Спутники связи

Wireless Communications

1. Introduction

Wireless сommunications are various telecommunications systems that use radio waves to carry signals and messages across distances. Wireless communications systems use devices called transmitters to generate radio waves. A microphone or other mechanism converts messages, like sounds or other data, into electronic impulses. The transmitters change, or modulate, the radio waves so they can carry the impulses, and then transmit the modulated radio signals across distances. Radio receivers pick up these signals and decode them back into original messages. Commercial radio and television are also wireless telecommunications system, but radio and television are mainly public broadcast services rather than personal communications systems.

Wireless communications allow people greater flexibility while communicating, because they do not need to remain at a fixed location, such as a home or office. Wireless technologies make communications services more readily available than traditional wire-based services (such as ordinary telephones), which require the installation of wires. This is useful in places where only temporary communications services are needed, such as at outdoor festivals or large sporting events. These technologies are also useful for communicating in remote locations, such as mountains, jungles, or deserts, where telephone service might not exist. Wireless services allow people to communicate while in a car, airplane, or other moving vehicle. Police, fire, and other emergency departments use two-way radio to communicate information between vehicles that are already responding to emergency calls, which saves valuable time. Construction and utility workers frequently use hand-held radios for short-range communication and coordination. Many businesspeople use wireless communications, particularly cellular radio telephones, to stay in contact with colleagues and clients while traveling.

All wireless communications devices use radio waves to transmit and receive signals. These devices operate on different radio frequencies so that signals from one device will not overlap and interfere with nearby transmissions from other devices.

2. Principles of Wireless Communications

Wireless communications begin with a message that is converted into an electronic signal by a device called a transmitter. The transmitter uses an oscillator to generate radio waves. The transmitter modulates the radio wave to carry the electronic signal and then sends the modified radio signal out through space, where it is picked up by a receiver. The receiver decodes, or demodulates, the radio wave and plays the decoded message over a speaker. Wireless communications provide more flexibility than wire-based means of communication. However, there are some drawbacks. Wireless communications are limited by the range of the transmitter (how far a signal can be sent), and since radio waves travel through the atmosphere, they can be disturbed by electrical interferences (such as lightning) that cause static.

Wireless communications systems involve either one-way transmissions, in which a person merely receives notice of a message, or two-way transmissions, such as a telephone conversation between two people. An example of a device that sends one-way transmission is a pager, which is a radio receiver. When a person dials a pager number, the pager company sends a radio signal to the desired pager. The encoded signal triggers the pager circuitry and notifies the customer carrying the pager of the incoming call with a tone or a vibration, and often the telephone number of the caller. Advanced pagers can display short messages from the caller, or provide news updates or sports scores.

Two-way transmissions require both a transmitter and a receiver for sending and receiving signals. A device that functions as both a transmitter and a receiver is called a transceiver. Cellular radio telephones and two-way radios use transceivers, so that back-and-forth communication between two people can be maintained. Early transceivers were very large, but they have decreased in size due to advances in technology. Fixed-base transceivers, such as those used at police stations, can fit on a desktop, and hand-held transceivers have shrunk in size as well. Several current models of hand-held transceivers weigh less than 0.2 kg (0.5 lb).

3. Modes of Wireless Communications

Wireless communications systems have grown and changed as technology has improved. Several different systems are used today, all of which operate on different radio frequencies. New technologies are being developed to provide greater service and reliability.

A Air Transceivers

Radio operators still monitor distress channels, but maritime and aviation telecommunications systems now use high-frequency radios and satellites capable of transmitting speech, rather than wireless telegraphy, to send messages. Aircraft pilots use radios to communicate with air traffic controllers at airports and also to communicate with other pilots. Navigation beacons are equipped with transmitters that send automated signals to help ships and aircraft in distress determine their positions. While high-frequency radio can transmit signals over long distances, the quality of these signals can be diminished by bad weather or by electrical interference in the atmosphere, which is often caused by radiation from the sun.

B Hand-Held Radio Transceivers

Police, fire, and other emergency organizations, as well as the military, have used two-way wireless radio communication since the 1930s. Early vehicle-based radios were large, heavy units. After the invention of the transistor in 1948, radios shrank in size to small hand-held radio transceivers, which civil authorities now use to communicate with each other directly. Public two-way radios with several frequency options are widely available as well. Usually limited in range to a few miles, these units are great aids for such mobile professionals as construction workers, film crews, event planners, and security personnel. Simpler two-way radios, called walkie-talkies, have been popular children’s toys for years.

