В радиосвязи , используемой в радио- и телевещании , сотовых телефонах, радиосвязях , беспроводных сетях и спутниковой связи , среди множества других применений, радиоволны используются для передачи информации в пространстве от передатчика к приемнику путем модуляции радиосигнал (наложение информационного сигнала на радиоволну путем изменения некоторых аспектов волны) в передатчике. В радаре, используемом для обнаружения и отслеживания объектов, таких как самолеты, корабли, космические корабли и ракеты, луч радиоволн, излучаемый радиолокационным передатчиком, отражается от целевого объекта, а отраженные волны показывают местоположение объекта приемнику, который обычно размещается рядом с ним. передатчик. В радионавигационных системах, таких как GPS и VOR , мобильный навигационный прибор принимает радиосигналы от нескольких навигационных радиомаяков , положение которых известно, и путем точного измерения времени прибытия радиоволн приемник может рассчитать свое положение на Земле. В устройствах беспроводного радиодистанционного управления , таких как дроны , устройства открывания гаражных ворот и системы бесключевого доступа , радиосигналы, передаваемые от устройства-контроллера, управляют действиями удаленного устройства.
Существование радиоволн было впервые доказано немецким физиком Генрихом Герцем 11 ноября 1886 года. [4] В середине 1890-х годов, основываясь на методах, которые физики использовали для изучения электромагнитных волн, Гульельмо Маркони разработал первый аппарат для радиосвязи на большие расстояния. , [5] отправка беспроводного сообщения азбукой Морзе получателю на расстоянии более километра в 1895 году, [6] и первый трансатлантический сигнал 12 декабря 1901 года. [7] Первая коммерческая радиопередача была передана 2 ноября 1920 года, когда Результаты президентских выборов Хардинга-Кокса в прямом эфире транслировались компанией Westinghouse Electric and Manufacturing Company в Питтсбурге под позывным KDKA . [8]
Излучение радиоволн регулируется законом, координируемым Международным союзом электросвязи (ITU), который распределяет полосы частот радиоспектра для различных целей.
Этимология
Слово радио происходит от латинского слова radius , что означает «спица колеса, луч света, луч». Впервые оно было применено к средствам связи в 1881 году, когда по предложению французского ученого Эрнеста Меркадье [ фр ] Александр Грэм Белл принял радиофон (что означает «излучаемый звук») в качестве альтернативного названия для своей системы оптической передачи фотофона . [9] [10]
После открытия Герцем существования радиоволн в 1886 году для этого излучения первоначально использовался термин «волны Герца» . [11] Первые практические системы радиосвязи, разработанные Маркони в 1894–1895 годах, передавали телеграфные сигналы с помощью радиоволн, [4] поэтому радиосвязь сначала называлась беспроводной телеграфией . Примерно до 1910 года термин « беспроводная телеграфия» также включал множество других экспериментальных систем для передачи телеграфных сигналов без проводов, включая электростатическую индукцию , электромагнитную индукцию , а также водную и земную проводимость , поэтому возникла необходимость в более точном термине, относящемся исключительно к электромагнитному излучению. . [12] [13]
Французский физик Эдуард Бранли , который в 1890 году разработал когерер , обнаруживающий радиоволны , назвал его по-французски радиопроводником . [14] [15] Радио - префикс позже использовался для образования дополнительных описательных составных слов и слов, написанных через дефис, особенно в Европе. Например, в начале 1898 года британское издание The Practice Engineer включило упоминание о радиотелеграфе и радиотелеграфии . [14] [16]
Использование радио как отдельного слова восходит как минимум к 30 декабря 1904 года, когда в инструкциях британской почтовой службы по передаче телеграмм указывалось, что «слово «Радио»... отправляется в служебных инструкциях». [14] [17] Эта практика была принята повсеместно, а слово «радио» было введено на международном уровне Берлинской радиотелеграфной конвенцией 1906 года, которая включала Положение о службе, в котором уточнялось, что «Радиотелеграммы должны указывать в преамбуле, что эта служба является «Радио » ». . [14]
Переход на радио вместо беспроводной связи в англоязычном мире происходил медленно и неравномерно. Ли де Форест помог популяризировать новое слово в Соединенных Штатах — в начале 1907 года он основал радиотелефонную компанию ДеФорест, а его письмо в журнале Electrical World от 22 июня 1907 года о необходимости юридических ограничений предупреждало, что «радиохаос, безусловно, будет результат до тех пор, пока не будет введено в действие такое строгое регулирование». [18] ВМС США также сыграют свою роль. Хотя в переводе Берлинской конвенции 1906 года использовались термины «беспроводной телеграф» и «беспроводная телеграмма» , к 1912 году вместо этого она начала пропагандировать использование радио . Широкая общественность стала отдавать предпочтение этому термину в 1920-х годах с появлением радиовещания.
История
Электромагнитные волны были предсказаны Джеймсом Клерком Максвеллом в его теории электромагнетизма 1873 года , теперь называемой уравнениями Максвелла , который предположил, что связанные колеблющиеся электрическое поле и магнитное поле могут распространяться в пространстве как волна, и предположил, что свет состоит из электромагнитных волн короткой длины . . 11 ноября 1886 года немецкий физик Генрих Герц , пытаясь подтвердить теорию Максвелла, впервые наблюдал радиоволны, которые он генерировал с помощью примитивного передатчика на искровом разряднике . [4] Эксперименты Герца и физиков Джагадиша Чандры Бозе , Оливера Лоджа , лорда Рэлея и Аугусто Риги , среди других, показали, что радиоволны, такие как свет, демонстрируют отражение, преломление , дифракцию , поляризацию , стоячие волны и распространяются с той же скоростью, что и свет. свет, подтверждая, что и свет, и радиоволны были электромагнитными волнами, различающимися только по частоте. [19] В 1895 году Гульельмо Маркони разработал первую систему радиосвязи, используя передатчик с искровым разрядником для передачи кода Морзе на большие расстояния. К декабрю 1901 года он пересек Атлантический океан. [4] [5] [6] [7] Маркони и Карл Фердинанд Браун разделили Нобелевскую премию по физике 1909 года «за вклад в развитие беспроводной телеграфии». [20]
В течение первых двух десятилетий радио, называемых эрой радиотелеграфии , примитивные радиопередатчики могли передавать только импульсы радиоволн, а не непрерывные волны, которые были необходимы для аудиомодуляции , поэтому радио использовалось для передачи коммерческих, дипломатических и военных текстов между людьми. обмен сообщениями. Примерно с 1908 года индустриальные страны построили всемирные сети мощных трансокеанских передатчиков для обмена телеграммами между континентами и связи со своими колониями и военно-морскими флотами. Во время Первой мировой войны разработка радиопередатчиков непрерывного действия , выпрямляющих электролитических и кристаллических радиоприемников-детекторов позволила Реджинальду Фессендену и другим реализовать радиотелефонию с амплитудной модуляцией (AM) , что позволило передавать звук . 2 ноября 1920 года компания Westinghouse Electric and Manufacturing Company в Питтсбурге под позывным KDKA передала первую коммерческую радиопередачу , включающую прямую трансляцию президентских выборов Хардинга-Кокса . [8]
Технологии
Радиоволны излучаются электрическими зарядами, претерпевающими ускорение . [21] [22] Они генерируются искусственно изменяющимися во времени электрическими токами , состоящими из электронов , текущих взад и вперед в металлическом проводнике, называемом антенной. [23] [24]
По мере удаления от передающей антенны радиоволны распространяются, поэтому мощность их сигнала ( интенсивность в ваттах на квадратный метр) уменьшается, поэтому радиопередачи могут быть приняты только в пределах ограниченного диапазона передатчика, причем расстояние зависит от мощности передатчика. диаграмма направленности антенны , чувствительность приемника, уровень шума и наличие препятствий между передатчиком и приемником. Всенаправленная антенна передает или принимает радиоволны во всех направлениях, тогда как направленная антенна или антенна с высоким коэффициентом усиления передает радиоволны лучом в определенном направлении или принимает волны только с одного направления. [25] [26] [27]
Радиоволны распространяются со скоростью света в вакууме. [28] [29]
Радиосвязь. Такая информация, как звук, преобразуется преобразователем, например микрофоном, в электрический сигнал, который модулирует радиоволну , создаваемую передатчиком . Приемник перехватывает радиоволну и извлекает информационный модуляционный сигнал, который преобразуется обратно в форму, пригодную для использования человеком, с помощью другого преобразователя, например громкоговорителя .Сравнение модулированных радиоволн AM и FM
В системах радиосвязи информация передается в пространстве с помощью радиоволн. На передающей стороне информация, подлежащая отправке, преобразуется преобразователем определенного типа в изменяющийся во времени электрический сигнал , называемый сигналом модуляции. [24] [30] Сигнал модуляции может быть аудиосигналом , представляющим звук из микрофона , видеосигналом , представляющим движущиеся изображения с видеокамеры , или цифровым сигналом , состоящим из последовательности битов , представляющей двоичные данные с компьютера. Сигнал модуляции подается на радиопередатчик . В передатчике электронный генератор генерирует переменный ток , колеблющийся на радиочастоте , называемый несущей волной , поскольку он служит для «переноса» информации по воздуху. Информационный сигнал используется для модуляции несущей, изменяя некоторые аспекты несущей волны и передавая информацию на несущую. В разных радиосистемах используются разные методы модуляции : [31]
FM ( частотная модуляция ) – в FM-передатчике частота несущей радиоволны изменяется сигналом модуляции; [31] : 33
FSK ( частотная манипуляция ) — используется в беспроводных цифровых устройствах для передачи цифровых сигналов , частота несущей волны смещается между частотами. [31] : 58
Также используются многие другие типы модуляции. В некоторых типах передается не несущая волна, а только одна или обе боковые полосы модуляции . [33]
Модулированная несущая усиливается в передатчике и подается на передающую антенну , которая излучает энергию в виде радиоволн. Радиоволны передают информацию к месту расположения приемника. [34]
В приемнике радиоволна индуцирует небольшое колебательное напряжение в приемной антенне, которое является более слабой копией тока в передающей антенне. [24] [30] Это напряжение подается на радиоприемник , который усиливает слабый радиосигнал, делая его сильнее, а затем демодулирует его, извлекая исходный сигнал модуляции из модулированной несущей волны. Сигнал модуляции преобразуется преобразователем обратно в форму, удобную для использования человеком: аудиосигнал преобразуется в звуковые волны с помощью громкоговорителя или наушников, видеосигнал преобразуется в изображения с помощью дисплея , а цифровой сигнал подается на компьютер. или микропроцессор, который взаимодействует с пользователями-людьми. [31]
Радиоволны от многих передатчиков проходят через воздух одновременно, не мешая друг другу, поскольку радиоволны каждого передатчика колеблются с разной скоростью, другими словами, каждый передатчик имеет разную частоту , измеряемую в герцах (Гц), килогерцах (кГц), мегагерц (МГц) или гигагерц (ГГц). Приемная антенна обычно принимает радиосигналы многих передатчиков. Приемник использует настроенные схемы для выбора желаемого радиосигнала из всех сигналов, принимаемых антенной, и отклонения остальных. Настроенный контур ( также называемый резонансным контуром или контуром резервуара) действует как резонатор , похожий на камертон . [30] Он имеет собственную резонансную частоту , на которой он колеблется. Резонансная частота настраиваемого контура приемника подстраивается пользователем под частоту нужной радиостанции; это называется "тюнинг". Колеблющийся радиосигнал от нужной станции заставляет настроенную схему резонировать , колебаться в сочувствии, и он передает сигнал остальной части приемника. Радиосигналы на других частотах блокируются настроенной схемой и не передаются дальше. [35]
Пропускная способность
Частотный спектр типичного модулированного радиосигнала AM или FM. Он состоит из компонента C на частоте несущей волны с информацией ( модуляцией ), содержащейся в двух узких полосах частот, называемых боковыми полосами ( SB ), чуть выше и ниже несущей частоты.
