TheInfoList

Radar (radio detection and ranging) is a detection system that uses
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
s to determine the distance (''
ranging Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which length Length is a measure of distance. In the International System of Quantities The International System of Quantities (ISQ) is a ...
''), angle, or velocity of objects. It can be used to detect
aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force), dynamic lift of an airfoil, or in ...

,
ships A ship is a large watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered structural and functional propertie ...
,
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite alt=, A full-size model of the Earth observation satellite ERS 2 ">ERS_2.html" ;"title="Earth observation satellite ERS 2">Earth obse ...

,
guided missiles In military terminology, a missile, also known as a guided missile or guided rocket, is a guided airborne ranged weapon capable of self-propelled flight usually by a jet engine or rocket motor. Missiles have four system components: targeti ...
,
motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles ...
s, , and
terrain Relief map of Sierra Nevada, Spain Terrain or relief (also topographical Topography is the study of the forms and features of land surfaces. The topography of an area could refer to the surface forms and features themselves, or a desc ...
. A radar system consists of a
transmitter In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particle ...
producing
electromagnetic wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

s in the
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...
or
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

s domain, a transmitting
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two col ...
, a receiving antenna (often the same antenna is used for transmitting and receiving) and a and processor to determine properties of the object(s). Radio waves (pulsed or continuous) from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed. Radar was developed secretly for
military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare War is an intense armed conflict between State (polity), states, governments, Society, societies, or pa ...

use by several countries in the period before and during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
. A key development was the
cavity magnetron The cavity magnetron is a high-power vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric volt ...
in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shorth ...

, which allowed the creation of relatively small systems with sub-meter resolution. The term ''RADAR'' was coined in 1940 by the
United States Navy ), (unofficial)."''Non sibi sed patriae''" ( en, "Not for self but for country") (unofficial). , colors = Blue and gold  , colors_label = Colors , march = "Anchors Aweigh" , mascot = , equipment = List of equipment of the United St ...
as an
acronym An acronym is a word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign langu ...
for "radio detection and ranging". The term ''radar'' has since entered
English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventually become the World language, leading lan ...

and other languages as a common noun, losing all capitalization. During
RAF "Through Adversity to the Stars" , colours = , colours_label = , march = Royal Air Force March Past , mascot = , anniversaries = , e ...
RADAR courses in 1954/5 at Yatesbury Training Camp "radio azimuth direction and
ranging Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which length Length is a measure of distance. In the International System of Quantities The International System of Quantities (ISQ) is a ...
" was suggested. The modern uses of radar are highly diverse, including air and terrestrial traffic control,
radar astronomy Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗm ...
, air-defense systems, antimissile systems,
marine radar Marine radars are X band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is r ...
s to locate landmarks and other ships, aircraft anti-collision systems,
ocean surveillance The ocean (also the sea or the world ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
systems, outer
space surveillance Space domain awareness is the study and monitoring of satellites orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a ...
and
rendezvous Rendezvous or rendez-vous () refers to a planned meeting between two or more parties at a specific time and place. The term may also refer to: In arts and entertainment In film and television * The Rendezvous (1923 film), ''The Rendezvous'' (1923 ...
systems,
meteorological Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting Weather forecasting is the application of science and technology to predict the ...
precipitation monitoring, altimetry and
flight control system A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft ...
s,
guided missile In military terminology, a missile is a guided airborne ranged weapon . English longbowmen figure prominently in the foreground at right where they drive away the French crossbow A crossbow is a ranged weapon using an elastic launching ...
target locating systems,
self-driving car A self-driving car, also known as an autonomous vehicle (AV), driverless car, or robotic car (robo-car), is a car A car (or automobile) is a wheeled motor vehicle Electric bicycles parked in Yangzhou's main street, Wenchang Lu. They a ...
s, and
ground-penetrating radar Ground-penetrating radar (GPR) is a geophysical Geophysics () is a subject of natural science Natural science is a Branches of science, branch of science concerned with the description, understanding and prediction of Phenomenon, natural ph ...
for geological observations. High tech radar systems are associated with
digital signal processing Digital signal processing (DSP) is the use of digital processing Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of ...
,
machine learning Machine learning (ML) is the study of computer algorithms that can improve automatically through experience and by the use of data. It is seen as a part of artificial intelligence. Machine learning algorithms build a model based on sample data ...

and are capable of extracting useful information from very high
noise Noise is unwanted sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and t ...
levels. Other systems similar to radar make use of other parts of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequency, frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energy, photon energies. The electromagnetic spectrum covers electromagnetic waves with f ...

