Tasting the Seagull. In the arsenal of radio hacker

Tasting the Seagull. In the arsenal of radio hacker
Here it is like this, all your life you study, you work, you think you are such a peppercorn, salt of the earth, and then you understand
It was so, I planned system of jamming navigation , made fee for her. I write, honestly, without guile, say, the synchronization input is, you can send there the signal of the exact PPS time and do the measurements.
And then it dawned on me: if there is no navigation, then there is no exact time either! (Here I also remembered old Munchausen, who also wanted to pull himself out by the hair from the water). Almost everything today, including me, still does not fully understand at the everyday level this technological and, at the same time, fundamental connection between space and time, and those troubles that bear a disregard for him.
But there is, of course, a way out. After all, everything good has been invented before us.
Seagull . In terms of positioning, it is not very global, it works in regions, but our country is covered. Its name includes the abbreviation of IFRS, which means a pulse-phase radio-navigation system. That is, first the position of the receiver is calculated roughly by the difference in the delay of the pulses, and then is specified by the phase difference.
In the US there is a similar system - Loran .
Now there is an increased interest in these systems, since satellite navigation signals in practice were easily susceptible to suppression and imitation. Particularly worried about the military. And it's not that these IFNSNs were somehow imitated or jam-protected. Just war involves the use of a set of tools, you need to use all possible means and process them together.
You will find in the network a lot of beautiful photos of transmitter masts, descriptions of the principles of work and much more. There are article on Habré to process the IFNSN signal. Cool!
My experience with the signal is still superficial. I just tried to see it and write it down. For this, I blew the dust off the old USB3 input board from the LTC2217 ADC.

The ADC here is stupidly wound up on the CYUSB301? it is clocked by a 40 MHz oscillator. This is quite enough to digitize the IFNSS signal with a carrier frequency of 100 kHz.
First I tried to see the structure of the signal by attaching a wire-antenna directly to the ADC. And I did it in the city. Nothing was found, although the antenna was about 15 meters long. It was anything, but not packs of nine or eight radio pulses with a repetition period of 1 millisecond and a time between packets of about 80 milliseconds. Removal from the residential quarters did not give anything: since there was no filter at the entrance, the necessary frequencies were clogged by the signals of the broadcasting stations. It became clear that anti-aliasing filter is necessary. And best of all with an amplifier.
And the world is not without good people. Dear FH gave me for temporary use some miracle of engineering thought, which not only misses frequencies from 9 kHz to 30 MHz, but also amplifies by 15 dB.

But in the city to see the signal still did not work. I had to go to the field, away from people, that already for certain. And, about a miracle, there the signal showed us its round barrel!


There were no big trees near, the antenna was thrown on small bushes. The antenna was symmetrical - a dipole, just two wires soldered to the shoulders of the input circuit of the filter-amplifier. First the shoulders of the dipole were completely dissolved, the length was 7 meters. The signal is excellent! Then I began to reduce my shoulders, twisting their ends with rings by the method of half-division. At a length of eight times less, the signal has already gone to noise.
I can not tell from the timing diagram of the signal-to-noise ratio (SNR), and I can not disclose the SNR link to positioning accuracy, it's a matter of experts, but I want to note that with a shoulder length of less than a meter, the signal was guided by the eye. This gives hope that you will be able to receive a signal on a small antenna that you can carry with you.
Like the gullible seagull in the photo below.

And indeed, there is a long-selling module from the St. Petersburg RIRV and antenna small size.
Now we have the recording of the signal and we can begin to understand how to regain the space and time lost in the zones of suppression and imitation of satellite navigation.
It seems to me that in the near future it will be possible to install an IFNSN receiver even for flying drones. And thus get away from the jamming of navigation.
I ask the specialists on IFNS not to kick me for simplicity of approach, but to promote the development of technology. Your opinion and assistance in mastering the reception will be very useful. I, for example, are interested in the resulting positioning accuracy in different conditions. They say that there are even differential solutions for IFRS.
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