Radar
25676
226093150
2008-07-16T20:09:17Z
Pierre cb
846678
Charging to position on an image recently added to the proper place
{{otheruses}}
{{Cleanup|date=January 2008}}
[[Image:Radar antenna.jpg|thumbnail|This long-range radar [[antenna (electronics)|antenna]], known as ALTAIR, is used to detect and track space objects in conjunction with [[Anti-ballistic missile|ABM]] testing at the [[Ronald Reagan Ballistic Missile Defense Test Site|Ronald Reagan Test Site]] on the [[Kwajalein]] [[atoll]].<ref>[http://www.smdc.army.mil/kwaj/rangeinst/altair.html Ronald Reagan Test Site] on the [[Kwajalein]] [[atoll]]</ref>]]
'''Radar''' is a system that uses [[electromagnetic radiation|electromagnetic]] waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as [[aircraft]], ships, motor vehicles, weather formations, and terrain. The term ''RADAR'' was coined in 1941 as an [[Acronym and initialism|acronym]] for '''Ra'''dio '''D'''etection '''a'''nd '''R'''anging. The term has since entered the English language as a standard word, ''radar'', losing the capitalization. Radar was originally called RDF (Radio Direction Finder) in the United Kingdom.
A radar system has a transmitter that emits either [[radio waves]] or (more usually these days) [[microwaves]] that are reflected by the target and detected by a receiver, typically in the same location as the transmitter. Although the signal returned is usually very weak, the signal can be amplified. This enables radar to detect objects at ranges where other emissions, such as [[sound]] or [[visible light]], would be too weak to detect. Radar is used in many contexts, including [[meteorology|meteorological]] detection of [[Precipitation (meteorology)|precipitation]], [[wave radar|measuring ocean surface waves]], [[air traffic control]], [[police]] detection of [[speeding]] [[traffic]], and by the military.
==History==
{{main|History of radar}}
Several [[inventor]]s, scientists, and [[engineer]]s contributed to the [[History of radar|development of radar]]. The first to use radio waves to detect "the presence of distant metallic objects" was Christian Hülsmeyer, who in 1904 demonstrated the feasibility of detecting the presence of a ship in dense fog, but not its distance.<ref name=RadarWorld>[http://www.radarworld.org/huelsmeyer.html Christian Hülsmeyer by Radar World]</ref><ref>(German) [http://www.dgpt.org/DE/Biografien/2007_biografien/Christian_Huelsmeyer.php '''Christian Hülsmeyer Biografie''']</ref> He received Reichspatent Nr. 165546<ref>[http://upload.wikimedia.org/wikipedia/commons/1/11/DE165546.pdf ''Patent DE165546; Verfahren, um metallische Gegenstände mittels elektrischer Wellen einem Beobachter zu melden.'']</ref> for his pre-radar device in April 1904, and later patent 169154<ref>[http://upload.wikimedia.org/wikipedia/commons/e/e9/DE169154.pdf ''Verfahren zur Bestimmung der Entfernung von metallischen Gegenständen (Schiffen o. dgl.), deren Gegenwart durch das Verfahren nach Patent 16556 festgestellt wird.'']</ref> for a related amendment for ranging. He also received a patent <ref>{{patent|GB|13170|''Telemobiloscope''}}</ref> in England for his ''telemobiloscope'' on [[September 22]] [[1904]].<ref name=RadarWorld>[http://www.radarworld.org/huelsmeyer.html Christian Hülsmeyer by Radar World]</ref><ref>(German) [http://www.100-jahre-radar.de/index.html?/gdr_2_huelsmeyer.html 100. Jahre Radar ''Improvement in Hertzian-wave Projecting and Receiving Apparatus for Locating the Position of Distant Metal Objects'' in 100 years of radar a German publication]</ref>
[[Nikola Tesla]], in August 1917, first established principles regarding frequency and power level for the first primitive radar units.<ref>[[The Electrical Experimenter]], 1917</ref> He stated, "[...] ''by their'' [standing electromagnetic waves] ''use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; ''[with which]'' we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed''."
