Wednesday, October 22, 2014

Magic of the SSB radio

I realize that many people may not find this interesting (as I was informed by one of my boatmates), but I was so pleased by this that I wanted to share it anyway!  A couple nights ago I tried for the first time to receive a weather fax chart over the radio, and it worked!  
To me it was almost like magic!  I mean, I am a very computer-savvy person and understand how data gets transmitted and all that, but I think I was most amazed that this very low-tech idea of using audible tones over radio waves, travelling hundreds of miles bounced off the ionosphere, could actually transmit a very nice image, albeit very slowly.

Here is the weather fax I received.  It happens to be the 96-hour forecast for the 500mB data (which indicates atmospheric pressure high up, at about 18000 feet or so)  for the north Pacific .  I wasn't particularly interested in this exact chart, but it was the next chart available in the transmission schedule at the time I was messing around, so that was a good enough test!  I had to fix up this image a little, because there was a data interruption (because I was messing around with settings) about half-way down, so it got out of register.  And there's some skewing, which is another setting I was messing with and maybe screwed up a bit. This image took maybe about 10 minutes to receive... something like that.
This chart was sent out from the station at Point Reyes, California, which is only about 340 miles.  But I was also easily hearing weather fax signals from New Orleans and Honolulu, and even New Zealand sometimes, so I am feeling confident that our radio setup is working well.

The setup in our boat is pretty simple: we have a very nice ICOM M802 Marine SSB radio (SSB is "single side-band") -- it seems to be the most common marine SSB nowadays.  I also got a little gizmo that has one end that plugs into the speaker output of the radio, and the other end that plugs into a USB port on a laptop computer.  The laptop runs a program called "WeatherFax 2000".  At the scheduled time, one just tunes into the desired NOAA frequency and listens for a start tone.  When you hear that, you just click the "Receive" button in the software, and the fax image starts appearing on screen, one raster line at a time.

The different weather stations all have different schedules of when they transmit the various "products" (as they are called).  The products include stuff like:  analysis of current surface conditions, 500mb analysis, wind/wave analysis, and forecasts of all those things for 24, 48, and 96 hours; some of those are updated daily, some twice a day; there are also satellite images available.  The stations each send out maps for their own area, plus some for neighboring areas (like, Point Reyes will broadcast eastern Pacific charts; but Honolulu will also broadcast eastern Pacific, as well as western Pacific charts).  Here's an example of the Point Reyes broadcast schedule.


  1. I only understand a minuscule portion - but think it's interesting anyway, cause it gives me a sense of all the information you guys are processing, and the knowledge base you need to have! "Fax" seems like a very outmoded term....

  2. Neat method for broadcasting useful data as widely as possible (especially with no need for minute-by-minute updates) - but a speaker jack to USB? The lower tech the solution, the better the bodge cabling!


    1. Apparently the common approach used to be to just plug the audio cable into the sound card of a PC, but it seems that does not work as well as the purpose-built DSP incorporated into the little black box that we are using.

    2. Well, quite - don't let the connector get in the way of using appropriate components!

  3. No way... low tech adapted...

  4. Tod, I think it's really cool! We ( horseback riders) would love to have data that accurate before we go out on things like The Moonlight Ride. Every bush is the Headless Horseman anyway, and at night it's worse! If we had wind in knots, we would know how much body arm our to wear. Who is cruiser? You guys or us gals? Alice


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