Модулированная радиоволна, несущая информационный сигнал, занимает диапазон частот . Информация ( модуляция ) в радиосигнале обычно сосредоточена в узких полосах частот, называемых боковыми полосами ( SB ), чуть выше и ниже несущей частоты. Ширина в герцах частотного диапазона, который занимает радиосигнал (самая высокая частота минус самая низкая частота), называется его полосой пропускания ( BW ). [31] [36] Для любого заданного отношения сигнал/шум полоса пропускания может нести один и тот же объем информации ( скорость передачи данных в битах в секунду) независимо от того, в каком месте радиочастотного спектра она расположена, поэтому полоса пропускания равна мера информационной емкости . Полоса пропускания, необходимая для радиопередачи, зависит от скорости передачи данных отправляемой информации (сигнала модуляции) и спектральной эффективности используемого метода модуляции ; сколько данных он может передать на каждом килогерце полосы пропускания. Различные типы информационных сигналов, передаваемых по радио, имеют разную скорость передачи данных. Например, телевизионный (видео) сигнал имеет большую скорость передачи данных, чем аудиосигнал . [31] [37]
Радиоспектр , общий диапазон радиочастот, который можно использовать для связи в данной местности, является ограниченным ресурсом . [36] [3] Каждая радиопередача занимает часть всей доступной полосы пропускания. Пропускная способность радиоканала рассматривается как экономический товар , имеющий денежную стоимость и пользующийся растущим спросом. В некоторых частях радиоспектра право использования полосы частот или даже одного радиоканала покупается и продается за миллионы долларов. Таким образом, существует стимул использовать технологии для минимизации полосы пропускания, используемой радиослужбами. [37]
Медленный переход от аналоговых к цифровым технологиям радиопередачи начался в конце 1990-х годов. [38] [39] Частично это связано с тем, что цифровая модуляция часто может передавать больше информации (более высокая скорость передачи данных) в заданной полосе пропускания, чем аналоговая модуляция , за счет использования алгоритмов сжатия данных , которые уменьшают избыточность отправляемых данных. и более эффективная модуляция. Другие причины перехода заключаются в том, что цифровая модуляция обладает большей помехоустойчивостью , чем аналоговая, микросхемы цифровой обработки сигналов обладают большей мощностью и гибкостью, чем аналоговые схемы, а с помощью одной и той же цифровой модуляции можно передавать самые разнообразные типы информации. [31]
МСЭ произвольно делит радиоспектр на 12 диапазонов, каждый из которых начинается с длины волны, равной степени десяти (10 н ) метров, с соответствующей частотой, равной 3-кратной степени десяти, и каждый охватывает декаду частоты или длины волны. [3] [40] Каждая из этих групп имеет традиционное название: [41]
Видно, что полоса пропускания , диапазон частот, содержащийся в каждой полосе, не одинакова, а увеличивается экспоненциально с увеличением частоты; каждая полоса содержит в десять раз большую полосу пропускания, чем предыдущая полоса. [42]
Термин «чрезвычайно низкая частота» (TLF) использовался для длин волн от 1 до 3 Гц (300 000–100 000 км), [43] , хотя этот термин не был определен МСЭ. [41]
Регулирование
Эфир — это ресурс, которым пользуются многие пользователи. Два радиопередатчика в одной и той же зоне, которые пытаются вести передачу на одной и той же частоте, будут создавать помехи друг другу, что приведет к искажению приема, поэтому ни одна из передач не может быть принята четко. [36] Помехи в радиопередачах могут не только иметь большие экономические издержки, но и быть опасными для жизни (например, в случае создания помех экстренной связи или управлению воздушным движением ). [44] [45]
Чтобы предотвратить помехи между различными пользователями, излучение радиоволн строго регулируется национальным законодательством и координируется международным органом, Международным союзом электросвязи (ITU), который распределяет полосы радиоспектра для различных целей. [36] [3] Радиопередатчики должны быть лицензированы правительством по различным классам лицензий в зависимости от использования и ограничены определенными частотами и уровнями мощности. В некоторых классах, например радио- и телевещательных станциях, передатчику присваивается уникальный идентификатор, состоящий из строки букв и цифр, называемый позывным , который должен использоваться во всех передачах. [46] Для настройки, обслуживания или внутреннего ремонта радиотелефонных передатчиков люди должны иметь государственную лицензию, например, общую лицензию оператора радиотелефонной связи в США, полученную путем прохождения теста, демонстрирующего адекватные технические и юридические знания в области безопасной эксплуатации радиосвязи. [47]
Ниже приведены некоторые из наиболее важных применений радио, сгруппированные по функциям.
Вещание
Вещание – это односторонняя передача информации от передатчика к приемникам, принадлежащим общественной аудитории. [49] Поскольку радиоволны с расстоянием становятся слабее, радиовещательную станцию можно принимать только на ограниченном расстоянии от ее передатчика. [50] Системы, вещающие со спутников, обычно могут принимать сигнал на всю страну или континент. Старые наземные радио и телевидение оплачиваются коммерческой рекламой или правительством. В системах подписки, таких как спутниковое телевидение и спутниковое радио, клиент платит ежемесячную плату. В этих системах радиосигнал зашифрован и может быть расшифрован только приемником, который контролируется компанией и может быть отключен, если клиент не заплатит. [51]
Вещание использует несколько частей радиоспектра в зависимости от типа передаваемых сигналов и желаемой целевой аудитории. Длинноволновые и средневолновые сигналы могут обеспечить надежное покрытие территорий в несколько сотен километров в поперечнике, но имеют более ограниченную информационную способность и поэтому лучше всего работают с аудиосигналами (речь и музыка), а качество звука может ухудшаться из-за радиошумов естественного происхождения. и искусственные источники. Коротковолновые диапазоны имеют больший потенциальный диапазон, но более подвержены помехам со стороны удаленных станций и меняющимся атмосферным условиям, влияющим на прием . [52] [53]
В диапазоне очень высоких частот , превышающем 30 мегагерц, атмосфера Земли оказывает меньшее влияние на дальность действия сигналов, и основным режимом становится распространение в пределах прямой видимости . Эти более высокие частоты обеспечивают большую полосу пропускания, необходимую для телевизионного вещания. Поскольку на этих частотах меньше естественных и искусственных источников шума, возможна качественная передача звука с использованием частотной модуляции . [54] [55]
Аудио: Радиовещание
Радиовещание – передача звука (звука) на радиоприемники, принадлежащие общественной аудитории. Аналоговое аудио — самая ранняя форма радиовещания. AM-вещание началось примерно в 1920 году. FM-вещание было введено в конце 1930-х годов с улучшенной точностью воспроизведения . Вещательный радиоприемник называется радиоприемником . Большинство радиоприемников могут принимать как AM, так и FM. [56]
АМ ( амплитудная модуляция ) – в АМ амплитуда (сила) несущей радиоволны изменяется звуковым сигналом. AM-радиовещание , старейшая технология радиовещания, разрешено в диапазонах AM-вещания , между 148 и 283 кГц в низкочастотном (LF) диапазоне для длинноволновых передач и между 526 и 1706 кГц в среднечастотном (MF) диапазоне для средневолновых передач. трансляции. [57] Поскольку волны в этих диапазонах распространяются как земные волны, следующие за местностью, AM-радиостанции могут приниматься за горизонтом на расстоянии сотен миль, но AM имеет более низкую точность воспроизведения, чем FM. Излучаемая мощность ( ERP ) AM-станций в США обычно ограничивается максимум 10 кВт, хотя некоторым ( станциям со свободным каналом ) разрешено передавать на мощности 50 кВт. AM-станции вещают в монофоническом режиме; Стандарты стереовещания AM существуют в большинстве стран, но радиоиндустрия не смогла перейти на них из-за отсутствия спроса. [58]
Коротковолновое вещание . AM-вещание также разрешено в коротковолновых диапазонах устаревшими радиостанциями. Поскольку радиоволны в этих диапазонах могут распространяться на межконтинентальные расстояния, отражаясь от ионосферы с использованием космических волн или «пропускающего» распространения, короткие волны используются международными станциями, вещающими на другие страны. [58] [59]
Цифровое радио включает в себя множество стандартов и технологий для эфирного вещания цифровых радиосигналов. Некоторые системы, такие как HD Radio и DRM , работают в тех же диапазонах волн, что и аналоговое вещание, либо в качестве замены аналоговых станций, либо в качестве дополнительной услуги. Другие, такие как DAB/DAB+ и ISDB _Tsb, работают в диапазонах волн, традиционно используемых для телевидения или спутниковых услуг. [61]
Радио "Робертс" для DAB
Цифровое аудиовещание (DAB) дебютировало в некоторых странах в 1998 году. Оно передает звук в виде цифрового сигнала , а не аналогового сигнала , как это делают AM и FM. [62] DAB потенциально может обеспечить более высокое качество звука, чем FM (хотя многие станции не предпочитают передачу с таким высоким качеством), имеет большую устойчивость к радиошуму и помехам, лучше использует ограниченную полосу радиоспектра и предоставляет опытным пользователям такие функции, как электронные программы передач . Его недостатком является то, что он несовместим с предыдущими радиоприемниками, поэтому необходимо приобрести новый приемник DAB. [63] Несколько стран установили даты отключения аналоговых FM-сетей в пользу DAB / DAB+, в частности, Норвегия в 2017 году [64] и Швейцария в 2024 году. [65]
Одна станция DAB передает сигнал с полосой пропускания 1500 кГц , который передает 9–12 каналов цифрового звука, модулированного OFDM , из которых слушатель может выбирать. Вещательные компании могут передавать канал с разной скоростью передачи данных , поэтому разные каналы могут иметь разное качество звука. В разных странах станции DAB вещают либо в диапазоне III (174–240 МГц), либо в диапазоне L (1,452–1,492 ГГц) в диапазоне УВЧ, поэтому, как и прием FM, ограничен визуальным горизонтом примерно до 40 миль (64 км). [66] [63]
HD Radio — альтернативный стандарт цифрового радио, широко распространенный в Северной Америке. [67] HD Radio представляет собой внутриполосную внутриканальную технологию, которая передает цифровой сигнал на поднесущей аналогового FM- или AM-сигнала станции. Станции могут передавать более одного аудиосигнала на поднесущей, поддерживая передачу нескольких аудиоуслуг с разными скоростями передачи данных. [68] Цифровой сигнал передается с использованием OFDM с собственным форматом сжатия звука HDC ( кодирование высокой четкости ) . HDC основан на стандарте MPEG-4 HE-AAC , но не совместим с ним . [69] Он использует модифицированный алгоритм сжатия аудиоданных дискретного косинусного преобразования (MDCT) . [70]
Digital Radio Mondiale (DRM) — это конкурирующий стандарт цифрового наземного радио, разработанный в основном вещательными компаниями в качестве замены устаревшего AM- и FM-вещания с более высокой спектральной эффективностью . Mondiale означает «всемирный» на французском и итальянском языках; DRM был разработан в 2001 году и в настоящее время поддерживается 23 странами и принят некоторыми европейскими и восточными вещательными компаниями, начиная с 2003 года. Режим DRM30 использует полосы коммерческого вещания ниже 30 МГц и предназначен для замены стандартного AM-вещания на длинноволновые , средневолновые и коротковолновые диапазоны. В режиме DRM + используются частоты УКВ, сосредоточенные вокруг диапазона FM-вещания, и он предназначен для замены FM-вещания. Он несовместим с существующими радиоприемниками, поэтому слушателям необходимо приобрести новый приемник DRM. Используемая модуляция представляет собой форму OFDM , называемую COFDM , в которой до 4 несущих передаются по каналу, ранее занятому одним сигналом AM или FM, модулированным квадратурной амплитудной модуляцией (QAM). [71] [59]
Система DRM спроектирована так, чтобы быть максимально совместимой с существующими радиопередатчиками AM и FM, так что большая часть оборудования существующих радиостанций может продолжать использоваться, дополненная оборудованием модуляции DRM. [71] [59]
