Interference: as a tram, radar detector and poorly crimped cable can affect the satellite channel
In any radio communication systems, that cellular, that satellite, interference is an eternal problem. For satellite lines, all interference can be divided into two classes: local and airborne.
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Local are any radio signals in the working frequency band of a communication system that reach the receiver input from the local air. Satellite systems are much more sensitive than our usual cellular ones. If you shout in the ear of a person at a distance of 1 meter, the noise of the leaves in the yard does not bother you. But if you stand at a distance of a kilometer and whisper, then even if the person has the right receiver, the leaves will interfere. The power at the receiver input is about minus 110 dBm, or 1x10-14W. All the same, only instead of leaves - ground radio points, trams, car ignition systems, anti-radar.
Tram and his friends
The most typical and typical sources of local interference for satellite earth stations (JSSS) of the C band (reception frequency 3.4-4.2 GHz) are radio relay lines and radio access systems. Therefore, when choosing a place to accommodate a C-band satellite antenna, one should first of all pay attention to the presence of RRL antennas nearby that are directed either directly into the aperture of the antenna (the possibility of interference from this antenna) or just in the back of the antenna (the possibility of interference from the antenna RRL working together with this one on the other side of the span).
Reception in the Ku band (10.9–12.7 GHz) is subject to interference from electric transport (trams and trolley buses), old (induction) car ignition systems and automobile radars. Therefore, if you place a Ku-band HSSS antenna on the roof of a building in the center of the city and the antenna should look towards a busy street, it is wiser to arrange the antenna so that the antenna hides from the street behind the roof cut. Just do not rush to outweigh your plate, aimed at the satellite with the TV channels.
Satellite TV has a power margin, which is usually enough to compensate for such interference. And the short-term effect of interference on the received signal does not always lead to a noticeable eye “undermining” or “scattering” the image. But if the picture is constantly sprinkled after replacing the converter with a more sensitive one or after mounting a larger diameter antenna on the same place, think about it.
And of course, if after a preliminary survey of the intended place to install the antenna, doubts arose about the “purity of the ether”, then it is better and more correct to produce a so-called. radio intelligence: to see what is actually going on the air, whether there are powerful signals in the area of ​​the expected operating frequencies, and if so, from what direction they come.
And in fact, not everything is as sad as it may seem. Almost any local hindrance can be mastered in one way or another. At least in my memory there has not been a single case where we could not provide an acceptable quality of the channels and services due to interference. Although sometimes it was really necessary to apply very non-trivial solutions. For example, erecting a mesh fence on one side of the antenna (Pavlodar, Kazakhstan) or transferring the work of one of the network segments to “provide channels on demand” to extremely inefficient in terms of band usage, but very resistant to interference, modulation and coding (terminals at posts Traffic police in the Krasnodar Territory).
Interference from satellite communications equipment itself
Airborne interference is a much more complex and interesting topic. Any satellite in this regard can be compared with a swimming pool: the cleanliness of the water in it primarily depends on the swimmers. But if access to the pool can be fairly easily restricted, then theoretically any radio equipment inside the so-called can reach the satellite. the service area of ​​this satellite and radiating towards the satellite at its operating frequencies and with adequate power. The satellite basically can not separate the useful or its signals from interfering or alien; he just reemitted towards the Earth everything that somehow fell on him. And if, together with the useful signal emitted by one of the “law-abiding” ZSSS, the interference from the Earth comes at the same frequency, then the sum “signal plus interference” will go to the Earth. And it will look something like this:
The canopy trapezoid line is the envelope of the useful signal. High "stick" on top of it - a hindrance.
The presence in the signal band of such interference leads at least to the appearance of errors on the channel affected by the interference. And as a maximum - to the complete inoperability of the channel and all services going inside it.
The source of airborne interference, as a rule, are themselves ZSSS (although, of course, one should not completely exclude the possibility of malicious or targeted jamming using some "special means"). The most typical causes of interference (or, strictly speaking, any additional or unwanted signals) in the emission spectrum of an ESL are as follows:
- The cable braid is broken or there is an unauthorized entry (s) of the transmission path of the ESF. In this case, such a ZSSS can easily “bring on board” a piece of its local ether: or VHF / FM broadcasting (for ZSSS working with the so-called PCh70 / 140 MHz, i.e., with input signals in the 50–180 MHz frequency band ), or signals from local cellular networks (for FMSS operating with IF on the L-Band, i.e., with input signals in the frequency band 950–1950 MHz).