Before the [[Second World War]], developments by the Americans (Dr. Robert M. Page tested the first [[monopulse radar]] in 1934),<ref>{{cite web
| last=Goebel
| first=Greg
| title=The Wizard War: WW2 & The Origins Of Radar, Chapter 1: The British Invention of Radar
| url=http://www.vectorsite.net/ttwiz_01.html
| date=2007-01-01
| accessdate=2007-03-24
}}</ref>
the Germans, the French (French Patent n° 788795 in 1934)<ref>{{patent|FR|788795|''Nouveau système de repérage d'obstacles et ses applications''}}</ref><ref name="radarnet">{{fr icon}} [http://www.radar-france.fr/brevet%20radar1934.htm '''Copy of Patents for the invention of radar'''] on www.radar-france.fr</ref> and mainly the British who were the first to fully exploit it as a defence against aircraft attack (British Patent GB593017 by [[Robert Watson-Watt]] in 1935)<ref name="radarnet"/><ref>[http://www.patent.gov.uk/media/pressrelease/2001/1009.htm '''British man first to patent radar'''] official site of the ''Patent Office''</ref><ref>{{patent|GB|593017|Improvements in or relating to wireless systems}}</ref> led to the first real radars. Hungarian [[Zoltán Bay]] produced a working model by 1936 at the [[Tungsram]] laboratory in the same vein.
In 1934, Émile Girardeau, working with the first French radar systems, stated he was building radar systems "conceived according to the principles stated by Tesla". [http://www.teslasociety.com/time.jpg]
The war precipitated research to find better resolution, more portability and more features for the new defence technology. Post-war years have seen the use of radar in fields as diverse as [[air traffic control]], weather monitoring, [[astrometry]] and road speed control.
==Principles==
=== Reflection ===
[[Image:weather radar.jpg|thumb|Brightness can indicate reflectivity as in this 1960 weather radar image. The radar's frequency, pulse form, and antenna largely determine what it can observe.]]
[[Electromagnetic]] waves reflect (scatter) from any large change in the [[dielectric]] or [[diamagnetic]] constants. This means that a solid object in [[Earth's atmosphere|air]] or a [[vacuum]], or other significant change in atomic density between the object and what's surrounding it, will usually scatter radar (radio) waves. This is particularly true for [[electrical conduction|electrically conductive]] materials, such as metal and carbon fiber, making radar particularly well suited to the detection of [[aircraft]] and ships. [[Radar absorbing material]], containing [[Electrical resistance|resistive]] and sometimes [[magnetism|magnetic]] substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark color.
Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a [[mirror]]. If the wavelength is much longer than the size of the target, the target is [[polarization|polarized]] (positive and negative charges are separated), like a [[dipole antenna]]. This is described by [[Rayleigh scattering]], an effect that creates the Earth's blue sky and red [[sunset]]s. When the two length scales are comparable, there may be [[resonance]]s. Early radars used very long [[wavelength]]s that were larger than the targets and received a vague signal, whereas some modern systems use shorter [[wavelength]]s (a few [[centimeter]]s or shorter) 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 [[reflection (physics)|reflective surface]]s. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will always reflect waves entering its opening directly back at the source. These so-called [[corner reflector]]s are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are often found on boats in order to improve their detection in a rescue situation and to reduce collisions. For similar reasons, objects attempting to avoid detection will angle their surfaces in a way to eliminate inside corners and avoid 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 (radar countermeasure)|chaff]], are very reflective but do not direct the scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its [[radar cross section]].
===Radar equation===
The amount of power ''P<sub>r</sub>'' returning to the receiving antenna is given by the radar equation:
:<math>P_r = {{P_t G_t A_r \sigma F^4}\over{{(4\pi)}^2 R_t^2R_r^2}}</math>
where
*''P''<sub>t</sub> = transmitter power
*''G''<sub>t</sub> = [[gain]] of the transmitting antenna
*''A''<sub>r</sub> = effective aperture (area) of the receiving antenna
*''σ'' = [[radar cross section]], or scattering coefficient, of the target
*''F'' = pattern propagation factor
*''R''<sub>t</sub> = distance from the transmitter to the target
*''R''<sub>r</sub> = distance from the target to the receiver.
In the common case where the transmitter and the receiver are at the same location, ''R''<sub>t</sub> = ''R''<sub>r</sub> and the term ''R''<sub>t</sub>² ''R''<sub>r</sub>² can be replaced by ''R''<sup>4</sup>, where ''R'' is the range.
This yields:
:<math>P_r = {{P_t G_t A_r \sigma F^4}\over{{(4\pi)}^2 R^4}}.</math>
This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very, very small.
