Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
NEXRAD or Nexrad is a network of 159 high-resolution S-band Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration within the United States Department of Commerce, the Federal Aviation Administration within the Department of Transportation, the U. S. Air Force within the Department of Defense, its technical name is WSR-88D. NEXRAD detects atmospheric movement or wind, it returns data which when processed can be displayed in a mosaic map which shows patterns of precipitation and its movement. The radar system operates in two basic modes, selectable by the operator – a slow-scanning clear-air mode for analyzing air movements when there is little or no activity in the area, a precipitation mode, with a faster scan for tracking active weather. NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans. In the 1970s, the U. S. Departments of Commerce and Transportation, agreed that to better serve their operational needs, the existing national radar network needed to be replaced.
The radar network consisted of WSR-57 developed in 1957, WSR-74 developed in 1974. Neither system employed Doppler technology, which provides wind direction information; the Joint Doppler Operational Project was formed in 1976 at the National Severe Storms Laboratory to study the usefulness of using Doppler radar to identify severe and tornadic thunderstorms. Tests over the next three years, conducted by the National Weather Service and the Air Weather Service agency of the U. S. Air Force, found that Doppler radar provided much improved early detection of severe thunderstorms. A working group that included the JDOP published a paper providing the concepts for the development and operation of a national weather radar network. In 1979, the NEXRAD Joint System Program Office was formed to move forward with the development and deployment of the proposed NEXRAD radar network; that year, the NSSL completed a formal report on developing the NEXRAD system. When the proposal was presented to the Reagan administration, two options were considered to build the radar systems: allow corporate bids to build the systems based on the schematics of the developed prototype radar or seek contractors to build their own systems using predetermined specifications.
The JSPO group opted to select a contractor to develop and produce the radars that would be used for the national network. Radar systems developed by Raytheon and Unisys were tested during the 1980s. However, it took four years to allow the prospective contractors to develop their proprietary models. Unisys was selected as the contractor, was awarded a full-scale production contract in January 1990. Installation of an operational prototype was completed in the fall of 1990 in Oklahoma; the first installation of a WSR-88D for operational use in daily forecasting was in Sterling, Virginia on June 12, 1992. The last system deployed as part of the installation program was installed in North Webster, Indiana on August 30, 1997. In 2011, the new Langley Hill NEXRAD was added at Langley Hill, Washington to better cover the Pacific Coast of that area; the site locations were strategically chosen to provide overlapping coverage between radars in case one failed during a severe weather event. Where possible, they were co-located with NWS Weather Forecast Offices to permit quicker access by maintenance technicians.
The NEXRAD radars incorporated a number of improvements over the radar systems that were in use. The new system provided Doppler velocity, improving tornado prediction ability by detecting rotation present within the storm at different scan angles, it provided improved resolution and sensitivity, enabling operators to see features such as cold fronts, thunderstorm gust fronts, mesoscale to storm scale features of thunderstorms that had never been visible on radar. The NEXRAD radars provided volumetric scans of the atmosphere allowing operators to examine the vertical structure of storms and can act as wind profilers by providing detailed wind information for several kilometers above the radar site; the radars had a much increased range allowing detection of weather events at much greater distances from the radar site. WSR-88D development and training are coordinated by the NEXRAD Radar Operations Center located at the National Weather Center in Norman, Oklahoma. A standard WSR-88D operates in the S band, at a frequency of around 2800 MHz, with a typical gain around 53 dB using a center-fed parabolic antenna.
The pulse repetition frequency varies from 318 to 1300 Hz with a maximum power output of 700 kW at Klystron output, although dependent on the volume coverage pattern selected by the operator. All NEXRADs have a dish diameter of 9.1 m and an aperture diameter of 8.5 m. Using the predetermined VCPs, NEXRADs have a traditional elevation minimum and maximum ranging from 0.1 to 19.5 degrees, although the non-operational minimum and maximum spans from −1 to +45 degrees. Unlike its predecessor, the WSR-74, the antenna can not be manually steered by the operator. Spatial resolution varies with data type and scan angle – level III data has a resolution of 1 km x 1 degree in azimuth, while super-res level II, has a resolution of 250m by 0.5 degrees in azimuth below 2.4 degrees in elevation. The NEXRAD radar system continually refreshes its three-dimensional database via one of several predetermined scan patterns; these patterns have differing P
Weather radar called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type. Modern weather radars are pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather. During World War II, radar operators discovered that weather was causing echoes on their screen, masking potential enemy targets. Techniques were developed to filter them. Soon after the war, surplus radars were used to detect precipitation. Since weather radar has evolved on its own and is now used by national weather services, research departments in universities, in television stations' weather departments. Raw images are used and specialized software can take radar data to make short term forecasts of future positions and intensities of rain, snow and other weather phenomena.
Radar output is incorporated into numerical weather prediction models to improve analyses and forecasts. During World War II, military radar operators noticed noise in returned echoes due to rain and sleet. After the war, military scientists returned to civilian life or continued in the Armed Forces and pursued their work in developing a use for those echoes. In the United States, David Atlas at first working for the Air Force and for MIT, developed the first operational weather radars. In Canada, J. S. Marshall and R. H. Douglas formed the "Stormy Weather Group" in Montreal. Marshall and his doctoral student Walter Palmer are well known for their work on the drop size distribution in mid-latitude rain that led to understanding of the Z-R relation, which correlates a given radar reflectivity with the rate at which rainwater is falling. In the United Kingdom, research continued to study the radar echo patterns and weather elements such as stratiform rain and convective clouds, experiments were done to evaluate the potential of different wavelengths from 1 to 10 centimeters.
