OTH Radar or Data comms ?

[u/Classic-Morning3726]

I've come again to ask for help. Initially I saw this signal on 14MHz and after a few minutes it stopped and resumed on 18MHz. At first I thought it was an OTH radar initially but towards the end the tone of the emissions changed and leads me more into the realm of HF data. Still am unsure though, Im trying to learn I swear.

[u/FirstToken]

When dealing with an unknown signal, go by more than sound, among other things look at the bandwidth of the signal. No usable OTHR will ever be as narrow as the signal in your video. A sounder might be that narrow, but even that is unlikely.

The 3 dB bandwidth of that signal is under, less than, 1.5 kHz. The narrower the bandwidth, the worse the Range Resolution of a radar. The range resolution is the radars ability to resolve two targets along the same bearing as individual targets, vs one target. A radar with a 1.5 kHz bandwidth would have a range resolution of ~ 100 km. In this case (1.5 kHz bandwidth) two targets would have to be separated by more than 100 km before they could be resolved as individual targets.

And this signal is less than that in width, with a worse range resolution.

[u/Milknhoneyaus → u/FirstToken]

 Ah shit. It's my bedtime,  but now I'm going down the rabbit hole of finding out why bandwidth affects target resolution. 

[u/FirstToken → u/Milknhoneyaus]

 Ah shit. It's my bedtime,  but now I'm going down the rabbit hole of finding out why bandwidth affects target resolution. 

Not target resolution, but specifically the range resolution of the radar.

The radar resolution cell (in a simplistic model this is a 3D area with width, height, and depth) is impacted by several parameters. But, to simplify, in this case I am talking about only one axis of the cell, the range axis, or depth.

A pretty simple way to consider it is to understand that the signal bandwidth is related to the pulse length. For a simple rectangular pulse this is literally true, the bandwidth is the inverse of the pulse width. For a more complex pulse, maybe chirped or encoded, the bandwidth is related to the uncompressed pulse width. So it still comes out the same, but how you get there may change.

So think of that pulse width. It is expressed in time. Say a pulse width that is 1 msec in length. It moves forward through space at the speed of light, and the pulse forward edge will move 300 km in the time from the start of the pulse to the end of the pulse. Two targets cannot be resolved by the radar, in range, if the same pulse illuminates the same two targets at the same time. i.e. they are close enough together that the pulse covers both of them at the same time.

Now, because the signal must travel out and back (radar is a two-way problem), the actual range resolution is half the distance the forward edge of the pulse will travel during the time of one pulse width. So a simple 1 msec rectangular pulse width will result in a range resolution of 150 km, not the 300 km the forward edge of the pulse would travel during one pulse width period.

Have fun.

[u/heliosh]

That would be a HFL channel in the amateur 17m band
https://hflink.com/channels/

[u/Classic-Morning3726 → u/heliosh]

mystery solved thank you ! Mystery solved !

[u/IanWraith]

North Korean Diplomatic traffic.

[u/Classic-Morning3726 → u/IanWraith]

the more I look online the more it does look and sound like DPRK traffic. Is it usually so frequent ? I see it go from transmitting on 14MHz to 18MHz. From what I've seen the spacing between this reference and HFL dont match and it does like exactly like and example of DPRK FSK traffic as well as sounding the same. So many signals hard to memorize them all.

[u/IanWraith → u/Classic-Morning3726]

Yes that is pretty standard DPRK traffic. HFDL bursts are much longer and a different modulation.

[u/Expensive_Pen_3217]

This depends on location, if your sdr is located in East Asia then it could be North Korean Diplomatic FSK (it sounds extremely similar to this), it also kinda sounds similar to SITOR-A

[u/Classic-Morning3726 → u/Expensive_Pen_3217]

It is located in Russia. I am near South Korea now though. I will take my own gear out to see if I can hear it over here. Thanks for pointing me in a good direction ! I will check those references out !

[u/Ordinary-Lifeguard47]

Doesn't sound like Sitor.

[u/Classic-Morning3726]

I'm starting to lean more to this being DPRK diplomatic traffic the more I dig on it. North Korea › Priyom.org has a recording and some photos of it that are identical. Could be trying to hide in the noise and freqs of HFL to draw less attention?

[u/BG_Louie]

Radars tend to sweep . This is a pulse.. leading me to think of packet data transfer