The equation above with ''F'' = 1 is a simplification for [[vacuum]] without interference. The propagation factor accounts for the effects of [[multipath]] and shadowing and depends on the details of the environment. In a real-world situation, [[pathloss]] effects should also be considered.
Other mathematical developments in radar signal processing include [[time-frequency analysis]] ([[Weyl Heisenberg]] or [[wavelet]]), as well as the [[chirplet transform]] which makes use of the fact that radar returns from moving targets typically "chirp" (change their frequency as a function of time, as does the sound of a bird or bat).
===Polarization===
In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the [[polarization]] of the wave. 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.
===Interference===
Radar systems must overcome several different sources of unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its [[signal-to-noise ratio]] (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal.
In less technical terms, signal-to-noise ratio (SNR), compares the level of a desired signal (such as targets) to the level of background noise. The higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.
====Noise====
[[Signal noise]] is an internal source of random variations in the signal, which is inherently generated to some degree by all electronic components. 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 (similar to trying to hear a whisper while standing near a busy road). Therefore, the most important noise sources appear in the receiver and much effort is made to minimize these factors. [[Noise figure]] is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.
Noise is also generated by external sources, most importantly the natural thermal radiation of the background scene surrounding the target of interest. In modern radar systems, due to the high performance of their receivers, the internal noise is typically about equal to or lower than the external scene 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]].
There will be also [[Flicker noise]] due to electrons transit, but depending on 1/f, will be much lower than thermal noise when the frequency is high.
Hence, in pulse radar, the system will be always [[heterodyne]]. See [[intermediate frequency]].
====Clutter====
Clutter refers to actual radio frequency (RF) echoes returned from targets which are by definition uninteresting to the radar operators in general. Such targets mostly include natural objects such as ground, sea, [[Precipitation (meteorology)|precipitation]] (such as rain, snow or hail), [[sand storm]]s, animals (especially birds), atmospheric [[turbulence]], and other atmospheric effects, such as [[ionosphere]] reflections and [[meteor]] trails. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as [[Chaff (radar countermeasure)|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.
While some clutter sources may be undesirable for some radar applications (such as storm clouds for air-defence radars), they may be desirable for others ([[meteorological]] radars in this example). Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar.
There are several methods of detecting and neutralizing clutter. Many of these methods rely on the fact that clutter tends to appear static between radar scans. Therefore, when comparing subsequent scans echoes, desirable targets will appear to move and all stationary echoes can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with [[circular polarization]] (note that meteorological radars wish for the opposite effect, therefore using [[linear polarization]] the better to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.
[[CFAR]] (Constant False-Alarm Rate, a form of [[Automatic Gain Control]], or AGC) is a method relying on the fact that clutter returns far outnumber 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.
[[Image:Multichemin.svg|thumb|Radar [[multipath]] [[Light echo|echoes]] from an actual target cause [[ghost]]s to appear.]]
Clutter may also originate from [[multipath]] echoes from valid targets due to ground reflection, [[atmospheric ducting]] or [[ionospheric reflection]]/[[refraction]]. This specific clutter type is especially bothersome, since it appears to move and behave like other normal (point) targets of interest, thereby creating a ghost. In a typical scenario, an aircraft echo is multipath-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 - worse - eliminating it on the basis of [[jitter]] or a physical impossibility. 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. In newer Air Traffic Control (ATC) 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 due to calculated height, distance, and radar timing.
====Jamming====
[[Radar jamming]] refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional, as with an [[electronic warfare]] (EW) 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 (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the [[line of sight]] from the jammer to the radar (''Mainlobe Jamming''). Jammers have an added effect of affecting radars along other lines of sight, due to the radar receiver's [[sidelobe]]s (''Sidelobe Jamming'').
Mainlobe jamming can generally only be reduced by narrowing the mainlobe [[solid angle]], and can never fully be eliminated when directly facing a jammer which uses the same frequency and polarization as the radar. Sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using an [[omnidirectional antenna]] to detect and disregard non-mainlobe signals. Other anti-jamming techniques are [[frequency hopping]] and [[polarization]]. See [[Electronic counter-counter-measures]] for details.
