playing with psk-Sounder and Stanag-4285

I used the word "play" because this post does not claim to be a scientific treatment or a in-depth study on propagation mechanisms. HF propagation is not a sort of "deterministic machine" with a certain set of known rules to apply, is not so immediate as a multiplication table, but rather it's a very complex science which involves several disciplines; there is a lot of things to know and a lot of still not clear things... and something unpredictable. Almost casually I found the psk-Sounder software and decided to have just a close look at it: the results are shown in this "cheap and cheerful" post. I want to thanks Murray Greenman, a great expert on HF propagation (20 years experience) and co-developer of psk-sounder, for the precious help and clarifications.

Background
STANAG-4285 is basically a 2400 baud PSK-8 signal, transmitted in frames of 256 symbols in a frame. The intersting thing is an 80-symbol section of this frame which contains a repeated 31-bit pseudo-random binary sequence (PN). PSK-Sounder uses frames of the same length and its cross-correlator allows to locate each frame exactly in time. Since a PN sequence is transmitted every 256/2400 secs, by comparing the time of the frame with a clock in the computer, we can also measure changes in the propagation delay of the received signal over time and plot such measures in the so-termed Correlogram.
The Correlogram is then a representation of propagation delay (vertically), elapsed time (horizontally) and correlation significance (relative signal strength of each ray) as brightness. You can use the mouse to read off the delay between responses.

Fig. 1 - psk-Sounder monitoring a STANAG-4285 transmission on 17MHz from CTA Monsanto (Portugal)

"As an example, imagine we are receiving a signal from a station some distance away, which is providing both ground wave and F-layer ionospheric signals - the classic NVIS situation. The ground wave will be delayed by just a millisecond or two (300km per millisecond at the speed of light), while the F-layer signal, travelling a further 300km up to the ionosphere and another 300km back, will be delayed an extra 2ms. Both these signals arrive at the receiver, and the cross-correlator has to work with the combined signal. So, it will in fact find two peaks, about 2ms apart, and we can display or plot this information, and measure the delay. Very often the propagation is even more complex, especially at greater distances, and of course very often the ground wave signal is not received at all." 
PSK-Sounder, along with documentation, instructions and examples, can be downloaded from:
http://www.qsl.net/zl1bpu/SOFT/PSKSounder.htm

Notes
1) a serious approach requires the monitoring of several stations and frequencies for long periods, say one year at least, so to observe the changes in propagation in different seasons and in different hours of the day per each station/frequency (...the sun is not a light bulb). For my scope, I monitored only two S4285 stations at fixed times (for about a 5-minutes period) during the day:

- 5215.6 KHz French Navy Toulon F, 445 Km West from my QTH
- 8698.2 KHz Norwegian Navy Bodo NOR, 2721 Km North from my QTH

It's important to note that in the case of Toulon the path is in large part over the sea (83.5%).
 

Fig. 2 - the "monitored" STANAG-4285 stations

2) There are empty areas in some of the following correlograms which are due the lack of S4285 sync. It's always difficult to achieve really long monitoring times with PSK Sounder, as something inevitably disrupts the bitstream and consequently the synchronization with the incoming S4285 signal is lost. Obviously, real modems resynchronise to the PN sequence instantly, but PSK-Sounder is not locked at all and simply observes where the PN sequence is! (as said above).

3) I have no stations within ground wave range (say 50 km at 5 MHz, 100 at most), the closest is IDR  Italian Navy from Santa Rosa (Rome), so the lowest 0 seconds fligth-time track in Correlograms (that would be the ground wave) is here the E layer response, ie the region about 30-50 km up from earth. This means that the delays are not the real ones but are related to E responses, when present, or at least to the lower one.

4) Combined Correlograms (Toulon/Bodo) would be very interesting but unfortunately I do not have a such tool at my disposal.

One could say that similar tests do not make a great sense (and that's right), anyway, although the above limitations, some interesting observations can be noted.

Correlograms - Bodø (8698.2 KHz - 2721 Km North - 20 July)

Fig. 2a

Fig. 2b

Fig. 2c

Fig. 2d

I am not in a position to correctly describe the above scenarios, but I can give a rough description of what I see. While around 1000Z (Fig. 2b) the reception is sustained by E layer, around 1400Z (Fig. 2c) the propagation is mostly due to F layers: you can see the different heights (ie delays) of the thickest black line. Pobably, at that time (1420Z) the absorbing D layer is disappeared, signals have a greater strength and can penetrate E layer and reach the upper F region.

Correlograms - Toulon (5215.6 KHz - 445 Km West - 20 July)

Fig. 3a

Fig. 3b

Fig. 3c

 

Fig. 3d

In the morning (Fig. 3a) are visible two well distinct paths reception due to the refractions by E and F layers. In the central part of the day, the E response line is a less pronunciated line and delayed responses up to 4-5 msec begin to be visible (Figs. 3b, 3c); it's very interesting the short duration scatter in Figure 3c which causes a cloud of delay responses > 5 msec.
As said, I am not in a position to give an explanation to this, I asked Murray and I quote here the reply he sent me (it's worth reading it): "[...] It's very difficult to say from just the Correlogram. If you consider a triangle with a base of 450 km (the station's distance from you), which is a flight time of 1.5 ms, and a total 'distance' of the two sides of 4.5 ms (the delay added by the path you are interested in), then you can construct an isosceles triangle which will have the refractive point at the peak. The sides will each be 2.25 ms, and the vertical height to the peak can be calculated to be 4.44 ms, or a height of 1332 km above the base.
We know that the F layers are 200 - 300 km in altitude, so clearly the scenario I just described is not correct. There are two factors involved: (1) the possibility that the signal bounced around (is scatter); and (2) that the speed of light is much reduced at the refractive layer, since the refractive index is high here. So therefore the path is not a simple triangle, but has an area of multiple refractions with reduced propagation speed.
So the complicated answer is that I consider that area to be one of multiple scatter, the signal bouncing around between layers (probably F1 and F2) for some time before returning to earth. It is probably not a small  'cloud' as such, but a wide layer which was briefly in the right place for you to see the effect. Another point to consider is that the ground wave range on 5 MHz is about 50 km, 100 at most, so what looks like the ground wave is most likely the E layer response, the layer about 30 - 50 km up. [...]". Anyway, bounces between sky and sea are also probable.
Around 2000Z (Fig. 3d) the E response returns to be preminent along with the lower F layer, probably still many bounces cause delayed responses up to 5msec. 

Final note
HF propagation is a fascinating part of our hobby but things get very complicated if one want to deepen his knowledge about it. Unlike what it may seem from reading some websites, propagation is not summarizable in a few simple rules but there are a lot of things to study ...and "The trouble is that this sort of study raises more questions than we have answers for!", as Murray Greenman says.