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Satellite Communications Systems


In 1945, the article «Extraterrestrial repeaters» ( «Extra-terrestrial Relays»), published in the October issue of the journal «Wireless World», English scholar, writer and inventor Arthur C. Clarke proposed the idea of establishing a system of communications satellites in geostationary orbits, which would organize a global system of communication. Subsequently, Clark to question why he had not patented the invention (which was quite possible), answered that it was not believed in the possibility of implementing such a system, with its life, and thought that this idea should benefit all mankind.

The first studies in the field of satellite communications began to appear in the second half of the 50-ies of the XX century. The incentive to have increased demand for trans-Atlantic telephone service. August 20, 1964 11 countries signed an agreement establishing the International Telecommunications Satellite Organization Intelsat (International Telecommunications Satellite organization), but the Soviet Union in their number is not included. April 6, 1965, the program was launched the first commercial communications satellite Early Bird ( «early bird»), a corporation made COMSAT.

By today's standards satellite Early Bird (INTELSAT I) had more than modest: having bandwidth 50 MHz, it can provide up to 240 telephone channels. At any given time communication can take place between the earth station in the U.S. and only one of three earth stations in Europe (Britain, France or Germany), which were connected by cable lines.

In the future, technology stepped forward, and satellite INTELSAT IX has had a 3456 MHz bandwidth.
Despite the fact that the first artificial Earth satellite was launched in the USSR in 1957, the development of satellite communications in the socialist countries has been somewhat delayed. Agreement between the 9 countries of the socialist bloc to establish a communications system «INTERSPUTNIK» was signed only in 1971

Orbits of satellite transponders

Orbits, which are posted on the satellite transponder is divided into three classes:
1) equatorial,
2) slope,
3) polar.

An important variant of the equatorial orbit is a geostationary orbit at which the satellite is rotating with angular velocity equal to the angular velocity of the Earth in the direction coincides with the rotation of the Earth. The obvious advantage of the geostationary orbit is that the receiver is in the service area «sees» the satellite continuously.

However, a geostationary orbit, and satellites put into it is not possible. Another drawback is its high altitude, and therefore a great price withdrawal satellite into orbit. Furthermore, a geostationary satellite is unable to serve the earth stations in the circumpolar area.

Slant orbit allows to solve these problems, however, due to the movement of the satellite on the ground observer, it is necessary to run at least three satellites in one orbit, to provide round-the-clock access to communications.

Polar orbit - a limiting case of an inclined (with an inclination of 90 º).

When using inclined orbits of the earth stations are equipped with surveillance systems that undertake the antenna pointing at the moon. Stations operating with satellites in geostationary orbit, as a rule, are also equipped with such systems is to compensate for the deviation from the ideal geostationary orbit. The exception is the small antennas used to receive satellite television: the diagram is wide, so they do not feel the vibrations near the ideal point of the satellite.

Frequency reuse. Coverage.

Because radio frequencies are a limited resource, it must be possible using the same frequencies of the different earth stations. You can do this in two ways:

1) spatial separation - each satellite antenna takes the signal from a given area, with different areas may use the same frequency.

2) polarization separation - different take and pass the antenna signal into mutually perpendicular planes of polarization, with the same frequency can be applied twice (one for each of the planes).

A typical map coverage for satellites in geostationary orbit, includes the following components:

1) a global beam - makes communication with earth stations throughout the area covered, they are the frequencies do not overlap with other beams of the satellite.

2) the rays of the western and eastern hemisfer - these rays are polarized in the plane A, in the western and eastern hemisferah uses the same frequency band.

3) gap rays - polarized in the plane B (perpendicular to A) and use the same frequency as the rays hemisfer. Thus, the earth station, located in one of these areas can also use the rays hemisfer and the global beam.

When all frequencies (except for reserved for the global beam) are used repeatedly: in the western and eastern hemisferah in each of the zones.

for transmission over the satellite signal has to be to modulate. Modulation is performed in the earth station. Modulated signal is amplified, is carried to the desired frequency and enters the transmitter antenna. Satellite transponder can be neregenerativnymi and regenerative. Neregenerativny satellite, taking the signal from one earth station, transferring it to another frequency, amplifies and transfers to another earth station. The satellite can use several independent channels, carrying out these operations, each of which operates with a certain part of the spectrum (these channels are called transponders processing).

Frequency Bands

The choice of frequencies for the transmission of data from earth station to satellite and from satellite to earth station is not arbitrary. From the frequency dependent, for example, the absorption of radio waves in the atmosphere, as well as the required size of the transmitting and receiving antennas. The frequencies at which the transmission of satellite earth station to differ from the frequencies used for transmission from the satellite to earth station (usually the first above).

The frequencies used in satellite communications, is divided into bands, labeled. Unfortunately, the exact boundaries of the literature of different ranges can not overlap. Approximate values are given in Recommendation ITU-R V.431-6

Name range frequencies (according to ITU-R V.431-6) Application
L 1,5 GHz Mobile Satellite
S 2,5 GHz Mobile Satellite
P 4 GHz, 6 GHz Fixed satellite communications
X satellite recommendations ITU-R frequency is not defined. For radar applications, given the range of 8-12 GHz. Fixed satellite (for military purposes)
Ku 11 GHz, 12 GHz, 14 GHz Fixed satellite communications, satellite broadcasting
K 20 GHz Fixed satellite communications, satellite broadcasting
Ka 30 GHz Fixed satellite communications, intersatellite link


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