Sunday, December 1, 2013

NOAA Weather Satellites and RTL-SDR Dongles, Part II

Arrrggghhh!!!

I raised the antenna yesterday to 20', and now I'm getting a SOLID signal. Some directions I pick it up at 2*, and other directions from about 6*.

The spectrum display in SDR# looks solid, and I figured out you need the squelch DISABLED, other wise you get 'popping' noises that mess up the recorded audio, and the filter set to 48kHz.

Here's a screenshot of the last session I recorded:



The highlighted area of the spectrum is the filter width, and you can see in the display panel below it that all of the signal is within the filter's bandpass. I was cutting it off before, and losing some sync and telemetry information from the satellite.

With the antenna at 20', these birds are LOUD, well out of the noise by about 20dB.

With SDR# you manually have to adjust for Doppler Shift, but it's pretty easy to check it every minute or two, and 'nudge' the frequency a bit.

The recordings I've made so far sound great, BUT WXtoImg doesn't process them correctly. All I get is the proverbial "Black Cat In A Coal Bin" pictures, with a bunch of white noise thrown in for good measure.

So, I'm able to record the audio files, but I still have to figure out what I'm doing wrong to decode and display them.

I might have a sample rate error between how I'm recording them, and what WXtoImg expects, or some other problem. I'm still hesitant to setup my Virtual Audio Cable paths to feed WXtoImg directly, as if I mess it up, I'll wind up spending more time than I want to getting them squared away for use with my Flex 5000 SDR transceiver.

Oh, well.....I'm about done for this weekend.

See you all later.

3 comments:

  1. At least you've got something to keep you busy! ;-)

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  2. Well, I don't know anything about the modulation, but I think I recall it being WBFM. Any chance it's something silly, something like the mark/space being reversed?

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  3. It's "WBFM" in that it's wider (about 40kHz) than voice is, but not nearly so "WB" as FM Stereo (230kHz!!) is.

    WxtoImg was throwing some error messages about the bandwidth being too narrow, so that's when I spent the time and figured out how to increase the BW in HDSDR. SDR# has an "APT" setting if you know where to look, and I found it, and I'm getting some decoding, but not of acceptable quality.

    The preamp I'm using is exceptionally good (.8dB NF), BUT it has some filtering in it designed for the 2 Meter Amateur band. Using it makes the signal appear stronger on the spectrum displays of both HDSDR and SDR# compared to turning it off, but I'm going to try a pass with it turned off just to see if it matters.

    I also have yet to try the FUNCube dongle to record a pass with using the new antenna. Seeing as the FCD has built-in bandwidth limitations (only about 80kHz usable), I'm wondering if that might help. One thing I did manage to do was change the sample rate so that a smaller slice of spectrum is received by the RTL dongle, and that made it easier to correct the Doppler Shift.

    I might just make a simple turnstyle antenna (crossed dipoles fed 90* out of phase to get RHCP) for 137MHz, and gin up a simple preamp/filter for 137MHz, and use that for a dedicated WX_Sat antenna system. The only reason I started messing around with using the 2 Meter Eggbeater is that the VSWR curves looked excellent at that frequency, and M2 even said it was an acceptable antenna for that use.

    Still a Work-In-Progress I guess!

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Keep it civil, please....