[TangerineSDR] Compressed Spectrogram of VLF Spectrum

Jonathan emuman100 at gmail.com
Tue Sep 6 11:31:35 EDT 2022

Hi All,

Nathaniel was kind enough to let me use one of his older computers as a
signal processing server and NAS for the recorded VLF signal from our VLF
receiver and Raspberry Pi. I set it up to forward the timestamped 96k
stream to the server via a network socket, just like the Whistler Catcher
VLF module will do with the Tangerine SDR. I wrote a bash script to
automate everything and it currently works as a systemd service for
automatic startup. Here are a list of the programs running on the server:

  15871  -  1.6  0.0 vtcat -B -L /data/log/vtcat_in.log ++9000 @timed,f4
  15875  -  2.5  0.2 vtfilter -B -a th=5 @timed:1 @filtered
  15879  -  9.9  0.2 vtresample -B -r192000 @timed:1 @timed192k
  15884  -  0.7  0.0 vtcat -B @timed:1 -- -,f4
  15886  -  0.2  0.0 vtwrite -B -L /data/log/vtwrite_96k.log -G 3600
  15890  -  4.2  0.0 vtresample -B -r32000 @filtered:1
  15892  -  1.7  0.1 vtevent -B -d /data/events
  15895  - 15.3  0.1 vtsid -B -c /home/user/sid.conf @timed:1
  15898  -  3.1  0.2 vtsid -B -c /home/user/sid_resample192k.conf
  15901  -  2.2  0.1 vttoga -B -G 300 -i 25 -F 6000,16000 -d /data/togas

As you can see, the SID monitor, vtsid, uses quite a bit of the CPU
resources, but it's monitoring 17 frequencies for VLF transmitters as well
as the mains fundamental, mains 3rd harmonic, the sferic band, and
time-decimated spectrum data up to 16 kHz. I run a second instance of vtsid
to monitor NAA. Due to the nyquist boundary being a multiple of 24 NAA's
kHz, I need to resample the stream at a higher frequency to get proper
absolute phase measurements.

One of the really cool features of vtsid is that it can record
time-decimated spectrum data along with amplitude and phase measurements of
various signals. With this data, you can create a compressed spectrogram
showing long durations of a day or more. Here is a compressed spectrogram
of yesterday's UT day showing the daily sferic activity and the nighttime
effect of the earth-ionosphere waveguide cutoff:

[image: Screen Shot 2022-09-05 at 9.41.38 PM.png]

I cannot tell you how unbelievably cool this visualization is!

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