What is heard on the radio? Receive and decode the most interesting signals

What is heard on the radio? Receive and decode the most interesting signals


Hello Habr.

The 21st century is already in the courtyard, and it would seem that you can transfer data in HD quality even to Mars. However, there are still a lot of interesting devices on the radio, and you can hear a lot of interesting signals.



All of them are considered, of course, unreal, we will try to choose the most interesting ones that can be received and decoded by yourself using a computer. For receiving signals, we will use the Dutch online receiver WebSDR , the MultiPSK decoder and the Virtual Audio Cable program.


For convenience of consideration, we will give signals in increasing frequency. I will not consider broadcasting stations, it’s boring and trite, anyone who wants to can listen to the Radio of China at AM. And we move on to more interesting signals.

Time signals


At a frequency of 77.5 kHz (long wavelength range), the exact time signals of the German station DCF77 are transmitted. They already have a separate article , so we can only briefly repeat that this is a simple signal in amplitude modulation - different the lengths are encoded “1” and “0”, as a result, a 58-bit code is accepted in one minute.
image

130-140KHz - power grid telemetry


At these frequencies, if you believe the radioscanner site , the control signals of the German power grids are transmitted.


The signal is strong enough, and according to reviews, it is accepted even in Australia. You can decode it in MultiPSK, if you set the parameters as shown in the screenshot.


At the output, we will receive data packets, their structure is of course unknown, those who wish can experiment and do analysis at their leisure. Technically, the signal itself is very simple, the method is called FSK (Frequency Shift Keying) and consists in shaping the bit sequence by changing the transmission frequency. The same signal, in the form of a spectrum - the bits can be counted even manually.


Meteoteletype


On the spectrum above, very close by, at a frequency of 147 kHz, one more signal is visible. This (also German) station DWD (Deutscher Wetterdienst), transmitting weather reports for ships. In addition to this frequency, the signals are also transmitted to 11039 and 14467KHz.

The decoding result is shown in the screenshot.


The teletype coding principle is the same, FSK, the text encoding is of interest here. It is 5-bit using the Bodo code , and has almost 100 years of history.


It seems that a similar code has been used on punched tapes, well, weather teletypes have been sent somewhere since the 60s, and as you can see, they are still working. Of course, on a real ship, the signal is not decoded using a computer - there are special receivers that record the signal and display it on the screen.


In general, even with satellite communications and the Internet, transferring data in this way is still a simple, reliable and cheap means. Although of course, it can be assumed that sometime these systems will also go down in history and will be replaced by fully digital services.So, those who wish to receive such a signal should not be too delayed.

Meteofax


Another legacy signal with about the same long history. In this signal, the image is transmitted in analog form at a speed of 120 lines per minute (there are other values, for example, 60 or 240 LPM), frequency coding is used to encode the brightness - the brightness of each point of the image is proportional to the frequency change. Such a simple scheme made it possible to transmit images even in those times when very few people heard about “digital signals”.

Popular in the European part and convenient for reception is the already mentioned German station DWD (Deutche Wetterdienst), which transmits messages at frequencies of 3855, 7880 and 13882 kHz. Another organization whose faxes are easy to receive is the British Joint Operational Meteorology and Oceanography Center, which transmit signals at frequencies of 2618, 4610, 6834, 8040, 11086, 12390 and 18261 KHz.

To receive HF Fax signals, you need to use the USB receiver mode, you can use MultiPSK for decoding. The result of the reception via the websdr receiver is shown in the figure:


This picture was taken right at the time of writing. It is seen by the way that the vertical lines have moved out - the analog protocol, and the synchronization accuracy is critical here, even small sound delays cause an image shift. When using a “real” receiver, there will be no such effect.

Of course, as in the case of meteoteletype, nobody decodes faxes using computers on ships - there are specialized receivers (an example of a picture from the beginning of the article) that do all the work automatically.

