| Key Issues | | | | Mega respectively. For understanding, the table |
| Looking ahead of 1990's, one could observe a | | | | below provides the conversions: |
| very rapid expansion of global market in satellite | | | | Table 1 |
| communication into personal communication and | | | | I kHz 1000 Hz |
| new mobile satellite services, such as Personal | | | | 1MHz 1000 kHz |
| Communication System (PCS) and Mobile satellite | | | | 1 GHz 1000 MHz |
| Services (MSS) respectively, Low Earth Orbit | | | | Where,k = Kilo |
| (LEO) satellite systems, Global Positioning System | | | | M = Mega |
| (GPS) navigation, and new direct broadcast | | | | G = Giga |
| satellite services. LEO satellite services were | | | | Staying in the subject of bandwidth, generally |
| introduced towards the end of 1990's, and the | | | | three types of bandwidths are utilised in satellite |
| growth depended on the competitive factors. The | | | | communication and these are, Ku-band, L-band |
| conventional Fixed Satellite services (FSS) and | | | | and C-band. The Ku-band uses frequencies from |
| Maritime Mobile Satellite Services (MMSS) grew | | | | 14 Giga Hertz to 14.5 Giga Hertz (see Table 1), |
| steadily but not as before. | | | | for up-linking signals from the Earth stations to |
| | | | the satellite and 11.7GHz and 12.7GHz and for |
| It was clear by now that the world of satellite | | | | down-linking from the satellite to the Earth |
| communication was changing fast and threats | | | | stations. |
| existed for fixed satellite services, while new | | | | It has been mentioned above, that receiving and |
| opportunities opened up in mobile, broadcast and | | | | transmitting frequencies, to and from the satellite |
| personal services. Presently, the US leadership in | | | | are kept wide apart, to avoid any interference |
| satellite communication is being challenged, while it | | | | between the two. The higher frequencies, |
| was undoubtedly the leader of such technology | | | | Ku-band frequencies are significantly more |
| and was an agent of the changes in the past. | | | | susceptible to signal quality problems caused by |
| There are reasons as to why there has been a | | | | rainfalls. This is known as "rain-fading." |
| bleak assessment of the future of US in satellite | | | | L-band frequencies range from 390MHz to |
| communication technology. The important reasons | | | | 1,55GHz. Satellite communication and terrestrial |
| include, the governments reduced role, lagging | | | | communications between satellite equipment uses |
| R&D effort, lack of systems | | | | this band of frequencies. L-band higher frequencies |
| conceptualisation, non-focusing of effort in new | | | | are less susceptible to rain-fading compared to |
| applications, and lack of effective industrial liaison | | | | Ku-band signals. |
| and co-operation. On record, the assessment | | | | The original frequency band allocated for satellite |
| shows that during 1970's and 1980's there was | | | | communication is the C-band frequency, which |
| extremely limited activity in US in the area of | | | | uses 3.7GHz to 4.2Ghz for down-linking signals to |
| satellite communications projects, while there | | | | the Earth stations and 5.925GHz to 6.425Ghz for |
| were frequent diverse research programs that | | | | up-linking from the Earth stations. The lower |
| were going on in Europe and Japan. Although | | | | frequency ranges in this band have a better |
| these projects are of a different technology and | | | | performance under bad weather conditions than |
| much less budgeted than the US ones, the overall | | | | the Ku-band frequencies. Variations of C-band |
| impression of US losing ground in the area of | | | | frequencies are being used in different parts of |
| satellite communication is essentially correct. | | | | the world and these are classified as, Extended |
| The setting up of policy, planning, and supporting | | | | C-band, Super Extended C-Band, INSAT C-Band, |
| industrial development in different countries varies | | | | etc. C-band requires a larger Earth station dish |
| widely, with the governments of each country | | | | antenna, varying between 3 inches to 9 inches, |
| playing a key role in such activities. The policies | | | | depending on the design parameters. Reflector |
| and planning of the governments in Europe and | | | | antennas are mostly used in traditional |
| Japan are far more aggressive than that of US, | | | | geostationary satellite, having applications in fixed |
| with the resources for such development being | | | | satellite services (FSS) and maritime mobile |
| far more deployed. In-fact, in the last ten years, | | | | satellite service (MMSS). These are used to link |
| NASA has spent much less in satellite | | | | L-band, C-band and Ku-band, which require high |
| communication than its counterparts, the | | | | gain antennas with parabolic dish structure. A |
| Japanese National Space Development Agency | | | | reflector antenna is the one which has a spherical |
| (NASDA) or the European Space Agency (ESA), | | | | wave-front, which means that the radiations of |
| although NASA's total budget is many times | | | | the signals from the antenna are spherical in |
| greater. | | | | nature, one in which the energy spreads out in all |
| Satellite Communication Technologies | | | | directions away from the antenna and produces a |
| A brief discussion, relating to the assessment of | | | | pattern that is not very directional. A parabolic |
| satellite communication technology, is presented | | | | antenna is specifically used for high directivity. |
| here. | | | | These antennas are illuminated by a set of "feed" |
| The Antenna System | | | | antennas or indirectly through a system of |
| A component of active transmitter and receiver, | | | | sub-reflectors. A feed antenna will generally |
| the antenna is a transducer between | | | | consist of a horn type structure, having |
