab entry:

0 * * * * full_path_name/start_record

It is easy to configure the crontab to start and stop recording under virtually any imaginable condition, such as only during day time, or only during weekends.

The perl script metar.pl downloads hourly METAR weather data from the NOAA ftp site (tgftp.nws.noaa.gov). One could also use the METAR data in some intelligent fashion to control the recording program. The possibilities are endless.

If you need any more information, please send me an email, and I will be happy to send you all the relevant files.