Direct-broadcast satellite
A broadcast satellite, or television satellite ( also english Direct Broadcasting Satellite or Direct Broadcast Satellite, short DPS), is a communications satellite for the transmission of broadcast content. Broadcasting satellites are used both for satellite coverage as well as for the transmission of broadcast programs to the audience ( often by injection into a cable television network ).
The offer to transfer shipments directly to the final recipient, it is called english Direct to Home, DTH service shortly.
In Europe, almost all geostationary satellites are used as broadcast satellite. This is usually not a single satellite, but a satellite group, which consists of several satellites and is summarized in a satellite position, actually orbital position.
Technology
The width of each transponder channel is tuned in pure satellite television for analog reception to the bandwidth of an analog television program and therefore is typically 27 MHz. The Astra system, a channel width of 33 MHz is used in the digital -used 12 - GHz range, because it allows a higher data transfer rate than the narrower originally intended for analog broadcasts transponders in the 11 GHz range. However, these are now used digitally.
In television satellites, which were originally planned as a so-called Fernsehverteilsatelliten to supply cable networks with analog channels, a channel width of 36 MHz was chosen because of the inevitable loss of quality in these long nets with their many amplifiers to increase the image quality. However, some of these satellites was also connected for direct satellite reception significance. There, however, the higher bandwidth of the signals did not matter.
Have some of the satellite transponder as broadband channels (72 MHz or more), that on each channel even two analog television programs have been transmitted.
Use the TV satellites in order to exploit the available frequency range better as communications satellites in successive channels different polarization, whereby the channels may overlap partially. The rotation of the polarization plane by the Faraday effect when passing through the ionosphere is negligible because of the very high transmission frequency.
History
In Europe in the late 1970s was determined that broadcast satellite should provide only the respective country of origin with television and radio programs. Each country was assigned along with other countries one orbital position, exclusive five frequencies, the polarization direction, the transmit power and tailored to the size and shape of each country illumination zone. They had compared to communications satellites extremely high transmit power per channel possess, so that the viewer despite the then much worse receiver with only a 60 cm parabolic antenna could get along. The so- scheduled TV Satellite high power (so-called high-power DBS satellites) should radiate from the 1980s, five additional TV programs or a mixture of television and radio programs (several radio programs per channel) for each country.
Should broadcast satellite, the plans that have been set on the World Admistrative Radio Converence 1977 ( WARC77 ) send the so-called Broadcasting Satellite Services area of Ku- band and use circular polarization. In contrast, communications satellite or communications satellite use in the Fixed Satellite Services and SMS band called area of Ku- band linear polarization.
Each state was allocated (frequency range of 11.7 -12.5 GHz) for its DBS satellite on a satellite position five frequencies = transponder including a cut to size to his country illumination zone in the BSS band. The BSS band was again divided into a lower ( 11.7 to 12.1 GHz) and an upper half-band ( 12.1 to 12.5 GHz). Normally, a state was assigned his five frequencies in one of these two half- bands. About each transponder a TV program or several radio programs could be transferred. In order not to interfere with the reception of other DBS satellite positions, the positions of the satellite DBS satellite had to be at least six degrees of longitude apart. For up to eight different states at any given satellite position five frequencies were allocated, since as many satellites can be stationed on one position. In order to receive the technology at that time at all with the small satellite dishes of 90 cm diameter satellite, these extremely high transmit power per transponder had to use. In the center of the footprint, which surrounds the country, a power density of -100 dBW / m² and at the edge of the footprint should be -103 dBW / m² reached. The satellite could only be aligned with an alignment accuracy of 0.1 ° to the target area, which is why the safety the footprint state lines with a minimum distance of 70 km enclosed to prevent border areas of the state by inaccuracies of alignment no longer in the footprint were. In a zone around the footprint should rise to around 1.8 m at a Leistungsfußdichte of up to -111 dBW / m of the antenna diameter needed. ( Furthermore, it was also allows a state the frequencies used for satellite broadcasts terrestrially not used )
The required high transmit power for this high flux density at the soil led to Satellite with high power were very big, heavy and expensive and that most countries could not afford / wanted. Most of these satellites therefore already failed in the planning phase. Only a few, such as, financed by the German Federal Government and the French Government TV - SAT and TDF project were realized, but then failed in viewer popularity at the low program number and the new television standard D2 - MAC, which are introduced with the same should. The TV - SAT, TDF, Tele-X and Marco Polo satellites being tracked, the concept of Direct Broadcasting Satellite, described at the time in office as German broadcasting satellite.
Before these satellites were launched, much less noisy signal converter with HEMT amplifiers were developed. In the DBS satellite thereby also the reception at a window with 40 cm antenna was possible. But the reception with 60 to 90 cm antennas in the zone around the illumination area was thus possible.
Satellite low transmission power, such as DFS Copernicus, who could not prevail, because something larger antennas than needed for the satellite medium transmission power for their reception. In these, an antenna having a diameter of 60 to 90 cm for receiving was sufficient.
So finally put satellites average transmission power, such as Astra, by. They send in the frequency domain and with the polarization directions that were originally used by communications satellites. Therefore, they were referred to initially officially as communications satellites. Because the orbital positions for communications satellites are awarded only to a country, and thus the entire prescribed for communication satellite frequency range becomes available, this television satellites can radiate their television and radio programs across dozens of channels. However, this is only possible because the transmission power of each channel is not as high as for the high power satellites, and thus the power generating solar cell ranges for these many transponders.
Originally, communication satellites, who worked in the FSS and SMS area of Ku- band, be used only for message delivery, or for supplying programs on cable networks or SNG. Because of the larger program number, the radio reception via satellite in the FSS band prevailed. Today this Ku-band is being used by this communication satellite.