C Shortwave

Long-range broadcast services and frequencies, in what is known as the shortwave radio band (with frequencies of 3 to 30 megahertz), are available for amateur or ham radio operators. Shortwave radio broadcasts can travel long distances because of the concentration of ionized, or electrically charged, particles in the layer of the atmosphere known as the ionosphere. This layer reflects radio signals, sending signals that are transmitted upward back to earth. This skipping of waves against the ionosphere can greatly increase the range of the transmitter. The degree of reflectivity of the ionosphere depends on the time of day.

D Cellular Radio Telephones

Cellular radio telephones, or cell phones, combine their portable radio capability with the wired, or wireline, telephone network to provide mobile users with access to the rest of the public telephone system used by non-mobile callers. Modern cellular telephones use a network of several short-range antennas that connect to the telephone system. Because the antennas have a shorter range, frequencies can be reused a short distance away without interference.

E Satellite Communications

Satellite communications services connect users directly to the telephone network from almost anywhere in the world. Special telephones are available to consumers that communicate directly with communications satellites orbiting the earth. The satellites transmit these signals to ground stations that are connected to the telephone system. These satellite services, while more expensive than cellular or other wireless services, give users access to the telephone network in areas of the world where no telephone service exists.

The number of companies offering wireless communications services has grown steadily in recent years. In 1988 about 500 companies offered cellular radio telephone (cell phone) services. By 1995 that number had grown to over 1500 companies serving millions of subscribers. Wireless communication is becoming increasingly popular because of the convenience and mobility it affords, the expanded availability of radio frequencies for transmitting, and improvements in technology.

 

 

 

Communications Satellites

1. Introduction

A communications satellite is any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals.

2. History and Development

Some of the first communications satellites were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so they could be picked up by receiving stations around the world. Echo 1, launched by the United States in 1960, consisted of an aluminized plastic balloon 30 m (100 ft) in diameter. Launched in 1964, Echo 2, was 41 m (135 ft) in diameter. The capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas.

Satellite communications currently make exclusive use of active systems, in which each satellite carries it own equipment for reception and transmission. Score, launched by the United States in 1958, was the first active communications satellite. It was equipped with a tape recorder that stored messages received while passing over a transmitting ground station. These messages were retransmitted when the satellite passed over a receiving station. Telstar 1, launched by American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan and could also relay several hundred voice channels. Launched into an elliptical orbit inclined 45 ° to the equatorial plane, Telstar could only relay signals between two ground stations for a short period during each revolution, when both stations were in its line of sight.

Hundreds of active communications satellites are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. One frequency band used, 500 MHZ wide, is divided into repeater channels of various bandwidths (located at 6 GHZ for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission). A band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non-mobile) ground stations. An 80-MH Z-wide band at about 1.5 GHZ (up-and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission.

3. Geosynchronous Orbit

A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitude of 35,800 km (22,300 mi) completing one orbit every 24 hours, in the time that it takes the earth to rotate once. Moving in the same direction as the earth's rotation, the satellite remains in a fixed position over a point on the equator, thereby providing uninterrupted contact between ground stations in its line of sight. The first communications satellite to be placed in this type of orbit was Syncom 2, launched by the National Aeronautics and Space Administration (NASA) in 1963. Most of those that followed were also placed in geosynchronous orbit.

