Satellite communications, types, system, equipment, tools, orbit, ranges of satellite communication.
Satellite communication is one of the types of space radiocommunications based on the use of fast satellites, as a rule, specialized satellites.
Satellite communication marks a new stage in the development of advanced technologies, which is inextricably linked with space exploration.
The definition of satellite communication sounds convincing enough in the following terms: satellite communications must be equated with a variety of space radio, which is based on the use of special transponders – satellites communication.
Satellite communication is one of the types of space radiocommunications based on the use as relays of artificial satellites of the Earth, as a rule, dedicated satellites for communication.
The radio signal is relayed to a small spacecraft which move around the Earth along a certain trajectory.
Camera flying in an orbit in order to ensure retransmission of signal processing, called artificial communications satellite (abbreviated SSI). On Board an artificial satellite communication complex relay mounted equipment: units of transmission/reception signal, and a highly directional antenna, operating on specific frequencies. The work of an artificial communications satellite is receiving a signal, its amplification, frequency processing and relay in the direction of earth stations, staying in sight of the camera. The satellite repeater is a standalone device that is able to provide his whereabouts at a given point in space and consume power from the onboard power sources. Stabilization system ensures consistent orientation of the satellite antennas. The downlink transmission data on the status of the spacecraft, the reception of control commands provides telemetry equipment.
Retransmission of the received radio signal can be implemented with memory and without memory, because of the irregular stay of a satellite in sight of the earth stations.
Today satellite communication systems are an integral part of the telecommunications highways of the world, linking continents and countries.
The principle of satellite space communications involves the transmission/reception of a radio signal using a basic land or mobile stations via a satellite repeater. The specifics of ensuring the passage of radio waves due to the curvature of the earth, preventing the passage of the radio signal. In other words, in the line of sight the radio from one station to another is transmitted without delay. However, if the task is to get the signal over many thousands of kilometers from the transmitting station, the repeater is required, split the signal an appropriate angle to the receiving station.
In essence, a satellite communication device through the relay is a typical analogy of radio relay communication, only in this case, the repeater is located at a considerable distance (height) from the earth’s surface, thousands of kilometers. If to provide radio communications over long distances in different places of the globe required a lot of ground repeaters, with the advent of satellites, their number has decreased significantly. Now to broadcast radio signal from one mainland to another requires only one satellite.
Satellite communications, in General, is enforced through a set of interrelated elements of communication system: satellites-repeaters; stationary earth stations for satellite communications on the earth’s surface; the control center of the satellite (CUSS) and other elements of the system.
For the efficient transmission of radio signals over large distances, the analog signal is not suitable due to the high noise load, so it pre-digitized (i.e., a digital satellite connection) and then transmit to the satellite. Error correction schemes using error-correcting coding.
To date, the reception/transmission TV signal and radio broadcasting on the territory of the Russian Federation provide a satellite communication system (SSS). Satellite communicationsis a key element in the interconnected communications network of the Russian Federation. Part of satellite communications system includes two basic components are ground and space.
The first artificial Earth satellite was launched into orbit in 1957. The weight of the spacecraft was only 83,6 kg. control of the satellite was carried out through the miniature unit is a transmitter beacon. The successful outcome of the reception/transmission of a radio signal in the open space allowed us to implement ambitious plans, involving the use of ISS as active and passive repeater signal. However, to realize such long-term plans, it was necessary to create such spacecraft that could carry enough weight (various receiving and transmitting equipment). In addition, to put into orbit an artificial satellite, needed a powerful missile engines and equipment. After the Russian engineers have solved these problems, the opportunity to run into the open space of the ISS for conducting scientific and research works, the solution of navigational, meteorological, reconnaissance missions, and to ensure a stable channel of communication for transmission of radio signals over long distances. The process of formation satellite communications system (SSS) has intensified after the launch of the first artificial satellite. In the framework of the implementation of this concept on the earth’s surface began to build a base receiving-transmitting station, equipped with parabolic antennas. The diameter of the antenna up to 12 meters, which ensured a strong reception and transmission of radio signals. In 1965, the Russian engineers were able to provide television programmes in Vladivostok, broadcasting from Moscow via SSS.
In 1967, after testing and bring technical capacity to the required parameters have been commissioned satellite communication system “Orbit”. In 1975, on a circular orbit was launched the space satellite “rainbow”. Distance from earth’s surface to an artificial aircraft accounted for nearly 36 km. Direction of rotation of the planet and moon were almost identical, so the ISS is literally “floating” above the Ground, remaining motionless during the day. This technical solution simplifies the transmission of control commands to the spacecraft and guaranteed the stable operation of the channel of reception/transmission of radio waves. The subsequent orbit was launched more advanced ISS “Horizon”.
The operating results of ISS Orbit showed the inefficiency of the service signal in the interest of broadcast television in small communities numbering several tens of thousands of local residents. Therefore, priority was given to the compact ground stations, the reception-signal transmission serviced CCC “Screen”. Artificial satellite of the satellite communication system was launched into orbit in 1976. Now a program of the Central television could watch people even in remote places of Siberia and the Far East.
In the 80-ies of the last century through ISS “Horizon” is actively operated satellite communication system “Moscow”.
