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| | image1 = Radar antenna.jpg
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| | alt1 = A long-range radar [[antenna (electronics)|antenna]], known as ''ALTAIR'', used to detect and track space objects in conjunction with [[anti-ballistic missile|ABM]] testing at the [[Ronald Reagan Ballistic Missile Defense Test Site|Ronald Reagan Test Site]] on [[Kwajalein Atoll]].<!--Original text, please keep it simple in the lead-->
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| | caption1 = Long-range radar [[antenna (electronics)|antenna]], used to track space objects and ballistic missiles.
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| | image2 = Radar-hatzerim-1-1.jpg
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| | alt2 = Israeli military radar is typical of the type of radar used for [[air traffic control]]. The antenna rotates at a steady rate, sweeping the local airspace with a narrow vertical fan-shaped beam, to detect aircraft at all altitudes.<!--Original text-->
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| | caption2 = Radar of the type used for detection of aircraft. It rotates steadily sweeping the airspace with a narrow beam.
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| }}
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| '''Radar''' is an object-detection system that uses [[radio wave]]s to determine the range, altitude, direction, or speed of objects. It can be used to detect [[aircraft]], ships, [[spacecraft]], [[guided missiles]], [[motor vehicle]]s, [[Weather radar|weather formations]], and terrain. The radar dish or antenna transmits pulses of radio waves or [[microwave]]s that bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna that is usually located at the same site as the transmitter.
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| Radar was secretly developed by several nations before and during [[World War II]]. The term ''RADAR'' itself, not the actual development, was coined in 1940 by the [[United States Navy]] as an [[acronym and initialism|acronym]] for '''''RA'''dio '''D'''etection '''A'''nd '''R'''anging''.<ref>{{cite web | url = http://www.btb.termiumplus.gc.ca/tpv2alpha/alpha-fra.html?lang=fra&i=1&index=ent&__index=ent&srchtxt=radar&comencsrch.x=0&comencsrch.y=0 | title= Radar definition | publisher=Public Works and Government Services Canada | author = Translation Bureau | year= 2013 | accessdate= November 8, 2013}}</ref><ref>McGraw-Hill dictionary of scientific and technical terms / Daniel N. Lapedes, editor in chief. Lapedes, Daniel N. New York ; Montreal : McGraw-Hill, 1976. [xv], 1634, A26 p.</ref> The term ''radar'' has since entered [[English language|English]] and other languages as the common noun ''radar'', losing all capitalization.
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| The modern uses of radar are highly diverse, including air traffic control, [[radar astronomy]], [[antiaircraft warfare|air-defense systems]], [[close-in weapon system|antimissile systems]]; [[marine radar]]s to locate landmarks and other ships; aircraft anticollision systems; [[Research vessel|ocean surveillance]] systems, outer space surveillance and [[Space rendezvous|rendezvous]] systems; [[meteorology|meteorological]] precipitation monitoring; altimetry and [[flight control system]]s; [[Precision-guided munition|guided missile]] target locating systems; and [[ground-penetrating radar]] for geological observations. High tech radar systems are associated with [[digital signal processing]] and are capable of extracting useful information from very high [[noise (electronics)|noise]] levels.
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| Other systems similar to radar make use of other parts of the [[electromagnetic spectrum]]. One example is "[[lidar]]", which uses visible light from [[laser]]s rather than radio waves.
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| ==History==
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| {{Main|History of radar}}
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| As early as 1886, German physicist [[Heinrich Hertz]] showed that radio waves could be reflected from solid objects. In 1895, [[Alexander Stepanovich Popov|Alexander Popov]], 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.<ref>Kostenko, A. A., A. I. Nosich, and I. A. Tishchenko, "Radar Prehistory, Soviet Side," ''Proc. of IEEE APS International Symposium 2001,'' vol.4. p. 44, 2003</ref>
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| The German inventor [[Christian Hülsmeyer]] 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.<ref name="RadarWorld">[http://www.radarworld.org/huelsmeyer.html Christian Hülsmeyer by Radar World]</ref> He obtained a patent<ref>[[Media:DE165546.pdf|''Patent DE165546; Verfahren, um metallische Gegenstände mittels elektrischer Wellen einem Beobachter zu melden.'']]</ref> for his detection device in April 1904 and later a patent<ref>[[Media:DE169154.pdf|''Verfahren zur Bestimmung der Entfernung von metallischen Gegenständen (Schiffen o. dgl.), deren Gegenwart durch das Verfahren nach Patent 16556 festgestellt wird.'']]</ref> for a related amendment for estimating the distance to the ship. He also got a British patent on September 23, 1904<ref>{{patent|GB|13170|''Telemobiloscope''}}</ref> for a full system, that he called a ''telemobiloscope''.[[File:Chain home.jpg|thumb|upright|A [[Chain Home]] tower in Great Baddow, United Kingdom]]
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| In 1922 [[A. Hoyt Taylor]] and [[Leo C. Young]], researchers working with the U.S. Navy, had a transmitter and a receiver on opposite sides of the [[Potomac River]] and discovered that a ship passing through the beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this 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]] observed similar fading effects from a passing aircraft; this led to a patent application<ref>Hyland, L.A,, A.H. Taylor, and L.C. Young; "System for detecting objects by radio," U.S. Patent No. 1981884, granted 27 Nov. 1934</ref> as well as a proposal for serious work at the NRL (Taylor and Young were then at this laboratory) on radio-echo signals from moving targets.<ref>Howeth, Linwood S.; "Radar," Ch. XXXVIII in ''History of Communications -Electronics in the United States Navy'', 1963; [http://earlyradiohistory.us/1963hw38.htm Radar]</ref>
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| Before the [[Second World War]], researchers in [[France]], [[Germany]], [[Italy]], [[Japan]], the [[Netherlands]], the [[Soviet Union]], the [[United Kingdom]], and 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, and [[Hungary]] had similar developments during the war.<ref>{{cite book| author = Watson, Raymond C., Jr.| title = Radar Origins Worldwide: History of Its Evolution in 13 Nations Through World War II| url = http://books.google.com/?id=g-rQQgAACAAJ| date = 2009-11-25| publisher = Trafford Publishing| isbn = 978-1-4269-2111-7 }}</ref>
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| In France in 1934, following systematic studies on the magnetron, the research branch of the [[Thomson-CSF|Compagnie Générale de Télégraphie Sans Fil]] (CSF), headed by Maurice Ponte, with Henri Gutton, Sylvain Berline, and M. Hugon began developing an obstacle-locating radio apparatus, a part of which was installed on the [[SS Normandie|Normandie]] liner in 1935.<ref>{{cite book| author = Hearst Magazines| title = Popular Mechanics| url = http://books.google.com/?id=x98DAAAAMBAJ&pg=PA844| date = December 1935| publisher = Hearst Magazines| isbn = | page = 844 }}</ref><ref>Frederick Seitz, Norman G. Einspruch, Electronic Genie: The Tangled History of Silicon - 1998 - page 104</ref>
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| During the same time, the Soviet military engineer [[Pavel K. Oshchepkov|P. K. Oshchepkov]], in collaboration with [[Saint Petersburg State Electrotechnical University|Leningrad Electrophysical Institute]], produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver.<ref>John Erickson. Radio-Location and the Air Defence Problem: The Design and Development of Soviet Radar. ''Science Studies'', vol. 2, no. 3 (Jul., 1972), pp. 241-263</ref> The French and Soviet systems, however, had continuous-wave operation and could not give the full performance that was ultimately at the center of modern radar.
