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78-809: After World War II ended, the United States had two military organizations for the collection of signals intelligence (SIGINT): the Army Security Agency (ASA) and the Naval Communications Intelligence Organization ( OP-20-G ). The latter was deactivated and reorganized into the much smaller Communications Support Activities (CSA) in 1946, leaving ASA as the main SIGINT agency. Additionally, the United States Air Force established its own US Air Force Security Service (USAFSS) for

156-466: A fractal surface, such as rocks or soil, and are used by navigation radars. A radar beam follows a linear path in vacuum but follows a somewhat curved path in atmosphere due to variation in the refractive index of air, which is called the radar horizon . Even when the beam is emitted parallel to the ground, the beam rises above the ground as the curvature of the Earth sinks below the horizon. Furthermore,

234-404: A transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna , a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about

312-424: A transmitter that emits radio waves known as radar signals in predetermined directions. When these signals contact an object they are usually reflected or scattered in many directions, although some of them will be absorbed and penetrate into the target. Radar signals are reflected especially well by materials of considerable electrical conductivity —such as most metals, seawater , and wet ground. This makes

390-840: A common noun, losing all capitalization . The modern uses of radar are highly diverse, including air and terrestrial traffic control, radar astronomy , air-defense systems , anti-missile systems , marine radars to locate landmarks and other ships, aircraft anti-collision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring, radar remote sensing , altimetry and flight control systems , guided missile target locating systems, self-driving cars , and ground-penetrating radar for geological observations. Modern high tech radar systems use digital signal processing and machine learning and are capable of extracting useful information from very high noise levels. Other systems which are similar to radar make use of other parts of

468-482: A different dielectric constant or diamagnetic constant from the first, the waves will reflect or scatter from the boundary between the materials. This means that a solid object in air or in a vacuum , or a significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves from its surface. This is particularly true for electrically conductive materials such as metal and carbon fibre, making radar well-suited to

546-535: A full radar system, that he called a telemobiloscope . It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap. His system already used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning of thunderstorms to airmen and during

624-581: A live mission before, during and after the Cuban Missile Crisis . The US forced the Soviet Union to remove the weapons. The perception of the last ASAs being shuttered after the end of the Vietnam War is incorrect. In addition to the 407th ASA at Fort Bliss, deactivated in 1982...the last ASA was the 523rd based out of Fort Snelling, Minnesota. Between late 1986-early 1988, the 523rd was deactivated and

702-729: A physics instructor at the Imperial Russian Navy school in Kronstadt , developed an apparatus using a coherer tube for detecting distant lightning strikes. The next year, he added a spark-gap transmitter . In 1897, while testing this equipment for communicating between two ships in the Baltic Sea , he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation. The German inventor Christian Hülsmeyer

780-495: A proposal for further intensive research on radio-echo signals from moving targets to take place at NRL, where Taylor and Young were based at the time. Similarly, in the UK, L. S. Alder took out a secret provisional patent for Naval radar in 1928. W.A.S. Butement and P. E. Pollard developed a breadboard test unit, operating at 50 cm (600 MHz) and using pulsed modulation which gave successful laboratory results. In January 1931,

858-698: A pulsed system, and the first such elementary apparatus was demonstrated in December 1934 by the American Robert M. Page , working at the Naval Research Laboratory . The following year, the United States Army successfully tested a primitive surface-to-surface radar to aim coastal battery searchlights at night. This design was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and

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936-442: A rescue. For similar reasons, objects intended to avoid detection will not have inside corners or surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft . These precautions do not totally eliminate reflection because of diffraction , especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff , are very reflective but do not direct

1014-495: A road near the old French garrison of Cau Xang. He had been assigned to the 3rd Radio Research Unit at Tan Son Nhut Airport near Saigon , along with 92 other members of his unit. Davis Station, at Tan Son Nhut, was named after him. Most ASA personnel processed "in country" through Davis Station. Others attached to larger command structures prior to transport to Vietnam processed in with those units. ASA personnel were attached to Army infantry and armored cavalry units throughout

1092-662: A system might do, Wilkins recalled the earlier report about aircraft causing radio interference. This revelation led to the Daventry Experiment of 26 February 1935, using a powerful BBC shortwave transmitter as the source and their GPO receiver setup in a field while a bomber flew around the site. When the plane was clearly detected, Hugh Dowding , the Air Member for Supply and Research , was very impressed with their system's potential and funds were immediately provided for further operational development. Watson-Watt's team patented

