HFU HF Underground
Loggings => HF Mystery Signals => Topic started by: alleycat on December 27, 2020, 0346 UTC
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...but I can't find anything that looks remotely close in the READ ME for this group nor at www.sigidwiki.com. Hear these all the time all over the HF band - waterfall at https://www.lutins.org/temp/waterfall.jpg (https://www.lutins.org/temp/waterfall.jpg) & audio sample at https://www.lutins.org/temp/audio.mp3 (https://www.lutins.org/temp/audio.mp3)
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This is either a radar (OTHR, Over The Horizon Radar) or it is an ionospheric sounder. In this case probably a radar.
Why do I say it can be either?
Ionospheric sounders are, at their cores, radars. The target set is the ionosphere, instead of man made things like ships, aircraft, or missiles. Since they are radars they can use transmission waveforms that are very similar to OTHRs. In fact, they can use techniques that are identical to what an OTHR might use, although typically there are slight differences.
In this case your waveform appears to be FMCW, and that is used by both OTHRs and sounders. However, the burst looks a bit short and the rep rate a bit high to be an ionosonde, so it is most likely an OTHR.
One way to narrow down the possibilities is to look at how often the particular signal hits. Think about the target sets for each, an ionosonde and an OTHR. The OTHR looks for man made targets that tend to move fairly rapidly, while the sounder looks at the ionosphere which tends to move (or change) more slowly. So sounders tend to transmit less often, say a few times an hour, while OTHRs tend to transmit more often, either continuously or several times a minute (sometimes across multiple freqs) is not uncommon.
There are also other ways to ID the signals, such as frequency excursion limits, burst pre-tones, and rep rates / specific bandwidth combinations.
T!
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Excellent reply - thanks! Curious that, given how frequently I see these on the waterfall, I don't see good correlates at sig ID Wiki (Relocatable Over-the-Horizon Radar is close, but far from identical). I would have thought a radar system would be more continuous - surprised to learn that it is used in short bursts and that the frequency jumps around. Then again, I cut my monitoring teeth in the 1970's when the Russian Woodpecker (which I believe was an OTHR system) was continuously gobbling up megahertz of bandwidth :(
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The Woodpecker also moved around in frequency, just not as fast as some of todays OTHRS might. And the Woodpecker used multiple frequencies in sets, as many of todays radars do. A big difference is that the Woodpecker was a pulsed radar, that is one of the reasons it seemed so wide.
Some radars are more continuous, sometimes setting on one frequency for hours at a time. For example, the Chinese OTH-B radars, the French Nostrodamus, the Russian 29B6, and the British PLUTO, all might set on a freq for hours at a time. By the same token, all of those radars can, when needed, change frequency, and sometimes might do it in minutes.
OTHRs changing frequency is part of how they control what portion of the Earths surface they are illuminating. Think of it this way, OTHR antennas typically can steer the beam in Azimuth, but not in Elevation. So they can point the beam in the direction they want, but to control where on the Earth it is going to "come down" they must control the takeoff angle, and so the angle of reflection off the ionosphere. You could build an antenna that could control elevation, that is not problem technically, but that by itself would not do you much good, the ionosphere must also be reflective at that angle. Instead of building an array that steers in elevation if you just pick the frequency that currently reflects off the right altitude of ionosphere to give you the angle you want you can still achieve the same thing, in a more simple antenna design.
So that accounts for OTHRs gross frequency steps (think of this like bands), but not the small frequency steps around the same band segments (specific frequencies within the bands).
The small bursts and frequency steps serve a few different purposes. For example, jumping around like that might make you less susceptible to interference. Setting on one frequency if there is any interference in your bandpass it hits you all the time, jumping around it only hits you part of the time and you can integrate it out. Another issue is that a specific pulse repetition interval and a specific frequency will have a specific set of "blind speeds" and ranges. Jumping around in frequency and PRI will eliminate those blind speeds and ranges.
ANd of course there are other factors, but that is a 101 level partial introduction to why they hop and move.
T!