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Satellite Communication Overview of the Technology & the Antenna System Part IV
The key issues
In view of the 1990s, one could observe a very rapid expansion of the global market in satellite communication in personal communication and new mobile satellite services, such as the Personal Communication System (PCS) and mobile satellite services ( MSS) respectively, low Earth orbit (LEO). satellite systems, Global Positioning System (GPS) navigation, and new direct broadcast satellite services. LEO satellite services were introduced in the late 1990s, and growth depended on competitive factors. Conventional fixed satellite services (FSS) and maritime mobile satellite services (MMSS) have grown steadily, but not as before.
Fiber optic cables, which now form a larger part of this worldwide communications revolution, are severely challenging fixed satellite services. Very high data rates, similar to High Dynamic Range (HDR) graphics, requiring more than 155 Mb per second of data transfer, which required excellent signal conditioning, were carried by fiber optic cables. Fiber optic cables have better performance than satellites, having much less transmission delay. It was a time when satellite services needed to prove their advantage over HDR applications and the network, having a more modest data rate, for example T1 = 1.5Mb per second. A T-1 line actually consists of 24 individual channels, each of which supports up to 64Kbits per second data rate. Advantages include wide area coverage, distance insensitivity, flexibility, multiple access and targeting capabilities and economy. Although much of the HDR traffic, such as multichannel telephone trunks, from satellites to cables, will be transmitted via fiber optic cables, new opportunities are opening up for HDR satellites to bring the distribution of HDTV picture signals, and also supports the emerging field of Distributed. High Performance Computing (DHPC). To have access to this application market, HDR satellites needed to be developed and deployed commercially.
It was clear now that the world of satellite communication was changing rapidly and threats existed for fixed satellite services, while new opportunities were opening up in mobile, broadcast and personal services. Currently, the leadership of the United States in satellite communication is challenged, while he was undoubtedly the leader of such technology and was an agent of changes in the past.
There are reasons why there has been a gloomy assessment of the future of the United States in satellite communication technology. Important reasons include, the reduced role of governments, lagging R&D effort, lack of systems design, lack of concentration of effort in new applications, and lack of effective industrial linkage and cooperation. On the record, the assessment shows that during the 1970s and 1980s there was extremely limited activity in the United States in the area of satellite communications projects, while there were frequent various research programs that took place in Europe and in Japan. Although these projects are of a different technology and much less budgeted than those of the United States, the general impression that the United States is losing ground in the area of satellite communication is essentially correct.
The establishment of policy, planning and support for industrial development in different countries varies greatly, with the governments of each country playing a key role in such activities. The policies and planning of the governments in Europe and Japan are much more aggressive than those of the United States, with the resources for such development being much more implemented. In fact, over the past ten years, NASA has spent much less on satellite communications than its counterparts, the Japanese National Space Agency (NASDA) or the European Space Agency (ESA), even though the total budget of NASA is several times larger.
Satellite communication technologies
A brief discussion, related to the assessment of satellite communication technology, is presented here.
The Antenna System
An active transmitter and receiver component, the antenna is a transducer between electromagnetic waves in space and voltages or currents in a transmission line. The receiving antenna transforms the receiving radio waves into electrical signals that are processed for the necessary information. On the other hand, a transmitting antenna converts the electrical signal into radio waves and transmits them to the stations on Earth. The radio waves (signals) received and transmitted by the two antennas are based on certain frequencies and the receiving frequency is always different from the transmitted one. These two frequencies are kept separate because if they were the same, there would be a conflict between the received and transmitted signals. These antennas are usually directional antennas, transmitting more power in a certain direction than others. The directional property of an antenna is represented by its radiation pattern, which is usually three-dimensional.
An antenna needs power to transmit. This power allows the antenna to transmit over greater distances. This ability to transmit depends on the “gain” of the antenna. The higher the “gain”, the greater the distance the antenna can transmit. This power is derived from the generation of electricity on board a satellite. Here there is a limitation of this power. A bank of batteries and solar cell panels, provides energy to the satellite systems on board. The solar panels are active during sunny times, because they power the satellite systems and also charge the battery bank. In the dark, the solar system cannot work and the battery bank starts to provide the generation. A dark situation occurs when the Earth is between the satellite and the Sun, when the battery bank turns on to provide the necessary power.