- Malfunction or operation in the wrong mode of one of the active elements of the transmitting path of the SCSS (modems, frequency converters, a powerful amplifier). The “forest of sticks” in the picture above is one of the examples of what can be generated at the same time at the output of the transmission path of such FMJ system. Also a fairly typical example is the appearance of “shoulders” in the useful signal of the SCSS. This occurs when one of the active elements of the SCSS path is working in overload. In the picture of the signal in this case, a kind of stand appears from below, which is twice as wide as the main signal in the strip. "Shoulders" in the form of additional noise fall into the bands of adjacent channels and can affect (sometimes significantly) their work.
- Accidental reclosing of an ESZS (or a part of ESF equipment) incorrectly removed from service. If ZSSS is no longer needed, then, of course, it is turned off. But it’s not enough just to unplug the plug. First you need to give at least a command to the modem (and in some cases to the transmitter) so that it turns off its transmission. Then, if, for one reason or another, someone again provides power to the equipment, the JSSS will not be able to go on the air spontaneously. And accordingly, can not prevent anyone.
- More should be mentioned so-called. cross-polarized interference . Practically all modern satellites use “polarization compaction”: the work goes on the same frequencies, but with different polarization. For the C range, this is mostly circular polarization - right and left (RHCP and LHCP), for the Ku range - linear, vertical and horizontal (V and H). The overwhelming majority of problems arise with SCSS operating in linear polarization: the irradiating system of such SCSS must be well and properly configured during installation. If this is not done, then such a ZSSS begins to radiate both in its own and in the reverse (orthogonal) polarization, interfering with those who work in this polarization. But it is very clearly seen for the satellite segment operator (satellite owner), and such improperly working ZSSS are calculated at times.
The main and most difficult task in case of interference is to calculate who and from where this interference can radiate. But once again: the obstacle can fly to the satellite from any JSSS, at least somehow looking at this satellite and located inside the “service area”. And here, for example, a picture of the "service area" of the fixed beam of the Ku-band of the Express-AM33 satellite:
How to calculate where (or at least in the territory of what state) is the “problem” FMS?
Perhaps the simplest case is the ascent to board some kind of a ZSSS with PC 70/140 MHz of its VHF / FM broadcast. In most cases, the satellite network operator itself can calculate such a ZSSS, without the assistance of the satellite segment operator. After all, such a problem ZSSS definitely works in the same satellite transponder, where interference is observed. And for such large operators as Vimpelcom, this is most likely one of the JSSS of its own network. It is not so difficult to find out who exactly is: most modern spectrum analyzers have the FM demodulator mode. We tune in to interference, we listen - and very soon you can understand what kind of radio station it is. Well, after that, it's quite simple: we drive in the name of the radio station and its broadcasting frequency in the search, we find the city where it broadcasts on this frequency, and compare it with the list of the location of the AML with the IF.
Somewhat more complicated is the situation with the calculation of the source of interference in the form of lifting on board the signals of cellular communication. There can not do without the help of the operator of the satellite segment. They have equipment capable of pulling out all service information from the cellular network signal: network name, operator ID, BS number, etc. Having this information and having information about all the radio communications systems working via this satellite, the operator can calculate the problem radio communications system. Well, after that - inform the operator, the owner of the ZSSS that this station needs to be urgently put in order.
The worst case is the interference of unknown origin, arising due to malfunctions of the equipment of the SCSS. And very often such interferences are not constantly present, but they appear and disappear or, worse, they constantly change the shape, amplitude and frequency (so-called non-stationary interference).
A trivial, but very complex and long way of calculating the source of such interference is the sequential shut-off of all SSZS operating via this satellite. It is clear that this is a very difficult task: there can be hundreds of such ZSSS. And they should be turned off for a reason, just at the moment of presence of interference. And it is advisable not to turn off "soft", but by removing power from the transmitter. It is clear that to coordinate and successfully carry out such work is almost impossible.
But there is a way out. Modern methods of signal processing and the use of large computing power make it almost possible to work wonders. And to calculate such interference, special satellite geolocation systems are used. The task of any ZSSS antenna is to form a more or less narrow beam in the direction of the satellite. But the width of this ray is still finite. In addition, any antenna has a so-called. side lobes, where part of the energy goes. Very insignificant, but leaves. Satellites in orbit are quite dense, respectively, any JSSS, at least a small part of the emitted signal is sent not only to "its" satellite, but also to neighboring ones.