Телевещание — это передача по радио движущихся изображений, состоящих из последовательности неподвижных изображений, отображаемых на экране телевизионного приемника («телевизора» или телевизора) вместе с синхронизированным звуковым (звуковым) каналом. Телевизионные ( видео ) сигналы занимают более широкую полосу пропускания , чем радиовещательные ( аудио ) сигналы. Аналоговое телевидение , оригинальная телевизионная технология, требовало 6 МГц, поэтому полосы телевизионных частот разделены на каналы по 6 МГц, которые теперь называются «РЧ-каналами». [74]
The current television standard, introduced beginning in 2006, is a digital format called high-definition television (HDTV), which transmits pictures at higher resolution, typically 1080 pixels high by 1920 pixels wide, at a rate of 25 or 30 frames per second. Digital television (DTV) transmission systems, which replaced older analog television in a transition beginning in 2006, use image compression and high-efficiency digital modulation such as OFDM and 8VSB to transmit HDTV video within a smaller bandwidth than the old analog channels, saving scarce radio spectrum space. Therefore, each of the 6 MHz analog RF channels now carries up to 7 DTV channels – these are called "virtual channels". Digital television receivers have different behavior in the presence of poor reception or noise than analog television, called the "digital cliff" effect. Unlike analog television, in which increasingly poor reception causes the picture quality to gradually degrade, in digital television picture quality is not affected by poor reception until, at a certain point, the receiver stops working and the screen goes black.[75][76]
Terrestrial television, over-the-air (OTA) television, or broadcast television – the oldest television technology, is the transmission of television signals from land-based television stations to television receivers (called televisions or TVs) in viewer's homes. Terrestrial television broadcasting uses the bands 41 – 88 MHz (VHF low band or Band I, carrying RF channels 1–6), 174 – 240 MHz, (VHF high band or Band III; carrying RF channels 7–13), and 470 – 614 MHz (UHF Band IV and Band V; carrying RF channels 14 and up).[77] The exact frequency boundaries vary in different countries.[78] Propagation is by line-of-sight, so reception is limited by the visual horizon.[79] In the US, the effective radiated power (ERP) of television transmitters is regulated according to height above average terrain.[80] Viewers closer to the television transmitter can use a simple "rabbit ears" dipole antenna on top of the TV, but viewers in fringe reception areas typically require an outdoor antenna mounted on the roof to get adequate reception.[79]
(left) DISH Network's Super Dish 121 mounted on a rooftop. (right) A residential tower block with TV satellite dishes used by various users
Satellite television – a set-top box which receives subscription direct-broadcast satellite television, and displays it on an ordinary television. A direct broadcast satellite in geostationary orbit 22,200 miles (35,700 km) above the Earth's equator transmits many channels (up to 900) modulated on a 12.2 to 12.7 GHz Ku bandmicrowave downlink signal to a rooftop satellite dish antenna on the subscriber's residence. The microwave signal is converted to a lower intermediate frequency at the dish and conducted into the building by a coaxial cable to a set-top box connected to the subscriber's TV, where it is demodulated and displayed. The subscriber pays a monthly fee.[81][82]
Time and frequency
Government standard frequency and time signal services operate time radio stations which continuously broadcast extremely accurate time signals produced by atomic clocks, as a reference to synchronize other clocks.[83] Examples are BPC, DCF77, JJY, MSF, RTZ, TDF, WWV, and YVTO.[84] One use is in radio clocks and watches, which include an automated receiver that periodically (usually weekly) receives and decodes the time signal and resets the watch's internal quartz clock to the correct time, thus allowing a small watch or desk clock to have the same accuracy as an atomic clock. Government time stations are declining in number because GPS satellites and the Internet Network Time Protocol (NTP) provide equally accurate time standards.[85]
Two-way voice communication
A two-way radio is an audiotransceiver, a receiver and transmitter in the same device, used for bidirectional person-to-person voice communication with other users with similar radios. An older term for this mode of communication is radiotelephony. The radio link may be half-duplex, as in a walkie-talkie, using a single radio channel in which only one radio can transmit at a time, so different users take turns talking, pressing a "push to talk" button on their radio which switches off the receiver and switches on the transmitter. Or the radio link may be full duplex, a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.[86]
Cell phone – a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station (cell tower).[87] The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel transceiver. All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies. The purpose of cellular organization is to conserve radio bandwidth by frequency reuse. Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell, allowing the same frequencies to be reused in geographically separated cells. When a user carrying a cellphone crosses from one cell to another, his phone is automatically "handed off" seamlessly to the new antenna and assigned new frequencies. Cellphones have a highly automated full duplex digital transceiver using OFDM modulation using two digital radio channels, each carrying one direction of the bidirectional conversation, as well as a control channel that handles dialing calls and "handing off" the phone to another cell tower. Older 2G, 3G, and 4G networks use frequencies in the UHF and low microwave range, between 700 MHz and 3 GHz. The cell phone transmitter adjusts its power output to use the minimum power necessary to communicate with the cell tower; 0.6 W when near the tower, up to 3 W when farther away. Cell tower channel transmitter power is 50 W. Current generation phones, called smartphones, have many functions besides making telephone calls, and therefore have several other radio transmitters and receivers that connect them with other networks: usually a Wi-Fi modem, a Bluetooth modem, and a GPS receiver.[88][89][90]
5G cellular network – next-generation cellular networks which began deployment in 2019. Their major advantage is much higher data rates than previous cellular networks, up to 10 Gbps; 100 times faster than the previous cellular technology, 4G LTE. The higher data rates are achieved partly by using higher frequency radio waves, in the higher microwave band 3–6 GHz, and millimeter wave band, around 28 and 39 GHz. Since these frequencies have a shorter range than previous cellphone bands, the cells will be smaller than the cells in previous cellular networks which could be many miles across. Millimeter-wave cells will only be a few blocks long, and instead of a cell base station and antenna tower, they will have many small antennas attached to utility poles and buildings.[91][92]
Satellite phones, showing the large antennas needed to communicate with the satellite
Satellite phone (satphone) – a portable wireless telephone similar to a cell phone, connected to the telephone network through a radio link to an orbiting communications satellite instead of through cell towers. They are more expensive than cell phones; but their advantage is that, unlike a cell phone which is limited to areas covered by cell towers, satphones can be used over most or all of the geographical area of the Earth. In order for the phone to communicate with a satellite using a small omnidirectional antenna, first-generation systems use satellites in low Earth orbit, about 400–700 miles (640–1,100 km) above the surface. With an orbital period of about 100 minutes, a satellite can only be in view of a phone for about 4 – 15 minutes, so the call is "handed off" to another satellite when one passes beyond the local horizon. Therefore, large numbers of satellites, about 40 to 70, are required to ensure that at least one satellite is in view continuously from each point on Earth. Other satphone systems use satellites in geostationary orbit in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.[93][94]
Cordless phone – a landline telephone in which the handset is portable and communicates with the rest of the phone by a short-range full duplex radio link, instead of being attached by a cord. Both the handset and the base station have low-power radio transceivers that handle the short-range bidirectional radio link.[95] As of 2022[update], cordless phones in most nations use the DECT transmission standard.[96]
Motorola SCR-536 from WW2, the first walkie-talkie
Land mobile radio system – short-range mobile or portable half-duplex radio transceivers operating in the VHF or UHF band that can be used without a license. They are often installed in vehicles, with the mobile units communicating with a dispatcher at a fixed base station. Special systems with reserved frequencies are used by first responder services; police, fire, ambulance, and emergency services, and other government services. Other systems are made for use by commercial firms such as taxi and delivery services. VHF systems use channels in the range 30–50 MHz and 150–172 MHz. UHF systems use the 450–470 MHz band and in some areas the 470–512 MHz range. In general, VHF systems have a longer range than UHF but require longer antennas. AM or FM modulation is mainly used, but digital systems such as DMR are being introduced. The radiated power is typically limited to 4 watts.[87] These systems have a fairly limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain. Repeaters installed on tall buildings, hills, or mountain peaks are often used to increase the range when it is desired to cover a larger area than line-of-sight. Examples of land mobile systems are CB, FRS, GMRS, and MURS. Modern digital systems, called trunked radio systems, have a digital channel management system using a control channel that automatically assigns frequency channels to user groups.[97]
Firefighter using modern walkie-talkie
Walkie-talkie – a battery-powered portable handheld half-duplex two-way radio, used in land mobile radio systems.[98]
Airband – Half-duplex radio system used by aircraft pilots to talk to other aircraft and ground-based air traffic controllers. This vital system is the main communication channel for air traffic control. For most communication in overland flights in air corridors a VHF-AM system using channels between 108 and 137 MHz in the VHF band is used. This system has a typical transmission range of 200 miles (320 km) for aircraft flying at cruising altitude.[99][100] For flights in more remote areas, such as transoceanic airline flights, aircraft use the HF band or channels on the Inmarsat or Iridium satphone satellites.[101] Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95 MHz.[102]
VHF marine radio on a ship
Marine radio – medium-range transceivers on ships, used for ship-to-ship, ship-to-air, and ship-to-shore communication with harbormasters They use FM channels between 156 and 174 MHz in the VHF band with up to 25 watts power, giving them a range of about 60 miles (97 km). Some channels are half-duplex and some are full-duplex, to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.[103]
Amateur radio – long-range half-duplex two-way radio used by hobbyists for non-commercial purposes: recreational radio contacts with other amateurs, volunteer emergency communication during disasters, contests, and experimentation. Radio amateurs must hold an amateur radio license and are given a unique callsign that must be used as an identifier in transmissions. Amateur radio is restricted to small frequency bands, the amateur radio bands, spaced throughout the radio spectrum starting at 136 kHz. Within these bands, amateurs are allowed the freedom to transmit on any frequency using a wide variety of voice modulation methods, along with other forms of communication, such as slow-scan television (SSTV), and radioteletype (RTTY). Additionally, amateurs are among the only radio operators still using Morse coderadiotelegraphy.[104]
One-way voice communication
One way, unidirectional radio transmission is called simplex.