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| Full radar evolved as a pulsed system, and the first such elementary apparatus was demonstrated in December 1934 by the American [[Robert Morris Page|Robert M. Page]], working at the [[Naval Research Laboratory]].<ref>Page, Robert Morris, ''The Origin of Radar'', Doubleday Anchor, New York, 1962, p. 66</ref> The following year, the [[United States Army]] successfully tested a primitive surface-to-surface radar to aim coastal battery search lights at night.<ref>{{cite book| author = Bonnier Corporation| title = Popular Science| url = http://books.google.com/?id=bygDAAAAMBAJ&pg=PA29| date = October 1935| publisher = Bonnier Corporation| isbn = | page = 29 }}</ref> This was followed by a pulsed system demonstrated in May 1935 by [[Rudolf Kühnhold]] and the firm [[GEMA]] in Germany and then one in June 1935 by an [[Air Ministry]] team led by [[Robert Watson-Watt|Robert A. Watson Watt]] in Great Britain. 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 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]].
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| The British were the first to fully exploit radar as a defence against aircraft attack. This was spurred on by fears that the Germans were developing [[death ray]]s.<ref name=dora/> The Air Ministry asked British scientists in 1934 to investigate the possibility of propagating electromagnetic energy and the likely effect. Following a study, they concluded that a death ray was impractical but that detection of aircraft appeared feasible.<ref name="dora">{{cite web|url=http://www.doramusic.com/Radar.htm |title=The story of RADAR Development|author=Alan Dower Blumlein|year=2002|accessdate=2011-05-06}}</ref> Robert Watson Watt's team demonstrated to his superiors the capabilities of a working prototype and then patented the device.<ref name="radarnet">{{Fr icon}} [http://www.radar-france.fr/brevet%20radar1934.htm Copy of Patents an Obstacle-Locating Radio Apparatus] on www.radar-france.fr</ref><ref>[http://www.patent.gov.uk/media/pressrelease/2001/1009.htm British man first to patent radar] official site of the ''Patent Office'' {{dead link|date=September 2011}}</ref><ref>{{patent|GB|593017|''Improvements in or relating to wireless systems''}}</ref> It served as the basis for the [[Chain Home]] network of radars to defend Great Britain, which detected approaching German aircraft in the [[Battle of Britain]] in 1940.
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| In April 1940, ''[[Popular Science]]'' showed an example of a radar unit using the Watson-Watt patent in an article on air defence, but not knowing that the U.S. Army and U.S. Navy were working on radars with the same principle, stated under the illustration, "This is not U.S. Army equipment."<ref>{{cite book| author = Bonnier Corporation| title = Popular Science| url = http://books.google.com/?id=hCcDAAAAMBAJ&pg=PA56| date = December 1941| publisher = Bonnier Corporation| isbn = | page = 56 }}</ref> 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.<ref name="Hearst Magazines 26">{{cite book| author = Hearst Magazines| title = Popular Mechanics| url = http://books.google.com/?id=69kDAAAAMBAJ&pg=PA26| date = September 1941| publisher = Hearst Magazines| isbn = | page = 26 }}</ref> [[Alfred Lee Loomis]] organized the [[Radiation Laboratory]] at Cambridge, Massachusetts which developed the technology in the years 1941-45. Later, in 1943, Page greatly improved radar with the [[Monopulse radar|monopulse technique]] that was used for many years in most radar applications.<ref>{{cite web | last=Goebel | first=Greg | title=The Wizard War: WW2 & The Origins Of Radar | url=http://www.vectorsite.net/ttwiz_01.html | date=2007-01-01 | accessdate=2007-03-24}}</ref>
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| The war precipitated research to find better resolution, more portability, and more features for radar, including complementary navigation systems like [[Oboe (navigation)|Oboe]] used by the [[Pathfinder (RAF)|RAF's Pathfinder]].
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| ==Applications==
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| [[File:Radar antennas on USS Theodore Roosevelt SPS-64.jpg|thumb|left|Commercial marine radar antenna. The rotating antenna radiates a vertical fan-shaped beam.]]
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| 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 roads.
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| In [[aviation]], aircraft are 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 was a 1938 Bell Lab unit on some [[United Air Lines]] aircraft.<ref name="Hearst Magazines 26"/> Such aircraft can land in fog at airports equipped with radar-assisted [[ground-controlled approach]] systems in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot.
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| [[Marine radar]]s 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.
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| Meteorologists use radar to monitor [[Precipitation (meteorology)|precipitation]] and wind. It has become the primary tool for short-term [[weather forecast]]ing and watching for [[severe weather]] such as [[thunderstorm]]s, [[tornado]]es, [[winter storm]]s, precipitation types, etc. [[Geologist]]s use specialised [[ground-penetrating radar]]s to map the composition of [[Crust (geology)|Earth's crust]].
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| Police forces use [[radar gun]]s to monitor vehicle speeds on the roads.
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| ==Principles==
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| A radar system has a [[transmitter]] that emits [[radio waves]] called ''radar signals'' in predetermined directions. When these come into contact with an object they are usually [[reflection (physics)|reflected]] or [[scattering|scattered]] in many directions. Radar signals are reflected especially well by materials of considerable [[electrical conductivity]]—especially by most metals, by [[seawater]] and by wet lands. Some of these make the use of [[radar altimeter]]s possible. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is ''moving'' either toward or away from the transmitter, there is a slight equivalent change in the [[frequency]] of the radio waves, caused by the [[Doppler effect]].
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| Radar receivers are usually, but not always, in the same location as the transmitter. Although the reflected radar signals captured by the receiving antenna are usually very weak, they can be strengthened by [[Amplifier|electronic amplifiers]]. More sophisticated methods of [[signal processing]] are also used in order to recover useful radar signals.
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| The weak absorption of radio waves by the medium through which it passes 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. Such weather phenomena 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 vapor, raindrops, or atmospheric gases (especially oxygen) are avoided in designing radars, except when their detection is intended.
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| Radar relies on its own transmissions rather than light from the [[Sun]] or the [[Moon]], or from [[electromagnetic wave]]s emitted by the objects themselves, such as infrared wavelengths (heat). This process of directing artificial radio waves towards objects is called ''illumination'', although radio waves are invisible to the human eye or optical cameras.
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| ===Reflection===
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| [[File:weather radar.jpg|thumb|Brightness can indicate reflectivity as in this 1960 [[weather radar]] image (of [[1960 Atlantic hurricane season#Hurricane Abby|Hurricane Abby]]). The radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.]]