In the initial period of the development of near-earth space in the interests of relaying radio signals into space run simple satellites, with a minimum of equipment on Board (space satellites “ECHO” and “ECHO 2”). As a repeater used metal sphere enclosure with a reflecting effect. Often as a reflector was used a polymer sphere with a metallic coating. The efficiency of such devices was extremely low, so passive artificial satellites due development have not received. Their the complete opposite became active artificial satellites having complex electronic inside the filling intended for reception, processing, amplification and transmission of the signal at any point of the globe.
According to the method of signal processing in space satellites are classified into two types: regenerative and non-regenerative ISS.
Regenerative communication satellites carry out a more extensive set of operations – stage signal demodulation produces it, and at the time the relay carries out the modulation. This method of signal processing requires additional equipment and is characterized by sufficient complexity. Regenerative satellites are of high value.
Non-regenerative communication satellites provide a simple set of operations with signals. At the time of receiving a signal from the earth station – satellite communication provides reinforcement and transfer it to a different frequency. Subsequently, the radio signal is relayed to another ground station. The satellite can simultaneously receive and transmit multiple radio signals on different channels (transponders). Each channel is allocated a dedicated portion of the spectrum. The disadvantage is the noticeable delay of the retransmitted radio signal caused by regulations to correct the mistakes.
At the moment there is the following classification of orbits satellite repeaters.
Equatorial orbit satellite communications. A characteristic feature of the Equatorial geostationary orbit supports the approach underlying the proposed technology. The essence of the approach is that the angular velocity of the satellite repeater and Earth not only coincide, but are in the same direction. In other words, the direction of the satellite and the earth’s rotation are identical. The main advantage of the Equatorial orbit is that the earth receiver is constantly connected with the satellite. In this case, the satellite, if located in one place, so the radio waves do not meet obstacles.
The disadvantages of the proposed options for the treatment of satellite communications include the following:
– as to the orbit at the same time displays hundreds and thousands of different satellites, increases the risk of colliding them with each other, so you have to carefully calculate and control their trajectory.
– great height (about 36 thousand km) output satellites in orbit leads to significant delays in the transmission of useful information (the effect of delay of the radio signal);
– high altitude output of satellites into orbit requires significant material costs;
– the inability to service earth stations in the polar regions.
Inclined orbit satellite communication is a more complex movement in space and interaction of the satellite with earth stations.
Under the proposed scheme, earth stations are equipped with special tracking devices that facilitate search space of the repeater in earth orbit and provide correction of the rotation angle of the antenna mirrors. An important advantage of this approach is the option of continuous support of a companion. In other words, the earth station continuously monitors the location of the satellite and “leads” him in the sky. Innovation is fully justified in pre-emergency and force majeure situations, when the owners of satellites for various reasons do not control their location.
Polar orbit satellite is identified with a special case of the inclined orbit and involves a tilt to the plane of the equator of 90°.
The earth stations transmit a radio signal to the satellite within a certain range. The specificity of this process is because the range of frequencies for transmission of radio signal from earth station is different from the frequency spectrum of the signal retransmitted from the satellite. In other words, for transmission of a radio signal uses a single frequency band, and to relay another. This feature is explained by the fact that the layers of the atmosphere at different flow radio signal by activating the process of attenuation and the absorption of the signal. Frequency bands for satellite communications is determined by the “radio Regulations”, which takes into account the specificity of the “window of transparency for radio waves” the atmosphere, the level of interference and influence of other factors.
The frequency bands used in satellite communication, designated by special letters.
For L-band is allocated the frequency band 1, 5-1,6 GHz, the scope of mobile satellite communications (MSS).
For the S-band is allocated the frequency band 1, 9-2,2 and 2.4-2.5 GHz, the scope of use mobile satellite communications (MSS).
For C-band is allocated the frequency band 4-6 GHz, the scope is a fixed satellite communications (FSS).
For Ku-band is allocated the frequency band of 11, 12, 14 GHz, scope – fixed satellite communications (FSS) satellite broadcasting.
For K-band is allocated the frequency band of 20 GHz, the scope is a fixed satellite communications (FSS) satellite broadcasting.
For Ka-band is allocated the frequency band of 30 GHz, the scope is a fixed satellite communications (FSS) mobile satellite communications (MSS), the link between the satellites.
For the ENF-band allocated frequency band in 40-50 GHz, the scope is a fixed satellite communications (FSS) perspective.
Higher quality radio reception provides C-band, however, this requires an antenna with a larger diameter plates.
Model satellite transceiver operating in the range 4-6 GHz and covers the frequency band width of 36 MHz, which allows to provide relay 6 TV channels, or 3.6 thousand telephone channels. One satellite is usually set to 12 or 24 of the transceiver.
In the perspective of modern satellite communication system will consist of several subsystems:
– fixed satellite communications (FSS), designed to serve the interconnected communications network of the Russian Federation;
subsystem satellite television broadcasting and radio broadcasting;
subsystem mobile satellite communications (MSS), designed to service the needs of remote and mobile customers.
To satellite repeater could exploit many users adopt the technology of multiple access with frequency, code or time division.
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