4. Commercial Communications Satellites

Deployment and operation of communications satellites on a commercial basis began with the founding of the Communications Satellite Corporation (COMSAT) in 1963. When the International Telecommunications Satellite Organization (INTELSAT) was formed in 1964, COMSAT became the U.S. member. Based in Washington, D.C., INTELSAT is owned by more than 120 nations. Intelsat 1, known as Early Bird, launched in 1965, provided either 2400 voice circuits or one two-way television channel between the United States and Europe. During the 1960s and 1970s, message capacity and transmission power of the Intelsat 2, 3, and 4 generations were progressively increased by beaming the satellite power only to the earth and segmenting the broadcast spectrum into transponder units of a certain bandwidth. The first of the Intelsat 4s, launched in 1971, provided 4000 voice circuits. With the Intelsat 5 series (1980), introduction of multiple beam operation resulted in additional increases in capacity. A satellite's power could now be concentrated on small regions of the earth, making possible smaller-aperture, lower-cost ground stations. An Intelsat 5 satellite can typically carry 12,000 voice circuits. The Intelsat 6 satellites, which entered service in 1989, can carry 24,000 circuits and feature dynamic on-board switching of telephone capacity among six beams, using a technique called SS-TDMA (satellite-switched time division multiple access). By the early 1990s, Intelsat had 15 satellites in orbit, providing the world's most extensive telecommunications system. Other systems also provide international service in competition with Intelsat. By 1997, all regulatory restraints to such competition will have been lifted. The growth of international systems has been paralleled by domestic and regional systems, such as the U.S. Telstar, Galaxy, and Spacenet programs and Europe's Eutalsat and Telecom.

5. Services

Commercial satellites provide a wide range of communications services. Television programs are relayed internationally, giving rise to the phenomenon known as the "global village." Satellites also relay programs to cable television systems as well as to homes equipped with dish antennas. In addition, very small aperture terminals (VSATs) relay digital data for a multitude of business services. Intelsat satellites now carry over 100,000 telephone circuits, with growing use of digital transmission. Digital source coding methods (see Telecommunications) have resulted in a ten-fold reduction in the transmission rate needed to carry a voice channel, thus enhancing the capacity of existing facilities and reducing the size of ground stations that provide telephone service.

The International Mobile Satellite Organization (INMARSAT), founded in 1979 as the International Maritime Satellite Organization, is a mobile telecommunications network, providing digital data links, telephone, and facsimile transmission, or fax, service between ships, offshore facilities, and shore-based stations throughout the world. It is also now extending satellite links for voice and fax transmission to aircraft on international routes.

6. Recent Technical Advances

Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low-cost stations. The development of multiple access methods has both hastened and facilitated this transition. With TDMA, each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations monitor these slots and select the communications directed to them. By amplifying a single carrier frequency in each satellite repeater, TDMA ensures the most efficient use of the satellite's onboard power supply.

 

A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency by transmitting in narrow beams pointed toward each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector.

A new method for interconnecting many ground stations spread over great distances is scheduled to be tested in 1993, with the launch of NASA's ACTS (Advanced Communications Technology Satellite). Known as the hopping spot beam technique, it combines the advantages of frequency reuse, spot beams, and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations that have smaller antennas and reduced power requirements.

The concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Italsat, developed by the Italian Research Council. With six spot beams operating at 30 GHZ (uplink) and 20 GHZ (downlink), the satellite interconnects TDMA transmissions between ground stations in all the major economic centers of Italy. It does this by demodulating uplink signals routing them between up- and downlink beams, and combining and remodulating them for downlink transmission.

The application of laser technology to satellite communications has been studied for over a decade. Laser beams can be used to transmit signals between a satellite and earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the blue-green wavelength, which penetrates water, have been used for communication between satellites and submarines.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Спутники связи 

1. Введение 

Спутник связи - любой земной-орбитальный космический корабль, который обеспечивает коммуникацию по длинным расстояниям, размышляя или передавая сигналы радиочастоты.

2. История и Развитие 

Некоторые из первых спутников связи  были разработаны, чтобы работать в  пассивном способе. Вместо того, чтобы активно передавать радио-сигналы, они служили просто, чтобы отразить сигналы, которые сиялись до них, передавая станции на основании. Сигналы были отражены во всех указаниях, таким образом они могли быть собраны, получая станции во всем мире. Отзовитесь эхом 1, начатый Соединенными Штатами в 1960, состоял из алюминированного пластмассового воздушного шара 30 м. (100 футов) в диаметре. Начат в 1964, Эхо 2, был 41 м. (135 футов) в диаметре. Вместимость таких систем была строго ограничена потребностью в мощных передатчиках и больших антеннах основания.

Спутниковые коммуникации в настоящее  время делают исключительное использование  активных систем, в которых каждый спутник несет это собственное  оборудование для приема и передачи. Счет, начатый Соединенными Штатами  в 1958, был первым активным спутником  связи. Это было оборудовано магнитофоном, который хранил сообщения, полученные, передавая по передающей станции  основания. Эти сообщения были повторно переданы, когда спутник передавал  по станции получения. Telstar 1, начатый американской Компанией Телефона и Телеграфа в 1962, обеспеченный прямую телевизионную передачу между Соединенными Штатами, Европой, и Японией и мог также передать несколько сотен каналов голоса. Начатый краткая орбита склоняла 45 ° на экваториальный самолет, Telstar мог только передать сигналы между двумя станциями основания в течение короткого периода в течение каждой революции, когда обе станции были в ее луче обзора.