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| If [[Electromagnetic radiation|electromagnetic waves]] traveling through one material meet another, having a very different [[dielectric constant]] or [[diamagnetism|diamagnetic constant]] from the first,
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| the waves will reflect or scatter from the boundary between the materials. This means that a solid object in [[Earth's atmosphere|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 [[electrical conduction|electrically conductive]] materials such as metal and carbon fiber, making radar well-suited to the detection of aircraft and ships. [[Radar absorbing material]], containing [[Electrical resistance|resistive]] and sometimes [[magnetism|magnetic]] substances, is used on military vehicles to [[stealth technology|reduce radar reflection]]. This is the radio equivalent of painting something a dark color so that it cannot be seen by the eye at night.
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| Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of 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 [[sunset]]s. When the two length scales are comparable, there may be [[resonance]]s. Early radars used very long wavelengths that were larger than the targets and thus received a vague signal, whereas some modern systems use shorter wavelengths (a few [[centimeter]]s or less) that can image objects as small as a loaf of bread.
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| 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 the [[reflection (physics)|reflective surfaces]]. A [[corner reflector]] consists of three flat surfaces meeting like the inside corner of a box. 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 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 completely eliminate reflection because of [[diffraction]], especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as [[Chaff (countermeasure)|chaff]], are very reflective but do not direct the scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its [[radar cross section]].
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| ===Radar equation===
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| The power ''P<sub>r</sub>'' returning to the receiving antenna is given by the equation:
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| :<math>P_r = {{P_t G_t A_r \sigma F^4}\over{{(4\pi)}^2 R_t^2R_r^2}}</math>
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| where
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| * ''P''<sub>t</sub> = transmitter power
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| * ''G''<sub>t</sub> = [[gain]] of the transmitting antenna
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| * ''A''<sub>r</sub> = [[effective aperture]] (area) of the receiving antenna
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| * ''σ'' = [[radar cross section]], or scattering coefficient, of the target
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| * ''F'' = pattern propagation factor
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| * ''R''<sub>t</sub> = distance from the transmitter to the target
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| * ''R''<sub>r</sub> = distance from the target to the receiver.
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| In the common case where the transmitter and the receiver are at the same location, ''R''<sub>t</sub> = ''R''<sub>r</sub> and the term ''R''<sub>t</sub>² ''R''<sub>r</sub>² can be replaced by ''R''<sup>4</sup>, where ''R'' is the range.
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| This yields:
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| :<math>P_r = {{P_t G_t A_r \sigma F^4}\over{{(4\pi)}^2 R^4}}.</math>
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| 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.
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| Additional filtering and pulse integration modifies the radar equation slightly for [[Pulse-Doppler radar#Performance|pulse-Doppler radar performance]], which can be used to increase detection range and reduce transmit power.
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| The equation above with ''F'' = 1 is a simplification for transmission in a [[vacuum]] without interference. The propagation factor accounts for the effects of [[Multipath propagation|multipath]] and shadowing and depends on the details of the environment. In a real-world situation, [[pathloss]] effects should also be considered.
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| ===Doppler effect===
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| {{Main|Doppler radar|Pulse-Doppler radar}}
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| Frequency shift is caused by motion that changes the number of wavelengths between the reflector and the radar. That can degrade or enhance radar performance depending upon how that affects the detection process. As an example, [[Moving Target Indication]] can interact with Doppler to produce signal cancellation at certain radial velocities, which degrades performance.
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| Sea-based radar systems, [[semi-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 the detection process. This also allows small objects to be detected in an environment containing much larger nearby slow moving objects.
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| 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.
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| The Doppler frequency shift for active radar is as follows, where <math>F_D</math> is Doppler frequency, <math>F_T</math> is transmit frequency, <math>V_R</math> is radial velocity, and <math>C</math> is the speed of light:<ref>{{cite web|title=Exploration: The Doppler Effect|author=M. Castelaz|publisher=Pisgah Astronomical Research Institute}}</ref>
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| :<math>F_D = 2 \times F_T \times \left (\frac {V_R}{C} \right)</math>
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| Passive radar is applicable to [[electronic countermeasures]] and [[radio astronomy]] as follows:
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| :<math>F_D = F_T \times \left (\frac {V_R}{C} \right)</math>
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| Only the radial component of the speed is relevant. When the reflector is moving at right angle to the radar beam, it has no relative velocity. Vehicles and weather moving parallel to the radar beam produce the maximum Doppler frequency shift.
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| Doppler measurement is reliable only if the sampling rate exceeds the [[Nyquist frequency]] for the frequency shift produced by radial motion. As an example, Doppler weather radar with a pulse rate of 2 kHz and transmit frequency of 1 GHz can reliably measure weather up to 150 m/s (330 mile/hour), but cannot reliably determine radial velocity of aircraft moving 1,000 m/s (3,300 mile/hour).
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| ===Polarization===
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| In all [[electromagnetic radiation]], the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the [[Polarization (waves)|polarization]] of the wave. In the transmitted radar signal the polarization can be controlled for 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 a [[fractal]] surface, such as rocks or soil, and are used by navigation radars.
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| === Limiting factors ===
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| ==== Beam path and range ====
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| [[File:Radar-height.PNG|thumb|300px|right|Echo heights above ground]]
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| The radar beam would follow a linear path in vacuum, but it really follows a somewhat curved path in the atmosphere because of the variation of the [[refractive index]] of air, that is called the [[radar horizon]]. Even when the beam is emitted parallel to the ground, it will rise above it as the [[Figure of the Earth|Earth curvature]] sinks below the horizon. Furthermore, the signal is attenuated by the medium it crosses, and the beam disperses.
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| The maximum range of a conventional radar can be limited by a number of factors:
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| *Line of sight, which depends on height above ground. This means with out a direct line of sight the path of the beam is blocked.
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| *The maximum non-ambiguous range, which is determined by the [[pulse repetition frequency]]. The maximum non-ambiguous range is the distance the pulse could travel and return before the next pulse is emitted.
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| *Radar sensitivity and power of the return signal as computed in the radar equation. This includes factors such as environmental conditions and the size (or radar cross section) of the target.
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| ==== Noise ====
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| [[Signal noise]] is an internal source of random variations in the signal, which is generated by all electronic components.
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| Reflected signals decline rapidly as distance increases, so noise introduces a radar range limitation. The [[noise floor]] and [[signal to noise ratio]] are two different [[Test and evaluation master plan#Measures of Performance|measure of performance]] that impact range performance. Reflectors that are too far away produce too little signal to exceed the noise floor and cannot be detected. [[Detection]] requires a signal that exceeds the [[noise floor]] by at least the [[signal to noise ratio]].
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| Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal, the more difficult it is to discern it from the noise. [[Noise figure]] is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.
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| [[Shot noise]] is produced by electrons in transit across a discontinuity, which occurs in all detectors. Shot noise is the dominant source in most receivers. There will also be [[flicker noise]] caused by electron transit through amplification devices, which is reduced using [[heterodyne]] amplification. Another reason for heterodyne processing is that for fixed fractional bandwidth, the instantaneous bandwidth increases linearly in frequency. This allows improved range resolution. The one notable exception to heterodyne (downconversion) radar systems is [[ultra-wideband]] radar. Here a single cycle, or transient wave, is used similar to UWB communications, see [[List of UWB channels]].
