Satellite Communication Overview of the Technology & the Antenna System Part IV
Key Issues
Looking ahead of 1990′s, one could abide by a very rapid additional room of global market in satellite interaction into personal interaction and new mobile satellite air force, such as Personal Interaction System (PCS) and Mobile satellite Air force (MSS) respectively, Low Earth Orbit (LEO) satellite systems, Global Positioning System (GPS) navigation, and new direct announce satellite air force. LEO satellite air force were introduced towards the end of 1990′s, and the growth depended on the competitive factors. The square Fixed Satellite air force (FSS) and Maritime Mobile Satellite Air force (MMSS) grew steadily but not as before.
Optical fiber cables, now forming a greater part of this interaction revolution through out the world, severely challenged the fixed satellite air force. Very high data rates, similar to High Dynamic Range (HDR) graphics, which requires greater than 155Mb per second of data conveying, which required exceptional signal conditioning, were being carried by the fiber optics cables. Fiber optic cables have a better performance than satellites, having much less time delay in transmission. It was a time when satellite air force needed to prove its benefit on HDR applications and networking, having a more modest data rates, for example T1=1.5Mb per second. A T-1 line in fact consists of 24 individual channels, each of which supports up-to 64Kbits per second data rate. The advantages include, wide area coverage, distance insensitivity, flexibility, manifold access and destination capabilities and nation. Although much of the HDR conveying, such as multi-channel telephone trunks, from satellites to cables, will be transmitted through fiber optics cables, new opportunities opened up for HDR satellites to carry HDTV picture signal distribution, and also help the emerging field of Spread High Performance Computing (DHPC). To gain access to this attention market, HDR satellites needed to be developed and deployed commercially.
It was clear by now that the world of satellite interaction was varying quick and threats existed for fixed satellite air force, while new opportunities opened up in mobile, announce and personal air force. Presently, the US leadership in satellite interaction is being challenged, while it was undoubtedly the leader of such technology and was an agent of the changes in the past.
There are reasons as to why there has been a bleak assessment of the future of US in satellite interaction technology. The vital reasons include, the governments cut-rate role, sheathing R&D effort, lack of systems conceptualisation, non-focusing of effort in new applications, and lack of effectual industrial liaison and co-operation. On record, the assessment shows that during 1970′s and 1980′s there was extremely restricted activity in US in the area of satellite exchanges projects, while there were normal diverse research programs that were going on in Europe and Japan. Although these projects are of a different technology and much less budgeted than the US ones, the overall impression of US losing ground in the area of satellite interaction is essentially right.
The background up of policy, plotting, and supporting industrial development in different countries varies widely, with the governments of each people playing a key role in such activities. The policies and plotting of the governments in Europe and Japan are far more aggressive than that of US, with the resources for such development being far more deployed. In-fact, in the last ten years, NASA has spent much less in satellite interaction than its counterparts, the Japanese National Space Development Agency (NASDA) or the European Space Agency (ESA), although NASA’s total budget is many times greater.
Satellite Interaction Technologies
A brief discussion, concerning to the assessment of satellite interaction technology, is presented here.
The Antenna System
A element of active transmitter and receiver, 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 necessary in rank. On the other hand, a transmitting antenna converts electrical signal into radio waves and transmits them to the Earth stations. The radio waves (signals) received and transmitted by the two antennas are based on certain frequencies and the receive frequency is always different from the transmitted one. These two frequencies are kept break owing to the reason that if they were the same, there would conflict between the received and transmitted signals. These antennas are generally directional antenna, transmitting more power in some management than others. The directional material goods of an antenna is represented by its radiation sample, which are generally 3-dimensional.
An antenna needs power to transmit. This power lets the antenna transmit over greater distances. This ability to transmit depends on the “gain” of the antenna. The more the “gain,” the antenna can transmit a greater distance. This power is consequential from the onboard electrical power age group in a satellite. Here there is a limitation on this power. A battery bank and solar cell panels, grant power to the onboard satellite systems. The solar panels are active during the sun-light times, as it powers the satellite systems and charges the battery bank as well. In dark the solar system cannot work and the battery bank starts to grant the age group. A dark circumstances occurs when the Earth comes in-between the satellite and the Sun, when the battery bank switches on to supply the power required.
