Can anyone identify the attached mystery signal?
I've personally observed it only in the 11 meter citizen's band, though that doesn't mean I'm saying it never happens elsewhere. I often see it between 27.0 - 27.3 MHz, and only when skip/propagation is in.
It has been happening for decades (earliest memory: late 1980s).
It can be received with tuners in both Europe and North America (and possibly other locations).
I first assumed it was an ionosonde, but now have doubts because:
1) Instead of a sinewave (like an ionosonde), it consists of a "warbly" up-down-up-down sweep tone.
2) That warbling tone itself descends in frequency (rather than ascending, as normal chipsounders do).
3) And it only descends 40-50 kHz. I.e., it begins instantly (no fade-in), descends 40-50 kHz, then stops instantly (no fade out).
Here are several audio samples of it, edited together. Note that I used Twente's WebSDR in DSB reception mode. That allows it to be heard as it sounds when crossing through AM carriers.
http://s000.tinyupload.com/index.php?file_id=93766677053675299202Finally, I should note that there appear to be two variations of this mystery signal:
Variation 1: Same as pictured above. While observing it yesterday, it always lasted 4 seconds, and then reoccured every 13 seconds. With this variation, the rate of frequency descent slows just before the signal vanishes (i.e., you see a diagonal line slowly curve into a vertical line). Sometimes, the signal even reverses direction (goes back up in frequency) just before vanishing.
Variation 2: This variation always lasts about 15 seconds (i.e., it descends in frequency much more slowly). Unlike the first variation, its rate of descent does not slow before the signal vanishes, nor does it reverse direction and go back up. Therefore it always appears as a diagonal line on waterfalls. This variation also has a longer duration between occurences (30 to 60 seconds, guessing). Nonetheless, it sounds audible exactly the same as the first variation when it passes through your receiver's IF (same warbling sweep effect).
Any ideas?
