Radar

For other uses, seeRadar (disambiguation).

Long-range radarantenna,used to track space objects and ballistic missiles

Radar of the type used for detection of aircraft. It rotates steadily, sweeping the airspace with a narrow beam.

Radaris a system that usesradio wavesto determine the distance (ranging),direction(azimuthandelevation angles), andradial velocityof objects relative to the site. It is aradiodeterminationmethodused to detect and trackaircraft,ships,spacecraft,guided missiles,motor vehicles,mapweather formations,andterrain.

A radar system consists of atransmitterproducingelectromagnetic wavesin theradioormicrowavesdomain, a transmittingantenna,a receiving antenna (often the same antenna is used for transmitting and receiving) and areceiverandprocessorto determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds.

Radar was developed secretly formilitaryuse by several countries in the period before and duringWorld War II.A key development was thecavity magnetronin theUnited Kingdom,which allowed the creation of relatively small systems with sub-meter resolution. The termRADARwas coined in 1940 by theUnited States Navyas anacronymfor "radio detection and ranging".The termradarhas since entered English and other languages as ananacronym,a common noun,losing all capitalization.

The modern uses of radar are highly diverse, including air and terrestrial traffic control,radar astronomy,air-defense systems,anti-missile systems,marine radarsto locate landmarks and other ships, aircraft anti-collision systems,ocean surveillancesystems, outerspace surveillanceandrendezvoussystems,meteorologicalprecipitationmonitoring,radar remote sensing,altimetry andflight control systems,guided missiletarget locating systems,self-driving cars,andground-penetrating radarfor geological observations. Modern high tech radar systems usedigital signal processingandmachine learningand are capable of extracting useful information from very highnoiselevels.

Other systems which are similar to radar make use of other parts of theelectromagnetic spectrum.One example islidar,which uses predominantlyinfrared lightfromlasersrather 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

Main article:History of radar

First experiments

As early as 1886, German physicistHeinrich Hertzshowed that radio waves could be reflected from solid objects. In 1895,Alexander Popov,a physics instructor at theImperial Russian Navyschool inKronstadt,developed an apparatus using acoherertube for detecting distant lightning strikes. The next year, he added aspark-gap transmitter.In 1897, while testing this equipment for communicating between two ships in theBaltic Sea,he took note of aninterference beatcaused 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 inventorChristian Hülsmeyerwas 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 patentfor his detection device in April 1904 and later a patentfor a related amendment for estimating the distance to the ship. He also obtained a British patent on 23 September 1904for a full radar system, that he called atelemobiloscope.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 inCologneandRotterdamharbour but was rejected.

In 1915,Robert Watson-Wattused radio technology to provide advance warning of thunderstorms to airmenand during the 1920s went on to lead the U.K. research establishment to make many advances using radio techniques, including the probing of theionosphereand the detection oflightningat long distances. Through his lightning experiments, Watson-Watt became an expert on the use ofradio direction findingbefore turning his inquiry toshortwavetransmission. Requiring a suitable receiver for such studies, he told the "new boy"Arnold Frederic Wilkinsto conduct an extensive review of available shortwave units. Wilkins would select aGeneral Post Officemodel after noting its manual's description of a "fading" effect (the common term for interference at the time) when aircraft flew overhead.

By placing a transmitter and receiver on opposite sides of thePotomac Riverin 1922, U.S. Navy researchersA. Hoyt TaylorandLeo C. Youngdiscovered 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. Hylandat theNaval Research Laboratory(NRL) observed similar fading effects from passing aircraft; this revelation led to a patent applicationas 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. Butementand P. E. Pollard developed abreadboardtest 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 theInventions Bookmaintained 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 intoChain HomeasChain Home (low).

Before World War II

Experimental radar antenna, USNaval Research Laboratory,Anacostia, D. C., from the late 1930s (photo taken in 1945)

Before theSecond World War,researchers in the United Kingdom,France,Germany,Italy,Japan,the Netherlands, theSoviet Union,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's radar development,Hungaryand Sweden generated its radar technology during the war.

In France in 1934, following systematic studies on thesplit-anode magnetron,the research branch of theCompagnie générale de la télégraphie sans fil(CSF) headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locating radio apparatus, aspects of which were installed on the ocean linerNormandiein 1935.

During the same period, Soviet military engineerP.K. Oshchepkov,in collaboration with theLeningrad Electrotechnical Institute,produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver.The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 but further development was slowed following the arrest of Oshchepkov and his subsequentgulagsentence. In total, only 607 Redut stations were produced during the war. The first Russian airborne radar,Gneiss-2,entered into service in June 1943 onPe-2dive 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 AmericanRobert M. Page,working at theNaval Research Laboratory.The following year, theUnited States Armysuccessfully tested a primitive surface-to-surface radar to aimcoastal batterysearchlightsat night.This design was followed by a pulsed system demonstrated in May 1935 byRudolf Kühnholdand the firmGEMAin Germany and then another in June 1935 by anAir Ministryteam led byRobert Watson-Wattin Great Britain.

The first workable unit built byRobert Watson-Wattand his team

In 1935, Watson-Watt was asked to judge recent reports of a German radio-baseddeath rayand 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 theDaventry Experimentof 26 February 1935, using a powerfulBBCshortwave 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,theAir Member for Supply and Research,was very impressed with their system's potential and funds were immediately provided for further operational development.Watson-Watt's team patented the device in patent GB593017.

AChain Hometower in Great Baddow, Essex, United Kingdom

Memorial plaque commemorating Robert Watson-Watt andArnold Wilkins

Development of radar greatly expanded on 1 September 1936, when Watson-Watt became superintendent of a new establishment under the BritishAir Ministry,Bawdsey Research Station located inBawdsey 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 theBattle of Britain;without it, significant numbers of fighter aircraft, which Great Britain did not have available, would always have needed to be in the air to respond quickly. The radar formed part of the "Dowding system"for collecting reports of enemy aircraft and coordinating the response.

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

Main article:Radar in World War II

East CoastChain Homeradar operators in England

A key development was thecavity magnetronin the UK, which allowed the creation of relatively small systems with sub-meter resolution. Britain shared the technology with the U.S. during the 1940Tizard Mission.

In April 1940,Popular Scienceshowed an example of a radar unit using the Watson-Watt patent in an article on air defence.Also, in late 1941Popular Mechanicshad 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'sattack on Pearl Harbor.Alfred Lee Loomisorganized the secretMIT Radiation LaboratoryatMassachusetts Institute of Technology,Cambridge, Massachusetts which developed microwave radar technology in the years 1941–45. Later, in 1943, Page greatly improved radar with themonopulse techniquethat was used for many years in most radar applications.

The war precipitated research to find better resolution, more portability, and more features for radar, including small, lightweight sets to equipnight fighters(aircraft interception radar) andmaritime patrol aircraft(air-to-surface-vessel radar), and complementary navigation systems likeOboeused by theRAF's Pathfinder.

Applications

Commercial marine radar antenna. The rotating antenna radiates a vertical fan-shaped beam.

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.

Inaviation,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 someUnited Air Linesaircraft.Aircraft can land in fog at airports equipped with radar-assistedground-controlled approachsystems in which the plane's position is observed onprecision approach radarscreens 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 radarsare 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 serviceradar systems are used to monitor and regulate ship movements in busy waters.

Meteorologists use radar to monitorprecipitationand wind. It has become the primary tool for short-termweather forecastingand watching forsevere weathersuch asthunderstorms,tornadoes,winter storms,precipitation types, etc.Geologistsuse specializedground-penetrating radarsto map the composition ofEarth's crust.Police forces useradar gunsto monitor vehicle speeds on the roads. Automotive radars are used for adaptive cruise control and emergency breaking on vehicles by ignoring stationary roadside objects that could cause incorrect brake application and instead measuring moving objects to prevent collision with other vehicles. As part ofIntelligent Transport Systems,fixed-position stopped vehicle detection (SVD) radars are mounted on the roadside to detect stranded vehicles, obstructions and debris by inverting the automotive radar approach and ignoring moving objects.Smaller radar systems are used todetect human movement.Examples are breathing pattern detection for sleep monitoringand hand and fingergesture detectionfor computer interaction.Automatic door opening, light activation and intruder sensing are also common.

Principles

Radar signal

3D Doppler radar spectrum showing aBarker codeof 13

Further information:Radar signal characteristics

A radar system has atransmitterthat emitsradio wavesknown asradar signalsin predetermined directions. When these signals contact an object they are usuallyreflectedorscatteredin many directions, although some of them will be absorbed and penetrate into the target. Radar signals are reflected especially well by materials of considerableelectrical conductivity—such as most metals,seawater,and wet ground. This makes the use ofradar altimeterspossible 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 ismovingeither toward or away from the transmitter, there will be a slight change in thefrequencyof the radio waves due to theDoppler effect.

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 byelectronic amplifiers.More sophisticated methods ofsignal processingare 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 asvisible light,infrared light,andultraviolet light,are too strongly attenuated. Weather phenomena, such as fog, clouds, rain, falling snow, and sleet, that block visible light are usually transparent to radio waves. Certain radio frequencies that are absorbed or scattered by water vapour, raindrops, or atmospheric gases (especially oxygen) are avoided when designing radars, except when their detection is intended.

Illumination

Radar relies on its own transmissions rather than light from the Sun or the Moon, or fromelectromagnetic wavesemitted by the target objects themselves, such as infrared radiation (heat). This process of directing artificial radio waves towards objects is calledillumination,although radio waves are invisible to the human eye as well as optical cameras.

Reflection

Main article:Reflection (physics)

Brightness can indicate reflectivity as in this 1960weather radarimage (ofHurricane Abby). The radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.

Ifelectromagnetic wavestravelling through one material meet another material, having a differentdielectric constantordiamagnetic constantfrom the first, the waves will reflect or scatter from the boundary between the materials. This means that a solid object inairor in avacuum,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 forelectrically conductivematerials such as metal and carbon fibre, making radar well-suited to the detection of aircraft and ships.Radar absorbing material,containingresistiveand sometimesmagneticsubstances, is used on military vehicles toreduce 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 amirror.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 byRayleigh scattering,an effect that creates Earth's blue sky and red sunsets. When the two length scales are comparable, there may beresonances.Early radars used very long wavelengths that were larger than the targets and thus received a vague signal, whereas many modern systems use shorter wavelengths (a few centimetres or less) that can image objects as small as a loaf of bread.

Short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between thereflective surfaces.Acorner reflectorconsists of three flat surfaces meeting like the inside corner of a cube. The structure will reflect waves entering its opening directly back to the source. They are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect. Corner reflectors on boats, for example, make them more detectable to avoid collision or during 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" lookingstealth aircraft.These precautions do not totally eliminate reflection because ofdiffraction,especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such aschaff,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 itsradar cross-section.

Radar range equation

The powerP_rreturning to the receiving antenna is given by the equation:

$P_{r}={\frac {P_{t}G_{t}A_{r}\sigma F^{4}}{{(4\pi )}^{2}R_{t}^{2}R_{r}^{2}}}$

where

• P_t= transmitter power
• G_t=gainof the transmitting antenna
• A_r=effective aperture(area) of the receiving antenna; this can also be expressed as${G_{r}\lambda ^{2}} \over {4\pi }$,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_rand the termR_t²R_r² can be replaced byR⁴,whereRis the range. This yields:

$P_{r}={{P_{t}G_{t}A_{r}\sigma F^{4}} \over {{(4\pi )}^{2}R^{4}}}.$

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 forpulse-Doppler radar performance,which can be used to increase detection range and reduce transmit power.

The equation above withF= 1 is a simplification for transmission in a vacuum without interference. The propagation factor accounts for the effects ofmultipathand shadowing and depends on the details of the environment. In a real-world situation,pathlosseffects are also considered.

Doppler effect

Main articles:Doppler radarandPulse-Doppler radar

Change ofwavelengthcaused by motion of the source

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 indicationcan 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, andradar astronomyrely 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:

$F_{D}=2\times F_{T}\times \left({\frac {V_{R}}{C}}\right)$.

Passive radar is applicable toelectronic countermeasuresandradio astronomyas follows:

$F_{D}=F_{T}\times \left({\frac {V_{R}}{C}}\right)$.

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. Objects 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 theNyquist frequency,since the returned frequency otherwise cannot be distinguished from shifting of a harmonic frequency above or below, thus requiring:

$|F_{D}|<{\frac {F_{R}}{2}}$

Or when substituting with$F_{D}$:

$|V_{R}|<{\frac {F_{R}\times {\frac {C}{F_{T}}}}{4}}$

As an example, a Doppler weather radar with a pulse rate of 2 kHz and transmit frequency of 1 GHz can reliably measure weather speed up to at most 150 m/s (340 mph), thus cannot reliably determine radial velocity of aircraft moving 1,000 m/s (2,200 mph).

Polarization

Further information:Polarization (waves)

In allelectromagnetic radiation,the electric field is perpendicular to the direction of propagation, and the electric field direction is thepolarizationof 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 polarizationis used to minimize the interference caused by rain.Linear polarizationreturns usually indicate metal surfaces. Random polarization returns usually indicate afractalsurface, such as rocks or soil, and are used by navigation radars.

Limiting factors

Beam path and range

See also:Beam formingandOver-the-horizon radar

Echo heights above ground
$H=\left({\sqrt {r^{2}+(k_{e}a_{e})^{2}+2rk_{e}a_{e}sin(\theta _{e})}}\right)-k_{e}a_{e}+h_{a}$
Where:
r: distance radar-target
ke: 4/3
ae: Earth radius
θe: elevation angle above the radar horizon
ha: height of the feedhorn above ground

A radar beam follows a linear path in vacuum but follows a somewhat curved path in atmosphere due to variation in therefractive indexof air, which is called theradar horizon.Even when the beam is emitted parallel to the ground, the beam rises above the ground as thecurvature of the Earthsinks 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 thepulse 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

Main articles:Noise (electronics)andNoise (radio)

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. Thenoise floorandsignal-to-noise ratioare two differentmeasures of performancethat affect range performance. Reflectors that are too far away produce too little signal to exceed the noise floor and cannot be detected.Detectionrequires a signal that exceeds thenoise floorby 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. Thenoise figureis a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.

Shot noiseis 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 beflicker noisecaused by electron transit through amplification devices, which is reduced usingheterodyneamplification. 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 isultra-widebandradar. Here a single cycle, or transient wave, is used similar to UWB communications, seeList 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 littlethermal noise.The thermal noise is given byk_BTB,whereTis temperature,Bis bandwidth (post matched filter) andk_Bis theBoltzmann 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 appears that then within a fixed interval of time, perfect, error free, detection could be obtained. This is done by compressing 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 that can be done is to match filter all energy into a single electron. Since the electron is moving at a certain temperature (Planck spectrum) this noise source cannot be further eroded. Ultimately, 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 withmoving target indicationcan improve noise up to${\sqrt {2}}$for each stage. The signal can also be split among multiple filters forpulse-Doppler signal processing,which reduces the noise floor by the number of filters. These improvements depend uponcoherence.

Interference

Main article:Interference (wave propagation)

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 itssignal-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

Radar multipathechoesfrom a target cause ghosts to appear

Main article:Clutter (radar)

Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to radar operators. Such targets include man-made objects such as buildings and — intentionally — by radar countermeasures such aschaff.Such targets also include natural objects such as ground, sea, and — when not being tasked for meteorological purposes —precipitation,hail spike,dust storms,animals (especially birds), turbulence in theatmospheric circulation,andmeteortrails. Radar clutter can also be caused by other atmospheric phenomena, such as disturbances in theionospherecaused bygeomagnetic stormsor otherspace weatherevents. This phenomenon is especially apparent near thegeomagnetic poles,where the action of thesolar windon the earth'smagnetosphereproduces convection patterns in the ionosphericplasma.Radar clutter can degrade the ability ofover-the-horizon radarto detect targets.

Some clutter may also be caused by a long radarwaveguidebetween the radar transceiver and the antenna. In a typicalplan position indicator(PPI) radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the center 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.

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 withcircular polarization(meteorological radars wish for the opposite effect, and therefore uselinear polarizationto detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.

Clutter moves with the wind or is stationary. Two common strategies to improvemeasures of performancein a clutter environment are:

• Moving target indication, which integrates successive pulses
• Doppler processing, which uses filters to separate clutter from desirable signals

The most effective clutter reduction technique ispulse-Doppler radar.Doppler separates clutter from aircraft and spacecraft using afrequency 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 amoving target indicatorthat subtracts the received 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 astime-domain pulse-amplitude radar.

Constant false alarm rate,a form ofautomatic 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.

Clutter may also originate from multipath echoes from valid targets caused by ground reflection,atmospheric ductingorionospheric 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 ofjitteror a physical impossibility. Terrain bounce jamming exploits this response by amplifying the radar signal and directing it downward.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.

Jamming

Main article:Radar jamming and deception

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 anelectronic warfaretactic, 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.

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 in accordance withinverse-square law.Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along theline of sightfrom the jammer to the radar (mainlobe jamming). Jammers have an added effect of affecting radars along other lines of sight through the radar receiver'ssidelobes(sidelobe jamming).

Mainlobe jamming can generally only be reduced by narrowing the mainlobesolid angleand 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 anomnidirectional antennato detect and disregard non-mainlobe signals.Other anti-jamming techniquesarefrequency hoppingandpolarization.

Signal processing

Distance measurement

Transit time

Further information:Time of flight

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.

One way to obtain adistance measurement(ranging) is based on thetime-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 round trip time multiplied by the speed of the signal. The factor of one-half comes from the fact that the signal has to travel to the object and back again. Since radio waves travel at thespeed of light,accurate distance measurement requires high-speed electronics. 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. 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.

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 (about 10 km (6.2 miles)) and a separate signal for longer ranges (about 100 km (62 miles)).

Distance may also be measured as a function of time. Theradar mileis the time it takes for a radar pulse to travel onenautical mile,reflect off a target, and return to the radar antenna. Since a nautical mile is defined as 1,852 m, then dividing this distance by the speed of light (299,792,458 m/s), and then multiplying the result by 2 yields a result of 12.36 μs in duration.

Frequency modulation

Continuous wave (CW) radar. Using frequency modulation allows range to be extracted.

Main article: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 incontinuous wave radarand is often found in aircraftradar altimeters.In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with asine waveor 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 simplebeat frequencymodulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.

Themodulation indexriding 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 thetransponder.This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach areAZUSA,MISTRAM,andUDOP.

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

Main article: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 1960searly warning radarsoften 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

Speedis 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 makinggrease pencilmarks on the radar screen and then calculating the speed using aslide 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 theDoppler effect.Most modern radar systems use this principle intoDoppler radarandpulse-Doppler radarsystems (weather radar,military radar). 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'sazimuthover time.

It is possible to make a Doppler radar without any pulsing, known as acontinuous-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 the range is not important.

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. Other mathematical developments in radar signal processing includetime-frequency analysis(Weyl Heisenberg orwavelet), as well as thechirplet transformwhich makes use of the change of frequency of returns from moving targets ( "chirp" ).

Pulse-Doppler signal processing

Main article:Pulse-Doppler signal processing

Pulse-Doppler signal processing. TheRange Sampleaxis represents individual samples taken in between each transmit pulse. TheRange Intervalaxis 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 thebed of nails.

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 aspectrum analyzerto 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.

The primary purpose is to measure both the amplitude and frequency of the aggregate reflected signal from multiple distances. This is used withweather radarto 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 alow 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.

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.Pulse-doppler signal processing increases the maximum detection distance using less radiation close 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 usingnap-of-the-earthflying techniques andstealth technologyto 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 requiremedium PRFwhich prevents range from being determined directly, andrange ambiguity resolutionprocessing 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.

Reduction of interference effects

Signal processingis employed in radar systems to reduce theradar interference effects.Signal processing techniques includemoving target indication,Pulse-Doppler signal processing,moving target detection processors, correlation withsecondary surveillance radartargets,space-time adaptive processing,andtrack-before-detect.Constant false alarm rateanddigital terrain modelprocessing are also used in clutter environments.

Plot and track extraction

Main articles:Radar trackerandTrack algorithm

A track algorithm is a radar performance enhancement strategy. Tracking algorithms provide the ability to predict the 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
• 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, aradar trackeris used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.

Engineering

Main article:Radar engineering details

Radar components

A radar's components are:

• Atransmitterthat generates the radio signal with an oscillator such as aklystronor amagnetronand controls its duration by amodulator.
• Awaveguidethat links the transmitter and the antenna.
• Aduplexerthat serves as a switch between the antenna and the transmitter or the receiver for the signal when the antenna is used in both situations.
• Areceiver.Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using amatched filter.
• A display processor to produce signals for human readableoutput devices.
• An electronic section that controls all those devices and the antenna to perform the radar scan ordered by software.
• A link to end user devices and displays.

Antenna design

Main article:Antenna (radio)

AS-3263/SPS-49(V) antenna (US Navy)

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 useomnidirectional 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 atright anglesfor 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 byrotatingthe 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 isa small partof that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.

Parabolic reflector

Surveillance radar antenna

Main article:Parabolic antenna

More modern systems use a steerableparabolic"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, orradar 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. TheNEXRADPulse-Dopplerweather 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

Slotted waveguide antenna

Main article: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.

Phased array

Phased array:Not all radar antennas must rotate to scan the sky.

Main article:Phased array

Another method of steering is used in aphased arrayradar.

Phased 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 discrete 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 degrees away from the centerline perpendicular to the array face. Signals travelling along that beam will be reinforced. Signals offset from that beam will be cancelled. The amount of reinforcement isantenna gain.The amount of cancellation is side-lobe suppression.

Phased array radars have been in use since the earliest years of radar in World War II (Mammut radar), but electronic device limitations led to poor performance. Phased array radars were originally used for missile defence (see for exampleSafeguard Program). They are the heart of the ship-borneAegis Combat Systemand thePatriot 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. To a lesser extent, phased array radars have been used inweathersurveillance.As of 2017, NOAA plans to implement a national network of multi-function phased array radars throughout the United States within 10 years, for meteorological studies and flight monitoring.

Phased array antennas can be built to conform to specific shapes, like missiles, infantry support vehicles, ships, and aircraft.

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 and similar systems.

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 theB-1B Lancer.The first fighter aircraft to use phased array radar was theMikoyan MiG-31.The MiG-31M's SBI-16Zaslonpassive electronically scanned arrayradar was considered to be the world's most powerful fighter radar,until theAN/APG-77active electronically scanned arraywas introduced on theLockheed Martin F-22 Raptor.

Phased-arrayinterferometryoraperture synthesistechniques, 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 inradio astronomy.Because of thethinned 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.

Aperture synthesisby post-processing motion data from a single moving source, on the other hand, is widely used in space andairborne radar systems.

Frequency bands

Main article:Radio spectrum § IEEE

Antennas generally have to be sized similar to the wavelength of the operational frequency, normally within anorder of magnitude.This provides a strong incentive to use shorter wavelengths as this will result in smaller antennas. Shorter wavelengths also result in higher resolution due to diffraction, meaning the shaped reflector seen on most radars can also be made smaller for any desired beamwidth.

Opposing the move to smaller wavelengths are a number of practical issues. For one, the electronics needed to produce high power very short wavelengths were generally more complex and expensive than the electronics needed for longer wavelengths or did not exist at all. Another issue is that theradar equation's effective aperture figure means that for any given antenna (or reflector) size will be more efficient at longer wavelengths. Additionally, shorter wavelengths may interact with molecules or raindrops in the air, scattering the signal. Very long wavelengths also have additional diffraction effects that make them suitable forover the horizon radars.For this reason, a wide variety of wavelengths are used in different roles.

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 theInstitute of Electrical and Electronics Engineersand internationally by theInternational Telecommunication Union.Most countries have additional regulations to control which parts of each band are available for civilian or military use.

Other users of the radio spectrum, such as thebroadcastingandelectronic countermeasuresindustries, have replaced the traditional military designations with their own systems.

Radar frequency bands

Band name	Frequency range	Wavelength range	Notes
HF	3–30MHz	10–100m	Coastal radar systems,over-the-horizon(OTH) radars; 'high frequency'
VHF	30–300 MHz	1–10 m	Very long range, ground penetrating; 'very high frequency'. Early radar systems generally operated in VHF as suitable electronics had already been developed for broadcast radio. Today this band is heavily congested and no longer suitable for radar due to interference.
P	< 300 MHz	> 1 m	'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF. Often used for remote sensing because of good vegetation penetration.
UHF	300–1000 MHz	0.3–1 m	Very long range (e.g.ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'. Efficiently produced and received at very high energy levels, and also reduces the effects ofnuclear blackout,making them useful in the missile detection role.
L	1–2GHz	15–30cm	Long range air traffic control andsurveillance;'L' for 'long'. Widely used for long rangeearly warning radarsas they combine good reception qualities with reasonable resolution.
S	2–4 GHz	7.5–15 cm	Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'sentimetric', its code-name during WWII. Less efficient than L, but offering higher resolution, making them especially suitable for long-rangeground controlled interceptiontasks.
C	4–8 GHz	3.75–7.5 cm	Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
X	8–12 GHz	2.5–3.75 cm	Missileguidance,marine radar,weather, medium-resolution mapping and ground surveillance; in the United States the narrow range 10.525 GHz ±25 MHz is used forairportradar; short-range tracking. Named X band because the frequency was a secret during WW2. Diffraction off raindrops during heavy rain limits the range in the detection role and makes this suitable only for short-range roles or those that deliberately detect rain.
Ku	12–18 GHz	1.67–2.5 cm	High-resolution, also used for satellite transponders, frequency under K band (hence 'u')
K	18–24 GHz	1.11–1.67 cm	FromGermankurz,meaning 'short'. Limited use due to absorption bywater vaporat 22 GHz, so Kuand Kaon either side 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.
Ka	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.
mm	40–300 GHz	1.0–7.5mm	Millimetre band,subdivided as below. Oxygen in the air is an extremely effective attenuator around 60 GHz, as are other molecules at other frequencies, leading to the so-called propagation window at 94 GHz. Even in this window the attenuation is higher than that due to water at 22.2 GHz. This makes these frequencies generally useful only for short-range highly specific radars, likepower lineavoidance systems forhelicoptersor use in space where attenuation is not a problem. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
V	40–75 GHz	4.0–7.5 mm	Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
W	75–110 GHz	2.7–4.0 mm	Used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.

Modulators

Modulatorsact 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, apulse forming network,and a high voltage switch such as athyratron.They generate short pulses of power to feed, e.g., themagnetron,a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known aspulsed 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 butcoherentwaveform. 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., aklystronor a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must usepulse compressiontechniques.

Coolant

Main article:Coolant

Coherent microwave amplifiers operating above 1,000 watts microwave output, liketravelling wave tubesandklystrons,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 use Doppler processing.

Coolanol(silicateester) was used in several military radars in the 1970s. However, it ishygroscopic,leading tohydrolysisand 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 namedPollution 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 byarticle 1.100of theInternational Telecommunication Union's(ITU)ITU Radio Regulations(RR) as:

Aradiodetermination systembased on the comparison of reference signals with radio signals reflected, or retransmitted, from the position to be determined. Eachradiodetermination systemshall be classified by theradiocommunication servicein which it operates permanently or temporarily. Typical radar utilizations areprimary radarandsecondary radar,these might operate in theradiolocation serviceor theradiolocation-satellite service.

Configurations

Radar come in a variety of configurations in the emitter, the receiver, the antenna, wavelength, scan strategies, etc.

• Bistatic radar
• Continuous-wave radar
• Doppler radar
• Fm-cw radar
• Monopulse radar
• Passive radar
• Planar array radar
• Pulse-doppler
• Synthetic-aperture radar
  • Synthetically thinned aperture radar
• Over-the-horizon radarwithchirp transmitter

See also

Main listing:List of radar types

Main category:Radar

• Terrain-following radar
• Radar imaging
• Radar navigation
• Inverse-square law
• Wave radar
• Radar signal characteristics
• Pulse doppler radar
• Mmwave sensing
• Acronyms and abbreviations in avionics

Definitions

• Amplitude-comparison monopulse
• Constant false alarm rate
• Sensitivity time control

Application

• Proximity fuze

Hardware

• Cavity magnetron
• Crossed-field amplifier
• Gallium arsenide
• Klystron
• Omniview technology
• Radar engineering details
• Radar tower
• Radio
• Travelling-wave tube

Similar detection and ranging methods

• Acoustic location
• Lidar
• LORAN
• Sonar

Historical radars

• List of radars
• Chain HomeandChain Home Low
• Würzburg radar
• Hohentwiel 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, "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.)
• Ekco RadarWW2 Shadow FactoryArchived12 December 2005 at theWayback MachineThe secret development of British radar.
• ES310"Introduction to Naval Weapons Engineering.".(Radar fundamentals section)
• Hollmann, Martin, "Radar Family Tree".Radar World.
• Penley, Bill, and Jonathan Penley, "Early Radar History—an Introduction".2002.
• Pub 1310Radar 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, 1946"Radar – The Great Detective"Archived28 July 2020 at theWayback MachineEarly development and production by Chrysler Corp. during WWII.
• Swords, Seán S., "Technical History of the Beginnings of Radar",IEEHistory of Technology Series,Vol. 6, London: Peter Peregrinus, 1986

General

• Reg Batt (1991).The radar army: winning the war of the airwaves.R. Hale.ISBN978-0-7090-4508-3.
• E.G. Bowen (1 January 1998).Radar Days.Taylor & Francis.ISBN978-0-7503-0586-0.
• Michael Bragg (1 May 2002).RDF1: The Location of Aircraft by Radio Methods 1935–1945.Twayne Publishers.ISBN978-0-9531544-0-1.
• Louis Brown (1999).A radar history of World War II: technical and military imperatives.Taylor & Francis.ISBN978-0-7503-0659-1.
• Robert Buderi (1996).The invention that changed the world: how a small group of radar pioneers won the Second World War and launched a technological revolution.Simon & Schuster.ISBN978-0-684-81021-8.
• Burch, David F.,Radar For Mariners,McGraw Hill, 2005,ISBN978-0-07-139867-1.
• Ian Goult (2011).Secret Location: A witness to the Birth of Radar and its Postwar Influence.History Press.ISBN978-0-7524-5776-5.
• Peter S. Hall (March 1991).Radar.Potomac Books Inc.ISBN978-0-08-037711-7.
• Derek Howse; Naval Radar Trust (February 1993).Radar at sea: the royal Navy in World War 2.Naval Institute Press.ISBN978-1-55750-704-4.
• R.V. Jones (August 1998).Most Secret War.Wordsworth Editions Ltd.ISBN978-1-85326-699-7.
• Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994.
• Colin Latham; Anne Stobbs (January 1997).Radar: A Wartime Miracle.Sutton Pub Ltd.ISBN978-0-7509-1643-1.
• François Le Chevalier (2002).Principles of radar and sonar signal processing.Artech House Publishers.ISBN978-1-58053-338-6.
• David Pritchard (August 1989).The radar war: Germany's pioneering achievement 1904-45.Harpercollins.ISBN978-1-85260-246-8.
• Merrill Ivan Skolnik (1 December 1980).Introduction to radar systems.McGraw-Hill.ISBN978-0-07-066572-9.
• Merrill Ivan Skolnik (1990).Radar handbook.McGraw-Hill Professional.ISBN978-0-07-057913-2.
• George W. Stimson (1998).Introduction to airborne radar.SciTech Publishing.ISBN978-1-891121-01-2.
• Younghusband, Eileen.,Not an Ordinary Life. How Changing Times Brought Historical Events into my Life,Cardiff Centre for Lifelong Learning, Cardiff, 2009.,ISBN978-0-9561156-9-0(Pages 36–67 contain the experiences of a WAAF radar plotter in WWII.)
• Younghusband, Eileen.One Woman's War.Cardiff. Candy Jar Books. 2011.ISBN978-0-9566826-2-8
• David Zimmerman (February 2001).Britain's shield: radar and the defeat of the Luftwaffe.Sutton Pub Ltd.ISBN978-0-7509-1799-5.

Technical reading

• M I. Skolnik, ed. (1970).Radar Handbook(PDF).McGraw-Hill.ISBN0-07-057913-X.
• Nadav Levanon; Eli Mozeson (2004).Radar signals.John Wiley & Sons, Inc.ISBN9780471473787.
• Hao He;Jian Li;Petre Stoica(2012).Waveform design for active sensing systems: a computational approach.Cambridge University Press.ISBN978-1-107-01969-0.
• Solomon W. Golomb;Guang Gong(2005).Signal design for good correlation: for wireless communication, cryptography, and radar.Cambridge University Press.ISBN978-0521821049.
• M. Soltanalian (2014).Signal Design for Active Sensing and Communications.Elanders Sverige AB.ISBN978-91-554-9017-1.{{cite book}}:|work=ignored (help)
• Fulvio Gini; Antonio De Maio; Lee Patton, eds. (2012).Waveform design and diversity for advanced radar systems.London: The Institution of Engineering and Technology.ISBN978-1849192651.
• E. Fishler; A. Haimovich; R. Blum; D. Chizhik; L. Cimini; R. Valenzuela (2004).MIMO radar: an idea whose time has come.IEEE Radar Conference.
• Mark R. Bell (1993). "Information theory and radar waveform design".IEEE Transactions on Information Theory.39(5): 1578–1597.doi:10.1109/18.259642.
• Robert Calderbank; S. Howard; Bill Moran (2009). "Waveform diversity in radar signal processing".IEEE Signal Processing Magazine.26(1): 32–41.Bibcode:2009ISPM...26...32C.doi:10.1109/MSP.2008.930414.S2CID16437755.
• Mark A. Richards; James A. Scheer; William A. Holm (2010).Principles of Modern Radar: Basic Principles.SciTech Publishing.ISBN978-1891121-52-4.

External links

• MIT Video Course: Introduction to Radar SystemsA set of 10 video lectures developed at Lincoln Laboratory to develop an understanding of radar systems and technologies.
• A set of educational videos created for air traffic control (ATC) staff.
• Glossary of radar terminology