In order to know more about antenna, let us now look at some of the terms used in defining an antenna characteristic. First, the radio signals received or transmitted by an antenna is correlated as frequencies and expressed in Hertz (Hz). Frequency has been names as Hertz (Hz), after Heinrich Rudolf Hertz (1847-1894), who was first to transmit and receive radio waves. Hertz is a measure of the frequency and denotes the number of cycles that a signal undergoes in a second. For example, if a signal makes a complete cycle in one second, that is measured as 1Hz. As for the term Bandwidth in the concept of radio interaction, the difference between the peak frequency signal element and its lowest one, in terms of Hz, is the spectrum which is called the bandwidth of the signal. A predictable voice signal has a bandwidth of 3 kHz, that is to say that the frequency of a voice lies within 3 kilo hertz bandwidth, where-as the TV signal has a bandwidth of 6MHz, some 2,000 times as wide as the voice. In here, “k” and “M” denote kilo and Mega respectively. For understanding, the table below provides the conversions:
Table 1
I kHz 1000 Hz
1MHz 1000 kHz
1 GHz 1000 MHz
Where,
k = Kilo
M = Mega
G = Giga
Staying in the theme of bandwidth, generally three types of bandwidths are utilised in satellite interaction 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 up-between signals from the Earth stations to the satellite and 11.7GHz and 12.7GHz and for down-between from the satellite to the Earth stations.
It has been mentioned above, that receiving and transmitting frequencies, to and from the satellite are kept wide apart, to avoid any interference between the two. The higher frequencies, Ku-band frequencies are significantly more susceptible to signal feature problems caused by rainfalls. This is known as “rain-fading.”
L-band frequencies range from 390MHz to 1,55GHz. Satellite interaction and terrestrial exchanges between satellite equipment uses this band of frequencies. L-band higher frequencies are less susceptible to rain-fading compared to Ku-band signals.
The original frequency band allocated for satellite interaction is the C-band frequency, which uses 3.7GHz to 4.2Ghz for down-between signals to the Earth stations and 5.925GHz to 6.425Ghz for up-between from the Earth stations. The lower frequency ranges in this band have a better performance under terrible weather circumstances 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. C-band requires a larger Earth station dish antenna, varying between 3 inches to 9 inches, depending on the design parameters. Reflector antennas are mostly used in habitual geostationary satellite, having applications in fixed satellite air force (FSS) and maritime mobile satellite service (MMSS). These are used to link L-band, C-band and Ku-band, which require high gain antennas with parabolic dish organize. A reflector antenna is the one which has a spherical wave-front, which means that the radiations of the signals from the antenna are spherical in nature, one in which the energy spreads out in all directions away from the antenna and produces a sample 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 type organize, having electronics gears for signal amplifications and signal conditioning circuitry. This feed antenna is mounted at the pledge center of the dish reflector antenna, with the horn facing the center of the dish. There could be manifold horns in such feed antenna.
Most of the Low Earth Orbit satellites have space restriction to have any of the type of parabolic antennas. As a substitution for they have antennas which are known as “Whip Antenna.” There is ofcourse a shrink in the gain of the antenna in comparison to the reflector antenna as used with the geosynchronous satellites. This loss of gain is compensated by the saving in the distance that such satellites orbit the Earth, being just 2,000 kilo meters as compared to 40,000 kilo meters for the geosynchronous satellites.
The ground antennas for the low Earth orbiting satellites are of generally Yagi or Helix design. Low Earth orbiting satellites use very low frequencies in receiving and transmitting signals and the dish antennas would be impractically large. There is not much of a difference between the requirement of a low Earth orbiting satellite and a geosynchronous one and with the advent of modern systems, like Motorola’s IRIDIUM, that require refined sunny of signals, low Earth orbiting satellites may soon have phased arrays and reflector antennas.
The Yagi antenna derives its name from two Japanese inventors, Yagi and Uda. This is the reason why the antenna is also referred to as 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 a additional element, a reflector, slightly larger in length than that of a dipole. This arrangement gives antenna better directional characteristic than a single dipole antenna. Yagi antennas are directional, along the axis perpendicular to its plane of elements, from the reflector to the driven parasitic elements. It is fascinating to note that additional directors in these type of antennas increase directivity of the signals, where-as, addition of additional reflectors makes no noteworthy difference.
The gain of a Yagi antenna is restricted by the number of elements that it has. But, spacing the elements is also a design factor in terms of gain of such antenna. The design of the Yagi antenna has many inter-correlated variables, and before designs were not being able to achieve the full the makings or performance of these antennas. Today’s computer design has made a fantastic impact of the design characteristics and greater improvement in performance has been achieved.
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Author: John Dulaney
Article Source: EzineArticles.com
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