By 1950 the UK company EKCO was demonstrating its airborne'cloud and collision warning search radar equipment'. In 1953 Donald Staggs, an electrical engineer working for the Illinois State Water Survey, made the first recorded radar observation of a "hook echo" associated with a tornadic thunderstorm. Between 1950 and 1980, reflectivity radars, which measure position and intensity of precipitation, were incorporated by weather services around the world; the early meteorologists had to watch a cathode ray tube. During the 1970s, radars began to be organized into networks; the first devices to capture radar images were developed. The number of scanned angles was increased to get a three-dimensional view of the precipitation, so that horizontal cross-sections and vertical cross-sections could be performed. Studies of the organization of thunderstorms were possible for the Alberta Hail Project in Canada and National Severe Storms Laboratory in the US in particular; the NSSL, created in 1964, began experimentation on dual polarization signals and on Doppler effect uses.
In May 1973, a tornado devastated Union City, just west of Oklahoma City. For the first time, a Dopplerized 10 cm wavelength radar from NSSL documented the entire life cycle of the tornado; the researchers discovered a mesoscale rotation in the cloud aloft before the tornado touched the ground – the tornadic vortex signature. NSSL's research helped convince the National Weather Service that Doppler radar was a crucial forecasting tool; the Super Outbreak of tornadoes on 3–4 April 1974 and their devastating destruction might have helped to get funding for further developments. Between 1980 and 2000, weather radar networks became the norm in North America, Europe and other developed countries. Conventional radars were replaced by Doppler radars, which in addition to position and intensity could track the relative velocity of the particles in the air. In the United States, the construction of a network consisting of 10 cm radars, called NEXRAD or WSR-88D, was started in 1988 following NSSL's research.
In Canada, Environment Canada constructed the King City station, with a 5 cm research Doppler radar, by 1985. This led to a complete Canadian Doppler network between 1998 and 2004. France and other European countries had switched to Doppler networks by the early 2000s. Meanwhile, rapid advances in computer technology led to algorithms to detect signs of severe weather, many applications for media outlets and researchers. After 2000, research on dual polarization technology moved into operational use, increasing the amount of information available on precipitation type. "Dual polarization" means that microwave radiation, polarized both horizontally and vertically is emitted. Wide-scale deployment was done by the end of the decade or the beginning of the next in some countries such as the United States and Canada. In April 2013, all United States National Weather Service NEXRADs were dual-polarized. Since 2003, the U. S. National Oceanic and Atmospheric Administration has been experimenting with phased-array radar as a replacement for conventional parabolic antenna to provide more time resolution in atmospheric sounding.
This could be significant with severe thunderstorms, as their evolution can be better evaluated with more timely data. In 2003, the National Science Foundation established the Engineering Research Center for Collaborative Adaptive Sensing of the Atmosphere
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
In the field of antenna design the term radiation pattern refers to the directional dependence of the strength of the radio waves from the antenna or other source. In the fields of fiber optics and integrated optics, the term radiation pattern may be used as a synonym for the near-field pattern or Fresnel pattern; this refers to the positional dependence of the electromagnetic field in the near-field, or Fresnel region of the source. The near-field pattern is most defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it; the far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, the radiation pattern deduced from it by computation. The far-field radiation pattern can be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can compute the near field; the far field radiation pattern may be represented graphically as a plot of one of a number of related variables, including.
Only the relative amplitude is plotted, normalized either to the amplitude on the antenna boresight, or to the total radiated power. The plotted quantity may be shown on a linear scale, or in dB; the plot is represented as a three-dimensional graph, or as separate graphs in the vertical plane and horizontal plane. This is known as a polar diagram, it is a fundamental property of antennas that the receiving pattern of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is proved below. Therefore, in discussions of radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient. Note however that this applies only to the passive antenna elements. Active antennas that include amplifiers or other components are no longer reciprocal devices. Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.
The simplest antennas and dipole antennas, consist of one or two straight metal rods along a common axis. These axially symmetric antennas have radiation patterns with a similar symmetry, called omnidirectional patterns; this illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry. In most antennas, the radiation from the different parts of the antenna interferes at some angles; this results in zero radiation at certain angles where the radio waves from the different parts arrive out of phase, local maxima of radiation at other angles where the radio waves arrive in phase. Therefore, the radiation plot of most antennas shows a pattern of maxima called "lobes" at various angles, separated by "nulls" at which the radiation goes to zero; the larger the antenna is compared to a wavelength, the more lobes there will be. In a directive antenna in which the objective is to direct the radio waves in one particular direction, the lobe in that direction is larger than the others.
The axis of maximum radiation, passing through the center of the main lobe, is called the "beam axis" or boresight axis". In some antennas, such as split-beam antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe; the other lobes, representing unwanted radiation in other directions, are called "side lobes". The side lobe in the opposite direction from the main lobe is called the "back lobe". Minor lobes represent radiation in undesired directions, so in directional antennas a design goal is to reduce the minor lobes. Side lobes are the largest of the minor lobes; the level of minor lobes is expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is termed the side lobe ratio or side lobe level. Side lobe levels of −20 dB or greater are not desirable in many applications. Attainment of a side lobe level smaller than −30 dB requires careful design and construction. In most radar systems, for example, low side lobe ratios are important to minimize false target indications through the side lobes.
For a complete proof, see the reciprocity article. Here, we present a common simple proof limited to the approximation of two antennas separated by a large distance compared to the size of the antenna, in a homogeneous medium; the first antenna is the test antenna. The second antenna is a reference antenna; each antenna is alternately connected to a transmitter having a particular source impedance, a receiver having the same input impedance. It is assumed that the two antennas are sufficiently far apart that the properties of the transmitting antenna are not affected by the load placed upon it by the receiving antenna; the amount of power transferred from the transmitter to the receiver c