Monthly Archives: January 2022

WSPR & MH370: Facts against Fake News

How Aircraft Scatter generally works. This adaption from Gary S. Sales’ paper “OTH-B Radar System” (University of Masschusetts, Lowell/USA, 1992) should add to some other entries on my website. Double-click the picture to enlarge it.

Furthermore, there are people who claim against all facts and reason that they can prove aircraft movements with aircraft scattering of WSPR signals from their log data. Surprisingly or not, they find enthusiastic approval in the popular press, but also in technical-scientific organizations like many ham radio associations, first and foremost the notorious German DARC. Whether one deals with supporters of “conspiracy theories” at all (Nobel laureate Joe Taylor, K1JT, said having too little time for such obvious and non-scienctific nonsense), or whether one meets their convoluted theories with technical-scientific arguments, is quite controversial and a topic more of social psychology than one of physics.

Nevertheless, “Never Give a Sucker an Even Break” as the great comedian and juggler, W.C. Fields stated 1941. And that is why I would like to deal with some “arguments”, which would not be difficult because of the subject matter, but because of how these people “argue”. For the sake of clarity and brevity, let’s do this in the form of a question and answer game.

Do aircraft affect RF signals?
Certainly. HF signals are scattered on the electrically conductive metallic hull of aircraft.

How does Aircraft Scatter work at all?
The drawing at the top explains it: Radio waves from a transmitter reach the receiver directly on the one hand, and via aicraft scatter on the other. On the receiver side, both signals add up. Thanks to the Doppler effect, which the signal part scattered by the aircraft has, both signals can be separated from each other again with a method called FFT; see my website for a couple of examples. However, this is not possible with WSPR log data, here only the total signal is noted.

How big are these influences?
They mainly affect the signal strength and are around 35 to 50dB+ below the original signal. There are exceptions. Downward, there are far more cases than the exceedingly rare constellations where the scattered signal may be larger than the original signal. Above 30 MHz this occurs more often, below 30 MHz I have never observed it as there always was at least some backscatter of the original signal.
Signals or field strength can be measured and calculated. Generally speaking, a suitable form of the “Radar Equation” will do the calculation, see here. They largely match the values being measured by the method “separate original signal and scattered signal”.

Facts, please – how big … ?
Sorry, yes. Say, a booming signal by a broadcaster in the 19 meter band hits your antenna with a level of -40 dBm. Then a Boeing 747, flying over your house to touch down at your airport nearby (“in your backyard”, as they say) at a distance of 500m only, this will peak at -86dBm.
Not bad, and easily visible by FFT analysis.

How much does this scattered signal adds to the original signal?
Good, with this you steer to the central point, because WSPR measures only this total signal. You just have to add -40dBm and -86dBm and with this most favorable constellation you get a total signal of -39.999890911528446dBm.
Believe me: you cannot distinguish it from the level of the original signal, being -40dBm.

Oh, that’s disappointing … but they tell they can identfy aircraft not only 500m, but some/many 1000km away?
First, physiscs may be disappointing. Secondly, I took a most favourable case – booming broadcaster, short distance. The effective power of a stronger WSPR transmitter may reach 40dBm, compared to 100dBm+ of many broadcasters. The difference of 60dBm and more is whopping.

“Whopping” – what do you exactly mean by this?
Take the example of the broadcaster, reading -40dBm on my S-Meter. If the transmitter were an even above-average WSPR transmitter it reading of the S-Meter would be -100dBm. Still readable, and WSPR would give a decode.

So, it works?
Wait a moment, for introducing the scattered signal, also 60dB down. It will peak at -160dBm, and it reliably is eaten by noise which will start between -130 and -140dBm.
By this, the orginal signal of -100dBm will be enhanced and strengthed to -99.999995657057354dBm. Quite an achievement!

I understand, it cannot work. Does a greater distance improve things?!
By no means. A greater distance worsens things even exponentially.

OK, but what the hell are they measuring to come up with such far-reaching results?
They are measuring indeed fluctuations of the signal but without knowing the reason. And there are much more and of stronger influence to the received signal level than aircraft scatter. Prevailing is multi-path leading to near-normally distributed changes of the signal level of around ±8dB from second to second, and often more than 30dB within just a few seconds!

