Moving Pictures And Data From Old Motorola To New One Satellite Communication Overview of the Technology & the Antenna System Part IV

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Satellite Communication Overview of the Technology & the Antenna System Part IV

Key issues

Looking ahead to the 1990s, one can observe a very rapid expansion of the global satellite communication market in personal communication and new mobile satellite services, such as Personal Communication System (PCS) and Mobile Satellite Services (MSS), respectively, Low Earth Orbit (LEO) satellite systems, Global Positioning System (GPS) navigation and new live 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) grew steadily, but not as before.

Fiber optic cables, now a major part of this worldwide communications revolution, seriously challenged fixed satellite services. Very high data rates, similar to high dynamic range (HDR) graphics, which require more than 155 Mb per second of data transfer, which requires excellent signal conditioning, were carried by cables of optical fibers. Fiber optic cables have a better performance than satellites, having much less time delay in transmission. There was a time when satellite services had to prove their edge in HDR applications and networks by having a more modest data rate, for example T1=1.5 Mb per second. A T-1 line actually consists of 24 individual channels, each of which supports data rates of up to 64 Kbit per second. Advantages include, wide area coverage, distance insensitivity, flexibility, multiple access and destination capabilities, and economy. Although most HDR traffic, such as multi-channel telephone trunks from satellites to cables, will be transmitted over fiber optic cables, new opportunities open up for HDR satellites to perform HDTV picture signal distribution and also to support the emerging field of Distribution. High Performance Computing (DHPC). To gain access to this application market, HDR satellites had to be developed and commercially deployed.

It was already clear that the world of satellite communications was changing rapidly and there were threats to fixed satellite services, while new opportunities opened up in mobile, broadcast and personal services. Currently, US leadership in satellite communications is being challenged, while it has undoubtedly been a leader in such technology and an agent of change in the past.

There is a reason why there has been a gloomy assessment of the future of the US in satellite communications technology. Important reasons include the diminishing role of governments, lagging R&D efforts, lack of systems conceptualization, lack of focus of efforts on new applications, and lack of effective industrial linkage and collaboration. According to the data, the assessment shows that during the 1970s and 1980s there was extremely limited activity in the US in the field of satellite communications projects, while there were various frequent research programs being developed in Europe and Japan. Although these projects are of a different technology and much less budgeted than the American ones, the general impression of the US losing ground in the field of satellite communications is essentially correct.

Policy making, planning and support for industrial development in different countries varies widely, with each country’s governments 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 US, with the resources for such development being much more dispersed. In fact, in the past ten years, NASA has spent far less on satellite communications than its counterparts, the Japanese National Space Development Agency (NASDA) or the European Space Agency (ESA), although NASA’s total budget is many times larger.

Satellite communication technologies

A brief discussion, regarding the assessment of satellite communication technology, is presented here.

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 which 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) that are received and transmitted by the two antennas are based on certain frequencies, and the received frequency is always different from the transmitted one. These two frequencies are kept separate because if they were the same, there would be conflict between the received and transmitted signals. These antennas are generally directional antennas, transmitting more power in one direction than others. The directional property of an antenna is represented by its radiation pattern, which is generally 3-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 more “gain”, the antenna can transmit a greater distance. This power is derived from electricity generation on board a satellite. There is a limitation to this power. A battery bank and solar cell panels provide power for the on-board satellite systems. The solar panels are active during daylight hours, as 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 comes between the satellite and the Sun, when the battery bank is turned on to supply the required power.