Interference has recently become a problem for [[C-band]] (5.66 [[gigahertz|GHz]]) meteorological radars with the proliferation of 5.4 GHz band [[WiFi]] equipment.<ref>[http://ronja.twibright.com/interference.php Example of WiFi equipment jamming meteorological radars.]</ref>
==Radar signal processing==
===Distance measurement===
====Transit time====
[[Image:Radarops.gif|thumb|right|Pulse radar]][[Image:Sonar Principle EN.svg|thumb|right|]]
One way to measure the distance to an object is to transmit a short pulse of radio signal (electromagnetic radiation), and measure the time it takes for the reflection to return. The distance is one-half the product of round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal. Since radio waves travel at the speed of light (186,000 miles per second or 300,000,000 meters per second), accurate distance measurement requires high-performance electronics.
In most cases, the receiver does not detect the return while the signal is being transmitted. Through the use of a device called a ''duplexer'', the radar switches between transmitting and receiving at a predetermined rate. The minimum range is calculated by measuring the length of the pulse multiplied by the speed of light, divided by two. In order to detect closer targets one must use a shorter pulse length.
A similar effect imposes a maximum range as well. If the return from the target comes in when the next pulse is being sent out, once again the receiver cannot tell the difference. In order to maximize range, one wants to use longer times between pulses, or commonly referred to as a pulse repetition time (PRT), or its inverse, pulse repetition frequency (PRF).
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 again. 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. This pattern of pulses and pauses is known as the [[Radar signal characteristics#The radar signal in the time domain|pulse repetition frequency]] (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars now can change their PRF thereby changing their range. The newest radars actually fire 2 pulses during one cell, one for short range 10 km / 6 miles and a separate signal for longer ranges 100 km /60 miles.
The distance [[Optical resolution|resolution]] and the characteristics of the received signal as compared to noise depends heavily on the shape of the pulse. The pulse is often [[modulation|modulated]] to achieve better performance thanks to a technique known as [[pulse compression]].
Distance may also be measured as a function of time. The Radar Mile is the amount of time it takes for a radar pulse to travel one Nautical Mile, reflect off a target, and return to the radar antenna. Since a Nautical Mile is defined as ''exactly'' 1,852 meters, then dividing this distance by the speed of light (''exactly'' 299,792,458 meters per second), and then multiplying the result by 2 (round trip = twice the distance), yields a result of approximately 12.36 microseconds in duration.
====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 changing 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 altimeter]]s. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a [[sine wave]] or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared using a simple ''beat frequency'' modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.
Since the signal frequency is changing, by the time the signal returns to the aircraft the broadcast has shifted to some other frequency. The amount of that shift is greater over longer times, so greater frequency differences mean a longer distance, the exact amount being the "ramp speed" selected by the electronics. The amount of shift is therefore directly related to the distance traveled, and can be displayed on an instrument. This signal processing is similar to that used in speed detecting [[Doppler effect|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, comparable to that used by UHF television. This was important in the early development of this type when high frequency signal generation was difficult or expensive.
=== 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 [[crayon|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.
However, if the transmitter's output is coherent (phase synchronized), there is another effect that can be used to make almost instant speed measurements (no memory is required), known as the [[Doppler effect]]. Most modern radar systems use this principle in the [[pulse-doppler radar]] system. Return signals from targets are shifted away from this base frequency via the Doppler effect enabling the calculation of the speed of the object relative to the 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's [[azimuth]] over time. Additional information of the nature of the Doppler returns may be found in the [[Radar signal characteristics#The radar signal in the frequency domain|radar signal characteristics]] article.
It is also possible to make a radar without any pulsing, known as a [[continuous-wave radar]] (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal for determining the radial component of a target's velocity, but it cannot determine the target's range. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where range is not important.
=== Reduction of interference effects ===
[[Signal processing]] is employed in radar systems to reduce the [[#Interference|radar interference effects]]. Signal processing techniques include [[moving target indication]] (MTI), [[Pulse-doppler radar|pulse doppler]], moving target detection (MTD) processors, correlation with [[secondary surveillance radar]] (SSR) targets, [[space-time adaptive processing]] (STAP), and [[track-before-detect]] (TBD). [[Constant false alarm rate]] (CFAR) and [[digital terrain model]] (DTM) processing are also used in clutter environments.
=== Plot And Track Extraction ===
Radar video returns on 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. A sequence of plots can then be monitored and a 'track' formed, thus easing the identification of a genuine aircraft target through unwanted and non relevant radar returns.