1170-514: A wide region and direct fighter aircraft towards targets. Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate, and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters. Meteorologists use radar to monitor precipitation and wind. It has become

1248-855: A writeup on the apparatus was entered in the Inventions Book maintained by the Royal Engineers. This is the first official record in Great Britain of the technology that was used in coastal defence and was incorporated into Chain Home as Chain Home (low) . Before the Second World War , researchers in the United Kingdom, France , Germany , Italy , Japan , the Netherlands , the Soviet Union , and

1326-452: Is a simplification for transmission in a vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, pathloss effects are also considered. Frequency shift is caused by motion that changes the number of wavelengths between the reflector and the radar. This can degrade or enhance radar performance depending upon how it affects

1404-451: Is as follows, where F D {\displaystyle F_{D}} is Doppler frequency, F T {\displaystyle F_{T}} is transmit frequency, V R {\displaystyle V_{R}} is radial velocity, and C {\displaystyle C} is the speed of light: Passive radar is applicable to electronic countermeasures and radio astronomy as follows: Only

1482-735: Is eternal vigilance. " Although most ASA units focused upon SIGINT (signals intelligence) most if not all ASA units contained HUMINT (human intelligence) specialists as well, mostly interrogators and counter-intelligence specialists. At the end of the Cold War era, some ASA units also were staffed with ELINT (electronic intelligence) specialists and warrant officers, which incorporated field ECM (electronic counter-measures and field ECCM (electronic counter-countermeasures) such as tactical jammers, direction finders, electronic signal decoys, and captured/repurposed Warsaw Pact radio and communications equipment. The Agency existed between 1945 and 1987 and

1560-560: Is intended. Radar relies on its own transmissions rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the target objects themselves, such as infrared radiation (heat). This process of directing artificial radio waves towards objects is called illumination , although radio waves are invisible to the human eye as well as optical cameras. If electromagnetic waves travelling through one material meet another material, having

1638-518: Is the highest-ranking military official of these organizations. The current Chief of the CSS is General Timothy D. Haugh , USAF. A function specific for the CSS is that of Deputy Chief CSS (DCH/CSS), who is the principal advisor on military cryptologic issues to the Director of NSA in his role as Chief of CSS. The Deputy Chief oversees the military cryptology system and manages the partnerships between NSA/CSS and

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1716-417: Is the range. This yields: This shows that the received power declines as the fourth power of the range, which means that the received power from distant targets is relatively very small. Additional filtering and pulse integration modifies the radar equation slightly for pulse-Doppler radar performance , which can be used to increase detection range and reduce transmit power. The equation above with F = 1

1794-623: The Nyquist frequency , since the returned frequency otherwise cannot be distinguished from shifting of a harmonic frequency above or below, thus requiring: Or when substituting with F D {\displaystyle F_{D}} : As an example, a Doppler weather radar with a pulse rate of 2 kHz and transmit frequency of 1 GHz can reliably measure weather speed up to at most 150 m/s (340 mph), thus cannot reliably determine radial velocity of aircraft moving 1,000 m/s (2,200 mph). In all electromagnetic radiation ,

1872-563: The Panama Canal Zone . A former field station outside Harrogate, England , in what is now North Yorkshire, was a primary listening post that the US turned over to the British in the postwar years. They adapted it as a Royal Air Force (RAF) station. It is called RAF Menwith Hill and has been the site of peace protests. There was also a listening post set up at Schneeberg Mountain, Germany. During

1950-714: The RAF's Pathfinder . The information provided by radar includes the bearing and range (and therefore position) of the object from the radar scanner. It is thus used in many different fields where the need for such positioning is crucial. The first use of radar was for military purposes: to locate air, ground and sea targets. This evolved in the civilian field into applications for aircraft, ships, and automobiles. In aviation , aircraft can be equipped with radar devices that warn of aircraft or other obstacles in or approaching their path, display weather information, and give accurate altitude readings. The first commercial device fitted to aircraft

2028-440: The electromagnetic spectrum . One example is lidar , which uses predominantly infrared light from lasers rather than radio waves. With the emergence of driverless vehicles, radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents. As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov ,

2106-407: The reflective surfaces . A corner reflector consists of three flat surfaces meeting like the inside corner of a cube. The structure will reflect waves entering its opening directly back to the source. They are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect. Corner reflectors on boats, for example, make them more detectable to avoid collision or during

