Tom Duff recommended this blog for my perusal, and I filed it away. Today I discovered that it was linking back to one of my more popular posts: the one in which Tom Duff talks a little bit about Duff’s device. But there’s a lot of good reasons to check it out. I found this post about Unix viruses, which contained a link to one of Tom’s papers that I never read, although Tom and I have discussed at various times. Very neat. Lots of other good things in there too! Worth reading.

Saturday NOAA 17 pass..

Here’s another NOAA 17 pass, recorded on a bright and sunny Saturday morning. I tried a bit of a different setup this morning: I was using a small preamp between my Yagi and my trusty rusty Pro-60 scanner. As you can see, an otherwise gorgeous 86 degree pass was spoiled by some interference. Can anyone tell me what they think it is? It’s obviously synced on a 1 Hz period.

Feb 09, 2008, NOAA 17 on an 86 degree pass

The preamp might have actually helped in getting the last 100 or so scanlines.

Addendum: While the interference that we see is serious, the broad horizontal streaks that cross the entire image are actually due to a bug in my sync detector. I’ll have to work that out.

Addendum2: Mike, WA7QPC had an explanation for the interference patterns that I am seeing. He sent me the following picture:

Interference pattern, recorded by Mike, WA7QPC

Looks pretty familiar, right?

From Mike’s email:

I’m attaching an image I received with a Timestep Proscan weather satellite receiver, which is supposed to have the proper IF bandwidth, but even that is a little too wide, apparently. This image was received when there was no NOAA satellite in range, and the image was generated using WXSAT. I got around it by attaching an outboard IF amp and demod to the receiver. It has a narrower or sharper filter, supposedly a little too sharp for NOAA, but it works fine for me, and gets rid of the interference. Sorry the file is so large. For some strange reason, it doesn’t want to compress very well….can’t imagine why!

Apparently several Orbcomm satellites timeshare on one frequency, and every few seconds one will switch on or off, accounting for the pattern you see….and hear.

Thanks a lot Mike! Looking at the the table of frequencies that Mike sent me, the most likely interference source is the Orbcomm satellites, broadcasting on 137.663Mhz. If my scanner had the right bandwidth (say, around 50khz instead of the 200khz or so that the scanner has) then this noise source would be outside the band.

Addendum3: Thanks to all the members of the GEO subscribers group on Yahoo! for confirming Mike’s diagnosis. And yes, I know, I should be using a QFH antenna. 🙂