Thursday, January 12, 2012

Pager portable, lightweight receiver of coded radio signals

Pagers notify their owners that someone is trying to contact them, usually by beeping or vibrating. The liquid crystal display can display short messages or telephone numbers.

Pager, portable, lightweight receiver of coded radio signals that indicates to its user that a message is waiting or someone is trying to get in touch. When people want to page someone—that is, contact a person via the person’s pager—they telephone a paging service company, which then sends wireless radio signals to the pager. These signals, in turn, alert the pager’s user by triggering a tone or a vibration. Most pagers can also deliver brief messages on liquid crystal displays. Because pagers do not rely on telephone wires, a page can be transmitted from a central location and received anywhere within the range of the transmitting tower (see Wireless Communications). Pagers make their users easy to contact, even if the user is away from a telephone. As a result, pagers help increase productivity and improve responsiveness to emergencies and to business and personal requests.
A page is the coded radio signal sent from a transmitter to the pager. Pages usually begin with the dialing of a telephone number to the user’s paging service provider, a company that maintains the paging transmitters and radio equipment. Placing a telephone call to the service provider enables the caller to access a computerized terminal. The caller hears a tone or receives instructions on how to page a subscriber. If the caller wants to leave a telephone number for the paged party to call, the caller can enter that number. Pressing the telephone’s pound (#) or star (*) key informs the paging terminal that the message is complete.
The paging terminal automatically determines which pager corresponds to the telephone number dialed. It then routes a signal to one or more radio transmitters located throughout the paging service area. The area can vary; some services are local, while others are networked together via satellite to cover larger regions.
Pagers monitor specific radio frequencies used by the service providers. The transmission of the page is encoded so that only the intended recipient of the page can receive and decode the message. If the pager is switched on and is within range of the radio tower, the pager will recognize the coded signal. The pager then converts the signal to data, alerting the owner that a page has been sent with a beep, tone, or vibration. Most pagers have liquid-crystal displays that can show the number of the calling party, or short messages. The owner of the pager can telephone the paging party, or call the paging company to retrieve other messages.
The use of radio signals to perform one-way notifications began in the 1920s and 1930s. During the same period, mobile radio systems were being developed for police dispatch and public safety services. These early systems broadcast messages to all receivers on the band, and they could not be used to contact a specific party.
Paging later developed from a one-to-many dispatch service into a system for reaching a single address that corresponded to one pager. Pagers in the 1960s and 1970s were simple devices that used a tone or vibration to alert the subscriber to call a single predetermined number to get the message. Communications satellites are commonly used to route pager connections throughout the United States, and in 1998 a satellite failure temporarily silenced millions of pagers across the United States. Pager connections were transferred to another satellite to restore service.
Innovations in computer technology have improved pagers, making them smaller, more affordable, and loaded with new features. Modern pagers have screens that can display numbers or short messages, and they can store those messages for future referral. Pager users can subscribe to special services that broadcast information such as stock market quotations and up-to-the minute sports scores. The next generation of pagers will include the ability to acknowledge reception of a page and respond with a short, predetermined message.

Nuclear Magnetic Resonance

Nuclear Magnetic Resonance
Magnetic Resonance Imaging Scan
This Magnetic Resonance Imaging (MRI) scan through a normal adult head shows the brain, airways, and soft tissues of the face. MRI has become a valuable diagnostic tool, and is especially effective at imaging the brain, head, and neck.

Nuclear Magnetic Resonance or NMR, technique developed separately in 1945 by American physicists Felix Bloch and Edward Mills Purcell for the spectroscopic analysis of substances. In NMR, a substance is placed in a strong magnetic field that affects the spin of the atomic nuclei of certain isotopes of common elements. A radio wave passes through the substance then reorients these nuclei. When the wave is turned off, the nuclei release a pulse of energy that provides data on the molecular structure of the substance and that can be transformed into an image by computer techniques. In the early 1980s, NMR also became a diagnostic tool for obtaining more precise images of tissues within the human body than are possible with CAT scans or ultrasonics (see Radiology). In its medical applications, NMR is now more commonly referred to as magnetic resonance imaging (MRI). MRI is unsurpassed as an imaging technique for scans of the brain, head, and neck.