STANAG 4285


We now consider a more modern standard for transmitting data on shortwave - the Stanag 4285 modem. This format was developed for NATO, and it exists in various versions. The basis is phase modulation, signal parameters can vary, as can be seen from the table, the speed can be from 75 to 2400bit/s. This may seem a bit, but given the transmission medium - short waves, with their fading and interference, this is quite a good result.


The MultiPSK program can decode STANAG, but in 95% of cases only “garbage” will be the result of decoding - the format itself only provides a lower-level bit-by-bit protocol, and the data itself can be encrypted or have some kind of its own format. Some signals, however, can be decoded, for example, the record below at a frequency of 8453 kHz. I could not decode at least some signal through the websdr receiver, apparently, the online transmission still breaks the data structure. Those interested can download the file from the real receiver via the link cloud.mail.ru/public/JRZs/gH581X71s . The decoding results in MultiPSK are shown in the screenshot below. As you can see, the speed for this record is 600bps, apparently the text file is transmitted as the content.


Interestingly, as you can see in the panorama, there are really a lot of such signals on the air:


Of course, not all of them are possible, they belong to STANAG - there are other protocols on similar principles. For example, you can give an analysis of the signal Thales HF Modem .

As in the case of the other considered signals, specialized devices are used for actual reception and transmission. For example, for the modem shown in the photo NSGDatacom 4539 declared speed from 75 to 9600bps at a 3KHz signal bandwidth.


The speed of 9600 certainly doesn’t inspire much, but considering that signals can be transmitted even from the jungle or from a ship in the ocean, and without paying anything to the telecom operator, this is not so bad.

Let's see by the way, more carefully on the panorama above. On the left, we see ... correctly, the good old Morse code. So, go to the next signal.

Morse Code (CW)


At a frequency of 8423KHz, we hear precisely him. The art of hearing Morse code is practically lost, so we will use MultiPSK (however, it decodes it so-so, the CW Skimmer program does much better).


As you can see, the repeating text of the DE SVO is transmitted, according to radioscanner site , the station is located in Greece.

Of course, such signals are few, but they still exist. As an example, we can give a long-running station on 4331 kHz, transmitting repeating signals “VVV DE E4X4XZ”. As Google suggests, the station belongs to the Israeli Navy. Is something else transmitted on this frequency? The answer is unknown, anyone can listen and check on their own.

The Buzzer (UVB-76)


Our hit parade is most probably the most famous, the signal is known both in Russia and abroad, the signal at a frequency of 4625 KHz.


The signal is used to alert the troops, and is a repeated beeps, in the intervals between which the code phrases from the cipherblock note are sometimes transmitted (abstract words like “KROLIST” or “BRAMIRKA”). Some write that they saw such receivers in the military registration and enlistment offices, others say that it is part of the “dead hand” system, in general, the signal is a mecca for fans of Stalker, conspiracy theories, “cold war” and other things. Those interested can dial in the search for "UVB-76", and I am sure, an entertaining reading for the evening is guaranteed (however, you should not take everything written seriously). At the same time, the system is quite interesting, even if it is still working since the days of the Cold War, it is hard to say whether it is necessary for someone now.

Completion


This list is far from complete. With the help of a radio receiver one can hear (or rather see) communication signals with submarines, over-the-horizon radars, and rapidly changing frequency hopping signals, and much more.

Here, for example, a picture made right now at a frequency of 8 MHz, on it you can count at least 5 signals of various kinds.


What they are is often unknown, at least in open sources you can find far from all (although there are sites like www.sigidwiki .com/wiki/Signal_Identification_Guide and www.radioscanner.ru/base ). The study of such signals is quite interesting from the point of view of mathematics, programming and DSP, and simply as a way to learn something new about the world around us.

It is also interesting that, despite the development of the Internet and communications, radio not only does not give up its position, but perhaps even vice versa - the possibility of transmitting data directly from the sender to the recipient, without censorship, traffic control and packet tracking, can become (although let's hope that will not) again relevant ...

Source text: What is heard on the radio? Receive and decode the most interesting signals