Satellites for Transmission

Communication Satellites by Carmen Atwell Today, it is hard to imagine a life where people are not surfing the Internet, watching television, or using their cell phones to call or send text messages. However, none of these activities were possible before the existence of communication satellites. These satellites have made it possible to watch television and use devices like laptops, tablets, cell phones, and more. Before the use of satellites, transmissions were difficult or impossible for long distances. Signals, which travel in straight lines, could not bend around Earth to reach a destination far away. Because satellites orbit Earth, signals are sent immediately into space and are then redirected to another satellite or are sent directly to their destination. Unlike other man-made satellites, communication satellites remain stationary, which allows them to transmit signals in that area. The area to which the communication satellite can transmit signals is called a satellite's footprint. Communication satellites are important for remote areas that do not have access to traditional landlines for telephone or Internet services. These satellites provide faster Internet and access to information from all around the world. Communication satellites help transport telephone signals, data, or television transmissions to almost anywhere. These satellites are used for things like making an international phone call or enabling access to over 150 channels on television. People can access news from around the world or watch videos in an instant because of the power of these satellites. In a way, communication satellites are like Atlas carrying the weight of the world on their shoulders. Without them, the world would cease to function, and the easiest and best forms of communication would end completely. Can you imagine life without communication? I surely cannot. As a child, I often saw my grandparents and family communicating with each other through letters. However, writing and delivering letters was a slow and tedious process. Gradually, people began to use electronic devices like the television and radio for entertainment. I often wondered how those sounds and pictures appeared on the electronic devices. It was later that I understood that a special type of satellite launched into space made it possible to listen to the radio or watch TV. Indeed, communication satellites have become an essential part of our lives. 5 Read the sentence from the passage. "Communication satellites help transport telephone signals, data, or television transmissions to almost anywhere." What is the meaning of the word transmissions? A. images B. rays C. devices D. broadcasts

TranspondersA transponder is the series of interconnected units which forms a singlecommunications channel between the receive and transmit antennas in acommunications satellite. Some of the units utilized by a transponder in agiven channel may be common to a number of transponders. Thus, althoughreference may be made to a specific transponder, this must be thought ofas an equipment channel rather than a single item of equipment.Before describing in detail the various units of a transponder, theoverall frequency arrangement of a typical C-band communicationssatellite will be examined briefly. The bandwidth allocated for C-bandservice is 500 MHz, and this is divided into subbands, one for eachThe Space Segment 213transponder. A typical transponder bandwidth is 36 MHz, and allowingfor a 4-MHz guardband between transponders, 12 such transponders canbe accommodated in the 500-MHz bandwidth. By making use of polar-ization isolation, this number can be doubled. Polarization isolationrefers to the fact that carriers, which may be on the same frequency butwith opposite senses of polarization, can be isolated from one anotherby receiving antennas matched to the incoming polarization. With linearpolarization, vertically and horizontally polarized carriers can be sep-arated in this way, and with circular polarization, left-hand circularand right-hand circular polarizations can be separated. Because thecarriers with opposite senses of polarization may overlap in frequency,this technique is referred to as frequency reuse. Figure 7.12 shows partof the frequency and polarization plan for a C-band communicationssatellite.214 Chapter SevenFigure 7.11 Satellite control system. (Courtesy of Telesat Canada, 1983.)Frequency reuse also may be achieved with spot-beam antennas, andthese may be combined with polarization reuse to provide an effectivebandwidth of 2000 MHz from the actual bandwidth of 500 MHz.For one of the polarization groups, Fig. 7.13 shows the channelingscheme for the 12 transponders in more detail. The incoming, or uplink,frequency range is 5.925 to 6.425 GHz. The carriers may be receivedon one or more antennas, all having the same polarization. The inputfilter passes the full 500-MHz band to the common receiver whilerejecting out-of-band noise and interference such as might be causedby image signals. There will be many modulated carriers within this500-MHz passband, and all of these are amplified and frequency-converted in the common receiver. The frequency conversion shiftsthe carriers to the downlink frequency band, which is also 500 MHzwide, extending from 3.7 to 4.2 GHz. At this point the signals arechannelized into frequency bands which represent the individualtransponder bandwidths.A transponder may handle one modulated carrier, such as a TV signal,or it may handle a number of separate carriers simultaneously, eachmodulated by its own telephony or other baseband channel. explain

Which is the First Indian Communication Satellite?/

Microwaves are used in communication with satellites. Describe in detail how the microwaves work in satellites communication and why they are suitable?

______ satellites provide communications between one ground-based transmitter and a number of ground-based receivers.Select one:Point-to-pointBidirectionalBroadcastOmnidirectional