Orbita


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Orbita

 

the conventional name for the terrestrial space communications stations that form an integrated network throughout the USSR; they transmit and receive for relay black-and-white and color programs of the Central Television Studios through Molniia communications satellites. The first 20 stations in the network were put into service in 1967; by 1973 their number had increased to 40.

With the establishment of the Orbita system, television centers in many remote parts of the country were accorded the possibility of broadcasting the first or second program from the Central Television Studios in addition to programs received over cable and radio-relay links. Molniia 1 satellites operating on decimeter wavelengths were originally used in the Soviet space communications system. In 1972, Orbita 2 stations operating on centimeter wavelengths with Molniia 2 satellites were also put into service. By May 1973, broadcasts from Moscow were being received by 11 Orbita 2 stations (the construction of another 25 stations is planned for 1974–75). The current space communications system of the USSR is called Molniia-Orbita. In addition to relaying television programs, the system is also used for two-way (duplex) exchange or one-way transmission of other types of information. Orbita operates throughout the USSR. The length of communications sessions through each Molniia satellite is eight to ten hours per day.

Television signals broadcast to the Molniia satellites by the central earth stations of the Orbita network are received by the satellites, amplified, and rebroadcast to earth. The received signals travel over trunk lines to local television centers, from which they are broadcast over one of the channels allotted to the television center in the meter and decimeter bands. A single-hop radio-relay link is usually used as the connecting line. For distances less than 1 km, cable lines with matchers, correctors, and antihum devices are also used.

Orbita stations are housed in standard round reinforced-concrete structures that also serve as the antenna supports for the system. All receiving equipment, satellite tracking equipment, and trunk lines are concentrated in the central room. Adjacent rooms contain the ventilation and airconditioning systems, the electric antenna drive, power equipment, and so on. A parabolic reflector antenna 12 m in diameter is mounted on a rotator and is moved by drives with respect to azimuth and elevation, tracking the satellite with high precision (within a few minutes of arc). Control of satellite tracking may be automatic—according to the television signal from the satellite or by means of a programmer —or manual. The antenna is capable of operating normally under the severe climatic conditions of the Far North, Siberia, the Far East, and Middle Asia without a windbreak. The noise temperature of the antenna when aimed at the zenith does not exceed 10°K.

The frequency-modulated (FM) signal received by the station antenna is fed to the input of the receiving complex, a parametric amplifier. To achieve maximum sensitivity, the first stages of the amplifier are cooled to the temperature of liquid nitrogen (77°K). The signal travels from the parametric amplifier output to a frequency converter and then to a radio-frequency preamplifier. Primary amplification of the signals received (by a factor of up to 107), with retention of the linearity of phase characteristics, is accomplished subsequently in a highly selective radio-frequency amplifier tuned to an intermediate frequency of 70 megahertz (MHz). Subsequent detection of the FM signals is accomplished by an interference-free demodulator, a synchronous phase detector. Since the sound signals are transmitted by means of time-division multiplexing in the same frequency band as the video signal, the receiving complex contains video and sound signal separation equipment. The Orbita receiving complex also includes monitoring equipment for immediate testing of the operability of all components and for measuring qualitative indexes. The equipment of the receiving complex has a 100-percent reserve that makes it possible to switch automatically from the primary equipment to the back-up system in case of emergency.

N. V. TALYZIN

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