Сотни активных спутников связи  находятся теперь в орбите. Они  получают сигналы от одной станции  основания, усиливают их, и затем  повторно передают их в различной  частоте к другой станции. Одна используемая полоса частоты, 500 мгц шириной, разделена на каналы ретранслятора различных полос пропускания (расположенный в 6 GHZ для восходящего, или uplink, передаче и 4 GHZ для нисходящего, или передаче информации из космоса, передача). Полоса в 14 GHZ (uplink) и 11 или 12 GHZ (передача информации из космоса) - также очень в использовании, главным образом с неподвижными (немобильными) станциями основания. Полоса Z-wide на 80 МИЛЛИГЕНРИ приблизительно в 1.5 GHZ (-и передаче информации из космоса) используется с маленькими, мобильными станциями основания (суда, наземные транспортные средства, и самолет). Солнечные ячейки энергии, установленные на больших группах, приложенных к спутнику обеспечивают власть для приема и передачи.

3. Орбита Geosynchronous

Спутник в geosynchronous орбите следует за круглой орбитой по экватору в высоте 35 800 км (22 300 ми) завершение одной орбиты каждые 24 часа, во время, когда требуется земля, чтобы вращаться однажды. Перемещаясь в то же самое руководство как вращение земли, спутник остается в неподвижном положении по пункту на экваторе, таким образом обеспечивая непрерывный контакт между станциями основания в его луче обзора. Первый спутник связи, который будет помещен в этот тип орбиты был Syncom 2, начатый Национальной Аэронавтикой и Космической Администрацией (НАСА) в 1963. Большинство из тех, которые следовали, было также размещено в geosynchronous орбиту.

4. Коммерческие Спутники связи

Развертывание и операция спутников  связи на коммерческом основании  начались с основания Корпорации Спутника связи (СИСТЕМА СПУТНИКОВОЙ  СВЯЗИ КОМСАТ) в 1963. Когда Международная  Телекоммуникационная Организация  Спутника (ИНТЕЛСАТ) была сформирована в 1964, СИСТЕМА СПУТНИКОВОЙ СВЯЗИ  КОМСАТ стала американским членом. Базируемый в Вашингтоне, округ Колумбия, ИНТЕЛСАТ принадлежит больше чем 120 нациям. Интелсат 1, известный как Ранняя Птица, начатая в 1965, обеспеченный или 2400 кругооборотов голоса или один двухсторонний телевизионный канал между Соединенными Штатами и Европой. В течение 1960-ых и 1970-ых, вместимость сообщения и власть передачи Интелсата 2, 3, и 4 поколений были прогрессивно увеличены сияющим спутниковая власть только к земле и сегментации спектра радиопередачи в единицы приемоответчика определенной полосы пропускания. Первый из Интелсата 4s, начатый в 1971, обеспечил 4000 кругооборотов голоса. С Интелсатом 5 рядов (1980), введение многократной операции луча привело к дополнительным увеличениям вместимости. Власть спутника могла теперь быть сконцентрирована на маленьких областях земли, делая возможную меньшую апертуру, станции основания меньшей стоимости. Интелсат 5 спутников может типично нести 12 000 кругооборотов голоса. Интелсат 6 спутников, которые вошли в обслуживание в 1989, может нести 24 000 кругооборотов и показывать динамическое бортовое переключение телефонной вместимости среди шести лучей, используя технику, названную Секретной-службой-TDMA (переключенное спутником разделение времени многократный доступ). К началу 1990-ых, Интелсат имел 15 спутников в орбите, обеспечивая всемирную самую обширную телекоммуникационную систему. Другие системы также обеспечивают международное обслуживание на соревновании с Интелсатом. К 1997, все регулирующие ограничения на такое соревнование будут сняты. Рост международных систем был сравнен внутренними и региональными системами, типа США. Telstar, Галактика, и программы Spacenet и Eutalsat Европы и Телекоммуникация.