Loran phrase long range navigation


Loran, abbreviation of the phrase long range navigation, used to designate a radio navigation system developed during World War II. Loran is one of several systems that enable navigators to establish the position of their ships or aircraft by finding the difference in the time that it takes radio signals to reach them from two synchronized transmitters spaced some distance apart.
The loran transmitter system consists of one so-called master and one slave station. The master station emits a short pulse or signal at regular intervals, and this pulse is repeated by the slave station, which is controlled from the master station by radio. Both of these signals are received aboard the ship or aircraft, are amplified, and are recorded as small waves or irregularities on the screen of a cathode-ray tube. The circuits of the receiver are so arranged that the distance between the waves corresponds to the difference in time between the arrivals of the signals from the two stations. The receiver is also equipped with an electronic timing device so that this difference in time can be measured in microseconds (millionths of a second). Because radio waves travel at a uniform speed of about 300,000 km (about 186,000 mi) per sec, the location of all points where the signals from the two stations are separated by a given time interval can be represented by a definite curve, which is a hyperbola. The navigator is equipped with a map giving a series of these curves, called loran lines of position, and, after finding the time interval, for example, of 3 microseconds, knows that the position of the craft is somewhere on the 3-microsecond curve on the chart. By switching to another pair of loran transmitters and repeating the procedure, the navigator can find another curve representing the craft's position, and can then determine the craft's actual position, which is at the intersection of the two loran lines of position. Loran has a useful range of about 2250 km (about 1400 mi) by night and about 1200 km (about 750 mi) by day. The signal is usually transmitted in the frequency band of 1.8 to 2.0 MHz. Loran can be used for setting and holding a course as well as for determining position, and it has the advantage of being independent of weather conditions. The accuracy ranges from a few hundred meters to a few kilometers, depending on the equipment used and the distance of the craft from the transmitter. See Navigation; Radio.



Maser, acronym for microwave amplification by stimulated emission of radiation, a device that amplifies or generates microwaves or radio waves. A maser producing radiation in the optical region is called a laser.
As in lasers, amplification of radiation in masers is obtained by stimulated emission. This occurs when a photon induces an excited atom or molecule to fall to a lower energy state while emitting a photon of the same frequency as the incoming photon. The emitted photon travels in the same direction and in phase with the incoming photon, which is not absorbed during the interaction. The amplitudes of the two waves add up, and amplification of the incoming wave has taken place. Masers make use of those transitions in molecules or crystals that correspond to the energies of microwave or radio frequencies.
Charles Townes
American physicist Charles Townes won the 1964 Nobel Prize in physics. He made fundamental contributions in quantum theory and significantly improved radar technology.

The first maser oscillator was developed by the American physicists Charles Hard Townes (1915- ), James P. Gordon, and Herbert J. Zeiger in 1954, and made use of the frequency of the ammonia molecule. This frequency corresponds to the energy of the photon emitted when the nitrogen atom moves from one side to the other of the triangle formed by the three hydrogen atoms in an ammonia molecule. The hydrogen maser makes use of the frequency corresponding to that of the photon released when the spin of the proton in a hydrogen atom flips over with respect to the spin of the atom's electron. Paramagnetic masers use energy transitions corresponding to the orientations of the magnetic moments of paramagnetic ions in crystalline substances placed in an external magnetic field. Different frequencies can be obtained by varying the magnetic field, thus allowing the tuning of a paramagnetic maser from less than a megacycle to several hundreds of megacycles.
Because of the high stability of the generated frequencies, masers serve as time standards in atomic clocks. Masers are also used as low-noise radio frequency amplifiers in satellite communication and radio astronomy.



Journalism, gathering, evaluating, and distributing facts of current interest. In journalism, reporters research and write stories for print and electronic distribution, often with the guidance of editors or producers. The earliest journalists produced their stories for news sheets, circulars, newspapers, and periodicals. With technological advances, journalism came to include other media, such as radio, documentary or newsreel films, television, and the Internet.
The Ladies’ Home Journal
By the 19th century affordable production costs and a demand for national advertising led to an increase in the number of magazines available in the United States. The Ladies’ Home Journal was founded in 1883 and circulation quickly swelled to nearly 1 million readers. Other publications such as Life appeared at the same time. This is the 1929 cover of The Christmas Ladies’ Home Journal.