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| Noise is also generated by external sources, most importantly the natural thermal radiation of the background surrounding the target of interest. In modern radar systems, the internal noise is typically about equal to or lower than the external noise. An exception is if the radar is aimed upwards at clear sky, where the scene is so "cold" that it generates very little [[thermal noise]]. The thermal noise is given by ''k<sub>B</sub> T B'', where ''T'' is temperature, ''B'' is bandwidth (post matched filter) and ''k<sub>B</sub>'' is [[Boltzmann's constant]]. There is an appealing intuitive interpretation of this relationship in a radar. Matched filtering allows the entire energy received from a target to be compressed into a single bin (be it a range, Doppler,elevation, or azimuth bin). On the surface it would appear that then within a fixed interval of time one could obtain perfect, error free, detection. To do this one simply compresses all energy into an infinitesimal time slice. What limits this approach in the real world is that, while time is arbitrarily divisible, current is not. The quantum of electrical energy is an electron, and so the best one can do is match filter all energy into a single electron. Since the electron is moving at a certain temperature (Plank spectrum) this noise source cannot be further eroded. We see then that radar, like all macro-scale entities, is profoundly impacted by quantum theory.
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| Noise is random and target signals are not. Signal processing can take advantage of this phenomenon to reduce the noise floor using two strategies. The kind of signal integration used with [[Moving Target Indication]] can improve noise up to <math>\sqrt{2}</math> for each stage. The signal can also be split among multiple filters for [[pulse-Doppler signal processing]], which reduces the noise floor by the number of filters. These improvements depend upon [[Coherence (physics)|coherence]].
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| ==== Interference ====
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| Radar systems must overcome unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its [[signal-to-noise ratio]] (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal; it compares the level of a desired target signal to the level of background noise (atmospheric noise and noise generated within the receiver). The higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.
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| ==== Clutter ====
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| Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground, sea, [[Precipitation (meteorology)|precipitation]] (such as rain, snow or hail), [[sand storm]]s, animals (especially birds), atmospheric [[turbulence]], and other atmospheric effects, such as [[ionosphere]] reflections, [[meteor]] trails, and [[three body scatter spike]]. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as [[Chaff (countermeasure)|chaff]].
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| Some clutter may also be caused by a long radar [[waveguide]] between the radar transceiver and the antenna. In a typical [[plan position indicator]] (PPI) radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the centre of the display as the receiver responds to echoes from dust particles and misguided RF in the waveguide. Adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range, since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna. Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar.
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| Clutter is detected and neutralized in several ways. Clutter tends to appear static between radar scans; on subsequent scan echoes, desirable targets will appear to move, and all stationary echoes can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with [[circular polarization]] (note that meteorological radars wish for the opposite effect, and therefore use [[linear polarization]] to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.
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| Clutter moves with the wind or is stationary. Two common strategies to improve [[Test and evaluation master plan#Measures of Performance|measure or performance]] in a clutter environment are:
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| :* [[Moving Target Indication]], which integrates successive pulses and
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| :* Doppler processing, which uses filters to separate clutter from desirable signals.
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| The most effective clutter reduction technique is [[pulse-Doppler radar]]. Doppler separates clutter from aircraft and spacecraft using a [[Radar signal characteristics#The radar signal in the frequency domain|frequency spectrum]], so individual signals can be separated from multiple reflectors located in the same volume using velocity differences. This requires a coherent transmitter. Another technique uses a [[moving target indicator]] that subtracts the receive signal from two successive pulses using phase to reduce signals from slow moving objects. This can be adapted for systems that lack a coherent transmitter, such as [[Radar signal characteristics#Unambiguous range|time-domain pulse-amplitude radar]].
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| [[Constant False Alarm Rate]], a form of [[Automatic Gain Control]] (AGC), is a method that relies on clutter returns far outnumbering echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled and affected the gain with greater granularity in specific detection cells.
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| [[File:Multipath propagation diagram en.svg|thumb|Radar multipath [[Light echo|echoes]] from a target cause ghosts to appear.]]
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| Clutter may also originate from multipath echoes from valid targets caused by ground reflection, [[atmospheric ducting]] or [[ionospheric reflection]]/[[refraction]] (e.g., [[Anomalous propagation]]). This clutter type is especially bothersome since it appears to move and behave like other normal (point) targets of interest. In a typical scenario, an aircraft echo is reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or eliminating it on the basis of [[jitter]] or a physical impossibility. Terrain bounce jamming exploits this response by amplifying the radar signal and directing it downward.<ref>{{cite web|url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA101208 |title= Investigation of Terrain Bounce Electronic Countermeasure |author=Strasser, Nancy C.|publisher=DTIC}}</ref> These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height. Monopulse can be improved by altering the elevation algorithm used at low elevation. In newer air traffic control radar equipment, algorithms are used to identify the false targets by comparing the current pulse returns to those adjacent, as well as calculating return improbabilities.
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| ==== Jamming ====
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| [[Radar jamming]] refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional, as with an [[electronic warfare]] tactic, or unintentional, as with friendly forces operating equipment that transmits using the same frequency range. Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals.
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| Jamming is problematic to radar since the jamming signal only needs to travel one way (from the jammer to the radar receiver) whereas the radar echoes travel two ways (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the [[Line-of-sight propagation|line of sight]] from the jammer to the radar (''mainlobe jamming''). Jammers have an added effect of affecting radars along other lines of sight through the radar receiver's [[sidelobe]]s (''sidelobe jamming'').
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| Mainlobe jamming can generally only be reduced by narrowing the mainlobe [[solid angle]] and cannot fully be eliminated when directly facing a jammer which uses the same frequency and polarization as the radar. Sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using an [[omnidirectional antenna]] to detect and disregard non-mainlobe signals. [[Electronic counter-counter-measures|Other anti-jamming techniques]] are [[frequency hopping]] and [[Polarization (waves)|polarization]].
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| ==Radar signal processing==
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| ===Distance measurement===
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| ====Transit time====
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| [[File:Radaroperation.gif|thumb|right|Pulse radar: The round-trip time for the radar pulse to get to the target and return is measured. The distance is proportional to this time.]]
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| [[File:Sonar Principle EN.svg|thumb|right|Continuous wave (CW) radar]]
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| One way to obtain a [[distance measurement]] is based on the [[time-of-flight]]: transmit a short pulse of radio signal (electromagnetic radiation) and measure the time it takes for the reflection to return. The distance is one-half the product of the round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal. Since radio waves travel at the [[speed of light]], accurate distance measurement requires high-performance electronics.
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| In most cases, the receiver does not detect the return while the signal is being transmitted. Through the use of a duplexer, the radar switches between transmitting and receiving at a predetermined rate.
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| A similar effect imposes a maximum range as well. In order to maximize range, longer times between pulses should be used, referred to as a pulse repetition time, or its reciprocal, pulse repetition frequency.