. One example is
LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object with a laser A laser is a device that emits light Light or visible light is electromagnetic radiati ...
, which uses predominantly
infrared light Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...
from
laser A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as h ...

s 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.

# History

## First experiments

As early as 1886, German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empi ...

showed that radio waves could be reflected from solid objects. In 1895, , a physics instructor at the
Imperial Russian Navy The Imperial Russian Navy () operated as the navy A navy, naval force, or maritime force is the branch of a Nation's armed forces principally designated for naval warfare, naval and amphibious warfare; namely, lake-borne, riverine, littoral ...
school in
Kronstadt Kronstadt (russian: Кроншта́дт, Kronštádt ), also spelled Kronshtadt, Cronstadt or Kronštádt (from german: link=no, Krone for " crown" and ''Stadt'' for "city"; et, Kroonlinn) is a Russian port city in Kronshtadtsky District of t ...

, developed an apparatus using a
coherer. The coherer was a primitive form of radio signal Detector (radio), detector used in the first radio receivers during the wireless telegraphy era at the beginning of the 20th century. Its use in radio was based on the 1890 findings of French physi ...

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 The Baltic Sea is an arm of the Atlantic Ocean, enclosed by Denmark Denmark ( da, Danmark, ) is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that a ...

, 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 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 September 23, 1904 for 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 Cologne ( ; german: Köln ; ksh, Kölle ) is the largest city of Germany, Germany's most populous States of Germany, state of North Rhine-Westphalia (NRW) and the List of cities in Germany by population, fourth-most populous city and one of t ...

and
Rotterdam Rotterdam ( , , ) is the second largest city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2n ...

harbour but was rejected. In 1915,
Robert Watson-Watt Sir Robert Alexander Watson Watt, KCB, FRS, FRAeS (13 April 1892 – 5 December 1973) was a British pioneer of radio direction finding and radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity ...

used radio technology to provide advance warning to airmen and during the 1920s went on to lead the U.K. research establishment to make many advances using radio techniques, including the probing of the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
and the detection of
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of
radio direction finding Direction finding (DF), or radio direction finding (RDF), is the measurement of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication, including radar signals detection and mon ...
before turning his inquiry to
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the High frequency, high frequency band (HF), which extends from 3 to 30 MHz (100 ...

transmission. Requiring a suitable receiver for such studies, he told the "new boy" to conduct an extensive review of available shortwave units. Wilkins would select a
General Post Office The General Post Office (GPO) was the state postal system and telecommunications Telecommunication is the transmission of information by various types of technologies over , radio, , or other systems. It has its origin in the desire of ...

model after noting its manual's description of a "fading" effect (the common term for interference at the time) when aircraft flew overhead. Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the
Potomac River The Potomac River () is found within the Mid-AtlanticMid-Atlantic or Mid Atlantic can refer to: *The middle of the Atlantic Ocean *Mid-Atlantic English, a mix between British English and American English *Mid-Atlantic Region (Little League World ...

, U.S. Navy researchers A. Hoyt Taylor and discovered that ships passing through 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 The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy ), (unofficial)."''Non sibi sed patriae''" ( en, "Not for self but for country") (unofficial). , colors ...

(NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application as well as 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, 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 Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An ...

as Chain Home (low).

## Just before World War II

Before the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
, researchers in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shorth ...

,
France France (), officially the French Republic (french: link=no, République française), is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses ...
,
Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin Berlin (; ) is the Capital city, capital and List of cities in Germany by population, largest city of Germany by both area and population. Its 3,769,495 inh ...