The earliest known journalistic effort was the Acta Diurna (Daily Events) of ancient Rome. In the 1st century bc, statesman Julius Caesar ordered these handwritten news bulletins posted each day in the Forum, a large public space. The first distributed news bulletins appeared in China around 750 ad. In the mid-15th century, wider and faster dissemination of news was made possible by the development of movable metal type, largely credited to German printer Johannes Gutenberg. At first, newspapers consisted of one sheet and often dealt with a single event. Gradually a more complex product evolved.
Germany, The Netherlands, and England produced newsletters and newsbooks of varying sizes in the 16th and 17th centuries. Journals of opinion became popular in France beginning late in the 17th century. By the early 18th century, politicians had begun to realize the enormous potential of newspapers in shaping public opinion. Consequently the journalism of the period was largely political in nature; journalism was regarded as an adjunct of politics, and each political faction had its own newspaper. During this period the great English journalists flourished, among them Daniel Defoe, Jonathan Swift, Joseph Addison, and Sir Richard Steele. Also at this time the long struggle for freedom of the press began.
In the English colonies of North America, the first newspaper was Publick Occurrences Both Forreign and Domestick, published in Boston, Massachusetts, in 1690; it was suppressed, and its editor, Benjamin Harris, was imprisoned after having produced the first issue. The trial of publisher John Peter Zenger in 1735 set a key precedent regarding freedom of the press in America more than 50 years before the First Amendment to the United States Constitution would secure it. Zenger was acquitted of charges of criminal libel stemming from articles he printed that were critical of the colonial authorities in New York, his defense being that his reports were factual. Provisions for censorship of the press were, however, included in the Alien and Sedition Acts, passed in 1798. After provoking a great deal of opposition, these acts were allowed to expire. See also Trial of John Peter Zenger.
Journalism in the 19th century became more powerful due to the mass production methods arising from the Industrial Revolution and to the general literacy promoted by public education. The large numbers of people who had learned how to read demanded reading matter, and new printing machinery made it possible to produce this inexpensively and in great quantities. In the United States, for example, publishers Joseph Pulitzer, Edward Wyllis Scripps, and William Randolph Hearst established newspapers appealing to the growing populations of the big cities. In the late 19th and early 20th centuries, news agencies exploited the invention of the telegraph by using it for the rapid gathering and dissemination of world news via wire services. These services included Reuters, based in England; the Associated Press and United Press (later United Press International), based in the United States; and the Canadian Press, in Canada.
At the same time, new popular magazines were made possible by new technologies, improved transportation, low postal rates, and the emergence of national brands of consumer goods that required national media in which to advertise. The Ladies' Home Journal, founded by Cyrus H. K. Curtis in 1883, soon had a circulation of almost a million—a prodigious figure for that day. In 1897 Curtis bought for $1,000 the old Saturday Evening Post, which rapidly achieved a circulation in the millions. Numerous other magazines appealing to the general reader appeared in the 20th century, including Reader's Digest, Collier's, Life, and Look.
Over time, some general magazines became unprofitable and ceased publication when they lost advertising to television and to more specialized magazines, such as Sports Illustrated and TV Guide. The newsmagazines Time, Newsweek, Maclean’s, and U.S. News & World Report have continued to occupy an important place in journalism, as have The Ladies’ Home Journal and other so-called women's service magazines.
In the early 20th century two new forms of news media appeared: newsreels and radio. By the 1920s, newsreels in the United States alone reached about 40 million people a week in about 18,000 film theaters, but they were displaced by television in the 1950s. Radio news survived more successfully. Stations in the United States and Canada started to report current events in the 1920s, borrowing most of their information from local newspapers. They soon developed their own newsgathering facilities.
By World War II (1939-1945), radio had amassed a huge audience. American president Franklin Delano Roosevelt appealed to his nation through his “fireside chats,” and radio was usually the first to bring reports on the war to the public. Popular radio reporters and commentators were heard by millions of people. Television later attracted much of radio’s audience, but radio has retained a loyal following for music, news, and talk shows.
Television became commercially viable in the 1950s, and by the 1970s nearly every household that wanted a television had one. (In 2000 there were 835 televisions for every 1,000 people in the United States and 710 per 1,000 in Canada.) Network evening newscasts, originally 15 minutes long, were extended to 30 minutes, and local news broadcasts in major cities expanded to an hour or more. Network newscasters gradually became national figures. Since the introduction in 1951 of the first major documentary series, See It Now, featuring commentator Edward R. Murrow, television documentaries and video newsmagazines such as 60 Minutes have become important news sources. The Cable News Network (CNN), operating in a news-only format 24 hours a day, reached 77 million U.S. and Canadian households by 2000, and its CNN International broadcasts were relayed by satellite to more than 200 other countries.
Television Reporter
Reporters must gather the facts about a story through research, interviews, and first-hand observation. In this photograph, a television news reporter conducts an on-camera interview, which may be aired live or taped and edited for broadcast at a later time.