Baby monitor – a crib-side appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parent, so they can monitor the baby while they are in other parts of the house.[105] The wavebands used vary by region, but analog baby monitors generally transmit with low power in the 16, 9.3–49.9 or 900 MHz wavebands, and digital systems in the 2.4 GHz waveband.[106] Many baby monitors have duplex channels so the parent can talk to the baby, and cameras to show video of the baby.[107]
Wireless microphone – a battery-powered microphone with a short-range transmitter that is handheld or worn on a person's body which transmits its sound by radio to a nearby receiver unit connected to a sound system. Wireless microphones are used by public speakers, performers, and television personalities so they can move freely without trailing a microphone cord. Traditionally, analog models transmit in FM on unused portions of the television broadcast frequencies in the VHF and UHF bands. Some models transmit on two frequency channels for diversity reception to prevent nulls from interrupting transmission as the performer moves around.[108] Some models use digital modulation to prevent unauthorized reception by scanner radio receivers; these operate in the 900 MHz, 2.4 GHz or 6 GHz ISM bands.[109] European standards also support wireless multichannel audio systems (WMAS) that can better support the use of large numbers of wireless microphones at a single event or venue. As of 2021[update], U.S. regulators were considering adopting rules for WMAS.[110]
Data communication
Wireless networking – automated radio links which transmit digital data between computers and other wireless devices using radio waves, linking the devices together transparently in a computer network. Computer networks can transmit any form of data: in addition to email and web pages, they also carry phone calls (VoIP), audio, and video content (called streaming media). Security is more of an issue for wireless networks than for wired networks since anyone nearby with a wireless modem can access the signal and attempt to log in. The radio signals of wireless networks are encrypted using WPA.[111]
Wireless LAN (wireless local area network or Wi-Fi) – based on the IEEE 802.11 standards, these are the most widely used computer networks, used to implement local area networks without cables, linking computers, laptops, cell phones, video game consoles, smart TVs and printers in a home or office together, and to a wireless router connecting them to the Internet with a wire or cable connection. Wireless routers in public places like libraries, hotels and coffee shops create wireless access points (hotspots) to allow the public to access the Internet with portable devices like smartphones, tablets or laptops. Each device exchanges data using a wireless modem (wireless network interface controller), an automated microwave transmitter and receiver with an omnidirectional antenna that works in the background, exchanging data packets with the router. Wi-Fi uses channels in the 2.4 GHz and 5 GHz ISM bands with OFDM (orthogonal frequency-division multiplexing) modulation to transmit data at high rates. The transmitters in Wi-Fi modems are limited to a radiated power of 200 mW to 1 watt, depending on country. They have a maximum indoor range of about 150 ft (50 m) on 2.4 GHz and 50 ft (20 m) on 5 GHz.[112]
Neighborhood wireless WAN router on telephone pole
Wireless WAN (wireless wide area network, WWAN) – a variety of technologies that provide wireless internet access over a wider area than Wi-Fi networks do – from an office building to a campus to a neighborhood, or to an entire city. The most common technologies used are: cellular modems, that exchange computer data by radio with cell towers; satellite internet access; and lower frequencies in the UHF band, which have a longer range than Wi-Fi frequencies. Since WWAN networks are much more expensive and complicated to administer than Wi-Fi networks, their use so far has generally been limited to private networks operated by large corporations.[112]
Bluetooth – a very short-range wireless interface on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless headphones. In the most widely used mode, transmission power is limited to 1 milliwatt, giving it a very short range of up to 10 m (30 feet). The system uses frequency-hopping spread spectrum transmission, in which successive data packets are transmitted in a pseudorandom order on one of 79 1 MHz Bluetooth channels between 2.4 and 2.83 GHz in the ISM band. This allows Bluetooth networks to operate in the presence of noise, other wireless devices and other Bluetooth networks using the same frequencies, since the chance of another device attempting to transmit on the same frequency at the same time as the Bluetooth modem is low. In the case of such a "collision", the Bluetooth modem just retransmits the data packet on another frequency.[113]
Packet radio – a long-distance peer-to-peerwireless ad-hoc network in which data packets are exchanged between computer-controlled radio modems (transmitter/receivers) called nodes, which may be separated by miles, and maybe mobile. Each node only communicates with neighboring nodes, so packets of data are passed from node to node until they reach their destination using the X.25 network protocol. Packet radio systems are used to a limited degree by commercial telecommunications companies and by the amateur radio community.[114]
Text messaging (texting) – this is a service on cell phones, allowing a user to type a short alphanumeric message and send it to another phone number, and the text is displayed on the recipient's phone screen. It is based on the Short Message Service (SMS) which transmits using spare bandwidth on the control radio channel used by cell phones to handle background functions like dialing and cell handoffs. Due to technical limitations of the channel, text messages are limited to 160 alphanumeric characters.[115]
Microwave relay – a long-distance high bandwidth point-to-point digital data transmission link consisting of a microwave transmitter connected to a dish antenna that transmits a beam of microwaves to another dish antenna and receiver. Since the antennas must be in line-of-sight, distances are limited by the visual horizon to 30–40 miles (48–64 km). Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companies to transmit long-distance phone calls and television signals between cities.[116][117]
Telemetry – automated one-way (simplex) transmission of measurements and operation data from a remote process or device to a receiver for monitoring. Telemetry is used for in-flight monitoring of missiles, drones, satellites, and weather balloonradiosondes, sending scientific data back to Earth from interplanetary spacecraft, communicating with electronic biomedical sensors implanted in the human body, and well logging. Multiple channels of data are often transmitted using frequency-division multiplexing or time-division multiplexing.[118] Telemetry is starting to be used in consumer applications such as:
Automated meter reading – electric power meters, water meters, and gas meters that, when triggered by an interrogation signal, transmit their readings by radio to a utility reader vehicle at the curb, to eliminate the need for an employee to go on the customer's property to manually read the meter.[119]
Electronic toll collection – on toll roads, an alternative to manual collection of tolls at a toll booth, in which a transponder in a vehicle, when triggered by a roadside transmitter, transmits a signal to a roadside receiver to register the vehicle's use of the road, enabling the owner to be billed for the toll.[120]
RFID tag from a DVD
Radio Frequency Identification (RFID) – identification tags containing a tiny radio transponder (receiver and transmitter) which are attached to merchandise. When it receives an interrogation pulse of radio waves from a nearby reader unit, the tag transmits back an ID number, which can be used to inventory goods. Passive tags, the most common type, have a chip powered by the radio energy received from the reader, rectified by a diode, and can be as small as a grain of rice. They are incorporated in products, clothes, railroad cars, library books, airline baggage tags and are implanted under the skin in pets and livestock (microchip implant) and even people. Privacy concerns have been addressed with tags that use encrypted signals and authenticate the reader before responding. Passive tags use 125–134 kHz, 13, 900 MHz and 2.4 and 5 GHz ISM bands and have a short range. Active tags, powered by a battery, are larger but can transmit a stronger signal, giving them a range of hundreds of meters.[121]
Submarine communication – When submerged, submarines are cut off from all ordinary radio communication with their military command authorities by the conductive seawater. However radio waves of low enough frequencies, in the VLF (30 to 3 kHz) and ELF (below 3 kHz) bands are able to penetrate seawater. Navies operate large shore transmitting stations with power output in the megawatt range to transmit encrypted messages to their submarines in the world's oceans. Due to the small bandwidth, these systems cannot transmit voice, only text messages at a slow data rate. The communication channel is one-way, since the long antennas needed to transmit VLF or ELF waves cannot fit on a submarine. VLF transmitters use miles long wire antennas like umbrella antennas. A few nations use ELF transmitters operating around 80 Hz, which can communicate with submarines at lower depths. These use even larger antennas called ground dipoles, consisting of two ground (Earth) connections 23–60 km (14–37 mi) apart, linked by overhead transmission lines to a power plant transmitter.[122][123]
This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft. Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft. In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic "dish" antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwave band are used, since microwaves pass through the ionosphere without refraction, and at microwave frequencies the high-gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF, L, C, S, ku and ka band are allocated for space communication. A radio link that transmits data from the Earth's surface to a spacecraft is called an uplink, while a link that transmits data from the spacecraft to the ground is called a downlink.[125]
Communication satellite – an artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth. These are used because the microwaves used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth. As of 1 January 2021[update], there were 2,224 communications satellites in Earth orbit.[126] Most are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it. In a satellite ground station a microwave transmitter and large satellite dish antenna transmit a microwave uplink beam to the satellite. The uplink signal carries many channels of telecommunications traffic, such as long-distance telephone calls, television programs, and internet signals, using a technique called frequency-division multiplexing (FDM). On the satellite, a transponder receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first. There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines. Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.[127]
Direct broadcast satellite – a geostationary communication satellite that transmits retail programming directly to receivers in subscriber's homes and vehicles on Earth, in satellite radio and TV systems. It uses a higher transmitter power than other communication satellites, to allow the signal to be received by consumers with a small unobtrusive antenna. For example, satellite television uses downlink frequencies from 12.2 to 12.7 GHz in the ku band transmitted at 100 to 250 watts, which can be received by relatively small 43–80 cm (17–31 in) satellite dishes mounted on the outside of buildings.[128]
Radar
Military air traffic controller on US Navy aircraft carrier monitors aircraft on radar screen
Radar is a radiolocation method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain. A radar set consists of a transmitter and receiver.[129][130] The transmitter emits a narrow beam of radio waves which is swept around the surrounding space. When the beam strikes a target object, radio waves are reflected back to the receiver. The direction of the beam reveals the object's location. Since radio waves travel at a constant speed close to the speed of light, by measuring the brief time delay between the outgoing pulse and the received "echo", the range to the target can be calculated. The targets are often displayed graphically on a map display called a radar screen. Doppler radar can measure a moving object's velocity, by measuring the change in frequency of the return radio waves due to the Doppler effect.[131]
Radar sets mainly use high frequencies in the microwave bands, because these frequencies create strong reflections from objects the size of vehicles and can be focused into narrow beams with compact antennas.[130]Parabolic (dish) antennas are widely used. In most radars the transmitting antenna also serves as the receiving antenna; this is called a monostatic radar. A radar which uses separate transmitting and receiving antennas is called a bistatic radar.[132]
ASR-8 airport surveillance radar antenna. It rotates once every 4.8 seconds. The rectangular antenna on top is the secondary radar.