But – they mention “drift” … and “Doppler” means “drift”?!
Yes, but the “moon shapes” of a few signals surely have other reasons, much more obvious – just think of bad power supplies, meteor scatter (stronger and more often seen compared to aircraft scatter) and travelling waves within the ionosphere itself. Have you ever asked yourself, why in the presented cases the whole signal is shifted, instead of seeing a Doppler signal branching out from the original signal? „They don‘t know what they do“, says K1JT into their direction.

How much can I rely on the quality of WSPR signals?
Look yourself at the screenshot below, showing three hours of WSPR signals, showing drift, over-modulation, noisy signals. All fine for decoding WSPR but on only very few you consider those rocks where you want to build your church on (Mathew 16-18). You see instabilities at many scales, and also the duly repeating (!) half-moon footprints which for some ghostseers are the evidence of aircraft.

Drifting away: Three hours of WSPR signals on 20m. Their quality works for decoding WSPR, but it is difficult to use them as reference …

They work with the concept of “tripwire”. Any comment on this?
Well, they seem to consider propagation working by distinctive, laser-like “rays”, not fields of energy. (This is just a guess from this blog entry.) Each object crossing this ray causes a-normal propagation which they fail to precisely specify. This is a fundamental misconcept of how HF propagation works plus an incomprehensable application of PropLab Pro 3.1, the propagation software, which they seem not to understand. Propagation doesn’t produce “tripwires”. And if you need some parallel, you should more think of a booby trap, thanks to which not only signals are pulverized, but with them all the dream fantasies that this or that plane may have caused them to go off.
They must use “Broadcast Coverage Map” with PropLab Pro to get a realistic view of electromagnetic fields and their propagation, see secreenshot below.

No “tripwire”: HF propagation doesn’t work by laser-like rays, but by electromagnetic fields. This PropLab 3.1 Broadcasting Coverage Map screenshot gives a general impression of this – transmitter Tiganesti/Romania, simulated a sector of ±30° of the antenna’s direction. And you can try to get your own impression for free with e.g., VOACAP online.

Can I understand your assertions?
Absolutely! In theory, as well as in practice. You can find many examples on my website. A SDR and software are all you need. Oh, and, last but not least: and unbiased view not on the possibly desirable, but on the physically possible!

But why do they still spread their charlanteries with great success?
Look around you. The world is full of castles in the air. That’s actually not so bad. Here, however, they are built by those who could know better and they are spread with enthusiasm by those who know better. Or at least should know better.
But that is the usual pattern of Fake News. Only that it undermines the technical-scientific competence of the radio amateurs and makes them look ridiculous.

SDRC: New Bitmap Display helps to raise DX!

Bitmap Display showing 24 hours from 0 to 25MHz from a recording of 23NOV2021.

Simon Brown, G4ELI, author of free SDRC software to control (and much more …) most of the SDRs walking on earth, again surprised the community: he added a stunning fast “Bitmap Display” to get a literally overlook onto the content of a recording. The screenshot at the top shows a 25 MHz recording over 24 hours, made with Winradio’s Sigma SDR (16 bit), produced from a near-9TB file within only few seconds. It clearly shows how propagation follows the sun. Medium wave signals thin out after sunrise (06:56UTC here on 23NOV2021) to fade in just before sunset (15:16UTC). You also see the still active broadcasting bands, and, alas, also some interference from PVs at the higher end of the spectrum. You also see the power of s state-of-the-art SDR like this Winradio Sigma, at a professional wide-band active vertical dipole antenne MD-300DX.

See, for comparison, the range of 24MHz/24 hours on a summer day, namely 08JUN2021 (SR 04:00/SS 19:39 UTC), with Elad FDM-S3:

During a short June’s night, the lower frequencies are only sparsely populated And on the higher frequencies you see something of a “summer’s depression”, where in the late autumn’s screenshot they get some boos from the “winter anomaly”, but fading in later and fading out much earlier.