Largely for economic reasons, including competition from television, the number of local daily newspapers in the United States declined in number from 2,200 in 1910 to less than 1,500 in 2002. Canada, with just over one-tenth the population of the United States, had about 100 daily newspapers in 2002. Weekly newspapers, which generally have lower circulation numbers than daily newspapers, are more numerous: In 2002 more than 9,200 of them were published in the United States, and about 900 in Canada.
A major trend affecting newspapers in the 1980s was their incorporation into newspaper chains—ownership of a number of newspapers by a single company. By 2000 only about a dozen cities in the United States had separately owned competing newspapers, and in 2002 Canada had only eight cities with competing newspapers under different ownership. Similarly, major radio and television stations, even when independently owned, have become affiliated with networks that provide much of their news and other program materials.
The rise of cable television and public broadcasting has reduced uniformity of programming somewhat. By 2000, 67.7 million U.S. households and 11 million Canadian households were wired to receive cable television. Because cable can bring in more channels than are generally available over the air, opportunities for the expression of diverse viewpoints increased. Public television, also called educational television, is likewise gradually expanding its audience. In the mid-1970s it accounted for only a small part of the time Americans spent viewing television; by the 1990s, during the average week, public television was watched in more than half of all homes with television sets.
New technologies continue to bring about changes in journalism. Television satellites, for example, enable viewers in one part of the world to witness live events occurring in another (see Communications Satellite) and facilitate new forms of video news distribution. Reporters can summon from data banks information that previously would have taken them days or weeks to assemble. Wire-service copy can be set in type automatically at a subscribing newspaper without the services of a local editor or printer (see Office Systems).
Popular Magazines
Full-color weekly and monthly publications such as The New Yorker, Wired, and Newsweek have huge circulations through subscriptions and newsstand sales. Hundreds of publications targeting general topics as well as specific interests—from doll collecting to windsurfing—are regularly issued in the United States.

In the mid- and late 1990s the Internet became a major force in journalism. Most of the major journalism companies—including those involved in newspapers, periodicals, wire services, radio stations, and television stations—began to publish material on the World Wide Web. One of the advantages of the Internet is that readers can find continually updated information on a variety of subjects, without waiting several hours for a new edition or the next news broadcast. Another advantage is the ability of news organizations to publish more in-depth information on the Internet, such as background documents, detailed maps, or previous stories. One of the disadvantages of the Internet is that, because information can be published almost instantly, companies occasionally release stories without subjecting them to the same quality controls and fact-checking processes common in other media. Nevertheless, people have flocked to the Internet as a news source. The percentage of Americans getting news from the Internet at least once a week continues to grow, having surpassed 35 percent in 2000. More than 40 percent of those obtaining news from the Internet say they go online to get more information about stories they first encountered in other media.
Reporting the Watergate Scandal
Headlines from the Washington Post call attention to events of the Watergate scandal, which led to the resignation of President Richard M. Nixon in 1974. Investigation by journalists helped reveal the extent of the president's involvement in illegal activities and illustrated the importance of freedom of the press in a democratic society.

During the 19th century more and more newspapers and magazines began to campaign for social and political reforms as a method of attracting mass audiences. William Randolph Hearst and Joseph Pulitzer, while often engaging in sensationalism, also spoke out against social evils of their day. Some of the mass magazines of the time, such as McClure's Magazine and Everybody's, built their reputations largely on the exposure of abuses. Newspaper and magazine editorials exerted some influence, but even more important was the ability of news stories to focus public attention on social problems or political corruption. Crusading journalists, the so-called muckrakers, helped to bring about a number of reforms—for example, antitrust legislation (see Trusts) and the passage of pure food laws (see Pure Food and Drug Acts).
Journalists have continued to serve as watchdogs for the public. In the 1960s television brought civil rights demonstrations in the United States—and the brutal means sometimes used to control them—into people’s living rooms. Reporters covering the Vietnam War (1959-1975), having become convinced that officials were not telling the truth about U.S. involvement there, were instrumental in turning public opinion against the war.
In 1972 and 1973, led by investigative reporters from the Washington Post, the press exposed links between the administration of President Richard M. Nixon and a burglary of the Democratic Party national headquarters (known as the Watergate scandal, so-named for the building that housed the burglarized office). Senate hearings on the scandal and preparations by the House of Representatives for impeachment proceedings were carried live on television and attracted large audiences. President Nixon resigned soon thereafter. Some investigative reporters then turned their attention to alleged abuses by the Central Intelligence Agency (CIA) and the Federal Bureau of Investigation (FBI), charging, for instance, that these agencies had spied illegally on American citizens.
Except during World Wars I and II, freedom of the American press was not seriously abridged in the 20th century. Governmental efforts to prevent publication of the Pentagon Papers (a collection of secret documents on the Vietnam War) were struck down by the courts in 1971 as a violation of the First Amendment to the Constitution of the United States. Broadcasting stations, which must be licensed by the Federal Communications Commission (FCC) to operate, have generally been more cautious in their criticisms of government than have newspapers.
Traditionally, reporters had learned their skills on the job, but this began to change in the 20th century. The first school of journalism in the United States was established at the University of Missouri, in Columbia, Missouri, in 1908, and a bequest from Joseph Pulitzer led to the creation in 1912 of a graduate school of journalism at Columbia University, in New York City. More than 100 schools and departments of journalism now exist, and reporters frequently receive some of their early training on school or college newspapers.
Not all journalism graduates seek employment in the news media. A substantial proportion engages in public relations, advertising, teaching, or other communications occupations. Courses in journalism education programs frequently include reporting, newswriting, editing, broadcasting, new media, and related courses, as well as public relations, advertising, marketing, and social science research dealing with the process and effects of mass communications.