To learn more about the antenna, let’s now look at some of the terms used in the definition of an antenna characteristic. First, the radio signals received or transmitted by an antenna are related as frequencies and expressed in Hertz (Hz). The frequency was named Hertz (Hz), after Heinrich Rudolf Hertz (1847-1894), who was the first to transmit and receive radio waves. Hertz is a measure of frequency and denotes the number of cycles that a signal undergoes in one second. For example, if a signal makes one complete cycle in one second, it is measured as 1Hz. Regarding the term Bandwidth in the concept of radio communication, the difference between the component of the highest frequency signal and its lowest, in terms of Hz, is the spectrum that is called the bandwidth of the signal. A typical voice signal has a bandwidth of 3 kHz, meaning that the frequency of a voice is in a bandwidth of 3 kilo hertz, where the TV signal has a bandwidth of 6MHz, about 2000 times more wide of the voice. Here, “k” and “M” denote kilo and Mega respectively. For your understanding, the table below provides the conversions:
I kHz 1000 Hz
1 MHz 1000 kHz
1 GHz 1000 MHz
k = kilo
M = Mega
G = Giga
Staying in the subject of bandwidth, generally three types of bandwidths are used in satellite communication and these are, Ku-band, L-band and C-band. The Ku band uses frequencies from 14 Giga Hertz to 14.5 Giga Hertz (see Table 1), for the up-linking signals from the Earth stations to the satellite and 11.7GHz and 12.7GHz and for the down- link from the satellite to the Earth stations. .
It was mentioned above, that the reception and transmission frequencies, from and to the satellite are kept widely separated, to avoid any interference between the two. The higher frequencies, Ku band frequencies are significantly more susceptible to signal quality problems caused by precipitation. This is known as “rain-fading”.
L-band frequencies range from 390MHz to 1.55GHz. Satellite communication and terrestrial communication between satellite equipment use this frequency band. Higher L-band frequencies are less susceptible to rain extinction compared to Ku-band signals.
The original frequency band allocated for satellite communication is the C-band frequency, which uses 3.7GHz to 4.2Ghz for down-linking signals to Earth stations and 5.925GHz to 6.425Ghz for up-linking from the stations of the Earth. Lower frequency ranges in this band perform better in bad weather conditions than Ku-band frequencies. Variations of C-band frequencies are used in different parts of the world and these are classified as, extended C-band, super-extended C-band, INSAT C-band, etc. C band requires a larger satellite dish than the ground station, ranging from 3 inches to 9 inches, depending on the design parameters. Reflector antennas are mainly used in traditional geostationary satellites, having applications in fixed satellite services (FSS) and maritime mobile satellite services (MMSS). These are used to link L band, C band and Ku band, which require high gain antennas with parabolic structure. A reflector antenna is one that has a spherical wavefront, which means that the signal radiation from the antenna is spherical in nature, one in which the energy spreads in all directions away from the antenna and produces a model that is not very directional. A parabolic antenna is specifically used for high directivity. These antennas are illuminated by a set of “feed” antennas or indirectly through a system of sub-reflectors. A feed antenna will generally consist of a horn structure, which has electronic components for signal amplification and signal conditioning circuits. This feed antenna is mounted at the absolute center of the reflector antenna of the dish, with the horn facing the center of the dish. There could be several horns in such a feeding antenna.
Most Low Earth Orbit satellites have limited space to have any type of satellite dishes. Instead they have antenna that are known as “Whip Antenna”. There is of course a decrease in antenna gain compared to the reflector antenna as used with geosynchronous satellites. This loss of gain is compensated by the reduction of the distance that such satellites orbit the Earth, being only 2,000 kilo meters compared to 40,000 kilo meters for geosynchronous satellites.
Ground antennas for low Earth orbit satellites are usually of Yagi or Helix design. Satellites in low Earth orbit use very low frequencies to receive and transmit signals and the dish antennas will be impractically large. There is not much difference between the requirements of a satellite in low Earth orbit and a geosynchronous one and with the advent of modern systems, such as Motorola’s IRIDIUM, which require a sophisticated beam of signals, the satellites in low Earth orbit they could soon provide phased arrays and reflectors. antennas.
The Yagi antenna derives its name from two Japanese inventors, Yagi and Uda. That is why the antenna is also called a Yagi-Uda antenna. The invention was first published in 1928, which was presented by Yagi himself. This type of antenna consists of an array of a dipole and additional parasitic elements. There is another element, a reflector, slightly larger in length than that of a dipole. This arrangement gives the antenna a better directional characteristic than a single dipole antenna. The Yagi antennas are directional, along the axis perpendicular to the plane of its elements, from the reflector to the guided parasitic elements. It is interesting to note that the additional directors in these types of antennas increase the directivity of the signals, where the addition of more reflectors does not make a significant difference.
The gain of a Yagi antenna is controlled by the number of elements it has. However, the spacing of the elements is also a design factor regarding the gain of such antennas. The design of the Yagi antenna has many inter-related variables, and previous designs have not been able to achieve the full potential or performance of these antennas. The design of today’s computer has made a great impact of the design features and a greater improvement in performance has been achieved.
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