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Author Topic: Sounder or radar or..  (Read 471 times)

Offline dollydot

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Sounder or radar or..
« on: October 14, 2018, 2011 UTC »
This signal is often heard. This Sunday it was at 6815 kHz. I've switched modes (AM/USB) at minute 1.16.

It's quite loud so headphone users might wanna turn down the volume.
Thanks in advance.

https://vimeo.com/295056416

Offline Strange Beacons

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Re: Sounder or radar or..
« Reply #1 on: October 14, 2018, 2228 UTC »
This signal is often heard. This Sunday it was at 6815 kHz. I've switched modes (AM/USB) at minute 1.16.

It's quite loud so headphone users might wanna turn down the volume.
Thanks in advance.

https://vimeo.com/295056416

Nice clear copy of this signal. And yes, once you switched over to AM/USB, it took on the classic sound of RADAR.

Offline Token

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Re: Sounder or radar or..
« Reply #2 on: October 15, 2018, 0313 UTC »
Interesting signal.  It does not fall into any of the common sounders or radars.  I don't know what it is, for sure, but lets look at its features and see if we can narrow down indicators of what it might be.  I am going to use the term radar to mean something looking for man made targets, missiles, ships, aircraft, etc, and I will use sounders to mean something looking at natural phenomena, the ionsophere primarily.  CODAR, WERA, and SuperDARN are exceptions, they are generally called radars, but the are looking at natural targets (ocean waves and currents in the case of CODAR / WERA, and the ionopshere in the case of SuperDARN).  But really, sounders are radars, just radars optimized for a specific natural target set, so the waveforms used can be very similar, or even identical.

Do you have a time, UTC, when you heard this signal?  Also, if you had the swept bandwidth it might help a bit.

Lets look at the rep rate first, 5 Hz (200 msec).  The rep rate is much slower than any of the normal radars, excluding CODAR or WERA.  CODAR normally has rep rates of 0.5 to 2 Hz (2 to 0.5 sec), however it can do 5 Hz.  5 Hz is not something CODAR normally does, as it can reduce the low radial velocity performance of the CODAR.

While technically possible CODAR generally does not use this frequency range.

So the freq and the rep rate are both technically possible for CODAR, but not normally seen.

Most other radars, radars designed to track man made targets vs naturally occurring targets (ionosphere, ocean surface, etc), would use a faster rep rate.  Keep in mind HF OTHRs generally fall into two main categories, Surface Wave radars, which are short to medium ranged, and Sky Wave radars, which are medium to long ranged.

For an uncoded pulse, as this one appears to be (this appears to be, from your recording, FMCW, a compressed pulse), the rep rate determines the maximum possible unambiguous range.  The slower the rep rate the further the possible maximum unambiguous range.  This signal, at 5 Hz, could have a maximum unambiguous range of about 30,000 km.  But radars designed to track man made targets generally do not use such a slow rep rate.

Why don't they?

Use of too fast a rep rate reduces the maximum unambiguous range of the radar, not a great thing.  So why not just use a very slow one (like this signal) and have a way over long range?  Because it reduces your probability of detection.  The more hits you get on a target, the higher the average returned power, the more likely you are to detect it.  So to increase the probability of detection you want to have as high a rep rate as possible.  In general you cannot rely on propagation conditions to give you that kind of usable range (30,000 km).  Most long range HF radars anticipate a maximum usable range, restricted by propagation, of less than 15,000 km, and generally 12,000 km or less, around 3,000 and 6,000 km, or less, are most common.

So you design the radar to have the most possible hits (highest rep rate) for the maximum range you might be able to use.  Rep rates of 10 to 100 Hz are most common for Sky Wave, long range, HF radars designed to track man made targets.  25 and 50 Hz are the most common that I hear, but that is just an accident of what radars I hear most often.

Sounders (mini-radars, backscatter ionosondes, etc), on the other hand, often use lower rep rates, 10 Hz is common.  They use lower rep rates because they want very long ranges and their target sets do not move rapidly.

Frequency revisit rate.

Radars often change frequency regularly, they leverage changing propagation conditions to illuminate the area they want to observe.  Sounders change frequency also, but generally because they are trying to sample a wide range of frequencies, vs trying to illuminate a specific area.

Radars tend to transmit often, even continuously, around their selected frequencies.  This is because their targets move relatively rapidly.  Sounders may transmit in a given frequency range seldom, maybe only once or twice an hour.  This is because their target sets move slowly.

And so the revisit rate, how often the device looks at a specific frequency area, can be an indicator of its tasking, radar or sounder.  If it hits the freq for many minutes at a time, or short bursts but very often, it is likely a radar.  If it hits a given frequency area only a few times an hour it is likely a sounder.

So this signal has an odd rep rate, not radar like, more sounder like.  But the recording you have is a couple of minutes long, and that is unusual for a sounder.

It will take more observation to narrow down what it is.  But my first guess is a CODAR or WERA being tested on an unusual freq and with an unusual rep rate.  Or a sounder in a testing mode.

T!
« Last Edit: October 15, 2018, 0315 UTC by Token »
T!
Mojave Desert, California USA

Offline dollydot

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Re: Sounder or radar or..
« Reply #3 on: October 15, 2018, 1951 UTC »
Thank you Token for the comprehensive answer!
To answer your questions. The signal was there around 19:00 UCT and lasted at least 5,20 minutes. Its width was about 8 hertz. The image looked like the "29B6 sounder slow" as it does in this link, sound is similar too.

https://www.sigidwiki.com/wiki/29B6_%27Kontayner%27_OTH_Radar

Just now, it was on again, same time similar frequency and a bit later this digital noise was heard. Don't know whether it's coincindence  or if one has to do with the other.

https://vimeo.com/295241062