Global Positioning System

Global Positioning System

Launching a GPS Satellite
A Navstar global positioning system (GPS) satellite is launched into orbit by a Delta rocket. GPS satellites continuously transmit data about the satellite’s position and the current time. Military and civilian navigators use the information gathered from several satellites to compute their own position.

Global Positioning System (GPS), space-based radio-navigation system, consisting of 24 satellites and ground support. Operated by the United States military but open to civilian uses, GPS provides users with accurate information about their location and velocity anywhere in the world. GPS is one of three satellite-based radio-navigation systems. The Russian Federation operates the Global Orbiting Navigation Satellite System (GLONASS), which also uses 24 satellites and provides accuracy similar to GPS. The European Union (EU) launched the first satellite in its planned Galileo program, also known as the Global Navigation Satellite System (GNSS), in December 2005.
GPS, formally known as the Navstar Global Positioning System, is operated and maintained by the United States Department of Defense. The National Space-Based Position, Navigation, and Timing Executive Committee manages GPS. The deputy secretaries of the Departments of Defense and Transportation lead the committee, which has a permanent staff that is responsible for the development of GPS.
GPS was initiated in 1973 to reduce the proliferation of navigation aids. By overcoming the limitations of many existing navigation systems, GPS became attractive to a broad spectrum of users. It was initially used as a navigational aid by military ground, sea, and air forces. In more recent years, GPS has been used by civilians in many new ways, such as in automobile and boat navigation, hiking, emergency rescue, and precision agriculture and mining.
The GPS system was designed for 24 satellites. Each satellite lasts about ten years. Replacement satellites are placed in orbit regularly to ensure that at least 24 satellites are always functioning. The device that receives the GPS signal is known as a receiver. Handheld or wrist-mounted GPS receivers are available to the civilian population; GPS receivers can also be installed in automobiles and boats.
An atomic clock synchronized to GPS is required in order to compute ranges from these three signals. However, by taking a measurement from a fourth satellite, the receiver avoids the need for an atomic clock. Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and velocity.
GPS Receiver with Map
A Global Positioning System (GPS) receiver links with an array of satellites to give users their location. Many GPS units have enough memory capacity to store maps so that users can pinpoint their map location and use it to plot routes to their next destination.