| electromagnetic waves in space and voltages or | | | | electronics components for signal amplifications |
| currents in a transmission line. The receiving | | | | and signal conditioning circuitry. This feed antenna |
| antenna transforms the receiving radio waves into | | | | is mounted at the absolute center of the dish |
| electrical signals which are processed for | | | | reflector antenna, with the horn facing the center |
| necessary information. On the other hand, a | | | | of the dish. There could be multiple horns in such |
| transmitting antenna converts electrical signal into | | | | feed antenna. |
| radio waves and transmits them to the Earth | | | | Most of the Low Earth Orbit satellites have space |
| stations. The radio waves (signals) received and | | | | constraint to have any of the type of parabolic |
| transmitted by the two antennas are based on | | | | antennas. Instead they have antennas which are |
| certain frequencies and the receive frequency is | | | | known as "Whip Antenna." There is ofcourse a |
| always different from the transmitted one. These | | | | shrink in the gain of the antenna in comparison to |
| two frequencies are kept separate owing to the | | | | the reflector antenna as used with the |
| reason that if they were the same, there would | | | | geosynchronous satellites. This loss of gain is |
| conflict between the received and transmitted | | | | compensated by the reduction in the distance |
| signals. These antennas are generally directional | | | | that such satellites orbit the Earth, being just |
| antenna, transmitting more power in some | | | | 2,000 kilo meters as compared to 40,000 kilo |
| direction than others. The directional property of | | | | meters for the geosynchronous satellites. |
| an antenna is represented by its radiation pattern, | | | | The ground antennas for the low Earth orbiting |
| which are generally 3-dimensional. | | | | satellites are of generally Yagi or Helix design. Low |
| An antenna needs power to transmit. This power | | | | Earth orbiting satellites use very low frequencies in |
| lets the antenna transmit over greater distances. | | | | receiving and transmitting signals and the dish |
| This ability to transmit depends on the "gain" of | | | | antennas would be impractically large. There is not |
| the antenna. The more the "gain," the antenna | | | | much of a difference between the requirement of |
| can transmit a greater distance. This power is | | | | a low Earth orbiting satellite and a geosynchronous |
| derived from the onboard electrical power | | | | one and with the advent of modern systems, like |
| generation in a satellite. Here there is a limitation | | | | Motorola's IRIDIUM, that require sophisticated |
| on this power. A battery bank and solar cell | | | | beaming of signals, low Earth orbiting satellites |
| panels, provide power to the onboard satellite | | | | may soon have phased arrays and reflector |
| systems. The solar panels are active during the | | | | antennas. |
| sun-light times, as it powers the satellite systems | | | | The Yagi antenna derives its name from two |
| and charges the battery bank as well. In dark the | | | | Japanese inventors, Yagi and Uda. This is the |
| solar system cannot work and the battery bank | | | | reason why the antenna is also referred to as |
| starts to provide the generation. A dark situation | | | | Yagi-Uda antenna. The invention was first |
| occurs when the Earth comes in-between the | | | | published in 1928, which was presented by Yagi |
| satellite and the Sun, when the battery bank | | | | himself. This type of antenna consists of an array |
| switches on to supply the power required. | | | | of a dipole and additional parasitic elements. There |
| In order to know more about antenna, let us now | | | | is another element, a reflector, slightly larger in |
| look at some of the terms used in defining an | | | | length than that of a dipole. This arrangement |
| antenna characteristic. First, the radio signals | | | | gives antenna better directional characteristic than |
| received or transmitted by an antenna is related | | | | a single dipole antenna. Yagi antennas are |
| as frequencies and expressed in Hertz (Hz). | | | | directional, along the axis perpendicular to its plane |
| Frequency has been names as Hertz (Hz), after | | | | of elements, from the reflector to the driven |
| Heinrich Rudolf Hertz (1847-1894), who was first | | | | parasitic elements. It is interesting to note that |
| to transmit and receive radio waves. Hertz is a | | | | additional directors in these type of antennas |
| measure of the frequency and denotes the | | | | increase directivity of the signals, where-as, |
| number of cycles that a signal undergoes in a | | | | addition of further reflectors makes no significant |
| second. For example, if a signal makes a complete | | | | difference. |
| cycle in one second, that is measured as 1Hz. As | | | | The gain of a Yagi antenna is controlled by the |
| for the term Bandwidth in the concept of radio | | | | number of elements that it has. However, spacing |
| communication, the difference between the | | | | the elements is also a design factor in terms of |
| highest frequency signal component and its lowest | | | | gain of such antenna. The design of the Yagi |
| one, in terms of Hz, is the spectrum which is | | | | antenna has many inter-related variables, and |
| called the bandwidth of the signal. A typical voice | | | | earlier designs were not being able to achieve the |
| signal has a bandwidth of 3 kHz, that is to say | | | | full potential or performance of these antennas. |
| that the frequency of a voice lies within 3 kilo | | | | Today's computer design has made a great |
| hertz bandwidth, where-as the TV signal has a | | | | impact of the design characteristics and greater |
| bandwidth of 6MHz, some 2,000 times as wide as | | | | improvement in performance has been achieved. |
| the voice. In here, "k" and "M" denote kilo and | | | | |