Recently, I’ve been having some fun decoding High Frequency Data Links (HFDL), a protocol that is used to track aircraft as they fly around the world. While doing some research on how this is done, I stumbled across a great article by Nils Schiffhauer, DK8OK, on how to decode multiple HFDL signals at one time using the 4 sub receivers of the Elad FDM-S2. By following his tutorial, I was able to set up my Elad to decode three HFDL signals at one time, and plot them all on Google Earth. While this is pretty cool, and I encourage everyone reading this to give it a try, it got me to thinking about how I could apply Neil’s efforts to something a little closer to my own heart: weather faxes.
As you probably know, I have a, ahem, thing for weather faxes, especially the infrared satellite photos. If you haven’t tried to decode them before, I strongly encourage you to read this first. For those of you who have been doing this a while, keep in mind that this quick tutorial is written from the point of view of an Elad FDM-S2 user. I’m sure you can take the same general principles and apply to any SDR that allows multiple sub-receivers though. You may have to adapt the virtual audio cable (VAC) settings to whatever program you’re using as well, but again the underlying logic is the same.
Step 1: Make sure you have enough virtual audio cables. Usually, all of my projects come to a screeching halt when I realize I am a cable short. Fortunately for me, this one requires cables that you can create on the fly. You can see in this image that I have created five virtual audio cables: for are for our project, while the extra one is for the Perseus. That way I can use both SDRs without them getting each other’s way.
Step 2: Go to the Audio setup of the Elad. This is where we will put our VACs to good use. As you can see from the picture, I have assigned each of the Elad’s virtual receivers it’s own VAC, starting with cable two and going through cable five. Remember, I have one cable (cable 1) set aside for the Perseus, but you can change this to fit your needs.
Step 3: Set up the receivers and decoders. Starting with the first sub receiver, set it up to receive your weather fax station of choice. In my case, that’s Point Reyes on 12786. Next, I open up Fldigi and go to the sound card under the configure tab. There, change the capture pull down box to select Line 2 and hit close. Go ahead and make whatever fine tuning adjustments you need to make to either Fldigi or the Elad until you start to see a decode come through. Once you’re seeing something come through on the display, go ahead and hit save on the decode, and send it to whatever directory you’d like. In my case, I set up individual directories for Point Reyes, Kodiak, Boston, and New Orleans. Not only will it keep things organized, you’ll also avoid the unlikely risk of accidentally overwriting an existing file.
Step 4: Repeat step 3 for each of your virtual receivers. Go ahead and set up the second Elad receiver for your target station, and then open up a separate instance of Fldigi. Once it’s open, go to the sound card configuration and set it up for the corresponding audio cable. In my setup, the second receiver is paired up with cable 3, so I’d pull down the capture box and set it for cable 3. Once that’s done, make your fine tuning adjustments as needed, and save whatever’s come through on the decoder to the directory of your choice, just like you did with the first receiver. Then you can finish up by configuring receivers three and four the same way. Keep in mind that every instance of Fldigi will need to be paired up with a different receiver and VAC for this to work properly. I chose these four stations because they are close enough together to fit inside the bandwidth of the Elad. I would’ve loved to have chosen Honolulu over Kodiak, but I can’t grab their signal and Point Reyes/New Orleans at the same time. Depending upon your flavor of SDR, your mileage may vary.
Putting It All Together.
A couple of notes about getting this all to work. While I am sure other decoders besides Fldigi are capable of doing this, I haven’t ran across any just yet. Sorcerer, my go to for WEFAX decodes these days, and will allow you to open multiple sessions, but it will not allow you to set a different VAC feed for each of these sessions. If anyone knows of a work around for this, or a different decoder that will let me do this, I’d like to hear from you.
EDIT: Since I wrote this, I have come to realize that, when I am using Fldigi as my decoder, I seem to spend a lot less time in Photoshop piecing these things back together again. They always just seem to line up correctly in Fldigi, which is more than I can say for most of the other decoders out there. The only problem I have with Fldigi is that it tends to turn a single weather fax decode into several files that then have to be combined later. Not a huge deal, but sometimes there will be a gap between the two that cannot be filled. If the Fldigi team can get that worked out, your choice of decoder becomes a no brainer.
Also, since these signals are probably coming to you from different directions, you’re also going to want an antenna that is fairly omni-directional. I went ahead and connected the Elad directly to my long wire, which is far less directional than the SAL-20, for this very reason. It also lets me switch directional patterns without messing up any decodes that may be in progress, giving me the freedom to chase other things with the Perseus without messing up any decodes in progress on the Elad. How slick is that?
Here’s a couple of decodes taken simultaneously from New Orleans and Point Reyes:
As you can see, it’s working pretty well!
Again, many thanks to DK8OK for giving me the idea to try this out. Thanks to him I’ll never have to choose between the Boston sat photo at 1400 and the Point Reyes sat photo at 1403 ever again. Now if you’ll excuse me, I need to go buy a larger hard drive.