GPS has three components: the space component, control component, and user component. The space component includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral, in Florida. GPS satellites fly in circular orbits at an altitude of 20,100 km (12,500 mi) and with a period of 12 hours. The orbits are tilted to Earth's equator by 55 degrees to ensure coverage of polar regions. Powered by solar cells, the satellites continuously orient themselves to point their solar panels toward the Sun and their antennas toward Earth. Each satellite contains four atomic clocks.
The control component includes the master control station at Falcon Air Force Base in Colorado Springs, Colorado, and monitor stations at Falcon Air Force Base and on Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in the South Pacific Ocean. These stations monitor the GPS satellites. The control segment uses measurements collected by the monitor stations to predict the behavior of each satellite's orbit and clock. The prediction data is uplinked, or transmitted, to the satellites for transmission to the users. The control segment also ensures that the GPS satellite orbits and clocks remain within acceptable limits.
The user component includes the equipment, or receivers, used by military personnel and civilians to receive GPS signals. Military GPS receivers have been integrated into fighter aircraft, bombers, tankers, helicopters, ships, submarines, tanks, jeeps, and soldiers' equipment. In addition to basic navigation activities, military applications of GPS include target designation, close air support, “smart” weapons, and rendezvous.
The general population typically uses lightweight, handheld receivers or receivers that have been integrated into automobiles or boats. The general population uses GPS in many different ways. Surveyors use GPS to save time over standard survey methods. GPS is used in aircraft and ships for en route navigation and for airport or harbor approaches. GPS tracking systems are used to route and monitor delivery vans and emergency vehicles. In a method called precision farming, GPS is used to monitor and control the application of agricultural fertilizer and pesticides. GPS is available as an in-car navigation aid and as such, is used by vacationers and businesspeople who make frequent calls on clients or customers. Handheld GPS receivers are often used by hikers and hunters. Rescue crews use GPS to locate persons in emergency situations. A pastime known as caching or geocaching uses GPS to locate objects deliberately concealed in certain locations in an adult version of a child’s treasure hunt. GPS is also used on the space shuttle.
Global Positioning System (GPS)
The Navstar Global Positioning System (GPS) is a network of 24 satellites in orbit around the earth that provides users with information about their position and movement. A GPS receiver computes position information by comparing the time taken by signals from three or four different GPS satellites to reach the receiver.

GPS is available in two basic forms: the standard positioning service (SPS), or civilian signal, and the precise positioning service (PPS), or military signal. Prior to 2000 the U.S. military intentionally corrupted or degraded the SPS signal for national security purposes by using a process known as Selective Availability. As a result, the SPS signal was much less accurate than PPS. In May 2000 President Bill Clinton announced that the military would stop Selective Availability. This increased the accuracy and reliability of SPS by a factor of ten. Today, the military and civilian GPS signals are believed to be of the same accuracy. For national security reasons the Defense Department retained the ability to jam the SPS signal on a regional basis if necessary. Both the SPS and the PPS signals provide a horizontal position that is accurate to about 10 m (about 33 ft).
Finding Location with GPS
Global Positioning System (GPS) satellites orbit high above the surface of Earth at precise locations. They allow a user with a GPS receiver to determine latitude, longitude, and altitude. The receiver measures the time it takes for signals sent from the different satellites (A, B, and C) to reach the receiver. From this data, the receiver triangulates an exact position. At any given time there are multiple satellites within the range of any location on Earth. Three satellites are needed to determine latitude and longitude, while a fourth satellite (D) is necessary to determine altitude.

Several techniques have been developed to enhance the performance of GPS. One technique, known as differential GPS (DGPS), employs two fixed stations on Earth as well as satellites. DGPS provides a horizontal position accurate to about 3 m (about 10 ft). Another technique, known as WAAS, or Wide Area Augmentation System, was developed by the Federal Aviation Administration (FAA) to improve the safety of aircraft navigation. WAAS monitoring stations around the United States catch GPS signals, correct errors, and send out more-accurate signals. A technique involving the use of carrier frequency processing (see Carrier Wave), known as survey grade GPS, was pioneered by surveyors to compute positions to within about 1 cm (about 0.4 in). SPS, DGPS, WAAS, and carrier techniques are accessible to all users.
The European Union (EU) launched the first satellite in its planned space-based radio-navigation system in December 2005. The EU system will eventually have 30 satellites and is expected to become operational in 2009. It will have an accuracy of 1 m (3.3 ft) and will be operated as a strictly civilian enterprise. Other countries outside the EU, such as China and Israel, are contributing financially to the creation of the system. Known as the Galileo program and also as the Global Navigation Satellite System (GNSS), the system will be compatible with GPS. The United States negotiated an agreement under which satellite signals could be jammed over a battlefield area without shutting down the entire GNSS or GPS system. The two systems, however, were expected to compete for commercial applications.

Citizens Band Radio

Citizens Band Radio

Citizens Band Radio, radio channels used for two-way, short-distance business and personal communications. Citizens band radio, or CB as it is commonly known, was initiated in the United States in 1947 when the Federal Communications Commission (FCC) opened the UHF (ultrahigh frequency) 460-470 MHz band to licensees from the general public. In 1958 the FCC reallocated part of the 11-m band (27 MHz) previously shared by amateur radio and industrial users to the Citizens Radio Service. The band was divided into 29 channels, 6 of which were reserved for radio-control devices such as garage-door openers. By the early 1980s more than 20 million CB sets were operating on 40 channels in the U.S. The CB fad was waning by that time, however, and in 1983 the FCC ceased requiring licenses for CB operation.
Recent CB transceivers use a digital synthesis circuit with two crystals to generate the 80 basic frequencies of the 40-channel system by means of integrated-circuit silicon-chip technology.
Citizens band transmitters, which are restricted to a maximum power input of 5 watts and output of 4 watts, have ranges of up to about 24 km (about 15 mi). Citizens band radio continues to be used by the drivers of trucks and automobiles, by factory personnel for internal mobile communications, and by other workers and private citizens. Channel 9, the emergency channel, is monitored by highway patrol and police departments and by citizen volunteer groups.