This “Bitmap Display” is called via the tab “Rec/Playback“, then menu “Navigator“. It works on recorded HF files with a fixed width of 4096 data points. So, with a recording of 25MHz width you get a freqeuncy resolution of roughly 6kHz. This makes it ideal for AM broadcast under 30MHz, as well as for all wider modes above 30MHz, let it be the full FM band to identify even short openings, the airbands to check most active channels etc. The time resolution can be set between on second and 60 seconds, see screenshot below.

The time resolution can be set in seven steps.

This “Bitmap Display” adds to the alread known “Grid Display” which still is on board, see the two screenshots below.

Toggle between “Bitmap” and “Grid” display …
“Grid Display”: set ot 06:00 UTC.

Both displays set the recording to the matching time by just a mouseclick. The frequency, however, has to be set separately in the “Receive” Panel. You can switch beween this two windows with a tab at the left bottom, see the following two screenshots.

Toggle between Receive and Playback with Bitmap/Grid.
The reception frequency is shown on the “Bitmap Display” as a white dashed double arrow, pointing to this frequency on the scale at the bottom of the display (here: 9420kHz has been tuned).

The ingenious double function “Click and display time and frequency” is still reserved for the File Analyser module, which is somewhat more complex to operate.

More than just a consolation for this, however, is the loop function: here you set the times for the start and end of the loop by numerical input or simply by mouse click – and off you go! See the both screenshots below:

Start and end of the playback loop can be set either numerically, or …
… by a right mouse click which will duly transfers the time for starting and finishing into the numerical display shown above. This has the advantage to match start/end time visually to the footprint of a signal.

One very fine feature is zooming into the “Bitmap Display”. Even though this software zoom does not change the resolution, this function is an important tool for checking the occupancy of a broadcast band, for example, and for jumping specifically to the start of a broadcast.
Frequency-wise – by position and bandwidth – the slider below the running Spectrogram (“Waterfall”) of the main window is responsible for this. This can be moved as well as changed in its width, so that the corresponding area is displayed. The following two screenshots are more helpful than any quick guide.

The slider has three handles which a mouse click transform into a double arrow to change lower end, upper and end centre. For a better time resolution, this has been changed here from 60 seconds to 1 second per pixel. With the scrollbar on the right you may scroll through the whole “Bitmap Display”.
Here the zoom has been set to the 25 meter broadcast band (slider), and the time resolution set to 1 second/pixel. With the scroll bar, I scrolled the “Bitmap Display” to around 13:00 UTC, and I clicked to 13:26 UTCon 12’040kHz.

It is also possible to tune to a specific frequency when only the “Playback” window is open, and not the “Receive” window. This workaround-like procedure is done by the function/window “Frequency Database” which has to be filled with at least one set of channels. I use the voluminous ILG for this.
With this or another database already loaded, click View -> Frequency Database. Your SDRC window should look like the screenshot below:

How tuning is accomplished by the “Frequency Database”.

Then set all the demodulation controls to match the type of signal you want to recevie, i.e., AM and 5kHz etc. for broadcast. In the next step, simply double-click to the frequency entry in your “Frequency Database”. The “Receive” frequency changes (as you might hear). If your displays had been zoomed and the new frequency is out of focus, a simple trick brings the new channel to full glory: click to “Centre”, see screenshot below.

A click onto the “Centre” icon, and the zoomed “Bitmap Display” window etc. is changed to that channel.

Thanks, Simon, for another great feature of your software!

Q&A:

What are the main differences between the “Bitmap Display” and the “File Analyser”?
* The “Bitmap Display” is by far faster to build up a spectrogram. It also features the whole bandwidth of a recording.
* The “File Analyser” is more flexible in frequency resolution, offers “see, click, tune” when a spectrogram has been built up, and features flexible CSV export of data – up to the whole spectrogram. But it takes much looonger to build up.

Can the “Bitmap Display” also being used to raise short-living utility signals like ALE?
* It depends. Limiting factor is the frequency resolution. With some experience, I can clearly make out ALE signals in an 1MHz wide recording, 1 second time resolution.

Do you have a wishlist? Thanks for asking, but it is an only small one:
* It would be nice if there were several options for (higher) frequency resolution. OK, it will slow down processing, but …
* As I like to process spectrograms, a CSV export would be welcome (as with the File Analyser).
* Undoubtedly, to change not only time, but also frequency would be the ice on the cake.