Satellite Communication Overview of the Technology & the Antenna System Part IV
By John Dulaney
Key Issues
Looking ahead of 1990's, one could observe a very rapid expansion of global market in satellite communication into 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 direct broadcast satellite services. LEO satellite services were introduced towards the end of 1990's, and the growth depended on the competitive factors. The conventional Fixed Satellite services (FSS) and Maritime Mobile Satellite Services (MMSS) grew steadily but not as before.
Optical fiber cables, now forming a greater part of this communication revolution through out the world, severely challenged the fixed satellite services. Very high data rates, similar to High Dynamic Range (HDR) graphics, which requires greater than 155Mb per second of data transfer, which required excellent 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 services needed to prove its advantage on HDR applications and networking, having a more modest data rates, 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. The advantages include, wide area coverage, distance insensitivity, flexibility, multiple access and destination capabilities and economy. Although much of the HDR traffic, 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 support the emerging field of Distributed High Performance Computing (DHPC). To gain access to this application market, HDR satellites needed to be developed and deployed commercially.
It was clear by now that the world of satellite communication was changing fast and threats existed for fixed satellite services, while new opportunities opened up in mobile, broadcast and personal services. Presently, the US leadership in satellite communication 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 communication technology. The important reasons include, the governments reduced role, lagging R&D effort, lack of systems conceptualisation, non-focusing of effort in new applications, and lack of effective industrial liaison and co-operation. On record, the assessment shows that during 1970's and 1980's there was extremely limited activity in US in the area of satellite communications projects, while there were frequent 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 communication is essentially correct.
The setting up of policy, planning, and supporting industrial development in different countries varies widely, 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 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 communication 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 Communication Technologies
A brief discussion, relating to the assessment of satellite communication technology, is presented here.
The Antenna System
A component 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 information. 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 separate 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 direction than others. The directional property of an antenna is represented by its radiation pattern, 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 derived from the onboard electrical power generation in a satellite. Here there is a limitation on this power. A battery bank and solar cell panels, provide 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 provide the generation. A dark situation 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 related 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 communication, the difference between the highest frequency signal component and its lowest one, in terms of Hz, is the spectrum which is called the bandwidth of the signal. A typical 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 kHz1000 Hz
1MHz1000 kHz
1 GHz1000 MHz
Where,
k = Kilo
M = Mega
G = Giga
Staying in the subject of bandwidth, generally three types of bandwidths are utilised 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 up-linking signals from the Earth stations to the satellite and 11.7GHz and 12.7GHz and for down-linking 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 quality problems caused by rainfalls. This is known as ain-fading.
L-band frequencies range from 390MHz to 1,55GHz. Satellite communication and terrestrial communications 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 communication is the C-band frequency, which uses 3.7GHz to 4.2Ghz for down-linking signals to the Earth stations and 5.925GHz to 6.425Ghz for up-linking from the Earth stations. The lower frequency ranges in this band have a better performance under 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. 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 traditional geostationary satellite, having applications in fixed satellite services (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 structure. 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 pattern 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 structure, having electronics components for signal amplifications and signal conditioning circuitry. This feed antenna is mounted at the absolute center of the dish reflector antenna, with the horn facing the center of the dish. There could be multiple horns in such feed antenna.
Most of the Low Earth Orbit satellites have space constraint to have any of the type of parabolic antennas. Instead 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 reduction 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 sophisticated beaming 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 another 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 interesting to note that additional directors in these type of antennas increase directivity of the signals, where-as, addition of further reflectors makes no significant difference.
The gain of a Yagi antenna is controlled by the number of elements that it has. However, spacing the elements is also a design factor in terms of gain of such antenna. The design of the Yagi antenna has many inter-related variables, and earlier designs were not being able to achieve the full potential or performance of these antennas. Today's computer design has made a great impact of the design characteristics and greater improvement in performance has been achieved.
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