Today's NVIS

If you know what NVIS (Near Vertical Incidence Sky-wave) stands for, then you are probably a ham radio operator. It is a mode of propagation in which a radio wave is emitted skyward, and, if it is below a certain critical frequency, it will bounce off the ionosphere, and back to earth. This is effectively used to communicate over 0-400 miles.

There is a great site which provides an hourly update of the current NVIS propagation. It is very useful to be able to tell if you can hit certain parts of the United States at a given time. It is updated hourly, at about 20 to the top of the hour.

I thought it would be educational to create a time-lapse GIF of the NVIS changing over the hours. I wrote a script to do this, and it is updated, since midnight Pacific time, to show a nice cartoon of the NVIS ebbing and flowing with the day light hours.

Time-lapse NVIS propagation, since midnight, Pacific

Time-lapse NVIS propagation, since midnight, Pacific time.

Here is the script I wrote to accomplish it:


# Cron job entry: Runs hourly (edit with crontab -e)  
# 00 * * * * bash -l /home/user1/bin/

# Get time stamp.  
time_stamp=$(date '+%Y-%m-%d-%H%M%S')

# go into directory.  
cd /home/user1/nvis-snapshots

# Download image, and put a time stamp on it.  
wget -q  
mv 'San Francisco.gif' "SanFrancisco-${time_stamp}.gif"

# Next, Create a animated gif plot from today's files, and push it to
# web server.  
today=$(date '+%d')  
convert -delay 67 -loop 0 *-????-*-$today-*.gif today-nvis.gif  
convert today-nvis.gif \( +clone -set delay 300 \) +swap +delete

# Set up keys, so that rsync over ssh doesn't need a login.  
rsync -az -e "ssh -p 10022" $local_file1

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