To know more about the antenna, let’s now look at some of the terms used in defining an antenna characteristic. First, radio signals received or transmitted by an antenna are related as frequencies and expressed in Hertz (Hz). Frequency has been named as 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 indicates the number of cycles 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 signal component of the highest and the lowest frequency, in terms of Hz, is the spectrum which is called the bandwidth of the signal. A typical audio signal has a bandwidth of 3 kHz, which means that the frequency of a sound lies within the 3 kilohertz bandwidth, while a television signal has a bandwidth of 6 MHz, about 2000 times wider than sound. Here, “k” and “M” indicate Kilogram and Mega respectively. To understand, the table below provides the conversions:

Table 1

I kHz 1000 Hz

1 MHz 1000 kHz

1 GHz 1000 MHz

Where,

k = kilogram

M = Mega

G = Giga

Staying on the topic of bandwidth, generally three types of bandwidth 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 connecting signals from Earth stations to the satellite and 11.7 GHz and 12.7 GHz and for downlinking from the satellite to Earth stations.

It has been mentioned above that the receiving and transmitting frequencies to and from the satellite are kept far apart to avoid any interference between the two. The higher the frequencies, the 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 390 MHz to 1.55 GHz. Satellite communication and terrestrial communications between satellite devices use this frequency band. Higher L-band frequencies are less susceptible to rain attenuation compared to Ku-band signals.

The original frequency band assigned to satellite communication is the C-band frequency, which uses 3.7 GHz to 4.2 GHz for downlink signals to Earth stations and 5.925 GHz to 6.425 GHz for uplink from Earth stations. The lower frequency ranges in this band have a better performance in bad weather conditions than the Ku-band frequencies. Variations of C-band frequencies are being used in different parts of the world and these are classified as extended C-band, super-extended C-band, INSAT C-band, etc. varies from 3 inches to 9 inches, depending on design parameters. Reflector antennas are mostly used in traditional geostationary satellites, having applications in the fixed satellite service (FSS) and the maritime mobile satellite service (MMSS). These are used to connect L-band, C-band and Ku-band, which require high gain antennas with parabolic dish structure. A reflector antenna is one that has a spherical wavefront, meaning that the signal radiations from the antenna are spherical in nature, one in which the energy is dispersed in all directions away from the antenna and produces a pattern that is not highly directional. A parabolic antenna is used specifically for high direction. These antennas are illuminated by an array of “feeder” antennas or indirectly through a system of sub-reflectors. A feed antenna will generally consist of a horn-type structure, having electronic components for signal amplification and signal conditioning circuitry. This feeder antenna is mounted at the absolute center of the dish reflector antenna, with the horn facing the center of the dish. There can be many horns on such a food antenna.

Most Low Earth Orbit satellites have space limitations to have any type of parabolic antenna. Instead they have antennae which are known as “Whip Antenna”. Of course there is a reduction in antenna gain compared to the reflector antenna as used with geosynchronous satellites. This loss of gain is offset by the reduced distance that such satellites orbit the Earth, being only 2,000 kilometers compared to 40,000 kilometers for geosynchronous satellites.

Ground antennas for satellites in low Earth orbit are generally of the Yagi or Helix design. Satellites in low Earth orbit use very low frequencies to receive and transmit signals, and dish antennas would be practically huge. There is not much difference between the requirement for a low-Earth orbit satellite and a geosynchronous satellite, and with the advent of modern systems, such as Motorola’s IRIDIUM, which require sophisticated signal beams, low-Earth orbit satellites may soon have phased arrays and reflector. antennas.

The Yagi antenna gets its name from two Japanese inventors, Yagi and Uda. This is why the antenna is also called a Yagi-Uda antenna. The invention was first published in 1928, which was introduced 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 dipole antenna. Yagi antennas are directed, along the axis perpendicular to its element plane, from the reflector to the driven parasitic elements. It is interesting to note that the additional directors in these types of antennas increase the directivity of the signals, while the addition of further reflectors does not make any significant difference.

The gain of a Yagi antenna is controlled by the number of elements it has. However, the distance between the elements is also a design factor in terms of the gain of such an antenna. Yagi antenna design has many interrelated variables, and previous designs were unable to achieve the full potential or performance of these antennas. Today’s computer design has made a huge impact on design features and greater improvement in performance has been achieved.

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