XIV Международная студенческая научная конференция Студенческий научный форум - 2022

Scientific advisor: Strakatova O.N., Vladimir State University named after Alexander Grigoryevich and Nikolay Grigoryevich Stoletovs Vladimir, Russia

Satellite communication systems used to transmit various signals over a large area have been known for a long time. Since the advent of satellite communications, there has been a rapid development of technologies in this direction. And with the accumulation of experience, the equipment has been improved, methods of signal transmission have been developed, and there has also been a transition from separate satellite communication lines to local and global systems.

Let's consider the principles of organizing satellite communication channels and define what satellite communication is. SC is one of the types of radio communication that is based on the use of artificial earth satellites as retransmission stations. Retransmission stations are located in at a very high altitude, from hundreds to tens of thousands of kilometers, because the area of its ​​visibility in this case is almost half of the globe. To transmit a signal via a satellite, you need to produce modulation, and it is done at the earth stations. After modulation, the so-called modulated signal is amplified, transferred to the desired frequency and then fed to the transmitting antenna. Satellite retransmission stations can be non-regenerative and regenerative. A non-regenerative satellite, having received a signal from one earth station, transfers it to another frequency, amplifies and transmits it to another earth station. In this case, the satellite can use several independent channels that carry out these operations, each working with a certain part of the spectrum. These processing channels are called transponders. The regenerative satellite demodulates the received signal and then modulates it again. With this, errors are corrected twice. But there is also a disadvantage, the complexity of this method. Satellite transponders are placed in orbits that are divided into three classes. Let's consider each of them. The first is the equatorial orbit. An important variation is the geostationary orbit. The satellite rotates at an angular velocity in the direction of the Earth's rotation. The advantage of this orbit is that the receiver in the service area sees the satellite on a continuing basis. But there are also drawbacks, it is impossible to bring all satellites into one orbit, as well as the high cost of putting a satellite into the orbit, and the inability to serve earth stations in the circumpolar region. But an inclined orbit solves all these problems, however, due to the movement of the satellite relative to the ground observer, it is necessary to launch at least three satellites into one orbit in order to provide round-the-clock communication access. A polar orbit is the limiting case of the inclined orbit.

To fully disclose this topic, let us turn to the areas of the satellite communications application. There are satellite backbone networks, VSAT systems and mobile satellite systems. For a complete picture, we will consider each area in private. The main element of the VSAT satellite communication is the Digital Communication Center. It is the network control center that provides access to client equipment from the Internet, public telephone network, other terminals of the VSAT network, and implements traffic exchange within the client's corporate network.

NCC has a broadband connection to backbone communication channels. We will also consider the principles of organizing mobile satellite communications. To ensure that the signal strength reaching the mobile satellite receiver is sufficient, one of two solutions is used. The first solution is that the satellites are located on the geostationary orbit. Since this orbit is located relatively far from the Earth, a powerful transmitter is also installed on the satellite. Another solution is directly related to many satellites located in an inclined or polar orbits. At the same time, the power required for the transmitter is not so high, and the cost of launching a satellite into orbit is lower. But there is one peculiarity, it requires the launch of many satellites into orbit, but also an extensive network of the ground communicators. And the last solution sounds like this: the client's equipment, that is, satellite phones, mobile satellite terminals, interacts with the outside world or with each other through a satellite-relay and interface stations of a mobile satellite communication operator. Satellite communication systems are designed to meet the needs of communication and satellite Internet access anywhere in the world. They are needed where increased reliability and fault tolerance are required, and are used for high-speed data transmission when organizing multichannel telephone communications. Specialized communication systems have a number of advantages, but the key is the ability to implement high-quality telephony outside the coverage areas of cellular stations. Such communication systems allow you to work from an autonomous power supply for a long time and be in a call standby mode, this is due to the low energy performance of user equipment, light weight and an omnidirectional antenna. Currently, there are many different satellite communication systems. They all have their pros and cons. Additionally, each manufacturer offers users an individual set of services.

Satellite communication systems are designed to meet the needs of communication and satellite Internet access anywhere in the world. They are needed where increased reliability and fault tolerance are required, and are used for high-speed data transmission when organizing multichannel telephone communications. Specialized communication systems have a number of advantages, but the key is the ability to implement high-quality telephony outside the coverage areas of cellular stations. Such communication systems allow you to work from an autonomous power supply for a long time and be in a call standby mode, this is due to the low energy performance of user equipment, light weight and an omnidirectional antenna. Currently, there are many different satellite communication systems. They all have their pros and cons. Additionally, each manufacturer offers users an individual set of services (Internet, fax, telex), defines a set of functions for each area of ​​coverage, and also calculates the cost of satellite equipment and communication services. In Russia, the key ones are: Inmarsat, Iridium and Thuraya. Scope of use of the SCS (Satellite Communication Systems): navigation, ministries and departments, governing bodies of state structures and institutions, the Ministry of Emergency Situations and rescue units. As with any communication, the satellite has its drawbacks as weak noise immunity The huge distances between earth stations and a satellite cause the signal-to-noise ratio at the receiver to be very low (much less than for most microwave links). In order to provide an acceptable error probability under these conditions, it is necessary to use large antennas, low-noise elements and complex error-correcting codes. This problem is especially acute in mobile communication systems, since they have restrictions on the size of the antenna and, as a rule, on the power of the transmitter.

The quality of satellite communications is strongly influenced by effects in the troposphere and ionosphere. The absorption of a signal by the atmosphere depends on its frequency. The absorption maxima are at 22.3 GHz (water vapor resonance) and 60 GHz (oxygen resonance). In general, absorption significantly affects the propagation of signals above 10 GHz. In addition to absorption, during the propagation of radio waves in the atmosphere, there is a fading effect, which is caused by the difference in the refractive indices of different layers of the atmosphere.

The problem of signal propagation delay, one way or another, affects all satellite communication systems. Systems using a satellite transponder in geostationary orbit have the highest latency. In this case, the delay due to the finite speed of radio wave propagation is about 250 ms, and taking into account multiplexing, switching and signal processing delays, the total delay can be up to 400 ms. Propagation delay is most undesirable in real-time applications such as telephony. Moreover, if the signal propagation time through the satellite communication channel is 250 ms, the time difference between the subscribers' replicas cannot be less than 500 ms. In some systems (for example, VSAT systems using a star topology), the signal is transmitted twice over the satellite link (from a terminal to a central site, and from a central site to another terminal). In this case, the total delay is doubled.

The advantage of satellite communications is based on serving geographically distant users without the additional costs of intermediate storage and switching. SCS is constantly and jealously compared to fiber-optic communication networks. The adoption of these networks is accelerating due to the rapid technological development of the respective areas of fiber optics, which raises the question of the fate of the SCS. For example, development and planning, most importantly, the introduction of concatenating (composite) coding sharply reduces the likelihood of an uncorrected bit error, which, in turn, allows us to overcome the main problem of CVS - fog and rain.

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