Cellular Radio Telephone

Cellular Radio Telephone

Cellular Telephone
Portable cellular telephones have become an invaluable tool for people who need to stay in touch while on the move. Cellular telephone systems combine radio and television technology with computer systems. As a caller moves from one geographical cell (the name given to a specific part of the area being covered by the system) to another, computers in switching offices transfer calls among variously located antenna transmitters without interrupting service.

Cellular Radio Telephone, also called cellular telephone or cell phone, low-powered, lightweight radio transceiver (combination transmitter-receiver) that provides voice telephone and other services to mobile users. Cellular telephones primarily operate like portable or cordless telephones. However, unlike conventional wire-based cordless phones, cellular telephones are completely portable and do not require proximity to a jack to access the wire-based networks operated by local telephone companies. A new generation of services for cell phones includes videoconferencing and Internet access with the ability to send e-mail. Cellular telephones have become very popular with professionals and consumers as a way to communicate while away from their regular, wire-based phones—for example, while traveling or when in remote locations lacking regular phone service. As cellular radio service proliferates and achieves greater market penetration, some users have begun to consider it an alternative to conventional wire-based services.
Cellular Radio Telephone
Students use a cellular radio telephone, also known as a cell phone. As cell phones have grown in popularity, they have also decreased in size.

Cellular telephones work by transmitting radio signals to cellular towers. These towers vary in their capability to receive cellular telephone signals. Some towers can receive signals from distances of only 1.5 to 2.4 km (1.0 to 1.5 mi), while others can receive signals from distances as far as 48 to 56 km (30 to 35 mi). The area a tower can cover is referred to as a cell. However, more than one tower may exist in a given cell area. The cells overlap so that the system can handle increased telephone traffic volume. The towers within these cells are networked to a central switching station, usually by wire, fiber-optic cable, or microwave. The central switching station handling cellular calls in a given area is directly connected to the wire-based telephone system. Cellular calls are picked up by the towers and relayed to another cell telephone user or to a user of the conventional wire-based telephone network. Since the cells overlap, as a mobile caller moves from one cell into another, the towers “hand off” the call so communication is uninterrupted.
Cellular phone networks exist in nearly every metropolitan area throughout the world, and cellular coverage is expanding in rural areas. Due to the convenience and mobility of cellular telephones, users typically pay a higher fee than they would for normal telephone use. A newer generation of cellular radio technology, called Personal Communications Services (PCS), operates much like earlier cellular services, but at higher frequencies, the number of times a radio wave oscillates or completes a cycle, which is measured in a unit known as a hertz (Hz). (The higher frequencies of PCS operate at around 1900 megahertz [MHz] in the United States.) PCS also utilizes completely digital transmissions, rather than both the analog and digital transmissions that many current cellular telephones use. Digital transmissions convert sound into digital form, which can be transmitted more efficiently than analog signals. Digital technologies can also generate more channel capacity over the same amount of the radio spectrum.
Cell Phone Tower
Cell phone towers are part of the wireless communications network that makes cellular radio telephone calls possible. Antennae on each tower receive high-frequency radio waves from cell phones. Their range varies from distances as short as 1.5 to 2.4 km (1.0 to 1.5 mi) to distances as long as 48 to 56 km (30 to 35 mi). Cell phone towers are now commonly seen along roadways as cell phone communication has become increasingly popular.