,
Italy Italy ( it, Italia ), officially the Italian Republic ( it, Repubblica Italiana, links=no ), is a country consisting of a peninsula delimited by the Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest ...
,
Japan Japan ( ja, 日本, or , and formally ) is an island country An island country or an island nation is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the land of an in ...

, the
Netherlands ) , national_anthem = ( en, "William of Nassau") , image_map = EU-Netherlands.svg , map_caption = , image_map2 = BES islands location map.svg , map_caption2 = , image_map3 ...

, the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
, and the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

, independently and in great secrecy, developed technologies that led to the modern version of radar.
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

,
Canada Canada is a country in the northern part of North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, ...

,
New Zealand New Zealand ( mi, Aotearoa ''Aotearoa'' (; commonly pronounced by English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon Engl ...

, and
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is the world's 23rd-most populous nation and covers an area of . South Africa has three capital citie ...

followed prewar Great Britain's radar development, and
Hungary Hungary ( hu, Magyarország ) is a in . Spanning of the , it is bordered by to the north, to the northeast, to the east and southeast, to the south, and to the southwest and to the west. Hungary has a population of 10 million, mostl ...
generated its radar technology during the war. In France in 1934, following systematic studies on the , the research branch of the 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, aspects of which were installed on the ocean liner in 1935. During the same period, Soviet military engineer P.K. Oshchepkov, in collaboration with the
Leningrad Electrotechnical Institute Saint Petersburg Electrotechnical University "LETI" (ETU, ETU "LETI", russian: Санкт-Петербургский государственный электротехнический университет «ЛЭТИ» им. В.И. Ульяно ...
, 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 the arrest of Oshchepkov and his subsequent
gulag The Gulag, GULAG, or GULag (russian: ГУЛАГ, ГУЛаг, an acronym An acronym is a word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by h ...

sentence. In total, only 607 Redut stations were produced during the war. The first Russian airborne radar,
Gneiss-2Gneiss-2 was the first Soviet Union, Soviet produced airborne radar system. It was introduced in 1942, and was installed on Petlyakov Pe-2, Petlyakov Pe-3 and Douglas A-20 Havoc aircraft . Development During the Soviet-Finnish War, the chief of the ...
, entered into service in June 1943 on
Pe-2 The Petlyakov Pe-2 (russian: Петляков Пе-2) was a Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a federal socialist state in Northern Eurasia Eurasia () is the largest continenta ...
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 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 United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy ), (unofficial)."''Non sibi sed patriae''" ( en, "Not for self but for country") (unofficial). , colors ...

. The following year, the
United States Army The United States Army (USA) is the land Land is the solid surface of Earth that is not permanently submerged in water. Most but not all land is situated at elevations above sea level (variable over geologic time frames) and consists ma ...
successfully tested a primitive surface-to-surface radar to aim
coastal battery File:Redoubt of the Dardanelles Fortified Area.jpg, 250px, An Ottoman Empire, Ottoman redoubt of the Dardanelles Fortified Area. The weapon is possibly a German-made 28 cm SK L/40 gun on a coast defense mount. Coastal artillery is the branch of th ...
searchlight A searchlight (or spotlight) is an apparatus that combines an extremely bright source (traditionally a carbon arc lamp) with a mirrored parabolic reflector to project a powerful beam of light Light or visible light is electromagnetic radiat ...

s at night. This design was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and the firm in Germany and then another in June 1935 by an
Air Ministry The Air Ministry was a department of the Government of the United Kingdom ga, Rialtas na Ríochta Aontaithe sco, Govrenment o the Unitit Kinrick , image = HM Government logo.svg , image_size=220px, date_established = , state = United ...
team led by
Robert Watson-Watt Sir Robert Alexander Watson Watt, KCB, FRS, FRAeS (13 April 1892 – 5 December 1973) was a British pioneer of radio direction finding and radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity ...

in Great Britain. In 1935, Watson-Watt was asked to judge recent reports of a German radio-based
death ray The death ray or death beam was a theoretical particle beam or electromagnetic weapon first theorized around the 1920s and 1930s. Around that time, notable inventors such as Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of ...
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 a system might do, Wilkins recalled the earlier report about aircraft causing radio interference. This revelation led to the
Daventry Experiment Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An ...
of 26 February 1935, using a powerful
BBC The British Broadcasting Corporation (BBC) is a public service broadcaster, headquartered at Broadcasting House in Westminster, London. It is the world's oldest national broadcaster, and the largest broadcasting, broadcaster in the world by ...

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 Air Chief Marshal Hugh Caswall Tremenheere Dowding, 1st Baron Dowding, (24 April 1882 – 15 February 1970) was an officer in the Royal Air Force. He was Air Officer Commanding RAF Fighter Command during the Battle of Britain and is generally c ...

, the
Air Member for Supply and Research The Air Member for Materiel is the senior Royal Air Force officer responsible for procurement matters. The post-holder is a member of the Air Force Board and is in charge of all aspects of procurement and organisation for RAF regular, reserve and c ...
was very impressed with their system's potential and funds were immediately provided for further operational development. Watson-Watt's team patented the device in GB593017. Development of radar greatly expanded on 1 September 1936 when Watson-Watt became Superintendent of a new establishment under the British
Air Ministry The Air Ministry was a department of the Government of the United Kingdom ga, Rialtas na Ríochta Aontaithe sco, Govrenment o the Unitit Kinrick , image = HM Government logo.svg , image_size=220px, date_established = , state = United ...
, Bawdsey Research Station located in
Bawdsey Manor Bawdsey Manor stands at a prominent position at the mouth of the River Deben close to the village of Bawdsey in Suffolk Suffolk () is an East Anglia East Anglia is a geographical area in the East of England. The area included has v ...
, near Felixstowe, Suffolk. Work there resulted in the design and installation of aircraft detection and tracking stations called "
Chain Home Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An ...

" 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 The Battle of Britain (german: die Luftschlacht um England, "the Air Battle for England") was a military campaign A military campaign is large-scale long-duration significant military strategy Military strategy is a set of ideas implemented ...

; without it, significant numbers of fighter aircraft, which Great Britain did not have available, would always need to be in the air to respond quickly. If enemy aircraft detection had relied solely on the observations of ground-based individuals, Great Britain might have lost the Battle of Britain. Also vital was the "
Dowding system The Dowding system was the world's first wide-area ground-controlled interception network, controlling the airspace across the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom ( ...
" of reporting and coordination to provide the best use of radar information during the tests of early radar during 1936 and 1937. 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 Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An ...

(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 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.

## During World War II

A key development was the
cavity magnetron The cavity magnetron is a high-power vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric volt ...
in the UK, which allowed the creation of relatively small systems with sub-meter resolution. Britain shared the technology with the U.S. during the 1940
Tizard Mission The Tizard Mission, officially the British Technical and Scientific Mission, was a British delegation that visited the United States during the Second World War in order to obtain the industrial resources to exploit the military potential of the re ...
. In April 1940, ''
Popular Science Popular science (also called pop-science or popsci) is an interpretation of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes ...

'' 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 was sent to the U.S. in 1941 to advise on air defense after Japan's
attack on Pearl Harbor The Attack on Pearl HarborAlso known as the Battle of Pearl Harbor was a surprise military strike In the United States Armed Forces, military of the United States, strikes and raids are a group of military operations that, alongside quite ...

.
Alfred Lee Loomis Alfred Lee Loomis (November 4, 1887 – August 11, 1975) was an American attorney, investment banker An investment bank is a financial services Financial services are the Service (economics), economic services provided by the finance industry, ...
organized the secret
MIT Radiation Laboratory The Radiation Laboratory, commonly called the Rad Lab, was a microwave and radar research laboratory located at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts (US). It was first created in October 1940 and operated unti ...
at
Massachusetts Institute of Technology Massachusetts Institute of Technology (MIT) is a private Private or privates may refer to: Music * "In Private "In Private" was the third single in a row to be a charting success for United Kingdom, British singer Dusty Springfield, aft ...
, 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 complementary navigation systems like
Oboe The oboe ( ) is a type of double reed A double reed is a type of reed Reed or Reeds may refer to: Science, technology, biology, and medicine * Reed bird (disambiguation) * Reed pen, writing implement in use since ancient times * Reed ( ...
used by the RAF's Pathfinder.

# Applications

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 Aviation is the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes airplane, fixed-wing and helicopter, rotary-wing types, morphable wings, wing-less lifting bodies, as well as aerostat, lighter-than-air ...
, 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 was a 1938 Bell Lab unit on some
United Air Lines United Airlines, Inc. (commonly referred to as United) is a major American airline headquartered at Willis Tower The Willis Tower (formerly and informally: Sears Tower, its name until 2009) is a 108- story, skyscraper in Chicago ...
aircraft. Aircraft can land in fog at airports equipped with radar-assisted
ground-controlled approachIn aviation Aviation is the activities surrounding mechanical flight Flight or flying is the process by which an object (physics), object motion (physics), moves through a space without contacting any planetary surface, either within an atmos ...
systems in which the plane's position is observed on
precision approach radar Precision approach radar (PAR) is a type of radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight ...
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 a wide region and direct fighter aircraft towards targets.
Marine radar Marine radars are X band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is r ...
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 A vessel traffic service is a marine traffic monitoring system established by harbour or port authorities, similar to air traffic control for aircraft. The International Maritime Organization defines vessel traffic service as "a service implemented ...
radar systems are used to monitor and regulate ship movements in busy waters. Meteorologists use radar to monitor
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
and wind. It has become the primary tool for short-term
weather forecast ''Weather and Forecasting'' is a scientific journal In academic publishing Academic publishing is the subfield of publishing Publishing is the activity of making information, literature, music, software and other content available to the ...

ing and watching for
severe weather Severe weather is any dangerous meteorological Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting Weather forecasting is the ap ...
such as
thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden fl ...

s,
tornado A tornado is a violently rotating column of air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmos ...

es,
winter storm A winter storm is an event in which wind coincides with varieties of precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere g ...
s, precipitation types, etc.
Geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, although backgrounds in physics, chem ...

s use specialized
ground-penetrating radar Ground-penetrating radar (GPR) is a geophysical Geophysics () is a subject of natural science Natural science is a Branches of science, branch of science concerned with the description, understanding and prediction of Phenomenon, natural ph ...
s to map the composition of
Earth's crust 350px, Plates in the crust of Earth Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere, a division of Earth's layers that includes the Crust (geology), ...
. Police forces use
radar gun A radar speed gun (also radar gun and speed gun) is a device used to measure the speed of moving objects. It is used in law-enforcement to measure the speed of moving vehicle A vehicle (from la, vehiculum) is a machine that transports peopl ...

s to monitor vehicle speeds on the roads. Smaller radar systems are used to . Examples are breathing pattern detection for sleep monitoring and hand and finger for computer interaction. Automatic door opening, light activation and intruder sensing are also common. Radar technology has recently been used for vital sign monitoring and human activity monitoring. The heartbeat and respiration rate are estimated by measuring the human body movements caused by ejection of blood into the great vessels and inhalation and exhalation of air into and out of the lungs using radar. Human activities are detected by classifying the radar return patterns using machine learning algorithms.

# Principles

A radar system has a
transmitter In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particle ...
that emits
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
s known as ''radar signals'' in predetermined directions. When these signals contact an object they are usually reflected or 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 Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current An electric current is a stream of charged particle In p ...
—such as most metals,
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

, and wet ground. This makes the use of
radar altimeter A radar altimeter (RA), radio altimeter (RALT), electronic altimeter, or reflection altimeter measures altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between ...
s 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 Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

of the radio waves due to the
Doppler effect The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the ...

. Radar receivers are usually, but not always, in the same location as the transmitter. The reflected radar signals captured by the receiving antenna are usually very weak. They can be strengthened by
electronic amplifiers Electronic may refer to: *Electronics, the science of how to control electric energy in semiconductor *Electronics (magazine), ''Electronics'' (magazine), a defunct American trade journal *Electronic storage, the storage of data using an electronic ...

. More sophisticated methods of
signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetis ...

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 Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
,
infrared light Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...
, and
ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural s ...
, 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 is intended.

## Illumination

Radar relies on its own transmissions rather than light from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

or the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

, or from
electromagnetic wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

s 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.

## Reflection

If
electromagnetic waves In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through ...

travelling through one material meet another material, having a different
dielectric constant The dielectric constant (or relative permittivity) is the permittivity, electric permeability of a material expressed as a ratio with the vacuum permittivity, electric permeability of a vacuum. A dielectric is an insulating material, and the diele ...
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 The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (ph ...

or in a
vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Gr ...

, 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 Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resists electric current An electric current is a stream of charged particles, ...
materials such as metal and carbon fibre, making radar well-suited to the detection of aircraft and ships. Radar absorbing material, containing
resistive In electronics and electromagnetism, the electrical resistance of an object is a measure of its opposition to the flow of electric current. The Multiplicative inverse, reciprocal quantity is , and is the ease with which an electric current passe ...
and sometimes
magnetic Magnetism is a class of physical attributes that are mediated by s. s and the s of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Magnetism is one aspect of the combined phenomenon of . The ...

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 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 A mirror is an object that reflects an image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment Environment most often refers t ...

. 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 Rayleigh scattering ( ), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength o ...

, an effect that creates Earth's blue sky and red
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

s. When the two length scales are comparable, there may be
resonance Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelen ...

s. 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 A centimetre (international spelling) or centimeter (American spelling) (SI symbol cm) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit ...
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 the reflective surfaces. A
corner reflector A corner reflector is a retroreflector consisting of three mutually perpendicular, intersection (Euclidean geometry), intersecting flat surfaces, which reflects waves directly towards the source, but Translation (geometry), translated. The three ...

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 Stealth aircraft are designed to avoid detection using a variety of technologies that reduce reflection/emission of radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging' ...
. These precautions do not completely eliminate reflection because of
diffraction Diffraction refers to various phenomena that occur when a wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which r ...

, especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as
chaff Chaff (; ) is the dry, scaly protective of s or similar fine, dry, scaly plant material (such as scaly parts of s or finely chopped ). Chaff is indigestible by humans, but can eat it. In it is used as livestock , or is a waste material ed into ...
, 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 ) Radar cross-section (RCS) is a measure of how detectable an object is by radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a ...
.

## Radar range equation

The power ''Pr'' returning to the receiving antenna is given by the equation: :$P_=\frac$ where * ''P''t = transmitter power * ''G''t = Antenna gain, gain of the transmitting antenna * ''A''r = effective aperture (area) of the receiving antenna; this can also be expressed as $\over$, where :* $\lambda$ = transmitted wavelength :* ''G''r = gain of receiving antenna * ''σ'' = radar cross section, or scattering coefficient, of the target * ''F'' = pattern propagation factor * ''R''t = distance from the transmitter to the target * ''R''r = distance from the target to the receiver. In the common case where the transmitter and the receiver are at the same location, ''R''t = ''R''r and the term ''R''t² ''R''r² can be replaced by ''R''4, where ''R'' is the range. This yields: :$P_r = .$ 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, pulse-Doppler radar performance, which can be used to increase detection range and reduce transmit power. The equation above with ''F'' = 1 is a simplification for transmission in a
vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Gr ...

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.

## Doppler effect

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 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 Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗm ...
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. 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 is as follows, where $F_D$ is Doppler frequency, $F_T$ is transmit frequency, $V_R$ is radial velocity, and $C$ is the speed of light: :$F_D = 2 \times F_T \times \left \left(\frac \right\right)$. Passive radar is applicable to electronic countermeasures and radio astronomy as follows: :$F_D = F_T \times \left \left(\frac \right\right)$. Only 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. Vehicles and weather moving parallel to the radar beam produce the maximum Doppler frequency shift. When the transmit frequency ($F_T$) is pulsed, using a pulse repeat frequency of $F_R$, the resulting frequency spectrum will contain harmonic frequencies above and below $F_T$ with a distance of $F_R$. As a result, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of $F_R$, called the Nyquist frequency, since the returned frequency otherwise cannot be distinguished from shifting of a harmonic frequency above or below, thus requiring: :$, F_D, < \frac$ Or when substituting with $F_D$: :$, V_R, < \frac$ 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 , thus cannot reliably determine radial velocity of aircraft moving .

## Polarization

In all electromagnetic radiation, the electric field is perpendicular to the direction of propagation, and the electric field direction is the Polarization (waves), 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 a fractal surface, such as rocks or soil, and are used by navigation radars.

## Limiting factors

### Beam path and range

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 Figure of the Earth, curvature of the Earth sinks below the horizon. Furthermore, the signal is attenuated by the medium the beam crosses, and the beam disperses. The maximum range of conventional radar can be limited by a number of factors: * Line of sight, which depends on the height above the ground. Without a direct line of sight, the path of the beam is blocked. * The maximum non-ambiguous range, which is determined by the pulse repetition frequency. The maximum non-ambiguous range is the distance the pulse can travel to and return from before the next pulse is emitted. * Radar sensitivity and the power of the return signal as computed in the radar equation. This component includes factors such as the environmental conditions and the size (or radar cross section) of the target.

### Noise

Signal noise is an internal source of random variations in the signal, which is generated by all electronic components. 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, measures of performance that affect 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. 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. 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. 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 ''kB T B'', where ''T'' is temperature, ''B'' is bandwidth (post matched filter) and ''kB'' 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 (Black body, Planck spectrum) this noise source cannot be further eroded. We see then that radar, like all macro-scale entities, is profoundly impacted by quantum theory. 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, moving target indication can improve noise up to $\sqrt$ 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.

### Interference

Radar systems must overcome unwanted signals in order to focus on the 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 the 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 at discriminating actual targets from noise signals.

### Clutter

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, and when not being tasked for meteorological purposes,
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
(such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
reflections, meteor trails, and Hail spike. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as
chaff Chaff (; ) is the dry, scaly protective of s or similar fine, dry, scaly plant material (such as scaly parts of s or finely chopped ). Chaff is indigestible by humans, but can eat it. In it is used as livestock , or is a waste material ed into ...

# Radar signal processing

## Distance measurement

### Frequency modulation

Another form of distance measuring radar is based on frequency modulation. In these systems, the frequency of the transmitted signal is changed over time. Since the signal takes a finite time to travel to and from the target, the received signal is a different frequency than what the transmitter is broadcasting at the time the reflected signal arrives back at the radar. By comparing the frequency of the two signals the difference can be easily measured. This is easily accomplished with very high accuracy even in 1940s electronics. 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. This technique can be used in continuous wave radar and is often found in aircraft
radar altimeter A radar altimeter (RA), radio altimeter (RALT), electronic altimeter, or reflection altimeter measures altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between ...
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. 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 frequency shift becomes greater with greater time delay. The frequency shift is directly proportional to the distance travelled. That distance can be displayed on an instrument, and it may also be available via the Transponder (aeronautics), transponder. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are AZUSA, MISTRAM, and UDOP. 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.

### Pulse compression

The two techniques outlined above both have their disadvantages. The pulse timing technique has an inherent tradeoff in that the accuracy of the distance measurement is inversely related to the length of the pulse, while the energy, and thus direction range, is directly related. Increasing power for longer range while maintaining accuracy demands extremely high peak power, with 1960s early warning radars often operating in the tens of megawatts. The continuous wave methods spread this energy out in time and thus require much lower peak power compared to pulse techniques, but requires some method of allowing the sent and received signals to operate at the same time, often demanding two separate antennas. The introduction of new electronics in the 1960s allowed the two techniques to be combined. It starts with a longer pulse that is also frequency modulated. Spreading the broadcast energy out in time means lower peak energies can be used, with modern examples typically on the order of tens of kilowatts. On reception, the signal is sent into a system that delays different frequencies by different times. The resulting output is a much shorter pulse that is suitable for accurate distance measurement, while also compressing the received energy into a much higher energy peak and thus reducing the signal to noise ratio. The technique is largely universal on modern large radars.

## Speed measurement

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. 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 The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the ...

## Reduction of interference effects

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.

## Plot and track extraction

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. 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 by radar sensors over the span of a few minutes. There are four common track algorithms. * Nearest neighbour algorithm * Probabilistic data association filter, Probabilistic Data Association * Multiple Hypothesis Tracking * Interactive Multiple Model (IMM) 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. 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.

# Engineering

A radar's components are: * A
transmitter In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particle ...

## Antenna design

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. 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 angles 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. 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.

### Parabolic reflector

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''. Parabolic reflectors can be either symmetric parabolas or spoiled parabolas: 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.

### Types of scan

* Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan, etc. * 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. * 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. * Conical scanning: The radar beam is rotated in a small circle around the "boresight" axis, which is pointed at the target.

### Slotted waveguide

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.

## Modulators

Modulation, Modulators act to provide the waveform of the RF-pulse. There are two different radar modulator designs: * High voltage switch for non-coherent keyed power-oscillators 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. * Hybrid mixers, 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 or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use pulse compression techniques.

## Coolant

Coherent microwave amplifiers operating above 1,000 watts microwave output, like traveling wave tube, travelling 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 generate plasma. This plasma flows from the collector toward the cathode. The same magnetic focusing that guides the electron beam forces the plasma into the path of the electron beam but flowing in the opposite direction. This introduces FM modulation which degrades Doppler performance. To prevent this, liquid coolant with minimum pressure and flow rate is required, and deionized water is normally used in most high power surface radar systems that utilize Doppler processing. Coolanol (silicate ester) was used in several military radars in the 1970s. However, it is hygroscopic, leading to hydrolysis and formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire. Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named Pollution Prevention (P2) to eliminate or reduce the volume and toxicity of waste, air emissions, and effluent discharges. Because of this, Coolanol is used less often today.

# Regulations

''Radar'' (also: ''RADAR'') is defined by ''article 1.100'' of the International Telecommunication Union, International Telecommunication Union's (ITU) ITU Radio Regulations (RR) as:ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.100, definition: ''radar / RADAR''

# Configurations

Chain Home Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An ...

and Chain Home Low * Würzburg radar * FuG 200 Hohentwiel, Hohentwiel radar * H2S (radar), H2S radar * SCR-270 radar

# Bibliography

## References

* Barrett, Dick, "
All you ever wanted to know about British air defence radar
'". The Radar Pages. (History and details of various British radar systems) * Buderi, "

'". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.) * Ekco Rada
The secret development of British radar. * ES310 "

* Hollmann, Martin, "

'"
* Penley, Bill, and Jonathan Penley, "

an Introduction''". 2002. * 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.') * Wesley Stout, 194

Early development and production by Chrysler Corp. during WWII. * Swords, Seán S., "Technical History of the Beginnings of Radar", ''Institution of Electrical Engineers, IEE History of Technology Series'', Vol. 6, London: Peter Peregrinus, 1986

## General

* * * * * * Burch, David F., ''Radar For Mariners'', McGraw Hill, 2005, . * * * * * Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994. * * * * * * * Younghusband, Eileen., ''Not an Ordinary Life. How Changing Times Brought Historical Events into my Life'', Cardiff Centre for Lifelong Learning, Cardiff, 2009., (Pages 36–67 contain the experiences of a WAAF radar plotter in WWII.) * Younghusband, Eileen. ''One Woman's War''. Cardiff. Candy Jar Books. 2011. *