==Radar engineering==
[[Image:Radar composantes.png|thumb|right|Radar components]]
A radar has different components:
* 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 (radio)|receiver]]. Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a [[matched filter]].
* An electronic section that controls all those devices and the antenna to perform the radar scan ordered by a [[software]].
* A link to end users.
===Antenna design===
Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.
Early systems tended to use [[omnidirectional antenna|omni-directional broadcast antennas]], with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, [[Chain Home]], used two straight antennas at [[right angle]]s for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by [[rotation|rotating]] the antenna so one display showed a maximum while the other shows a minimum.
One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is [[inverse-square law|a small part]] of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.
====Parabolic reflector====
More modern systems use a steerable [[parabola|parabolic]] "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or '''radar lock'''
Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:
*Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The [[NEXRAD]] [[Pulse-Doppler]] [[weather radar]] uses a symmetric antenna to perform detailed volumetric scans of the atmosphere.
*Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.
====Types of scan====
# Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan etc
# Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching etc.
# Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.
[[Image:PAVE PAWS Radar Clear AFS Alaska.jpg|thumb|[[Phased array]]: Not all radar antennas must rotate to scan the sky.]]
====Slotted waveguide====
{{main|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 in preference to the parabolic antenna.
====Phased array====
{{main|Phased array}}
Another method of steering is used in a phased array radar. This uses an [[array]] of similar aerials suitably spaced, the phase of the signal to each individual aerial being controlled so that the signal is reinforced in the desired direction and cancels in other directions. If the individual aerials are in one plane and the signal is fed to each aerial in phase with all others then the signal will reinforce in a direction perpendicular to that plane. By altering the relative phase of the signal fed to each aerial the direction of the beam can be moved because the direction of constructive interference will move. Because phased array radars require no physical [[Motion (physics)|movement]] the beam can scan at thousands of degrees per second, fast enough to irradiate and track many individual targets, and still run a wide-ranging search periodically. By simply turning some of the antennas on or off, the beam can be spread for searching, narrowed for tracking, or even split into two or more virtual radars. However, the beam cannot be effectively steered at small angles to the plane of the array, so for full coverage multiple arrays are required, typically disposed on the faces of a triangular pyramid (see picture).
Phased array radars have been in use since the earliest years of radar use in [[World War II]], but limitations of the electronics led to fairly poor accuracy. Phased array radars were originally used for [[missile]] [[defense (military)|defense]]. They are the heart of the ship-borne [[Aegis combat system]], and the [[MIM-104 Patriot|Patriot Missile System]], and are increasingly used in other areas because the lack of moving parts makes them more reliable, and sometimes permits a much larger effective antenna, useful in fighter aircraft applications that offer only confined space for mechanical scanning.
As the price of electronics has fallen, phased array radars have become more and more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is far offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance, weather radars and similar systems.
Phased array radars are also valued for use in aircraft, since they can track multiple targets. The first aircraft to use a phased array radar is the B-1B Lancer. The first aircraft fighter to use phased array radar was the [[Mikoyan MiG-31]]. The MiG-31M's SBI-16 [[Mikoyan-Gurevich MiG-31#Electronics Suite|Zaslon]] phased array radar is considered to be the world's most powerful fighter radar [http://www.globalsecurity.org/military/world/russia/mig-31.htm].
Phased-array [[interferometry]] or, [[aperture synthesis]] techniques, using an array of separate dishes that are phased into a single effective aperture, are not typically used for radar applications, although they are widely used in [[radio astronomy]]. Because of the [[Thinned array curse]], such arrays of multiple apertures, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques used could increase the spatial resolution, but the lower power means that this is generally not effective. Aperture synthesis by post-processing of motion data from a single moving source, on the other hand, is widely used in space and airborne radar systems (see [[Synthetic aperture radar]]).
===Frequency bands ===
The traditional band names originated as code-names during [[World War II]] and are still in military and aviation use throughout the world in the 21st century. They have been adopted in the United States by the [[IEEE]], and internationally by the [[International Telecommunication Union|ITU]]. 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 ([[Electronic counter-measures|ECM]]) industries, have replaced the traditional military designations with their own systems.