Airport surveillance radar – In aviation, radar is the main tool of air traffic control. A rotating dish antenna sweeps a vertical fan-shaped beam of microwaves around the airspace and the radar set shows the location of aircraft as "blips" of light on a display called a radar screen. Airport radar operates at 2.7 – 2.9 GHz in the microwave S band. In large airports the radar image is displayed on multiple screens in an operations room called the TRACON (Terminal Radar Approach Control), where air traffic controllers direct the aircraft by radio to maintain safe aircraft separation.[133]
Secondary surveillance radar – Aircraft carry radar transponders, transceivers which when triggered by the incoming radar signal transmit a return microwave signal. This causes the aircraft to show up more strongly on the radar screen. The radar which triggers the transponder and receives the return beam, usually mounted on top of the primary radar dish, is called the secondary surveillance radar. Since radar cannot measure an aircraft's altitude with any accuracy, the transponder also transmits back the aircraft's altitude measured by its altimeter, and an ID number identifying the aircraft, which is displayed on the radar screen.[134]
Electronic countermeasures (ECM) – Military defensive electronic systems designed to degrade enemy radar effectiveness, or deceive it with false information, to prevent enemies from locating local forces. It often consists of powerful microwave transmitters that can mimic enemy radar signals to create false target indications on the enemy radar screens.[135]
Rotating marine radar antenna on a ship
Marine radar – an S or X band radar on ships used to detect nearby ships and obstructions like bridges.[136] A rotating antenna sweeps a vertical fan-shaped beam of microwaves around the water surface surrounding the craft out to the horizon.
Phased-array radar – a radar set that uses a phased array, a computer-controlled antenna that can steer the radar beam quickly to point in different directions without moving the antenna. Phased-array radars were developed by the military to track fast-moving missiles and aircraft. They are widely used in military equipment and are now spreading to civilian applications.[138]
Synthetic aperture radar (SAR) – a specialized airborne radar set that produces a high-resolution map of ground terrain. The radar is mounted on an aircraft or spacecraft and the radar antenna radiates a beam of radio waves sideways at right angles to the direction of motion, toward the ground. In processing the return radar signal, the motion of the vehicle is used to simulate a large antenna, giving the radar a higher resolution.[139]
Ground-penetrating radar – a specialized radar instrument that is rolled along the ground surface in a cart and transmits a beam of radio waves into the ground, producing an image of subsurface objects. Frequencies from 100 MHz to a few GHz are used. Since radio waves cannot penetrate very far into earth, the depth of GPR is limited to about 50 feet.[140]
Collision avoidance system – a short range radar or LIDAR system on an automobile or vehicle that detects if the vehicle is about to collide with an object and applies the brakes to prevent the collision.[141]
Radar fuze – a detonator for an aerial bomb which uses a radar altimeter to measure the height of the bomb above the ground as it falls and detonates it at a certain altitude.[142]
Radiolocation
Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects, or for navigation.[143]
An early iPhone with its GPS navigation app in use.
Global Navigation Satellite System (GNSS) or satnav system – A system of satellites which allows geographical location on Earth (latitude, longitude, and altitude/elevation) to be determined to high precision (within a few metres) by small portable navigation instruments, by timing the arrival of radio signals from the satellites. These are the most widely used navigation systems today. The main satellite navigation systems are the US Global Positioning System (GPS), Russia's GLONASS, China's BeiDou Navigation Satellite System (BDS) and the European Union's Galileo.[144]
Global Positioning System (GPS) – The most widely used satellite navigation system, maintained by the US Air Force, which uses a constellation of 31 satellites in low Earth orbit. The orbits of the satellites are distributed so at any time at least four satellites are above the horizon over each point on Earth. Each satellite has an onboard atomic clock and transmits a continuous radio signal containing a precise time signal as well as its current position. Two frequencies are used, 1.2276 and 1.57542 GHz. Since the velocity of radio waves is virtually constant, the delay of the radio signal from a satellite is proportional to the distance of the receiver from the satellite. By receiving the signals from at least four satellites a GPS receiver can calculate its position on Earth by comparing the arrival time of the radio signals. Since each satellite's position is known precisely at any given time, from the delay the position of the receiver can be calculated by a microprocessor in the receiver. The position can be displayed as latitude and longitude, or as a marker on an electronic map. GPS receivers are incorporated in almost all cellphones and in vehicles such as automobiles, aircraft, and ships, and are used to guide drones, missiles, cruise missiles, and even artillery shells to their target, and handheld GPS receivers are produced for hikers and the military.[145][146]
Radio beacon – a fixed location terrestrial radio transmitter which transmits a continuous radio signal used by aircraft and ships for navigation. The locations of beacons are plotted on navigational maps used by aircraft and ships.[147]
VHF omnidirectional range (VOR) – a worldwide aircraft radio navigation system consisting of fixed ground radio beacons transmitting between 108.00 and 117.95 MHz in the very high frequency (VHF) band. An automated navigational instrument on the aircraft displays a bearing to a nearby VOR transmitter. A VOR beacon transmits two signals simultaneously on different frequencies. A directional antenna transmits a beam of radio waves that rotates like a lighthouse at a fixed rate, 30 times per second. When the directional beam is facing north, an omnidirectional antenna transmits a pulse. By measuring the difference in phase of these two signals, an aircraft can determine its bearing (or "radial") from the station accurately. By taking a bearing on two VOR beacons an aircraft can determine its position (called a "fix") to an accuracy of about 90 metres (300 ft). Most VOR beacons also have a distance measuring capability, called distance measuring equipment (DME); these are called VOR/DME's. The aircraft transmits a radio signal to the VOR/DME beacon and a transponder transmits a return signal. From the propagation delay between the transmitted and received signal the aircraft can calculate its distance from the beacon. This allows an aircraft to determine its location "fix" from only one VOR beacon. Since line-of-sight VHF frequencies are used VOR beacons have a range of about 200 miles for aircraft at cruising altitude. TACAN is a similar military radio beacon system which transmits in 962–1213 MHz, and a combined VOR and TACAN beacon is called a VORTAC. The number of VOR beacons is declining as aviation switches to the RNAV system that relies on Global Positioning System satellite navigation.[148][149]
Non-directional beacon (NDB) – Legacy fixed radio beacons used before the VOR system that transmit a simple signal in all directions for aircraft or ships to use for radio direction finding. Aircraft use automatic direction finder (ADF) receivers which use a directional antenna to determine the bearing to the beacon. By taking bearings on two beacons they can determine their position. NDBs use frequencies between 190 and 1750 kHz in the LF and MF bands which propagate beyond the horizon as ground waves or skywaves much farther than VOR beacons. They transmit a callsign consisting of one to 3 Morse code letters as an identifier.[150]
EPIRB emergency locator beacon on a ship
Emergency locator beacon – a portable battery powered radio transmitter used in emergencies to locate airplanes, vessels, and persons in distress and in need of immediate rescue. Various types of emergency locator beacons are carried by aircraft, ships, vehicles, hikers and cross-country skiers. In the event of an emergency, such as the aircraft crashing, the ship sinking, or a hiker becoming lost, the transmitter is deployed and begins to transmit a continuous radio signal, which is used by search and rescue teams to quickly find the emergency and render aid. The latest generation Emergency Position Indicating Rescue Beacons (EPIRBs) contain a GPS receiver, and broadcast to rescue teams their exact location within 20 meters.[151]
Cospas-Sarsat – an international humanitarian consortium of governmental and private agencies which acts as a dispatcher for search and rescue operations. It operates a network of about 47 satellites carrying radio receivers, which detect distress signals from emergency locator beacons anywhere on Earth transmitting on the international Cospas distress frequency of 406 MHz. The satellites calculate the geographic location of the beacon within 2 km by measuring the Doppler frequency shift of the radio waves due to the relative motion of the transmitter and the satellite, and quickly transmit the information to the appropriate local first responder organizations, which perform the search and rescue.[152][153]
Radio direction finding (RDF) – this is a general technique, used since the early 1900s, of using specialized radio receivers with directional antennas (RDF receivers) to determine the exact bearing of a radio signal, to determine the location of the transmitter. The location of a terrestrial transmitter can be determined by simple triangulation from bearings taken by two RDF stations separated geographically, as the point where the two bearing lines cross, this is called a "fix". Military forces use RDF to locate enemy forces by their tactical radio transmissions, counterintelligence services use it to locate clandestine transmitters used by espionage agents, and governments use it to locate unlicensed transmitters or interference sources. Older RDF receivers used rotatable loop antennas, the antenna is rotated until the radio signal strength is weakest, indicating the transmitter is in one of the antenna's two nulls. The nulls are used since they are sharper than the antenna's lobes (maxima). More modern receivers use phased array antennas which have a much greater angular resolution.[154][155]
Animal migration tracking – a widely used technique in wildlife biology, conservation biology, and wildlife management in which small battery-powered radio transmitters are attached to wild animals so their movements can be tracked with a directional RDF receiver. Sometimes the transmitter is implanted in the animal. The VHF band is typically used since antennas in this band are fairly compact. The receiver has a directional antenna (typically a small Yagi) which is rotated until the received signal is strongest; at this point the antenna is pointing in the direction of the animal. Sophisticated systems used in recent years use satellites to track the animal, or geolocation tags with GPS receivers which record and transmit a log of the animal's location.[156]
Remote control
US Air Force MQ-1 Predator drone flown remotely by a pilot on the ground