2184-527: The "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect (the common term for interference at the time) when aircraft flew overhead. By placing a transmitter and receiver on opposite sides of the Potomac River in 1922, U.S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through

2262-638: The 147th MI Bn (CEWI) under the 88th USARCOM. Composed of soldiers trained in radio communication, cryptography, military intelligence and linguists trained at the Defense Language Institute located at the Presidio of Monterey , California, the ASA was tasked with monitoring and interpreting military communications of the Soviet Union , the People's Republic of China , and their allies and client states around

2340-413: The 1920s went on to lead the U.K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told

2418-633: The 1973 pullout of US Army combat forces; they were finally withdrawn with other US personnel at the Fall of Saigon in April 1975. Bird, Kenneth L. (February 1997). "Menwith Hill Station: A Case Study in Signal Intelligence Gathering During the Cold War" (PDF). Monitoring Times. pp. 16–19. Archived from the original (PDF) on 14 July 2019. Retrieved 30 April 2009. Former Country singer Don Williams

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2496-537: The ASA was merged with the US Army's Military Intelligence component to create the United States Army Intelligence and Security Command (INSCOM). The last ASA field unit was the 407th ASA Company attached to the 3rd Armored Cavalry Regiment inactivated 15 NOV 1982 at Fort Bliss, Texas. However the 523rd ASA based out of Fort Snelling, Minnesota was active until 1987, when it was deactivated and reformed as

2574-623: The Air Force operates aircraft with sophisticated antennas and processing gear to listen to enemy radar and radio; and on the ground, the Army operates similar eavesdropping equipment. After its creation, the CSS had no emblem of its own for many years, so in 1996, a seal for the Central Security Service was created upon request of NSA Director Kenneth A. Minihan . The blue background of the CSS emblem represents "fidelity" and "steadfastness", with

2652-711: The Service Cryptologic Elements. He also ensures military capabilities to fulfill the National Cryptologic Strategy. The current Deputy Chief of the CSS is Brig. Gen. D. Scott George, USAF. The Central Security Service comprises the following Service Cryptoligic Components: These Service Cryptologic Components consist of a range of subordinate units for specific tasks, stationed both domestically and overseas. INSCOM has over 15 brigades, groups and centers with presence at over 180 locations worldwide; MCSB consists of seven companies, five of which are in

2730-712: The United States Armed Forces. Other SIGINT-related units and resources from the military can be subordinated to the Chief of the CSS by the Secretary of Defense with the advice of the Joint Chiefs of Staff. The day-to-day work of the CSS is to capture enemy signals ( radar , telemetry , radio/satellite communications) using the means of the involved service. For example, the Navy has special submarines for tapping undersea cables ;

2808-732: The United States Fleet Cyber Command, the United States Marine Corps Director of Intelligence, the United States Army's Intelligence and Security Command, the 16th Air Force representing both United States Air Force and United States Space Force services, and the US Coast Guard Deputy Assistant Commandant for Intelligence. Each are equally balanced around a six point star on which is centered the symbol of NSA/CSS, who provides

2886-714: The United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, Canada, New Zealand, and South Africa followed prewar Great Britain's radar development, Hungary and Sweden generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron , the research branch of the Compagnie générale de la télégraphie sans fil (CSF) headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locating radio apparatus, aspects of which were installed on

2964-512: The Vietnam War. Some teams were also attached to Special Forces units. Assigned to the 5th Special Forces Group (Airborne) based out of Nha Trang was the 403rd Radio Research Group, Special Operations Detachment (SOD). SOD forces were deployed to Operational Detachment base camps throughout South Vietnam. Other teams, such as the 313th Radio Research Battalion at Nha Trang , were independent of other army units. ASA personnel were kept in Vietnam after

3042-593: The armed forces. The new NSA/CSS solution increased performance standards and training and was the foundation for further centralization of NSA and the various military cryptologic elements and capabilities. The Chief of the Central Security Service (CH/CSS) is the Director of the National Security Agency (DIRNSA), who also serves as Commander of the United States Cyber Command (USCYBERCOM), and

3120-533: The arrest of Oshchepkov and his subsequent gulag sentence. In total, only 607 Redut stations were produced during the war. The first Russian airborne radar, Gneiss-2 , entered into service in June 1943 on Pe-2 dive bombers. More than 230 Gneiss-2 stations were produced by the end of 1944. The French and Soviet systems, however, featured continuous-wave operation that did not provide the full performance ultimately synonymous with modern radar systems. Full radar evolved as

3198-462: The authority of the newly created AFSA. In trying to get control over the military SIGINT elements, AFSA was replaced by the new and more powerful National Security Agency (NSA) on October 24, 1952. Tactical military intelligence was traditionally collected by specialized soldiers, sailors, airmen, Marines, and coast guardsmen deployed around the world. For example, during the Vietnam War , each of

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3276-428: The beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not immediately continue the work. Eight years later, Lawrence A. Hyland at the Naval Research Laboratory (NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application as well as

3354-530: The collection of communications intelligence in 1948. On May 20, 1949, the Secretary of Defense created the Armed Forces Security Agency (AFSA), which became responsible for the direction and control of all US communications intelligence (COMINT) and communications security (COMSEC) activities. However, at the tactical level these tasks continued to be performed by the respective army, navy, and air force agencies, which were not willing to accept

3432-603: The continental US, one in Hawaii and one in England; and AFISRA has five major wings and four supported ones with about 17,000 people at 65 locations worldwide. For all SIGINT-related activities, these military intelligence and security organizations are part of the Central Security Service and therefore subordinate to the Director of NSA in his role as Chief of the CSS. For administrative matters and logistic support, these cryptologic organizations are part of their respective parent service of

3510-408: The detection of aircraft and ships. Radar absorbing material , containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection . This is the radio equivalent of painting something a dark colour so that it cannot be seen by the eye at night. Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of

3588-471: The detection process. As an example, moving target indication can interact with Doppler to produce signal cancellation at certain radial velocities, which degrades performance. Sea-based radar systems, semi-active radar homing , active radar homing , weather radar , military aircraft, and radar astronomy rely on the Doppler effect to enhance performance. This produces information about target velocity during

3666-411: The detection process. This also allows small objects to be detected in an environment containing much larger nearby slow moving objects. Doppler shift depends upon whether the radar configuration is active or passive. Active radar transmits a signal that is reflected back to the receiver. Passive radar depends upon the object sending a signal to the receiver. The Doppler frequency shift for active radar

3744-606: The device in patent GB593017. Development of radar greatly expanded on 1 September 1936, when Watson-Watt became superintendent of a new establishment under the British Air Ministry , Bawdsey Research Station located in Bawdsey Manor , near Felixstowe, Suffolk. Work there resulted in the design and installation of aircraft detection and tracking stations called " Chain Home " along the East and South coasts of England in time for

3822-538: The electric field is perpendicular to the direction of propagation, and the electric field direction is the polarization of the wave. For a transmitted radar signal, the polarization can be controlled to yield different effects. Radars use horizontal, vertical, linear, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate

3900-473: The entire area in front of it, and then used one of Watson-Watt's own radio direction finders to determine the direction of the returned echoes. This fact meant CH transmitters had to be much more powerful and have better antennas than competing systems but allowed its rapid introduction using existing technologies. A key development was the cavity magnetron in the UK, which allowed the creation of relatively small systems with sub-meter resolution. Britain shared

3978-461: The firm GEMA  [ de ] in Germany and then another in June 1935 by an Air Ministry team led by Robert Watson-Watt in Great Britain. In 1935, Watson-Watt was asked to judge recent reports of a German radio-based death ray and turned the request over to Wilkins. Wilkins returned a set of calculations demonstrating the system was basically impossible. When Watson-Watt then asked what such

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4056-495: The funding, direction, and guidance to all of America's SIGINT activities. Army Security Agency The United States Army Security Agency (ASA) was the United States Army's signals intelligence branch from 1945 to 1987. The Latin motto of the Army Security Agency was Semper Vigilis (Vigilant Always), which echoes the declaration, often mistakenly attributed to Thomas Jefferson , that " The price of liberty

4134-531: The height of the Cold War, personnel from the 326 ASA Company stationed at Ft. Bragg, North Carolina , relocated classified mobile communications equipment to Homestead Air Force Base in Miami–Dade County, Florida . In 1962, they developed the precursor to the 6th USASA Field Station (Seminole Station). U.S. overflights photographed and discovered offensive nuclear weapons placed in Cuba by Soviet allies. Cuba became

4212-586: The military services deployed its own cryptologic units, supported by the NSA, which set up a number of SIGINT Support Groups (SSGs) as merging points for signal intelligence. With increasing cryptologic requirements, the military SIGINT systems had to be updated and unified and therefore it was planned to integrate NSA and the Service Cryptologic Agencies (SCAs) into a new unified command, with NSA absorbing SCA functions. According to James Bamford , NSA/CSS

4290-623: The objects' locations and speeds. Radar was developed secretly for military use by several countries in the period before and during World War II . A key development was the cavity magnetron in the United Kingdom , which allowed the creation of relatively small systems with sub-meter resolution. The term RADAR was coined in 1940 by the United States Navy as an acronym for "radio detection and ranging". The term radar has since entered English and other languages as an anacronym ,

4368-494: The ocean liner Normandie in 1935. During the same period, Soviet military engineer P.K. Oshchepkov , in collaboration with the Leningrad Electrotechnical Institute , produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver. The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 but further development was slowed following

4446-520: The outbreak of World War II in 1939. This system provided the vital advance information that helped the Royal Air Force win the Battle of Britain ; without it, significant numbers of fighter aircraft, which Great Britain did not have available, would always have needed to be in the air to respond quickly. The radar formed part of the " Dowding system " for collecting reports of enemy aircraft and coordinating

4524-706: The primary tool for short-term weather forecasting and watching for severe weather such as thunderstorms , tornadoes , winter storms , precipitation types, etc. Geologists use specialized ground-penetrating radars to map the composition of Earth's crust . Police forces use radar guns to monitor vehicle speeds on the roads. Automotive radars are used for adaptive cruise control and emergency breaking on vehicles by ignoring stationary roadside objects that could cause incorrect brake application and instead measuring moving objects to prevent collision with other vehicles. As part of Intelligent Transport Systems , fixed-position stopped vehicle detection (SVD) radars are mounted on

4602-432: The radial component of the velocity is relevant. When the reflector is moving at right angle to the radar beam, it has no relative velocity. Objects moving parallel to the radar beam produce the maximum Doppler frequency shift. When the transmit frequency ( F T {\displaystyle F_{T}} ) is pulsed, using a pulse repeat frequency of F R {\displaystyle F_{R}} ,

4680-414: The response. Given all required funding and development support, the team produced working radar systems in 1935 and began deployment. By 1936, the first five Chain Home (CH) systems were operational and by 1940 stretched across the entire UK including Northern Ireland. Even by standards of the era, CH was crude; instead of broadcasting and receiving from an aimed antenna, CH broadcast a signal floodlighting

4758-410: The resulting frequency spectrum will contain harmonic frequencies above and below F T {\displaystyle F_{T}} with a distance of F R {\displaystyle F_{R}} . As a result, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of F R {\displaystyle F_{R}} , called

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4836-427: The roadside to detect stranded vehicles, obstructions and debris by inverting the automotive radar approach and ignoring moving objects. Smaller radar systems are used to detect human movement . Examples are breathing pattern detection for sleep monitoring and hand and finger gesture detection for computer interaction. Automatic door opening, light activation and intruder sensing are also common. A radar system has

4914-407: The scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its radar cross-section . The power P r returning to the receiving antenna is given by the equation: where In the common case where the transmitter and the receiver are at the same location, R t = R r and the term R t ² R r ² can be replaced by R , where R

4992-505: The symbols for the cryptologic service elements provided shown clockwise from top right as follows: Army Intelligence and Security Command , Marine Corps Intelligence , Naval Security Group , Coast Guard Intelligence and Air Force Intelligence, Surveillance and Reconnaissance Agency with the symbol of the NSA in the center. The seal was updated in 2022 to display all six of the Service Cryptologic Components, which are

5070-491: The target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a mirror . If the wavelength is much longer than the size of the target, the target may not be visible because of poor reflection. Low-frequency radar technology is dependent on resonances for detection, but not identification, of targets. This is described by Rayleigh scattering , an effect that creates Earth's blue sky and red sunsets. When

5148-519: The technology with the U.S. during the 1940 Tizard Mission . In April 1940, Popular Science showed an example of a radar unit using the Watson-Watt patent in an article on air defence. Also, in late 1941 Popular Mechanics had an article in which a U.S. scientist speculated about the British early warning system on the English east coast and came close to what it was and how it worked. Watson-Watt

5226-879: The transmitter. The reflected radar signals captured by the receiving antenna are usually very weak. They can be strengthened by electronic amplifiers . More sophisticated methods of signal processing are also used in order to recover useful radar signals. The weak absorption of radio waves by the medium through which they pass is what enables radar sets to detect objects at relatively long ranges—ranges at which other electromagnetic wavelengths, such as visible light , infrared light , and ultraviolet light , are too strongly attenuated. Weather phenomena, such as fog, clouds, rain, falling snow, and sleet, that block visible light are usually transparent to radio waves. Certain radio frequencies that are absorbed or scattered by water vapour, raindrops, or atmospheric gases (especially oxygen) are avoided when designing radars, except when their detection

5304-487: The two length scales are comparable, there may be resonances . Early radars used very long wavelengths that were larger than the targets and thus received a vague signal, whereas many modern systems use shorter wavelengths (a few centimetres or less) that can image objects as small as a loaf of bread. Short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between

5382-467: The use of radar altimeters possible in certain cases. The radar signals that are reflected back towards the radar receiver are the desirable ones that make radar detection work. If the object is moving either toward or away from the transmitter, there will be a slight change in the frequency of the radio waves due to the Doppler effect . Radar receivers are usually, but not always, in the same location as

5460-644: The world. The agency was established after World War II, when the Soviet Union and the US had been allies. In the postwar years, after the Soviet Union and communist governments gained power in eastern Europe and China, they became enemies in the Cold War between Communist states and allies, and the US and western nations. The ASA was directly subordinate to the National Security Agency , and all major field stations had NSA technical representatives present. All gathered information had time-sensitive value, depending on its importance and classification. Information

5538-608: Was a 1938 Bell Lab unit on some United Air Lines aircraft. Aircraft can land in fog at airports equipped with radar-assisted ground-controlled approach systems in which the plane's position is observed on precision approach radar screens by operators who thereby give radio landing instructions to the pilot, maintaining the aircraft on a defined approach path to the runway. Military fighter aircraft are usually fitted with air-to-air targeting radars, to detect and target enemy aircraft. In addition, larger specialized military aircraft carry powerful airborne radars to observe air traffic over

5616-528: Was initially conceived as a separate "fourth department" beside the three that make up the Department of Defense . The latter resisted this idea, and therefore the CSS was founded as an inter-service organization. The Central Security Service was established by Presidential Directive in February 1972 to promote full partnership between the National Security Agency (NSA) and the Service Cryptologic Elements (SCEs) of

5694-462: Was part of the United States Army security agency. Radar Radar is a system that uses radio waves to determine the distance ( ranging ), direction ( azimuth and elevation angles ), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircraft , ships , spacecraft , guided missiles , motor vehicles , map weather formations , and terrain . A radar system consists of

5772-560: Was passed through intelligence channels within hours of intercept for the lowest-priority items, but in as little as 10 minutes for the most highly critical information. ASA personnel were stationed at locations around the globe, wherever the United States had a military presence. They were sometimes publicly acknowledged. In some cases, such as in Asmara, Eritrea , they constituted the primary US military presence. Other sites included Chitose , Japan; Sinop , Turkey; Kagnew Station , Ethiopia, and

5850-426: Was reformed as the 147th MI Bn (CEWI.) ASA personnel of the 3rd Radio Research Unit were covertly designated as Radio Research and were among the earliest U.S. military personnel in Vietnam. The 3rd later expanded to become the 509th Radio Research Group. The first ASA soldier to be killed on the battlefield in Vietnam was Specialist 4 James T. Davis (from Livingston, Tennessee). He was killed on 22 December 1961, on

5928-748: Was sent to the U.S. in 1941 to advise on air defense after Japan's attack on Pearl Harbor . Alfred Lee Loomis organized the secret MIT Radiation Laboratory at Massachusetts Institute of Technology , Cambridge, Massachusetts which developed microwave radar technology in the years 1941–45. Later, in 1943, Page greatly improved radar with the monopulse technique that was used for many years in most radar applications. The war precipitated research to find better resolution, more portability, and more features for radar, including small, lightweight sets to equip night fighters ( aircraft interception radar ) and maritime patrol aircraft ( air-to-surface-vessel radar ), and complementary navigation systems like Oboe used by

6006-411: Was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter. He obtained a patent for his detection device in April 1904 and later a patent for a related amendment for estimating the distance to the ship. He also obtained a British patent on 23 September 1904 for

6084-560: Was the successor to the Army Signals Intelligence Service , operations that dated to World War I . ASA was under the operational control of the Director of the National Security Agency (DIRNSA), located at Fort Meade , Maryland. It had its own tactical commander at Headquarters, ASA, at Arlington Hall Station , Virginia. Besides intelligence gathering, it had responsibility for the security of Army communications and for electronic countermeasures operations. In 1977,

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