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| These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast have less total energy, making the returns much smaller and the target harder to detect. This could be offset by using more pulses, but this would shorten the maximum range. So each radar uses a particular type of signal. Long-range radars tend to use long pulses with long delays between them, and short range radars use smaller pulses with less time between them. As electronics have improved many radars now can change their pulse repetition frequency, thereby changing their range. The newest radars fire two pulses during one cell, one for short range (10 km / 6 miles) and a separate signal for longer ranges (100 km /60 miles).
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| The distance [[Optical resolution|resolution]] and the characteristics of the received signal as compared to noise depends on the shape of the pulse. The pulse is often [[modulation|modulated]] to achieve better performance using a technique known as [[pulse compression]].
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| Distance may also be measured as a function of time. The [[radar mile]] is the amount of time it takes for a radar pulse to travel one [[nautical mile]], reflect off a target, and return to the radar antenna. Since a nautical mile is defined as 1,852 meters, then dividing this distance by the speed of light (299,792,458 meters per second), and then multiplying the result by 2 yields a result of 12.36 microseconds in duration.
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| ====Frequency modulation====
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| Another form of distance measuring radar is based on [[frequency modulation]]. Frequency comparison between two signals is considerably more accurate, even with older electronics, than timing the signal. By measuring the frequency of the returned signal and comparing that with the original, the difference can be easily measured.
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| This technique can be used in [[continuous wave radar]] and is often found in aircraft [[radar altimeter]]s. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a [[sine wave]] or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared using a simple ''beat frequency'' modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.
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| Since the signal frequency is changing, by the time the signal returns to the aircraft the transmit frequency has changed. The amount of frequency shift is used to measure distance.
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| The [[Frequency modulation#Modulation index|modulation index]] riding on the receive signal is proportional to the time delay between the radar and the reflector. The amount of that frequency shift becomes greater with greater time delay. The measure of the amount of frequency shift is directly proportional to the distance traveled. That distance can be displayed on an instrument, and it may also be available via the transponder. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are [[AZUSA]], [[MISTRAM]], and [[UDOP]].
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| A further advantage is that the radar can operate effectively at relatively low frequencies. This was important in the early development of this type when high frequency signal generation was difficult or expensive.
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| Terrestrial radar uses low-power FM signals that cover a larger frequency range. The multiple reflections are analyzed mathematically for pattern changes with multiple passes creating a computerized synthetic image. Doppler effects are used which allows slow moving objects to be detected as well as largely eliminating "noise" from the surfaces of bodies of water.
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| ===Speed measurement===
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| [[Speed]] is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making [[grease pencil]] marks on the radar screen and then calculating the speed using a [[slide rule]]. Modern radar systems perform the equivalent operation faster and more accurately using computers.
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| If the transmitter's output is coherent (phase synchronized), there is another effect that can be used to make almost instant speed measurements (no memory is required), known as the [[Doppler effect]]. Most modern radar systems use this principle into [[Doppler radar]] and [[pulse-Doppler radar]] systems ([[weather radar]], military radar, etc...). The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's [[azimuth]] over time.
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| It is possible to make a Doppler radar without any pulsing, known as a [[continuous-wave radar]] (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal for determining the radial component of a target's velocity. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where range is not important.
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| When using a pulsed radar, the variation between the phase of successive returns gives the distance the target has moved between pulses, and thus its speed can be calculated.
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| Other mathematical developments in radar signal processing include [[time-frequency analysis]] (Weyl Heisenberg or [[wavelet]]), as well as the [[chirplet transform]] which makes use of the change of frequency of returns from moving targets ("chirp").
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| ===Pulse-Doppler signal processing===
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| [[File:Pulse doppler signal processing.png|thumb|Pulse-Doppler signal processing. The ''Range Sample'' axis represents individual samples taken in between each transmit pulse. The ''Range Interval'' axis represents each successive transmit pulse interval during which samples are taken. The Fast Fourier Transform process converts time-domain samples into frequency domain spectra. This is sometimes called the ''bed of nails''.]]
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| [[Pulse-Doppler signal processing]] includes frequency filtering in the detection process. The space between each transmit pulse is divided into range cells or range gates. Each cell is filtered independently much like the process used by a [[spectrum analyzer]] to produce the display showing different frequencies. Each different distance produces a different spectrum. These spectra are used to perform the detection process. This is required to achieve acceptable performance in hostile environments involving weather, terrain, and electronic countermeasures.
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| The primary purpose is to measure both the amplitude and frequency of the aggregate reflected signal from multiple distances. This is used with [[weather radar]] to measure radial wind velocity and precipitation rate in each different volume of air. This is linked with computing systems to produce a real-time electronic weather map. Aircraft safety depends upon continuous access to accurate weather radar information that is used to prevent injuries and accidents. Weather radar uses a [[Pulse repetition frequency#Low PRF|low PRF]]. Coherency requirements are not as strict as those for military systems because individual signals ordinarily do not need to be separated. Less sophisticated filtering is required, and range ambiguity processing is not normally needed with weather radar in comparison with military radar intended to track air vehicles.
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| The alternate purpose is "[[look-down/shoot-down]]" capability required to improve military air combat survivability. Pulse-Doppler is also used for ground based surveillance radar required to defend personnel and vehicles.<ref name="mit1">{{cite web|url=http://www.mit.edu/~lrv/cornell/publications/Ground%20Surveillance%20Radars%20and%20Military%20Intelligence.pdf|title=Ground Surveillance Radars and Military Intelligence|publisher=Syracuse Research Corporation; Massachusetts Institute of Technology}}</ref><ref>{{cite web|url=http://www.youtube.com/watch?v=B0q1Pgz6Cm8|title=AN/PPS-5 Ground Surveillance Radar|publisher=YouTube; jaglavaksoldier's Channel}}</ref> Pulse-Doppler signal processing increases the maximum detection distance using less radiation in close proximity to aircraft pilots, shipboard personnel, infantry, and artillery. Reflections from terrain, water, and weather produce signals much larger than aircraft and missiles, which allows fast moving vehicles to hide using [[nap-of-the-earth]] flying techniques and [[stealth technology]] to avoid detection until an attack vehicle is too close to destroy. Pulse-Doppler signal processing incorporates more sophisticated electronic filtering that safely eliminates this kind of weakness. This requires the use of medium pulse-repetition frequency with phase coherent hardware that has a large dynamic range. Military applications require [[Pulse repetition frequency#Medium PRF|medium PRF]] which prevents range from being determined directly, and [[range ambiguity resolution]] processing is required to identify the true range of all reflected signals. Radial movement is usually linked with Doppler frequency to produce a lock signal that cannot be produced by radar jamming signals. Pulse-Doppler signal processing also produces audible signals that can be used for threat identification.<ref name="mit1"/>
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| ===Reduction of interference effects===
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| [[Signal processing]] is employed in radar systems to reduce the [[#Interference|radar interference effects]]. Signal processing techniques include [[moving target indication]], [[Pulse-Doppler signal processing]], moving target detection processors, correlation with [[secondary surveillance radar]] targets, [[space-time adaptive processing]], and [[track-before-detect]]. [[Constant false alarm rate]] and [[digital terrain model]] processing are also used in clutter environments.
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| ===Plot and track extraction===
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| {{Main|Track_algorithm}}
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| A Track algorithm is a radar performance enhancement strategy. Tracking algorithms provide the ability to predict future position of multiple moving objects based on the history of the individual positions being reported by sensor systems.
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| Historical information is accumulated and used to predict future position for use with air traffic control, threat estimation, combat system doctrine, gun aiming, and missile guidance. Position data is accumulated radar sensors over the span of a few minutes.
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| There are four common track algorithms.<ref>{{cite web|url=http://www.aticourses.com/fundamentals_radar_tracking.htm|title=Fundamentals of Radar Tracking|publisher=Applied Technology Institute}}</ref>
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| * Nearest Neighbor
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| * Probabilistic Data Association
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| * Multiple Hypothesis Tracking
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| * Interactive Multiple Model (IMM)
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| Radar video returns from aircraft can be subjected to a plot extraction process whereby spurious and interfering signals are discarded. A sequence of target returns can be monitored through a device known as a plot extractor.
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| The non-relevant real time returns can be removed from the displayed information and a single plot displayed. In some radar systems, or alternatively in the command and control system to which the radar is connected, a [[radar tracker]] is used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.
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| ==Engineering==
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| [[File:Radar composantes.svg|thumb|right|Radar components]]
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| A radar's components are:
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| * A [[transmitter]] that generates the radio signal with an oscillator such as a [[klystron]] or a [[magnetron]] and controls its duration by a [[modulator]].
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| * A [[waveguide]] that links the transmitter and the antenna.
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| * A [[duplexer]] that serves as a switch between the antenna and the transmitter or the receiver for the signal when the antenna is used in both situations.
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| * A [[Receiver (radio)|receiver]]. Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a [[matched filter]].
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| * A display processor to produce signals for human readable [[Radar display|output devices]].
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| * An electronic section that controls all those devices and the antenna to perform the radar scan ordered by software.
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| * A link to end user devices and displays.
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| ===Antenna design===
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| Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.
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| Early systems tended to use [[omnidirectional antenna|omnidirectional broadcast antennas]], with directional receiver antennas which were pointed in various directions. For instance, the first system to be deployed, Chain Home, used two straight antennas at [[right angle]]s for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by [[rotation|rotating]] the antenna so one display showed a maximum while the other showed a minimum.
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| One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is [[inverse-square law|a small part]] of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.
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| ====Parabolic reflector====
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| More modern systems use a steerable [[parabola|parabolic]] "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or '''radar lock'''.
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| Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:
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| Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The [[NEXRAD]] [[Pulse-Doppler]] weather radar uses a symmetric antenna to perform detailed volumetric scans of the atmosphere. Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.
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| [[File:SPS-10 radar antenna on a Knox class frigate.jpg|thumb|right|Surveillance radar antenna]]
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| ====Types of scan====
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| * Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan, etc.
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| * Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching, etc.
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| * Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.
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| ====Slotted waveguide====
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| [[File:Radar antennas on USS Theodore Roosevelt SPS-64.jpg|right|thumb|Slotted waveguide antenna]]
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| {{Main|Slotted waveguide}}
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| Applied similarly to the parabolic reflector, the slotted waveguide is moved mechanically to scan and is particularly suitable for non-tracking surface scan systems, where the vertical pattern may remain constant. Owing to its lower cost and less wind exposure, shipboard, airport surface, and harbour surveillance radars now use this approach in preference to a parabolic antenna.
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| ====Phased array====
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| [[File:PAVE PAWS Radar Clear AFS Alaska.jpg|thumb|left|[[Phased array]]: Not all radar antennas must rotate to scan the sky.]]
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| {{Main|Phased array antenna}}
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| Another method of steering is used in a [[phased array]] radar.
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| Phase array antennas are composed of evenly spaced similar antenna elements, such as aerials or rows of slotted waveguide. Each antenna element or group of antenna elements incorporates a discreet phase shift that produces a phase gradient across the array. For example, array elements producing a 5 degree phase shift for each wavelength across the array face will produce a beam pointed 5 degree away from the centerline perpendicular to the array face. Signals traveling along that beam will be reinforced. Signals offset from that beam will be canceled. The amount of reinforcement is [[antenna gain]]. The amount of cancellation is side-lobe suppression.<ref>{{cite web|url=http://mit.edu/6.933/www/Fall2000/mode-s/sidelobe.html|title=Side-Lobe Supression|publisher=MIT}}</ref>
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| Phased array radars have been in use since the earliest years of radar in World War II, but electronic device limitations led to poor performance. Phased array radars were originally used for missile defense. They are the heart of the ship-borne [[Aegis combat system]] and the [[MIM-104 Patriot|Patriot Missile System]]. The massive redundancy associated with having a large number of array elements increases reliability at the expense of gradual performance degradation that occurs as individual phase elements fail.
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| Phased array antenna can be built to conform to specific shapes, like missiles, infantry support vehicles, ships, and aircraft.
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| As the price of electronics has fallen, phased array radars have become more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance, weather radars and similar systems.
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| Phased array radars are valued for use in aircraft since they can track multiple targets. The first aircraft to use a phased array radar was the [[B-1B Lancer]]. The first aircraft fighter to use phased array radar was the [[Mikoyan MiG-31]]. The MiG-31M's SBI-16 [[Mikoyan-Gurevich MiG-31#Electronics suite|Zaslon]] phased array radar is considered to be the world's most powerful fighter radar.<ref>[http://www.globalsecurity.org/military/world/russia/mig-31.htm MiG-31 FOXHOUND]</ref>
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| Phased-array [[interferometry]] or [[aperture synthesis]] techniques, using an array of separate dishes that are phased into a single effective aperture, are not typical for radar applications, although they are widely used in [[radio astronomy]]. Because of the [[thinned array curse]], such multiple aperture arrays, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques could increase spatial resolution, but the lower power means that this is generally not effective.
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| [[Synthetic aperture radar|Aperture synthesis]] by post-processing motion data from a single moving source, on the other hand, is widely used in space and [[airborne radar system]]s .
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| ===Frequency bands===
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| {{Main|Radio spectrum#IEEE US}}
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| The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world. They have been adopted in the United States by the [[Institute of Electrical and Electronics Engineers]] and internationally by the [[International Telecommunication Union]]. Most countries have additional regulations to control which parts of each band are available for civilian or military use.
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| Other users of the radio spectrum, such as the [[broadcasting]] and [[electronic countermeasures]] industries, have replaced the traditional military designations with their own systems.
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| {| class="wikitable"
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| |+ '''Radar frequency bands'''
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| |- style="background:#ccc;"
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| !Band name!!Frequency range!!Wavelength range!!Notes
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| |-
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| |[[High frequency|HF]]||3–30 [[Megahertz|MHz]]||10–100 [[metre|m]]||coastal radar systems, [[over-the-horizon radar]] (OTH) radars; 'high frequency'
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| |-
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| |P||< 300 MHz||1 m+||'P' for 'previous', applied retrospectively to early radar systems
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| |-
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| |[[VHF]]||30–300 MHz||1–10 m||Very long range, ground penetrating; 'very high frequency'
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| |-
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| |[[UHF]]||300–1000 MHz||0.3–1 m||Very long range (e.g. [[Ballistic Missile Early Warning System|ballistic missile early warning]]), ground penetrating, foliage penetrating; 'ultra high frequency'
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| |-
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| |[[L band|L]]||1–2 [[Gigahertz|GHz]]||15–30 [[centimetre|cm]]||Long range air traffic control and [[surveillance]]; 'L' for 'long'
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| |-
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| |[[S band|S]]||2–4 GHz||7.5–15 cm||Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
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| |-
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| |[[C band|C]]||4–8 GHz||3.75–7.5 cm||Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
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| |-
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| |[[X band|X]]||8–12 GHz||2.5–3.75 cm||[[Missile]] guidance, [[marine radar]], weather, medium-resolution mapping and ground surveillance; in the [[USA]] the narrow range 10.525 GHz ±25 MHz is used for [[airport]] radar; short range tracking. Named X band because the frequency was a secret during WW2.
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| |-
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| ||[[Ku band|K<sub>u</sub>]]||12–18 GHz||1.67–2.5 cm||high-resolution, also used for satellite transponders, frequency under K band (hence 'u')
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| |-
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| |[[K band|K]]||18–24 GHz||1.11–1.67 cm||from [[German language|German]] ''kurz'', meaning 'short'; limited use due to absorption by [[water vapor|water vapour]], so K<sub>u</sub> and K<sub>a</sub> were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
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| |-
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| |[[Ka band|K<sub>a</sub>]]||24–40 GHz||0.75–1.11 cm||mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.
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| |-
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| |mm||40–300 GHz||7.5 mm – 1 mm ||[[millimetre band]], subdivided as below. The frequency ranges depend on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
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| |-
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| |[[V band|V]]||40–75 GHz||4.0–7.5 mm || Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
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| |-
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| |[[W band|W]]||75–110 GHz||2.7–4.0 mm||used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
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| |-
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| |[[Ultra-wideband|UWB]]||1.6–10.5 GHz||18.75 cm – 2.8 cm||used for through-the-wall radar and imaging systems.
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| |}
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| ===Radar modulators===
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| [[Modulation|Modulators]] act to provide the waveform of the RF-pulse. There are two different radar modulator designs:
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| * high voltage switch for non-coherent keyed power-oscillators<ref>[http://www.radartutorial.eu//08.transmitters/tx06.en.html Radartutorial]</ref> These modulators consist of a high voltage pulse generator formed from a high voltage supply, a [[pulse forming network]], and a high voltage switch such as a [[thyratron]]. They generate short pulses of power to feed, e.g., the [[Cavity magnetron|magnetron]], a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as [[pulsed power]]. In this way, the transmitted pulse of RF radiation is kept to a defined and usually very short duration.
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| * hybrid mixers,<ref>[http://www.radartutorial.eu//08.transmitters/tx10.en.html Radartutorial]</ref> fed by a waveform generator and an exciter for a complex but [[Coherence (physics)|coherent]] waveform. This waveform can be generated by low power/low-voltage input signals. In this case the radar transmitter must be a power-amplifier, e.g., a [[klystron tube]] or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use [[pulse compression]] techniques.
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| ===Radar coolant===
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| Coolanol (silicate ester) was used in several military radars in the 1970s. However, it is [[hygroscopic]], leading to formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire.<ref>{{cite web | url = http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA250517&Location=U2&doc=GetTRDoc.pdf | author= Stropki, Michael A. | year = 1992 | title = Polyalphaolefins: A New Improved Cost Effective Aircraft Radar Coolant | publisher = Aeronautical Research Laboratory, Defense Science and Technology Organisation, Department of Defense | location = Melbourne, Australia | accessdate = 2010-03-18}}</ref> Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named [[Pollution Prevention]] (P2) to reduce or eliminate the volume and toxicity of waste, air emissions, and effluent discharges. Because of this, Coolanol is used less often today.
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| Coherent microwave amplifiers operating above 1,000 watts microwave output, like traveling wave tubes and klystrons, require liquid coolant. The electron beam must contain 5 to 10 times more power than the microwave output, which can produce enough heat to corrupt the vacuum with plasma. This flows from the collector toward the cathode. Magnetic focusing for the electron beam forces ionized gas atoms into the same location as the electron beam. Plasma ions flow in the opposite direction of the electron beam. This introduces FM modulation, and that degrades Doppler performance. Liquid coolant with minimum pressure and flow rate is required to control collector gassing, and deionized water is normally used with most high power surface radar systems that utilize Doppler processing.<ref>{{cite web | url = http://www.cientificosaficionados.com/libros/CERN/vacio9-CERN.pdf | author = J.L. de Segovia | title = Physics of Outgassing | publisher = Instituto de Física Aplicada, CETEF “L. Torres Quevedo”, CSIC | location = Madrid, Spain | accessdate = 2012-08-12
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| }}</ref>
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| ==See also==
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| {{Portal|Electronics|Nautical}}
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| {{Main|Radar configurations and types}}
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| * [[Acronyms and abbreviations in avionics]]
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| ;Definitions
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| * [[Amplitude-Comparison Monopulse|Amplitude-comparison monopulse]]
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| * [[Constant false alarm rate]]
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| * [[Sensitivity time control]]
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| ;Hardware
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| * [[Radar engineering details]]
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| * [[Klystron]]
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| * [[Cavity magnetron]]
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| * [[Radio]]
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| * [[Traveling-wave tube]]
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| * [[Crossed-field amplifier]]
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| * [[Gallium(III) arsenide|Gallium arsenide]]
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| ;Similar detection and ranging methods
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| * [[LIDAR]]
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| * [[LORAN]]
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| * [[Sonar]]
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| ;Historical radars
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| * [[List of radars]]
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| * [[Chain Home]] and [[Chain Home Low]]
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| * [[Hohentwiel (Radar)|Hohentwiel radar]]
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| * [[H2S radar]]
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| * [[SCR-270 radar]]
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| ==Notes==
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| {{reflist|colwidth=30em}}
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| ==References==
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| * Barrett, Dick, "''[http://www.radarpages.co.uk/index.htm All you ever wanted to know about British air defence radar]''". The Radar Pages. (History and details of various British radar systems)
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| * Buderi, "''[http://www.privateline.com/TelephoneHistory3/radarhistorybuderi.html Telephone History: Radar History]''". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.)
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| * Ekco Radar [http://www.ekco-radar.co.uk/ WW2 Shadow Factory] The secret development of British radar.
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| * ES310 "''[http://www.fas.org/man/dod-101/navy/docs/es310/syllabus.htm Introduction to Naval Weapons Engineering.''". (Radar fundamentals section)]
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| * Hollmann, Martin, "''[http://www.radarworld.org/index.html Radar Family Tree]''". [http://www.radarworld.org/ Radar World].
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| * Penley, Bill, and Jonathan Penley, "''[http://www.penleyradararchives.org.uk/history/introduction.htm Early Radar History]—an Introduction''". 2002.
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| * Pub 1310 ''Radar Navigation and Maneuvering Board Manual'', National Imagery and Mapping Agency, Bethesda, MD 2001 (US govt publication '...intended to be used primarily as a manual of instruction in navigation schools and by naval and merchant marine personnel.')
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| * Swords, Seán S., "Technical History of the Beginnings of Radar", ''[[IEE]] History of Technology Series'', Vol. 6, London: Peter Peregrinus, 1986''
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| ==Further reading==
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| * {{cite book| author = Reg Batt| title = The radar army: winning the war of the airwaves| year = 1991| isbn = 978-0-7090-4508-3 }}
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| * {{cite book| author = E. G. Bowen| title = Radar Days| date = 1998-01-01| publisher = Taylor & Francis| isbn = 978-0-7503-0586-0 }}
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| * {{cite book| author = Michael Bragg| title = RDF1: The Location of Aircraft by Radio Methods 1935-1945| date = 2002-05-01| publisher = Twayne Publishers| isbn = 978-0-9531544-0-1 }}
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| * {{cite book| author = Louis Brown| title = A radar history of World War II: technical and military imperatives| year = 1999| publisher = Taylor & Francis| isbn = 978-0-7503-0659-1 }}
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| * {{cite book| author = Robert Buderi| title = The invention that changed the world: how a small group of radar pioneers won the Second World War and launched a technological revolution| year = 1996| isbn = 978-0-684-81021-8 }}
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| * Burch, David F., ''Radar For Mariners'', McGraw Hill, 2005, ISBN 978-0-07-139867-1.
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| * {{cite book| author = Ian Goult| title = Secret Location: A witness to the Birth of Radar and its Postwar Influence| year = 2011| publisher = History Press | isbn = 978-0-7524-5776-5 }}
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| * {{cite book| author = Peter S. Hall| title = Radar| date = March 1991| publisher = Potomac Books Inc| isbn = 978-0-08-037711-7 }}
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| * {{cite book| author = Derek Howse| coauthors = Naval Radar Trust| title = Radar at sea: the royal Navy in World War 2| date = February 1993| publisher = Naval Institute Press| isbn = 978-1-55750-704-4 }}
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| * {{cite book| author = R. V. Jones| title = Most Secret War| date = August 1998| publisher = Wordsworth Editions Ltd| isbn = 978-1-85326-699-7 }}
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| * Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994.
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| * Kouemou, Guy (Ed.): ''Radar Technology.'' InTech, 2010, ISBN 978-953-307-029-2, ([http://www.intechopen.com/books/show/title/radar-technology Radar Technology - Free Open Access Book | InTechOpen]).
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| * {{cite book| author = Colin Latham| coauthors = Anne Stobbs| title = Radar: A Wartime Miracle| date = January 1997| publisher = Sutton Pub Ltd| isbn = 978-0-7509-1643-1 }}
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| * {{cite book| author = François Le Chevalier| title = Principles of radar and sonar signal processing| year = 2002| publisher = Artech House Publishers| isbn = 978-1-58053-338-6 }}
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| * {{cite book| author = David Pritchard| title = The radar war: Germany's pioneering achievement 1904-45| date = August 1989| publisher = Harpercollins| isbn = 978-1-85260-246-8 }}
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| * {{cite book| author = Merrill Ivan Skolnik| title = Introduction to radar systems| date = 1980-12-01| isbn = 978-0-07-066572-9 }}
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| * {{cite book| author = Merrill Ivan Skolnik| title = Radar handbook| year = 1990| publisher = McGraw-Hill Professional| isbn = 978-0-07-057913-2 }}
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| * {{cite book| author = George W. Stimson| title = Introduction to airborne radar| year = 1998| publisher = SciTech Publishing| isbn = 978-1-891121-01-2 }}
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| * Younghusband, Eileen., ''Not an Ordinary Life. How Changing Times Brought Historical Events into my Life'', Cardiff Centre for Lifelong Learning, Cardiff, 2009., ISBN 9780956115690 (Pages 36–67 contain the experiences of a WAAF radar plotter in WWII.)
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| * Younghusband, Eileen., ''One Woman's War''. Cardiff. Candy Jar Books. 2011. ISBN 978-0-9566826-2-8
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| * {{cite book| author = David Zimmerman| title = Britain's shield: radar and the defeat of the Luftwaffe| date = February 2001| publisher = Sutton Pub Ltd| isbn = 978-0-7509-1799-5 }}
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| ==External links==
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| {{Wiktionary}}
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| {{Commons}}
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| *[http://ocw.mit.edu/resources/res-ll-001-introduction-to-radar-systems-spring-2007/ MIT Video Course: Introduction to Radar Systems] A set of 10 video lectures developed at Lincoln Laboratory to develop an understanding of radar systems and technologies.
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| *[http://books.google.com/books?id=_yYDAAAAMBAJ&pg=PA66&dq=popular+science+june+1941&hl=en&ei=cT2TTNqUB9Ofnwfn49ywCA&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=true ''Popular Science'', August 1943, ''What Are the Facts About RADAR''] one of the first detailed factual articles on radar history, principles and operation published in the US
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| *[http://imperialclub.com/Yr/1945/46Radar/Cover.htm "The Great Detective", 1946. Story of the development of radar by the Chrysler Corporation]
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| *[http://www.xs4all.nl/~aobauer/Huelspart1def.pdf Christian Hülsmeyer and the early days of radar]
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| *[http://www.warmuseum.ca/cwm/exhibitions/radar/index_e.shtml Radar: The Canadian History of Radar - Canadian War Museum]
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| *[http://www.radartutorial.eu/index.en.html Radar technology principles]
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| *[http://math.la.asu.edu/~kuang/LM/030902-Radar_History10.pdf History of radar]
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| *[http://www.metamaterials.net Radar invisibility with metamaterials]
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| *[http://crr.sesm.it Radar Research Center-Italy]
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| *[http://www.purbeckradar.org.uk/ Early radar development in the UK]
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| *[http://ourworld.compuserve.com/homepages/edperry/ewtutor1.htm Principles of radar target acquisition and weapon guidance systems]
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| *[http://www.radartechnology.eu/ Cloaking and radar invisibility]
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| *[http://www.radarmuseum.co.uk/ RAF Air Defence Radar Museum]
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| *[http://www.iop.org/publications/iop/2011/page_47522.html Radar - A case study highlighting the vital contribution physics research has made to major technological development]
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| *{{Internet Archive short film|id=gov.archives.arc.892095|name=Radar and Its Applications (1962)}}
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| *[http://www.fhr.fraunhofer.de/en.html/ Fraunhofer Institute for High Frequency Physics and Radar Techniques FHR]
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| *[http://www.fhr.fraunhofer.de/en/the_institute/technical-equipment/Space-observation-radar-TIRA.html Space Observation Radar TIRA (Tracking and Imaging Radar) of Fraunhofer FHR]
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| [[Category:Avionics]]
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| [[Category:Aircraft instruments]]
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| [[Category:Radar]]
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| [[Category:Microwave technology]]
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| [[Category:Measuring instruments]]
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| [[Category:Navigational equipment]]
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| [[Category:Air traffic control]]
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| [[Category:Science and technology during World War II]]
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| [[Category:Targeting (warfare)]]
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| {{Link FA|sr}}
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