Since discovering that my magnetic loop antenna works miracles on the VLF region, I find myself now browsing regularly through the very low frequencies, from the 10kHz to 20kHz region on my case. Its a strange world of all sorts of noise, but also with surprisingly large number of signals.
The most exciting discovery from last night was another buzzer-like signal found at around 15kHz. Apparently, this is a keying/sync signal for the chirp sounder. You may have noticed on the waterfall screen of your software radio a passing narrow stripe, what looks like someone is quickly scanning the band with the transmitter. Which is exactly what is happening. The chirp sounding is used for determining the propagation quality and delay of the signals what travel by reflecting off the different layers of the atmosphere. It is pretty common technology and there are many such transmitters all around the world, but I have always been wondering, on what frequency and how the scan cycle starts. Not sure if it has not been there before, or have I just missed it earlier, but here it is - the frequency where the (possibly closest to me) chirp sounder signal starts from:
The buzzer-like signal is clearly visible on the 15.612kHz, followed by the chirp pulse (the slightly diagonal line scanning away from the signal). The horizontal lines are some sort of digital downconverter artifacts and I have yet to figure out where they are coming from. Please also pay attention to the nice submarine communication session ending on 18.2kHz.
It seems that for a reason or another, atmospheric sounders seem to love buzzer-like signals, so may be there are some grounds for UVB-76 and atmospheric research theory. On the other hand, the similarity may also have led to speculations - go figure ..
And as an eye-candy and a proof that the VLF band is worth looking at, here is another picture from the same evening: