radar for other uses see radar disambiguation media all and radar antenna used to track space objects and ballistic missilesradar of the type used for detection of aircraft it rotates steadily sweeping the airspace with a narrow beam 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 ships spacecraft guided missiles motor vehicles weather formations and terrain a radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain a transmitting antenna a receiving antenna often the same antenna is used for transmitting and receiving and a receiver and processor to determine properties of the objects radio waves pulsed or continuous from the transmitter reflect off the object and return to the receiver giving information about the objects location and speed radar was developed secretly for military use by several nations in the period before and during world war ii a key development was the cavity magnetron in the uk which allowed the creation of relatively small systems with submeter resolution the term radar was coined in 1940 by the united states navy as an acronym for radio detection and ranging the term radar has since entered english and other languages as a common noun losing all capitalization the modern uses of radar are highly diverse including air and terrestrial traffic control radar astronomy airdefence systems antimissile systems marine radars to locate landmarks and other ships aircraft anticollision systems ocean surveillance systems outer space surveillance and rendezvous systems meteorological precipitation monitoring altimetry and flight control systems guided missile target locating systems groundpenetrating radar for geological observations and rangecontrolled radar for public health surveillance high tech radar systems are associated with digital signal processing machine learning and are capable of extracting useful information from very high noise levels other systems similar to radar make use of other parts of the electromagnetic spectrum one example is lidar which uses predominantly infrared light from lasers rather than radio waves history history of radar first experiments as early as 1886 german physicist heinrich hertz showed that radio waves could be reflected from solid objects in 1895 alexander popov a physics instructor at the imperial russian navy school in kronstadt developed an apparatus using a coherer tube for detecting distant lightning strikes the next year he added a sparkgap transmitter in 1897 while testing this equipment for communicating between two ships in the baltic sea he took note of an interference beat caused by the passage of a third vessel in his report popov wrote that this phenomenon might be used for detecting objects but he did nothing more with this observation the german inventor christian hlsmeyer 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 in 1915 robert watsonwatt used radio technology to provide advance warning to airmen and during the 1920s went on to lead the uk research establishment to make many advances using radio techniques including the probing of the ionosphere and the detection of lightning at long distances through his lightning experiments watsonwatt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission requiring a suitable receiver for such studies he told the new boy arnold frederic wilkins to conduct an extensive review of available shortwave units wilkins would select a general post office model after noting its manuals 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 us navy researchers a hoyt taylor and leo c young 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 nrl observed similar fading effects from passing aircraft this revelation led to a patent application just before world war ii experimental radar antenna us naval research laboratory anacostia d c late 1930s before the second world war researchers in the united kingdom france germany italy japan the netherlands the soviet 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 britains radar development and hungary generated its radar technology during the war in france in 1934 following systematic studies on the splitanode magnetron the research branch of the compagnie gnrale de tlgraphie sans fil csf headed by maurice ponte with henri gutton sylvain berline and m hugon began developing an obstaclelocating radio apparatus aspects of which were installed on the ocean liner normandie in 1935 during the same period soviet military engineer p k oshchepkov in collaboration with leningrad electrophysical institute produced an experimental apparatus rapid capable of detecting an aircraft within 3km of a receiver the french and soviet systems however featured continuouswave 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 this design was followed by a pulsed system demonstrated in may 1935 by rudolf khnhold and the firm gema in germany and then another in june 1935 by an air ministry team led by robert a watsonwatt in great britain the first workable unit built by robert watsonwatt and his team a chain home tower in great baddow essex united kingdom memorial plaque commemorating robert watsonwatt and arnold wilkins in 1935 watsonwatt was asked to judge recent reports of a german radiobased death ray and turned the request over to wilkins wilkins returned a set of calculations demonstrating the system was basically impossible when watsonwatt 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 of 26 february 1935 using a powerful bbc shortwave transmitter as the source and their gpo receiver setup in a field while a bomber flew around the site when the plane was clearly detected hugh dowding the air member for supply and research was very impressed with their systems potential and funds were immediately provided for further operational development development of radar greatly expanded on 1 september 1936 when watsonwatt became superintendent of a new establishment under the british air ministry bawdsey research station located in bawdsey manor near felixstowe suffolk work there resulted in the design and installation of aircraft detection and tracking stations called chain home along the east and south coasts of england in time for the outbreak of world war ii in 1939 this system provided the vital advance information that helped the royal air force win the battle of britain without it significant numbers of fighter aircraft would always need to be in the air to respond quickly enough if enemy aircraft detection relied solely on the observations of groundbased individuals also vital was the dowding system of reporting and coordination to make best use of the radar information during tests of early deployment of radar in 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 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 watsonwatts 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 radar in world war ii a key development was the cavity magnetron in the uk which allowed the creation of relatively small systems with submeter resolution britain shared the technology with the us during the 1940 tizard mission in april 1940 popular science showed an example of a radar unit using the watsonwatt patent in an article on air defence the war precipitated research to find better resolution more portability and more features for radar including complementary navigation systems like oboe used by the rafs pathfinder applications commercial marine radar antenna the rotating antenna radiates a vertical fanshaped 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 roads in aviation aircraft can be equipped with radar devices that warn of aircraft or other obstacles in or approaching their path display weather information and give accurate altitude readings the first commercial device fitted to aircraft was a 1938 bell lab unit on some united air lines aircraft marine radars are used to measure the bearing and distance of ships to prevent collision with other ships to navigate and to fix their position at sea when within range of shore or other fixed references such as islands buoys and lightships in port or in harbour vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters meteorologists use radar to monitor precipitation and wind it has become the primary tool for shortterm weather forecasting and watching for severe weather such as thunderstorms tornadoes winter storms precipitation types etc geologists use specialized groundpenetrating radars to map the composition of earths crust police forces use radar guns to monitor vehicle speeds on the roads smaller radar systems are used to detect human movement examples are breathing pattern detection for sleep monitoring automatic door opening light activation and intruder sensing are also common principles radar signal further information radar signal characteristics a radar system has a transmitter that emits radio waves called radar signals in predetermined directions when these come into contact with an object they are usually reflected or scattered in many directions but some of them absorb and penetrate into the target to some degree radar signals are reflected especially well by materials of considerable electrical conductivityespecially by most metals by seawater and by wet ground some of these make the use of radar altimeters possible the radar signals that are reflected back towards the transmitter are the desirable ones that make radar work if the object is moving either toward or away from the transmitter there is a slight equivalent change in the frequency of the radio waves caused by the doppler effect radar receivers are usually but not always in the same location as the transmitter although the reflected radar signals captured by the receiving antenna are usually very weak they can be strengthened by electronic amplifiers more sophisticated methods of signal processing are also used in order to recover useful radar signals the weak absorption of radio waves by the medium through which it passes is what enables radar sets to detect objects at relatively long rangesranges at which other electromagnetic wavelengths such as visible light infrared light and ultraviolet light are too strongly attenuated such weather phenomena as fog clouds rain falling snow and sleet that block visible light are usually transparent to radio waves certain radio frequencies that are absorbed or scattered by water vapour raindrops or atmospheric gases especially oxygen are avoided in 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 from electromagnetic waves emitted by the objects themselves such as infrared wavelengths heat this process of directing artificial radio waves towards objects is called illumination although radio waves are invisible to the human eye or optical cameras reflection reflection physics brightness can indicate reflectivity as in this 1960 weather radar image of hurricane abby the radars frequency pulse form polarization signal processing and antenna determine what it can observe if electromagnetic waves travelling through one material meet another material having a different dielectric constant or diamagnetic constant from the first the waves will reflect or scatter from the boundary between the materials this means that a solid object in air or in a vacuum or a significant change in atomic density between the object and what is surrounding it will usually scatter radar radio waves from its surface this is particularly true for electrically conductive materials such as metal and carbon fibre making radar wellsuited to the detection of aircraft and ships radar absorbing material containing resistive and sometimes magnetic substances is used on military vehicles to reduce radar reflection this is the radio equivalent of painting something a dark colour so that it cannot be seen by the eye at night radar waves scatter in a variety of ways depending on the size wavelength of the radio wave and the shape of the target if the wavelength is much shorter than the targets size the wave will bounce off in a way similar to the way light is reflected by a mirror if the wavelength is much longer than the size of the target the target may not be visible because of poor reflection lowfrequency radar technology is dependent on resonances for detection but not identification of targets this is described by rayleigh scattering an effect that creates earths blue sky and red sunsets when the two length scales are comparable there may be resonances early radars used very long wavelengths that were larger than the targets and thus received a vague signal whereas many modern systems use shorter wavelengths a few centimetres or less that can image objects as small as a loaf of bread short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass the most reflective targets for short wavelengths have 90 angles between the reflective surfaces a corner reflector consists of three flat surfaces meeting like the inside corner of a box the structure will reflect waves entering its opening directly back to the source they are commonly used as radar reflectors to make otherwise difficulttodetect objects easier to detect corner reflectors on boats for example make them more detectable to avoid collision or during a rescue for similar reasons objects intended to avoid detection will not have inside corners or surfaces and edges perpendicular to likely detection directions which leads to odd looking stealth aircraft these precautions do not completely eliminate reflection because of diffraction especially at longer wavelengths half wavelength long wires or strips of conducting material such as chaff 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 equation the power pr returning to the receiving antenna is given by the equation p r p t g t a r f 4 4 2 r t 2 r r 2 displaystyle prfrac ptgtarsigma f44pi 2rt2rr2 where pt transmitter power gt gain of the transmitting antenna ar effective aperture area of the receiving antenna this can also be expressed as g r 2 4 displaystyle grlambda 2 over 4pi where displaystyle lambda transmitted wavelength gr gain of receiving antenna radar cross section or scattering coefficient of the target f pattern propagation factor rt distance from the transmitter to the target rr distance from the target to the receiver in the common case where the transmitter and the receiver are at the same location rt rr and the term rt rr can be replaced by r4 where r is the range this yields p r p t g t a r f 4 4 2 r 4 displaystyle prptgtarsigma f4 over 4pi 2r4 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 pulsedoppler 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 without interference the propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment in a realworld situation pathloss effects should also be considered doppler effect doppler radar and pulsedoppler radar 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 seabased radar systems semiactive radar homing active radar homing weather radar military aircraft and radar astronomy rely on the doppler effect to enhance performance this produces information about target velocity during 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 displaystyle fd is doppler frequency f t displaystyle ft is transmit frequency v r displaystyle vr is radial velocity and c displaystyle c is the speed of light f d 2 f t v r c displaystyle fd2times fttimes leftfrac vrcright passive radar is applicable to electronic countermeasures and radio astronomy as follows f d f t v r c displaystyle fdfttimes leftfrac vrcright 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 displaystyle ft is pulsed using a pulse repeat frequency of f r displaystyle fr the resulting frequency spectrum will contain harmonic frequencies above and below f t displaystyle ft with a distance of f r displaystyle fr as a result the doppler measurement is only nonambiguous if the doppler frequency shift is less than half of f r displaystyle fr 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 f r 2 displaystyle fdfrac fr2 or when substituting with f d displaystyle fd v r f r c f t 4 displaystyle vrfrac frtimes frac cft4 as an example a doppler weather radar with a pulse rate of 2khz and transmit frequency of 1ghz can reliably measure weather speed up to at most 150ms 340mph thus cannot reliably determine radial velocity of aircraft moving 1000ms 2200mph polarization further information polarization waves in all electromagnetic radiation the electric field is perpendicular to the direction of propagation and the electric field direction is the polarization of the wave for a transmitted radar signal the polarization can be controlled to yield different effects radars use horizontal vertical linear and circular polarization to detect different types of reflections for example circular polarization is used to minimize the interference caused by rain linear polarization returns usually indicate metal surfaces random polarization returns usually indicate a fractal surface such as rocks or soil and are used by navigation radars limiting factors beam path and range see also beam forming and overthehorizon radar echo heights above ground the radar beam would follow a linear path in vacuum but it really follows a somewhat curved path in the atmosphere because of the variation of the refractive index of air that is called the radar horizon even when the beam is emitted parallel to the ground it will rise above it as the earth curvature sinks below the horizon furthermore the signal is attenuated by the medium it crosses and the beam disperses the maximum range of a conventional radar can be limited by a number of factors line of sight which depends on height above ground this means without a direct line of sight the path of the beam is blocked the maximum nonambiguous range which is determined by the pulse repetition frequency the maximum nonambiguous range is the distance the pulse could travel and return before the next pulse is emitted radar sensitivity and power of the return signal as computed in the radar equation this includes factors such as environmental conditions and the size or radar cross section of the target noise noise electronics and noise 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 the noise floor and signal to noise ratio are two different 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 ultrawideband 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 boltzmanns constant there is an appealing intuitive interpretation of this relationship in a radar matched filtering allows the entire energy received from a target to be compressed into a single bin be it a range doppler elevation or azimuth bin on the surface it would appear that then within a fixed interval of time one could obtain perfect error free detection to do this one simply compresses all energy into an infinitesimal time slice what limits this approach in the real world is that while time is arbitrarily divisible current is not the quantum of electrical energy is an electron and so the best one can do is match filter all energy into a single electron since the electron is moving at a certain temperature plank spectrum this noise source cannot be further eroded we see then that radar like all macroscale 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 can improve noise up to 2 displaystyle sqrt 2 for each stage the signal can also be split among multiple filters for pulsedoppler signal processing which reduces the noise floor by the number of filters these improvements depend upon coherence interference 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 its signaltonoise 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 systems snr the better it is at discriminating actual targets from noise signals clutter clutter radar 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 such as rain snow or hail sand storms animals especially birds atmospheric turbulence and other atmospheric effects such as ionosphere reflections meteor trails and hail spike clutter may also be returned from manmade objects such as buildings and intentionally by radar countermeasures such as chaff some clutter may also be caused by a long radar waveguide between the radar transceiver and the antenna in a typical plan position indicator ppi radar with a rotating antenna this will usually be seen as a sun or sunburst in the centre of the display as the receiver responds to echoes from dust particles and misguided rf in the waveguide adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna clutter is considered a passive interference source since it only appears in response to radar signals sent by the radar clutter is detected and neutralized in several ways clutter tends to appear static between radar scans on subsequent scan echoes desirable targets will appear to move and all stationary echoes can be eliminated sea clutter can be reduced by using horizontal polarization while rain is reduced with circular polarization meteorological radars wish for the opposite effect and therefore use linear polarization to detect precipitation other methods attempt to increase the signaltoclutter ratio clutter moves with the wind or is stationary two common strategies to improve measure or performance in a clutter environment are moving target indication which integrates successive pulses and doppler processing which uses filters to separate clutter from desirable signals the most effective clutter reduction technique is pulsedoppler radar doppler separates clutter from aircraft and spacecraft using a frequency spectrum so individual signals can be separated from multiple reflectors located in the same volume using velocity differences this requires a coherent transmitter another technique uses a moving target indicator that subtracts the receive signal from two successive pulses using phase to reduce signals from slow moving objects this can be adapted for systems that lack a coherent transmitter such as timedomain pulseamplitude radar constant false alarm rate a form of automatic gain control agc is a method that relies on clutter returns far outnumbering echoes from targets of interest the receivers 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 computersoftware controlled and affected the gain with greater granularity in specific detection cells radar multipath echoes from a target cause ghosts to appear clutter may also originate from multipath echoes from valid targets caused by ground reflection atmospheric ducting or ionospheric reflectionrefraction eg anomalous propagation this clutter type is especially bothersome since it appears to move and behave like other normal point targets of interest in a typical scenario an aircraft echo is reflected from the ground below appearing to the receiver as an identical target below the correct one the radar may try to unify the targets reporting the target at an incorrect height or eliminating it on the basis of jitter or a physical impossibility terrain bounce jamming exploits this response by amplifying the radar signal and directing it downward these problems can be overcome by incorporating a ground map of the radars 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 radar jamming and deception radar jamming refers to radio frequency signals originating from sources outside the radar transmitting in the radars frequency and thereby masking targets of interest jamming may be intentional as with an electronic warfare tactic or unintentional as with friendly forces operating equipment that transmits using the same frequency range jamming is considered an active interference source since it is initiated by elements outside the radar and in general unrelated to the radar signals 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 radartargetradar and are therefore significantly reduced in power by the time they return to the radar receiver jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the line of sight from the jammer to the radar mainlobe jamming jammers have an added effect of affecting radars along other lines of sight through the radar receivers sidelobes sidelobe jamming mainlobe jamming can generally only be reduced by narrowing the mainlobe solid angle and cannot fully be eliminated when directly facing a jammer which uses the same frequency and polarization as the radar sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using an omnidirectional antenna to detect and disregard nonmainlobe signals other antijamming techniques are frequency hopping and polarization radar signal processing distance measurement transit time further information time of flight pulse radar the roundtrip time for the radar pulse to get to the target and return is measured the distance is proportional to this time continuous wave cw radar one way to obtain a distance measurement is based on the timeofflight transmit a short pulse of radio signal electromagnetic radiation and measure the time it takes for the reflection to return the distance is onehalf the product of the round trip time because the signal has to travel to the target and then back to the receiver and the speed of the signal since radio waves travel close to the speed of light accurate distance measurement requires highspeed 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 longrange 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 10km 62mi and a separate signal for longer ranges about 100km 62mi the distance resolution and the characteristics of the received signal as compared to noise depends on the shape of the pulse the pulse is often modulated to achieve better performance using a technique known as pulse compression distance may also be measured as a function of time the radar mile is the time it takes for a radar pulse to travel one nautical mile reflect off a target and return to the radar antenna since a nautical mile is defined as 1852m then dividing this distance by the speed of light 299792458ms and then multiplying the result by 2 yields a result of 1236s in duration frequency modulation frequency modulation another form of distance measuring radar is based on frequency modulation frequency comparison between two signals is considerably more accurate even with older electronics than timing the signal by measuring the frequency of the returned signal and comparing that with the original the difference can be easily measured this technique can be used in continuous wave radar and is often found in aircraft radar altimeters 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 since the signal frequency is changing by the time the signal returns to the aircraft the transmit frequency has changed the frequency shift is used to measure distance the 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 this signal processing is similar to that used in speed detecting doppler radar example systems using this approach are azusa mistram and udop 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 terrestrial radar uses lowpower 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 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 transmitters output is coherent phase synchronized there is another effect that can be used to make almost instant speed measurements no memory is required known as the doppler effect most modern radar systems use this principle into doppler radar and pulsedoppler radar systems 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 targets azimuth over time it is possible to make a doppler radar without any pulsing known as a continuouswave 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 targets velocity cw radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where 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 include timefrequency analysis weyl heisenberg or wavelet as well as the chirplet transform which makes use of the change of frequency of returns from moving targets chirp pulsedoppler signal processing pulsedoppler signal processing pulsedoppler signal processing the range sample axis represents individual samples taken in between each transmit pulse the range interval axis represents each successive transmit pulse interval during which samples are taken the fast fourier transform process converts timedomain samples into frequency domain spectra this is sometimes called the bed of nails pulsedoppler signal processing includes frequency filtering in the detection process the space between each transmit pulse is divided into range cells or range gates each cell is filtered independently much like the process used by a spectrum analyzer to produce the display showing different frequencies each different distance produces a different spectrum these spectra are used to perform the detection process this is required to achieve acceptable performance in hostile environments involving weather terrain and electronic countermeasures the primary purpose is to measure both the amplitude and frequency of the aggregate reflected signal from multiple distances this is used with weather radar to measure radial wind velocity and precipitation rate in each different volume of air this is linked with computing systems to produce a realtime electronic weather map aircraft safety depends upon continuous access to accurate weather radar information that is used to prevent injuries and accidents weather radar uses a low prf coherency requirements are not as strict as those for military systems because individual signals ordinarily do not need to be separated less sophisticated filtering is required and range ambiguity processing is not normally needed with weather radar in comparison with military radar intended to track air vehicles the alternate purpose is lookdownshootdown capability required to improve military air combat survivability pulsedoppler is also used for ground based surveillance radar required to defend personnel and vehicles reduction of interference effects signal processing is employed in radar systems to reduce the radar interference effects signal processing techniques include moving target indication pulsedoppler signal processing moving target detection processors correlation with secondary surveillance radar targets spacetime adaptive processing and trackbeforedetect constant false alarm rate and digital terrain model processing are also used in clutter environments plot and track extraction track algorithm 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 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 nonrelevant 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 radar engineering details radar components a radars components are a transmitter that generates the radio signal with an oscillator such as a klystron or a magnetron and controls its duration by a modulator a waveguide that links the transmitter and the antenna a duplexer that serves as a switch between the antenna and the transmitter or the receiver for the signal when the antenna is used in both situations a receiver knowing the shape of the desired received signal a pulse an optimal receiver can be designed using a matched filter a display processor to produce signals for human readable output 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 antenna radio as3263sps49v 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 use 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 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 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 parabolic antenna more modern systems use a steerable 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 pulsedoppler 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 socalled 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 surveillance radar antenna 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 slotted waveguide applied similarly to the parabolic reflector the slotted waveguide is moved mechanically to scan and is particularly suitable for nontracking 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 phased array another method of steering is used in a phased array radar 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 centreline 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 is antenna gain the amount of cancellation is sidelobe 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 example safeguard program they are the heart of the shipborne aegis combat system and the patriot missile system the massive redundancy associated with having a large number of array elements increases reliability at the expense of gradual performance degradation that occurs as individual phase elements fail to a lesser extent phased array radars have been used in weather surveillance as of 2017 noaa plans to implement a national network of multifunction 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 movingantenna 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 the b1b lancer the first fighter aircraft to use phased array radar was the mikoyan mig31 the mig31ms sbi16 zaslon passive electronically scanned array radar was considered to be the worlds most powerful fighter radar until the anapg77 active electronically scanned array was introduced on the lockheed martin f22 raptor phasedarray interferometry or aperture synthesis techniques using an array of separate dishes that are phased into a single effective aperture are not typical for radar applications although they are widely used in radio astronomy because of the thinned array curse such multiple aperture arrays when used in transmitters result in narrow beams at the expense of reducing the total power transmitted to the target in principle such techniques could increase spatial resolution but the lower power means that this is generally not effective aperture synthesis by postprocessing motion data from a single moving source on the other hand is widely used in space and airborne radar systems frequency bands radio spectrum ieee the traditional band names originated as codenames during world war ii and are still in military and aviation use throughout the world they have been adopted in the united states by the institute of electrical and electronics engineers and internationally by the international telecommunication union most countries have additional regulations to control which parts of each band are available for civilian or military use other users of the radio spectrum such as the broadcasting and electronic countermeasures industries have replaced the traditional military designations with their own systems radar frequency bands band name frequency range wavelength range notes hf 330 mhz 10100 m coastal radar systems overthehorizon radar oth radars high frequency vhf 30300mhz 110 m very long range ground penetrating very high frequency p 300mhz 1m p for previous applied retrospectively to early radar systems essentially hf vhf uhf 3001000mhz 031 m very long range eg ballistic missile early warning ground penetrating foliage penetrating ultra high frequency l 12 ghz 1530 cm long range air traffic control and surveillance l for long s 24ghz 7515cm moderate range surveillance terminal air traffic control longrange weather marine radar s for short c 48ghz 37575cm satellite transponders a compromise hence c between x and s bands weather long range tracking x 812ghz 25375cm missile guidance marine radar weather mediumresolution mapping and ground surveillance in the united states the narrow range 10525ghz 25mhz is used for airport radar short range tracking named x band because the frequency was a secret during ww2 ku 1218ghz 16725cm highresolution also used for satellite transponders frequency under k band hence u k 1824ghz 111167cm from german kurz meaning short limited use due to absorption by water vapour so ku and ka were used instead for surveillance kband is used for detecting clouds by meteorologists and by police for detecting speeding motorists kband radar guns operate at 24150 0100ghz ka 2440ghz 075111cm 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 34300 0100ghz mm 40300ghz 1075mm millimetre band subdivided as below the frequency ranges depend on waveguide size multiple letters are assigned to these bands by different groups these are from baytron a now defunct company that made test equipment v 4075ghz 4075mm very strongly absorbed by atmospheric oxygen which resonates at 60ghz w 75110ghz 2740mm used as a visual sensor for experimental autonomous vehicles highresolution meteorological observation and imaging radar modulators modulators act to provide the waveform of the rfpulse there are two different radar modulator designs high voltage switch for noncoherent keyed poweroscillators 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 eg the 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 coherent waveform this waveform can be generated by low powerlowvoltage input signals in this case the radar transmitter must be a poweramplifier eg a klystron or a solid state transmitter in this way the transmitted pulse is intrapulsemodulated and the radar receiver must use pulse compression techniques radar coolant coolant coherent microwave amplifiers operating above 1000 watts microwave output like 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 us navy aircraft in 1978 was attributed to a silicate ester fire coolanol is also expensive and toxic the us 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 1100 of the international telecommunication unions itu itu radio regulations rr as a radiodetermination system based on the comparison of reference signals with radio signals reflected or retransmitted from the position to be determined each radiodetermination system shall be classified by the radiocommunication service in which it operates permanently or temporarily typical radar utilizations are primary radar and secondary radar these might operate in the radiolocation service or the radiolocationsatellite service electronics portal nautical portal radar configurations and types main category radar radar imaging radar navigation wave radar acronyms and abbreviations in avionics definitions amplitudecomparison monopulse constant false alarm rate sensitivity time control application proximity fuze hardware cavity magnetron crossedfield amplifier gallium arsenide klystron omniview technology radar engineering details radar tower radio travellingwave tube similar detection and ranging methods lidar loran sonar historical radars list of radars chain home and chain home low hohentwiel radar h2s radar scr270 radar notes and references bibliography 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 privatelinecom anecdotal account of the carriage of the worlds first high power cavity magnetron from britain to the us during ww2 ekco radar ww2 shadow factory the 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 historyan 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 1946 radar the great detective early development and production by chrysler corp during wwii swords sen s technical history of the beginnings of radar iee history of technology series vol 6 london peter peregrinus 1986 general reg batt 1991 the radar army winning the war of the airwaves isbn9780709045083 e g bowen 19980101 radar days taylor francis isbn9780750305860 michael bragg 20020501 rdf1 the location of aircraft by radio methods 19351945 twayne publishers isbn9780953154401 louis brown 1999 a radar history of world war ii technical and military imperatives taylor francis isbn9780750306591 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 isbn9780684810218 burch david f radar for mariners mcgraw hill 2005 isbn9780071398671 ian goult 2011 secret location a witness to the birth of radar and its postwar influence history press isbn9780752457765 peter s hall march 1991 radar potomac books inc isbn9780080377117 derek howse naval radar trust february 1993 radar at sea the royal navy in world war 2 naval institute press isbn9781557507044 r v jones august 1998 most secret war wordsworth editions ltd isbn9781853266997 kaiser gerald chapter 10 in a friendly guide to wavelets birkhauser boston 1994 kouemou guy ed radar technology intech 2010 isbn9789533070292 radar technology free open access book intechopen colin latham anne stobbs january 1997 radar a wartime miracle sutton pub ltd isbn9780750916431 franois le chevalier 2002 principles of radar and sonar signal processing artech house publishers isbn9781580533386 david pritchard august 1989 the radar war germanys pioneering achievement 190445 harpercollins isbn9781852602468 merrill ivan skolnik 19801201 introduction to radar systems isbn9780070665729 merrill ivan skolnik 1990 radar handbook mcgrawhill professional isbn9780070579132 george w stimson 1998 introduction to airborne radar scitech publishing isbn9781891121012 younghusband eileen not an ordinary life how changing times brought historical events into my life cardiff centre for lifelong learning cardiff 2009 isbn9780956115690 pages 3667 contain the experiences of a waaf radar plotter in wwii younghusband eileen one womans war cardiff candy jar books 2011 isbn9780956682628 david zimmerman february 2001 britains shield radar and the defeat of the luftwaffe sutton pub ltd isbn9780750917995 technical reading skolnik m i radar handbook mcgrawhill 1970 nadav levanon and eli mozeson radar signals wiley com 2004 hao he jian li and petre stoica waveform design for active sensing systems a computational approach cambridge university press 2012 solomon w golomb and guang gong signal design for good correlation for wireless communication cryptography and radar cambridge university press 2005 m soltanalian signal design for active sensing and communications uppsala dissertations from the faculty of science and technology printed by elanders sverige ab 2014 fulvio gini antonio de maio and lee patton eds waveform design and diversity for advanced radar systems institution of engineering and technology 2012 e fishler a haimovich r blum d chizhik l cimini r valenzuela mimo radar an idea whose time has come ieee radar conference 2004 mark r bell information theory and radar waveform design ieee transactions on information theory 395 1993 15781597 robert calderbank s howard and bill moran waveform diversity in radar signal processing ieee signal processing magazine 261 2009 3241 look up radar in wiktionary the free dictionary wikimedia commons has media related to radar mit video course introduction to radar systems a set of 10 video lectures developed at lincoln laboratory to develop an understanding of radar systems and technologies authority 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