Radio remote control is the use of electronic control signals sent by radio waves from a transmitter to control the actions of a device at a remote location. Remote control systems may also include telemetry channels in the other direction, used to transmit real-time information on the state of the device back to the control station. Uncrewed spacecraft are an example of remote-controlled machines, controlled by commands transmitted by satellite ground stations. Most handheld remote controls used to control consumer electronics products like televisions or DVD players actually operate by infrared light rather than radio waves, so are not examples of radio remote control. A security concern with remote control systems is spoofing, in which an unauthorized person transmits an imitation of the control signal to take control of the device.[157] Examples of radio remote control:
Unmanned aerial vehicle (UAV, drone) – A drone is an aircraft without an onboard pilot, flown by remote control by a pilot in another location, usually in a piloting station on the ground. They are used by the military for reconnaissance and ground attack, and more recently by the civilian world for news reporting and aerial photography. The pilot uses aircraft controls like a joystick or steering wheel, which create control signals which are transmitted to the drone by radio to control the flight surfaces and engine. A telemetry system transmits back a video image from a camera in the drone to allow the pilot to see where the aircraft is going, and data from a GPS receiver giving the real-time position of the aircraft. UAVs have sophisticated onboard automatic pilot systems that maintain stable flight and only require manual control to change directions.[158]
Remote keyless entry fob for a car
Keyless entry system – a short-range handheld battery powered key fob transmitter, included with most modern cars, which can lock and unlock the doors of a vehicle from outside, eliminating the need to use a key. When a button is pressed, the transmitter sends a coded radio signal to a receiver in the vehicle, operating the locks. The fob must be close to the vehicle, typically within 5 to 20 meters. North America and Japan use a frequency of 315 MHz, while Europe uses 433.92 and 868 MHz. Some models can also remotely start the engine, to warm up the car. A security concern with all keyless entry systems is a replay attack, in which a thief uses a special receiver ("code grabber") to record the radio signal during opening, which can later be replayed to open the door. To prevent this, keyless systems use a rolling code system in which a pseudorandom number generator in the remote control generates a different random key each time it is used. To prevent thieves from simulating the pseudorandom generator to calculate the next key, the radio signal is also encrypted.[159]
Garage door opener – a short-range handheld transmitter which can open or close a building's electrically operated garage door from outside, so the owner can open the door upon arrival, and close it after departure. When a button is pressed the control transmits a coded FSK radio signal to a receiver in the opener, raising or lowering the door. Modern openers use 310, 315 or 390 MHz. To prevent a thief using a replay attack, modern openers use a rolling code system.[160][161]
Radio-controlled models – a popular hobby is playing with radio-controlled model boats, cars, airplanes, and helicopters (quadcopters) which are controlled by radio signals from a handheld console with a joystick. Most recent transmitters use the 2.4 GHz ISM band with multiple control channels modulated with PWM, PCM or FSK.[162]
Wireless doorbell – A residential doorbell that uses wireless technology to eliminate the need to run wires through the building walls. It consists of a doorbell button beside the door containing a small battery powered transmitter. When the doorbell is pressed it sends a signal to a receiver inside the house with a speaker that sounds chimes to indicate someone is at the door. They usually use the 2.4 GHz ISM band. The frequency channel used can usually be changed by the owner in case another nearby doorbell is using the same channel.[163][164]
Jamming
Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals. Jamming devices are called "signal suppressors" or "interference generators" or just jammers.[165]
During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, some totalitarian countries which practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries. Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter.[166][167]
US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.[168]
Scientific research
Radio astronomy is the scientific study of radio waves emitted by astronomical objects. Radio astronomers use radio telescopes, large radio antennas and receivers, to receive and study the radio waves from astronomical radio sources. Since astronomical radio sources are so far away, the radio waves from them are extremely weak, requiring extremely sensitive receivers, and radio telescopes are the most sensitive radio receivers in existence. They use large parabolic (dish) antennas up to 500 meters (2,000 ft) in diameter to collect enough radio wave energy to study. The RF front end electronics of the receiver is often cooled by liquid nitrogen to reduce thermal noise. Multiple antennas are often linked together in arrays which function as a single antenna, to increase collecting power. In Very Long Baseline Interferometry (VLBI) radio telescopes on different continents are linked, which can achieve the resolution of an antenna thousands of miles in diameter.[169][170]
Remote sensing – in radio, remote sensing is the reception of electromagnetic waves radiated by natural objects or the atmosphere for scientific research. All warm objects emit microwaves and the spectrum emitted can be used to determine temperature. Microwave radiometers are used in meteorology and earth sciences to determine temperature of the atmosphere and earth surface, as well as chemical reactions in the atmosphere.[171][172]
^"Radio". Oxford Living Dictionaries. Oxford University Press. 2019. Archived from the original on 24 March 2019. Retrieved 26 February 2019.
^"Definition of radio". Encyclopedia. PCMagazine website, Ziff-Davis. 2018. Retrieved 26 February 2019.
^ a b c dEllingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press. pp. 1–4. ISBN 978-1316785164.
^ a b c d"125 Years Discovery of Electromagnetic Waves". Karlsruhe Institute of Technology. 16 May 2022. Archived from the original on 14 July 2022. Retrieved 14 July 2022.
^ a bBondyopadhyay, Prebir K. (1995) "Guglielmo Marconi – The father of long distance radio communication – An engineer's tribute", 25th European Microwave Conference: Volume 2, pp. 879–85
^ a b"1890s – 1930s: Radio". Elon University. Archived from the original on 8 June 2022. Retrieved 14 July 2022.
^ a b"History of Commercial Radio". Federal Communications Commission. 23 October 2020. Archived from the original on 1 January 2022. Retrieved 14 July 2022.
^"radio (n.)". Online Etymology Dictionary. Retrieved 13 July 2022.
^Bell, Alexander Graham (July 1881). "Production of Sound by Radiant Energy". Popular Science Monthly. pp. 329–330. [W]e have named the apparatus for the production and reproduction of sound in this way the "photophone", because an ordinary beam of light contains the rays which are operative. To avoid in future any misunderstandings upon this point, we have decided to adopt the term "radiophone", proposed by M. Mercadier, as a general term signifying the production of sound by any form of radiant energy...
^Manning, Trevor (2009). Microwave Radio Transmission Design Guide. Artech House. p. 2.
^Maver, William Jr. (1903). American Telegraphy and Encyclopedia of the Telegraph: Systems, Apparatus, Operation. New York: Maver Publishing Co. p. 333. wireless telegraphy.
^Steuart, William Mott; et al. (1906). Special Reports: Telephones and Telegraphs 1902. Washington D.C.: U.S. Bureau of the Census. pp. 118–119.
^ a b c dhttps://earlyradiohistory.us/sec022.htm Thomas H. White, United States Early Radio History, Section 22
^Collins, A. Frederick (10 May 1902). "The Genesis of Wireless Telegraphy". Electrical World and Engineer. p. 811.
^"Wireless Telegraphy". The Practical Engineer. 25 February 1898. p. 174. Dr. O. J. Lodge, who preceded Marconi in making experiments in what may be called "ray" telegraphy or radiotelegraphy by a year or two, has devised a new method of sending and receiving the messages. The reader will understand that in the radiotelegraph electric waves forming the signals of the message starting from the sending instrument and travel in all directions like rays of light from a lamp, only they are invisible.
^"Wireless Telegraphy", The Electrical Review (London), 20 January 1905, page 108, quoting from the British Post Office's 30 December 1904 Post Office Circular.
^"Interference with Wireless Messages", Electrical World, 22 June 1907, page 1270.
^Sungook Hong (2001), Wireless: From Marconi's Black-box to the Audion, MIT Press, pp. 5–10
^"The Nobel Prize in Physics 1909". NobelPrize.org. 2023. Archived from the original on 31 July 2023. Retrieved 31 July 2023.
^Kraus, John D. (1988). Antennas (2nd ed.). Tata-McGraw Hill. p. 50. ISBN 0074632191.
^Serway, Raymond; Faughn, Jerry; Vuille, Chris (2008). College Physics, 8th Ed. Cengage Learning. p. 714. ISBN 978-0495386933.
^Balanis, Constantine A. (2005). Antenna theory: Analysis and Design, 3rd Ed. John Wiley and Sons. p. 10. ISBN 978-1118585733.
^ a b c dEllingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press. pp. 16–17. ISBN 978-1316785164.
^Visser, Hubregt J. (2012). Antenna Theory and Applications. John Wiley & Sons. ISBN 978-1119990253. Retrieved 29 August 2022.
^Zainah Md Zain; Hamzah Ahmad; Dwi Pebrianti; Mahfuzah Mustafa; Nor Rul Hasma Abdullah; Rosdiyana Samad; Maziyah Mat Noh (2020). Proceedings of the 11th National Technical Seminar on Unmanned System Technology 2019: NUSYS'19. Springer Nature. p. 535. ISBN 978-9811552816. Extract of pp. 535–536
^"Omnidirectional Antenna - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 5 September 2022.
^"Electromagnetic Radiation". NASA. Archived from the original on 23 May 2016. Retrieved 18 August 2022.
^"How far can radio waves travel in vacuum? and light waves?". Physics Stack Exchange. July 2019. Archived from the original on 18 August 2022. Retrieved 18 August 2022.
^ a b cBrain, Marshall (7 December 2000). "How Radio Works". HowStuffWorks.com. Retrieved 11 September 2009.
^ a b c d e f g hFaruque, Saleh (2016). Radio Frequency Modulation Made Easy. Springer Publishing. ISBN 978-3319412023. Retrieved 29 August 2022.
^Mustafa Ergen (2009). Mobile Broadband: including WiMAX and LTE. Springer Science+Business Media. doi:10.1007/978-0-387-68192-4. ISBN 978-0387681894.
^Tony Dorbuck (ed.), The Radio Amateur's Handbook, Fifty-Fifth Edition, American Radio Relay League, 1977, p. 368
^John Avison, The World of Physics, Nelson · 2014, page 367
^C-W and A-M Radio Transmitters and Receivers, United States. Department of the Army – 1952, pp. 167–168
^ a b c d"Spectrum 101" (PDF). US National Aeronautics and Space Administration (NASA). February 2016. Archived (PDF) from the original on 11 February 2017. Retrieved 2 December 2019., p. 6
^ a b cPogorel, Girard; Chaduc, Jean-Marc (2010). The Radio Spectrum: Managing a Strategic Resource. Wiley). ISBN 978-0470393529. Retrieved 29 August 2022.
^Norberg, Bob (27 November 2022). "Digital Radio Is Coming, But Analog Isn't Dead Yet". The Ledger. Archived from the original on 3 September 2022. Retrieved 3 September 2022.
^"Analogue To Digital: Radio Slow To Tune Into Transition". Financial Express. 13 October 2005. Archived from the original on 3 September 2022. Retrieved 3 September 2022.
^"Radio Regulations, 2016 Edition" (PDF). International Telecommunication Union. 3 November 2016. Retrieved 9 November 2019. Article 2, Section 1, p.27
^ a bNomenclature of the frequency and wavelength bands used in telecommunications (PDF) (Report). Geneva: International Telecommunications Union. 2015. ITU-R V.431-8. Retrieved 6 April 2023.
^Communications-electronics Management of the Electromagnetic Spectrum (Report). Headquarters, Department of the Army. United States Department of the Army. 1973. p. 2.
^Duncan, Christopher; Gkountouna, Olga; Mahabir, Ron (2021). "Theoretical Applications of Magnetic Fields at Tremendously Low Frequency in Remote Sensing and Electronic Activity Classification". In Arabnia, Hamid R.; Deligiannidis, Leonidas; Shouno, Hayaru; Tinetti, Fernando G.; Tran, Quoc-Nam (eds.). Advances in Computer Vision and Computational Biology. Transactions on Computational Science and Computational Intelligence. Cham: Springer International Publishing. pp. 235–247. doi:10.1007/978-3-030-71051-4_18. ISBN 978-3030710507. S2CID 238934419.
^"Radio Frequency Interference Best Practices Guidebook - CISA - Feb. 2020" (PDF). Cybersecurity and Infrastructure Security Agency SAFECOM/National Council of Statewide Interoperability Coordinators. USDepartment of Homeland Security. Retrieved 29 August 2022.
^Mazar (Madjar), Haim (2016). Radio Spectrum Management: Policies, Regulations and Techniques. Wiley. ISBN 978-1118511794. Retrieved 29 August 2022.
^"ARTICLE 19 Identification of stations" (PDF). International Telecommunication Union. Retrieved 29 August 2022.
^"Commercial Radio Operator Types of Licenses". Federal Communications Commission. 6 May 2016. Retrieved 29 August 2022.
^Dichoso, Joe (October 9, 2007). "FCC Basics of Unlicensed Transmitters" (PDF). Federal Communications Commission. Retrieved 29 August 2022.
^Pizzi, Skip; Jones, Graham (2014). A Broadcast Engineering Tutorial for Non-Engineers, 4th Ed. National Association of Broadcasters, Taylor and Francis. ISBN 978-0415733397.
^Witten, Alan Joel (2017). Handbook of Geophysics and Archaeology. Routledge. ISBN 978-1351564588. Retrieved 30 August 2022.
^Bonsor, Kevin (26 September 2001). "How Satellite Radio Works". howstuffworks.com. HowStuffWorks. Retrieved 30 August 2022.
^Gosling, William (1998). Radio Antennas and Propagation: Radio Engineering Fundamentals. Newnes. ISBN 978-0750637411. Retrieved 30 August 2022.
^Griffin, B. Whitfield (2000). Radio-electronic Transmission Fundamentals. SciTech Publishing/Noble. ISBN 978-1884932137. Retrieved 30 August 2022.
^Pizzi, Skip; Jones, Graham (2014). A Broadcast Engineering Tutorial for Non-Engineers. CRC Press/Focal Press. ISBN 978-1317906834. Retrieved 30 August 2022.
^Perez, Reinaldo (2013). Handbook of Electromagnetic Compatibility. Academic Press. ISBN 978-1483288970. Retrieved 30 August 2022.
^Green, Clarence R.; Bourque, Robert M. (1980). The Theory and Servicing of AM, FM, and FM Stereo Receivers. Prentice-Hall. p. 6.
^"Appendix C: Glossary" (PDF). Radio – Preparing for the Future (Report). London: Ofcom. October 2005. p. 2.
^ a bGupta, Rakesh (2021). Education Technology in Physical Education and Sports. Audio Visual Media in Physical Education. India: Friends Publications. ISBN 978-9390649808. Retrieved 30 August 2022.
^ a b cBerg, Jerome S. (2008). Broadcasting on the Short Waves: 1945 to today. McFarland. ISBN 978-0786451982. Retrieved 30 August 2022.
^Sterling, Christopher H.; Kieth, Michael C. (2009). Sounds of Change: A history of FM broadcasting in America. University of North Carolina Press. ISBN 978-0807877555. Retrieved 30 August 2022.
^Digital Radio Guide (PDF) (Report). Switzerland: World Broadcasting Unions. 2017.
^Baker, William (2020). "DAB vs. FM: The differences between analog and digital radio". Radio Fidelity online magazine. Retrieved 14 September 2020.
^ a bHoeg, Wolfgang; Lauterbach, Thomas (2004). Digital Audio Broadcasting: Principles and applications of digital radio. John Wiley & Sons. ISBN 978-0470871423. Retrieved 30 August 2022.
^Revel, Timothy (10 January 2017). "Norway is first country to turn off FM radio and go digital-only". New Scientist. Retrieved 4 September 2022.
^McLane, Paul (30 August 2021). "Swiss FM shutdown reverts to original 2024 date". Radio World. Retrieved 4 September 2022.
^Trends in Radio Research: Diversity, innovation, and policies. Cambridge Scholars Publishing. 2018. p. 263.
^Bortzfield, Bill (27 November 2017). The state of HD Radio in Jacksonville and nationwide. WJCT Public Media (Report). Retrieved 4 September 2022.
^Hadfield, Marty (15 August 2016). Transmitter & programming considerations for HD Radio. RBR + TVBR (rbr.com) (Report). Retrieved 4 September 2022.
^"Receiving NRSC‑5". theori.io. 9 June 2017. Archived from the original on 20 August 2017. Retrieved 14 April 2018.
^ a bDRM System Specification (PDF) (vers. 4.2.1). Geneva, CH: European Broadcasting Union. January 2021. p. 178. ETSI ES 201 980. Retrieved 19 April 2018 – via ETSI.org.
^Satellite S‑band radio frequency table (Report). 15 August 2011. Retrieved 23 April 2013 – via CSG Network.
^Bonsor, Kevin (26 September 2001). "How satellite radio works". HowStuffWorks. Retrieved 1 May 2013.
^Enticknap, Leo Douglas Graham (2005). Moving Image Technology: From Zoetrope to Digital. Wallflower Press (Columbia University Press). ISBN 978-1904764069. Retrieved 31 August 2022.
^Starks, M. (2013). The Digital Television Revolution: Origins to Outcomes. Springer. ISBN 978-1137273345. Retrieved 31 August 2022.
^Brice, Richard (2002). Newnes Guide to Digital TV. Newnes. ISBN 978-0750657211. Retrieved 31 August 2022.
^Bartlet, George W., Ed. (1975). NAB Engineering Handbook, 6th Ed. Washington, D.C.: National Association of Broadcasters. p. 21.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Lundstrom, Lars-Ingemar (2012). Understanding Digital Television: An Introduction to DVB Systems with Satellite, Cable, Broadband and Terrestrial TV Distribution. CRC Press. ISBN 978-1136032820.
^ a bIngram, Dave (1983). Video Electronics Technology. TAB Books. ISBN 978-0830614745. Retrieved 1 September 2022.
^Benoit, Herve (1999). Satellite Television: Analogue and Digital Reception Techniques. Butterworth-Heinemann/Arnold. ISBN 978-0340741085. Retrieved 1 September 2022.
^Long, Mark (1999). The Digital Satellite TV Handbook. Newnes. ISBN 978-0750671712. Retrieved 1 September 2022.
^Weik, Martin H. (2000). "standard frequency and time signal". Computer Science and Communications Dictionary. Computer Science and Communications Dictionary. Springer. p. 1649. doi:10.1007/1-4020-0613-6_18062. ISBN 978-0792384250. Retrieved 1 September 2022.
^Radio Aids to Navigation, Publication 117, Chapter 2, Radio Time Signals. Lighthouse Press. 2005. ISBN 978-1577855361. Retrieved 1 September 2022.
^"What Closing A Government Radio Station Would Mean For Your Clocks". National Public Radio, Weekend Edition. Retrieved 1 September 2022.
^Frenzel, Louis (2017). Electronics Explained: Fundamentals for Engineers, Technicians, and Makers. Newnes. ISBN 978-0128118795. Retrieved 2 September 2022.
^ a bBrain, Marshall; Tyson, Jeff; Layton, Julia (2018). "How Cell Phones Work". How Stuff Works. InfoSpace Holdings LLC. Retrieved 31 December 2018.
^Lawson, Stephen. "Ten Ways Your Smartphone Knows Where You Are". PCWorld. Retrieved 2 September 2022.
^"Cellular Telephone Basics". Privateline.com. 1 January 2006. p. 2. Archived from the original on 17 April 2012. Retrieved 2 September 2022.
^Brown, Sara. "5G, explained". mitsloan.mit.edu. MIT Sloan School of Management. Retrieved 2 September 2022.
^Osseiran, Afif; Monserrat, Jose F.; Marsch, Patrick (2016). 5G Mobile and Wireless Communications Technology. Cambridge University Press. ISBN 978-1107130098. Retrieved 2 September 2022.
^Chandler, Nathan (13 February 2013). "How Satellite Phones Work". howstuffworks.com. HowStuffWorks. Retrieved 2 September 2022.
^"Satellite Phone : Functioning/Working Of Satellite Phone". tutorialsweb.com. Tutorials Web. Retrieved 2 September 2022.
^McComb, Gordon (October 1982). "Never Miss a Call: PS Buyer's Guide to Cordless Phones". Popular Science. pp. 84–85 – via Google Books.
^Guy, Nick (13 July 2022). "Wirecutter: The Best Cordless Phone". The New York Times. ISSN 0362-4331. Retrieved 7 September 2022.
^U.S. Fire Administration (June 2016). Voice Radio Communications Guide for the Fire Service (PDF) (Report). Washington, D.C.: Federal Emergency Management Agency. pp. 33–34. Retrieved 7 September 2022.
^Sterling, Christopher H. (2008). Military Communications: From Ancient Times to the 21st Century. ABC-CLIO. pp. 503–504. ISBN 978-1851097326.
^Aeronautical Frequency Committee Manual (PDF) (Report). Aviation Spectrum Resources Inc. 2012.
^"Aviation Radio Bands and Frequencies". Smeter network 2011. Archived from the original on 12 February 2004. Retrieved 16 February 2011.
^North Atlantic Operations and Airspace Manual (PDF) (Report). ICAO European and North Atlantic Office. 28 March 2019.
^Van Horn, Larry. "The Military VHF/UHF Spectrum". Monitoring Times.
^Fletcher, Sue (2002). A Boater's Guide to VHF and GMDSS. Camden, Maine: International Marine/McGraw-Hill. ISBN 0071388028. OCLC 48674566.
^The ARRL Handbook for Radio Communications 2017 (94th ed.). Newington, Connecticut: American Radio Relay League. 2016. ISBN 978-1625950628. OCLC 961215964.
^Brain, Marshall (11 February 2021). "Radio basics: Real life examples". How radio works. How Stuff Works website. Retrieved 27 August 2022.
^Radiofrequency Toolkit for Environmental Health Practitioners (PDF) (Report). Vancouver, British Columbia, Canada: British Columbia Centre for Disease Control/National Collaborating Centre for Environmental Health. p. 26. ISBN 978-1926933481.
^"Best Baby Monitor Buying Guide". Consumer Reports. 24 April 2016. Retrieved 9 September 2022.
^Eargle, John (2005). "Overview of Wireless Microphone Technology". The Microphone Book (2nd ed.). Oxford: Focal Press. pp. 142–151. ISBN 978-1136118067 – via Google Books.