The satellites and their “service areas” are initially designed and placed so that the two neighboring satellites do not interfere with each other. Accordingly, that very small part of the energy of the SCSS radiated to the “alien” satellite becomes even smaller. And during normal operation, the signal from the satellite of an “alien” satellite is deep under the natural noise level of this satellite. Nevertheless, this signal is somehow present on neighboring satellites, which means that you can try to pull it out. This is done through the use of antennas of sufficiently large diameter (larger diameter - more sensitivity and a narrower radiation pattern) and rather complex equipment of the correlation reception of a satellite geolocation system. Weak echoes of interference are searched for among the noise of neighboring satellites, and then the parameters of these found echoes are calculated: time difference, frequency, phase between the “exemplary” interference signal and echoes and change of this difference with time. The satellites - even geostationary - are not nailed to the sky, they move, respectively, the difference of times, amplitudes and frequencies also changes. Well, after that, the geolocation system, based on the measurement results, builds several lines on the surface of the Earth, the intersection of which indicates the area of ​​the most probable position of the ESN, radiating interference to the satellite. Thus, the task, in essence, is similar to the one that is solved by any navigator or smartphone when navigating on the map, but for its solution we need noticeably more serious tools.
It is clear that a successful search for such interference strongly depends on the availability of suitable "neighboring" satellites at the desired point. But often the suitable "neighboring" satellite belongs to a completely different operator. And since with the growth of the total number of JSSS, the problem of searching for such interference became quite acute and relevant for each operator, in 2009 the “Satellite Data Association” (Space Data Association, SDA; www.space-data.org ) was created. The association was created by three of the world's leading satellite operators: Inmarsat (www.inmarsat.com), Intelsat (www.intelsat.com) and SES (www.ses.com), a little later joined by Eutelsat (www.eutelsat.com) ).
Standardization of methods, systems, equipment and data format of geolocation systems, the exchange of data and databases between members of the Association allows you to quite successfully calculate and eliminate interference on satellites that interfere with the normal operation of channels and services. However, no guarantee can be given that the interference will be uniquely or quickly calculated and eliminated. Especially if this interference is of the most complex type - “non-stationary interference”. For example, SES “calculated” and eliminated the source of “non-stationary interference”, which significantly influenced a significant part of the channels / services of the NSS-703 satellite. But for this they needed two years of intensive work. We, in turn, provide all possible assistance in the search for interference: at the request of the operator, we lift the carrier from one of our JSSS, which is closest to the area of ​​possible finding of the source of interference. This allows the operator to more accurately calibrate geolocation systems.
And finally - about the "pirates"
In the previous post, the question was asked: “can a mere mortal connect two points together without the participation of third parties?” Or, paraphrasing, “is it possible to make a satellite channel and not pay satellite owners for onboard resources?”
Theoretically, it is possible if both of your stations are inside the “service area”, and if the power line from your stations is sufficient for the channel to work. But let's face it:
- Satellite communications equipment is not only specific enough, and so is not cheap. And if we are talking about a more or less wide channel, then small “basins” are not enough here (see the previous post, “power of satellite lines”). Therefore, the alleged “pirate” needs at least communications among the suppliers of satellite communications equipment. Well, our “pirate” should have a good start-up capital.
- Installation and configuration - require specific knowledge. Calculation of energy, at least approximate - even more so. Hence, our "pirate" - should not be simple, but a "learned pirate."
- Owners of satellites rather closely monitor the frequency resource. And in order to remain unnoticed for a long time - the “pirate” will either have to raise a very narrow channel, or turn it on for a very short time. Spending a lot of effort and money, and be content with channel 9.6k for a few hours - this is somehow not piratical ...
- Well, if our “pirate” is still calculated, then you can assign a lot to him, starting with “illegal business activities”. Does he need it?
So, there is a lot of effort, but practically no benefits. Yes, and practice confirms: "space pirate" - rather, it is something from the field of non-scientific fiction. And if the operator detects a certain illegally operating channel, then usually it is not someone’s malicious intent, but quite familiar “crooked hands”: lured the board, trunk, polarization, frequencies (underline the appropriate).
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Source: https://habr.com/ru/post/256411/
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