Both cellular radio and PCS use high-frequency radio waves to transmit calls. High-frequency waves have short wavelengths that pass by a given point at a very high rate. High-frequency waves can provide better sound quality and more reliable short-distance transmission than lower-frequency waves (such as AM radio) as they are less susceptible to sound degradation caused by the noise generated by weather, such as lightning which causes static, and other noise generators such as motors. However, high-frequency signals cannot effectively travel as far as low-frequency signals can.
For cellular networks, the limited range of high-frequency waves is actually advantageous because it means the same frequencies can be reused at nearby locations. Cell phone calls connect with short-range antennas known as towers. If there were only one tower for a large area, more customers would be trying to use the same high-frequency waves, and these waves would tend to overlap and cause interference. But because cell phone networks establish many towers covering small areas, a smaller number of customers access a given tower, and frequencies can be reused when a cell phone call is handed off from one tower to another as a mobile cell phone user travels. This ability to reuse frequencies is helpful because there are a limited number of radio frequencies available to cell phone companies. It also allows cellular network providers to accommodate a larger number of users.
How Wireless Communications Work
Cellular radio telephones, also known as cell phones, communicate by sending radio signals to a cell tower. Each cell tower has a certain range within which it can receive the radio signals. The range of each tower overlaps with that of another tower so as a mobile cell phone user travels, communication is uninterrupted. To communicate with the user of a wired telephone, the cell phone radio signals are routed from the cell tower to a mobile switching center, which in turn routes the signals to the telephone company. The signals then travel over telephone lines to reach a wired telephone.

The transceiver inside a cellular phone is a much more complex device than a conventional phone used over the wire-based network. A cellular telephone has circuitry that creates a unique identity code that is used to locate and track the telephone. This identity code is necessary for coordinating calls to and from the telephone, and for billing such calls. Because a cellular telephone user may move quite a distance during the duration of a call, the cellular radio network must manage calls from different tower sites as the telephone moves out of the range of one tower and into the range of another tower.
Current cellular telephones offer such features as a memory database for storing frequently called numbers and a lock to deter theft. Most cell phones, whether old or new, also have a small liquid crystal screen to display the telephone number being called or the number from which an incoming call originated. Many newer cell phones can display a short text message, much like a pager displays this information. Some cellular phones can also access the Internet and display text from Web sites, such as stock quotes and news stories. Internet-capable cell phones can also send and receive e-mail. Because mobile telephones use radio waves to send and receive calls, the device must include a power source. Rechargeable batteries provide the usual source of power, but most cell phones can also be attached to the cigarette lighter in a vehicle or to some other external power device.
The cells in a cellular radio network refer to the coverage area of each tower that receives and transmits calls from mobile telephones. The cells are arranged in a honeycomb pattern, and they overlap so that the system can handle increases in anticipated telephone traffic volume. Network management functions, performed by computers at a central facility known as a Mobile Telephone Switching Office (MTSO), include the ability to measure and compare the transmission quality between a single handset and multiple towers. This function is important so that the MTSO can select the best transmission link between mobile telephones and towers. This optimal link is then used to pass transmissions from one tower to another as the mobile telephone moves between cells.
All cell towers in a given area connect with the MTSO, which in turn has links to the wire-based local exchange carrier that handles normal telephone calls. The link between the MTSO and the wire-based local telephone company is essential for connecting wireless and wire-based calls. The vast majority of calls handled by a cellular radio network either begin on the wire-based network or end there.
The first commercial cellular telephones were tested in the late 1970s by Illinois Bell in Chicago, Illinois, and they were a great success. Cellular service carriers began nationwide operations in the mid-1980s operating in the 800- to 900-MHz frequency band. Before the availability of cellular radio service, mobile telephone service consisted of bulky mobile telephone radio units. These two-way radio units communicated with a single antenna in a city or area. The radio signals often interfered with those of other commercial radios, and due to the technology of the day the frequencies could not be reused as they can today. Therefore, limited numbers of channels were available for callers, and the service was unreliable and costly. Because of the consumer demand for cellular telecommunications, the Federal Communications Commission (FCC) in the mid-1990s authorized up to six additional mobile telephone service providers in each service area.
There were more than 120 million wireless subscribers in the United States in 2001, according to the Cellular Telecommunications and Internet Association, with the number of new users increasing significantly each year. Forecasters and regulators did not anticipate this growth, however. Network equipment and start-up costs were substantial, and the cost to consumers was high. Providers had limited their networks to a small group of high-volume business users. Eventually, the providers recognized that their businesses would be more profitable if they created innovative service packages and aggressively marketed their services to the general public. As a result, prices for cellular telephones and network access have dropped considerably.
Cellular radio telephone service has achieved great commercial success because users recognize that mobile telephone access can improve productivity and enhance safety. Delivery drivers, repair technicians, lawyers, and other professionals were early adopters of mobile telephone service. As more geographic areas are covered by cellular networks and as rates drop, new subscribers are buying cellular services for personal security, safety on the road, and general convenience. See also Telecommunications; Wireless Communications.

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