^Bell, Dee Ana (1 November 2012). "Avoiding Audio Problems with Wireless Microphone Systems". TV Technology. Retrieved 10 September 2022.
^Vernon, Tom (28 August 2021). "Wireless Mic Industry Debates WMAS Technology". Radio World. Retrieved 10 September 2022.
^Lewis, Barry D.; Davis, Peter T. (2004). Wireless Networks For Dummies. John Wiley & Sons. ISBN 978-0764579776. Retrieved 12 September 2022.
^ a bLowe, Doug (2020). Networking For Dummies. John Wiley & Sons. ISBN 978-1119748670. Retrieved 12 September 2022.
^Muller, Nathan J. (2002). Networking A to Z. McGraw-Hill Professional. pp. 45–47. ISBN 978-0071429139. Archived from the original on 24 June 2021. Retrieved 12 September 2022.
^Silver, H. Ward (2008). The ARRL Extra Class License Manual for Ham Radio. American Radio Relay League. ISBN 978-0872591356. Retrieved 12 September 2022.
^Hillebrand, Friedhelm (2010). Short Message Service (SMS): The Creation of Personal Global Text Messaging. John Wiley & Sons. ISBN 978-0470689936. Retrieved 12 September 2022.
^McGregor, Michael A.; Driscoll, Paul D.; Mcdowell, Walter (2016). Head's Broadcasting in America: A Survey of Electronic Media. Routledge. ISBN 978-1317347927. Retrieved 12 September 2022.
^Radio-Electronics-Television Manufacturers Association. Engineering Department (1955). "Microwave Relay Systems for Communications". Electronic Industries Association. Retrieved 12 September 2022.
^Bailey, David (2003). Practical Radio Engineering and Telemetry for Industry. Elsevier. ISBN 978-0080473895. Retrieved 12 September 2022.
^Arafath, Yeasin; Mazumder, Debabrata; Hassan, Rakib (2012). Automatic Meter Reading by Radio Frequency Technology. Lap Lambert Academic Publishing GmbH KG. ISBN 978-3847372219. Retrieved 12 September 2022.
^Bonsor, Kevin (28 August 2001). "How E-ZPass Works". howstuffworks.com. HowStuff Works. Retrieved 12 September 2022.
^Hunt, V. Daniel; Puglia, Albert; Puglia, Mike (2007). RFID: A Guide to Radio Frequency Identification. John Wiley & Sons. ISBN 978-0470112243. Retrieved 12 September 2022.
^White, Ryan (17 December 2021). "How do submarines communicate with the outside world?". navalpost.com. Naval Post. Retrieved 12 September 2022.
^"Naval Research Reviews, Vol. 27". Superintendent of Government Documents. 1974. Retrieved 12 September 2022.
^"State-of-the-Art of Small Spacecraft Technology, 9.0 - Communications". nasa.gov. National Aeronautics and Space Administration. 16 October 2021. Retrieved 11 September 2022.
^"UCS Satellite Database". Union of Concerned Scientists. 1 January 2021. Retrieved 21 May 2021.
^Marsten, Richard B. (2014). Communication Satellite Systems Technology. Academic Press. ISBN 978-1483276816. Retrieved 11 September 2022.
^Brain, Marshall (2020). "How radar works". How Stuff Works. Retrieved 3 September 2022.
^ a bSkolnik, Merrill (2021). "Radar". Encyclopædia Britannica online. Encyclopædia Britannica Inc. Retrieved 3 September 2022.
^"JetStream". www.noaa.gov.
^Chernyak, Victor S. (1998). Fundamentals of multisite radar systems: multistatic radars and multiradar systems. CRC Press. pp. 3, 149. ISBN 9056991655.
^"Airport Surveillance Radar". Air traffic control, technology. US Federal Aviation Administration website. 2020. Retrieved 3 September 2022.
^Binns, Chris (2018). Aircraft Systems: Instruments, Communications, Navigation, and Control. Wiley. ISBN 978-1119259541. Retrieved 11 September 2022.
^International Electronic Countermeasures Handbook. Artech/Horizon House. 2004. ISBN 978-1580538985. Retrieved 11 September 2022.
^Bhattacharjee, Shilavadra (2021). "Marine Radars and Their Use in the Shipping Industry". Marine Insight website. Retrieved 3 September 2022.
^"Using and Understanding Doppler Radar". US National Weather Service website. US National Weather Service, NOAA. 2020. Retrieved 3 September 2022.
^Fenn, Alan J. (2007). Adaptive Antennas and Phased Arrays for Radar and Communications. Artech House. ISBN 978-1596932739. Retrieved 11 September 2022.
^Teeuw, R.M. (2007). Mapping Hazardous Terrain Using Remote Sensing. Geological Society of London. ISBN 978-1862392298. Retrieved 11 September 2022.
^Jol, Harry M. (2008). Ground Penetrating Radar Theory and Applications. Elsevier. ISBN 978-0080951843. Retrieved 10 September 2022.
^Grosch, Theodore O. (30 June 1995). Verly, Jacques G. (ed.). "Radar sensors for automotive collision warning and avoidance". Synthetic Vision for Vehicle Guidance and Control. 2463. Society of Photo-Optical Instrumentation Engineers: 239–247. Bibcode:1995SPIE.2463..239G. doi:10.1117/12.212749. S2CID 110665898. Retrieved 10 September 2022.
^Brodie, Bernard; Brodie, Fawn McKay (1973). From Crossbow to H-bomb. Indiana University Press. ISBN 0253201616. Retrieved 10 September 2022.
^Sharp, Ian; Yu, Kegen (2018). Wireless Positioning: Principles and Practice, Navigation: Science and Technology. Springer. ISBN 978-9811087912. Retrieved 10 September 2022.
^Teunissen, Peter; Montenbruck, Oliver (2017). Springer Handbook of Global Navigation Satellite Systems. Springer. ISBN 978-3319429281. Retrieved 10 September 2022.
^El-Rabbany, Ahmed (2002). Introduction to GPS: The Global Positioning System. Artech House. ISBN 978-1580531832. Retrieved 10 September 2022.
^Kiland, Taylor Baldwin; Silverstein Gray, Judy (15 July 2016). The Military GPS: Cutting Edge Global Positioning System. Enslow Publishing. ISBN 978-0766075184. Retrieved 10 September 2022.
^Deltour, B.V. (August 1960). "A Guide To Nav-Com Equipment". Flying Magazine Aug 1960. Retrieved 10 September 2022.
^"2008 Federal Radionavigation Plan". U.S. Department of Defense. 2009. Retrieved 10 September 2022.
^Martin, Swayne. "How A VOR Works". boldmethod.com. Boldmethod -Digital Aviation Content. Retrieved 10 September 2022.
^"Non-Directional Beacon (NDB)". systemsinterface.com. Systems Interface. Retrieved 10 September 2022.
^"How does an emergency beacon work?". cbc.ca. CBC News. Retrieved 10 September 2022.
^"What is a Cospas-Sarsat Beacon?". cospas-sarsat.int. International Cospas-Sarsat Programme. Retrieved 10 September 2022.
^"Scientific and Technical Aerospace Reports, Volume 23, Issue 20". NASA, Office of Scientific and Technical Information. 1985. Retrieved 10 September 2022.
^"An Introduction to Radio Direction Finding". defenceweb.co.za. defenceWeb. 8 January 2021. Retrieved 10 September 2022.
^Moell, Joseph D.; Curlee, Thomas N. (1987). Transmitter Hunting: Radio Direction Finding Simplified. McGraw Hill Professional. ISBN 978-0830627011. Retrieved 10 September 2022.
^Layton, Julia (10 November 2005). "How Remote Controls Work". HowStuff Works. Retrieved 10 September 2022.
^Sadraey, Mohammad H. (2020). Design of Unmanned Aerial Systems. Wiley. ISBN 978-1119508694. Retrieved 10 September 2022.
^Smith, Craig (2016). The Car Hacker's Handbook: A Guide for the Penetration Tester. No Starch Press. ISBN 978-1593277703. Retrieved 10 September 2022.
^Pinkerton, Alasdair (15 June 2019). Radio: Making Waves in Sound. Reaktion Books. ISBN 978-1789140996. Retrieved 9 September 2022.
^Biffl, Stefan; Eckhart, Matthias; Lüder, Arndt; Weippl, Edgar (2019). Security and Quality in Cyber-Physical Systems Engineering. Springer Nature. ISBN 978-3030253127. Retrieved 9 September 2022.
^Boukerche, Azzedine (2008). Algorithms and Protocols for Wireless and Mobile Ad Hoc Networks. Wiley. ISBN 978-0470396377. Retrieved 9 September 2022.
^Wonning, Paul R. (12 May 2021). "A Guide to the Home Electric System". Mossy Feet Books. Retrieved 9 September 2022.
^Chatterjee, Jyotir Moy; Kumar, Abhishek; Jain, Vishal; Rathore, Pramod Singh (2021). Internet of Things and Machine Learning in Agriculture: Technological Impacts and Challenges. Walter de Gruyter GmbH & Co KG. ISBN 978-3110691283. Retrieved 9 September 2022.
^"What jamming of a wireless security system is and how to resist it | Ajax Systems Blog". Ajax Systems. April 2019. Retrieved 18 January 2020.
^"Remedial Electronic Counter-Countermeasures Techniques". FM 24-33 — Communications Techniques: Electronic Counter-Countermeasures (Report). Department of the Army. July 1990.
^Varis, Tapio (1970). "The Control of Information by Jamming Radio Broadcasts". Cooperation and Conflict. 5 (3): 168–184. doi:10.1177/001083677000500303. ISSN 0010-8367. JSTOR 45083158. S2CID 145418504.
^"Jammer Enforcement". Federal Communications Commission. 3 March 2011. Retrieved 18 January 2020.
^Yeap, Kim Ho; Hirasawa, Kazuhiro (2020). Analyzing the Physics of Radio Telescopes and Radio Astronomy. IG Global. ISBN 978-1799823834. Retrieved 9 September 2022.
^Joardar, Shubhendu; Claycomb, J. R. (2015). Radio Astronomy: An Introduction. Mercury Learning and Information. ISBN 978-1937585624.
^Chapman, Rick; Gasparovic, Richard (2022). Remote Sensing Physics: An Introduction to Observing Earth from Space. Wiley. ISBN 978-1119669074. Retrieved 9 September 2022.
^Pampaloni, Paulo; Paloscia, S. (2000). Microwave Radiometry and Remote Sensing of the Earth's Surface and Atmosphere. ISBN 9067643181. Retrieved 9 September 2022.
General references
Basic Radio Principles and Technology – Elsevier Science
The Electronics of Radio – Cambridge University Press
Radio Systems Engineering – Cambridge University Press
.jpg/1280px-HA0478-006_(6011470974).jpg)
.jpg/1280px-EPIRB_(1).jpg)
