Decode the helicopter position signal from youtube videos!

helicopter-decode.sh

#!/bin/sh

# Decode the helicopter position signal from youtube videos!
# It makes a KML file. You can open it in Google Earth or many other GIS programs.
# Requires sox, perl, and minimodem, and optionally youtube-dl / ffmpeg

# https://www.windytan.com/2014/02/mystery-signal-from-helicopter.html

# !!!
# !!! Don't just run any random script you find online! Read what it does first!
# !!!

# Download the audio track (uncomment if you don't have the audio yet)
# youtube-dl --extract-audio --audio=format wav ....

# Or if you have just a video file:
# ffmpeg -i video-file.mp4 helicopter-audio.wav

# Extract the right channel (if the signal is on left, change the 2 to 1)
# Replace file names with yours
sox helicopter-audio.wav helicopter-audio-mono.wav remix 2 sinc 500-5000

echo '<?xml version="1.0" encoding="UTF-8"?>'
echo '<kml xmlns="http://www.opengis.net/kml/2.2">'
echo '<Document>'

# Make tr accept non-ascii bytes
export LC_CTYPE=C

# Decode FSK to coordinates
minimodem --rx 1200 -f helicopter-audio-mono.wav 2>/dev/null | \
  tr '\200-\377\r' '\000-\177\n' | \
  perl -ne 'if (/^#L (N|S)(..)(....) (W|E)(...)(....)/) {
    print("<Placemark><Point><coordinates>".
      (($4 eq "E") ? 1 : -1) * ($5 + $6 / 6000).",".
      (($1 eq "N") ? 1 : -1) * ($2 + $3 / 6000).",0".
      "</coordinates></Point></Placemark>\n"); }'

echo '</Document>'
echo '</kml>'