{| class="wikitable"
|+ '''Radar frequency bands'''
|- bgcolor="#CCCCCC"
!Band Name!!Frequency Range!!Wavelength Range!!Notes
|-
|[[High frequency|HF]]||3–30 [[Megahertz|MHz]]||10–100 [[metre|m]]||coastal radar systems, [[over-the-horizon radar]] (OTH) radars; 'high frequency'
|-
|P||< 300 MHz||1 m+||'P' for 'previous', applied retrospectively to early radar systems
|-
|[[VHF]]||50–330 MHz||0.9–6 m||very long range, ground penetrating; 'very high frequency'
|-
|[[UHF]]||300–1000 MHz||0.3–1 m||very long range (e.g. [[Ballistic Missile Early Warning System|ballistic missile early warning]]), ground penetrating, foliage penetrating; 'ultra high frequency'
|-
|[[L band|L]]||1–2 [[Gigahertz|GHz]]||15–30 [[centimetre|cm]]||long range [[air traffic control]] and [[surveillance]]; 'L' for 'long'
|-
|[[S band|S]]||2–4 GHz||7.5–15 cm||terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
|-
|[[C band|C]]||4–8 GHz||3.75–7.5 cm||Satellite transponders; a compromise (hence 'C') between X and S bands; weather
|-
|[[X band|X]]||8–12 GHz||2.5–3.75 cm||[[missile]] guidance, [[marine radar]], weather, medium-resolution mapping and ground surveillance; in the [[USA]] the narrow range 10.525 GHz ±25 MHz is used for [[airport]] radar. Named X band because the frequency was a secret during WW2.
|-
|[[Ku band|K<sub>u</sub>]]||12–18 GHz||1.67–2.5 cm||high-resolution mapping, satellite altimetry; frequency just under K band (hence 'u')
|-
|[[K band|K]]||18–27 GHz||1.11–1.67 cm||from [[German language|German]] ''kurz'', meaning 'short'; limited use due to absorption by [[water vapor|water vapour]], so K<sub>u</sub> and K<sub>a</sub> were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
|-
|[[Ka band|K<sub>a</sub>]]||27–40 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||7.5 mm – 1 mm ||[[millimetre band]], subdivided as below. The frequency ranges depend on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
|-
|Q||40–60 GHz||7.5 mm – 5 mm || Used for Military communication.
|-
|[[V band|V]]||50–75 GHz||6.0–4 mm || Very strongly absorbed by the atmosphere.
|-
|E||60–90 GHz||6.0–3.33 mm
|-
|[[W band|W]]||75–110 GHz||2.7 – 4.0 mm||used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
|}
=== Radar modulators ===
[[Modulation|Modulators]] act to provide the short pulses of power to the [[Cavity magnetron|magnetron]], a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as [[Pulsed power]]. In this way, the transmitted pulse of RF radiation is kept to a defined, and usually, very short duration. Modulators consist of a high voltage pulse generator formed from an HV supply, a [[pulse forming network]], and a high voltage switch such as a [[thyratron]].
A [[klystron tube]] may also be used as a modulator because it is an amplifier, so it can be modulated by its low power input signal.
=== Radar coolant ===
''[[Coolanol]]'' and [[Polyolefin|PAO]] (poly-alpha olefin) are the two main coolants used to cool airborne radar equipment today.{{Fact|date=February 2007}}
The [[U.S. Navy]] has instituted a program named [[Pollution Prevention]] (P2) to reduce or eliminate the volume and toxicity of waste, air emissions, and effluent discharges. Because of this Coolanol is used less often today.
PAO is a synthetic lubricant composition is a blend of a polyol [[ester]] admixed with effective amounts of an [[antioxidant]], yellow metal pacifier and rust inhibitors. The polyol ester blend includes a major proportion of poly (neopentyl polyol) ester blend formed by reacting poly([[pentaerythritol]]) partial esters with at least one C7 to C12 [[carboxylic acid]] mixed with an ester formed by reacting a polyol having at least two hydroxyl groups and at least one C8-C10 carboxylic acid. Preferably, the acids are linear and avoid those which can cause odours during use. Effective additives include secondary arylamine antioxidants, [[triazole]] derivative yellow metal pacifier and an [[amino acid]] derivative and substituted primary and secondary [[amine]] and/or diamine rust inhibitor.
A synthetic coolant/lubricant composition, comprising an ester mixture of 50 to 80 weight percent of poly (neopentyl polyol) ester formed by reacting a poly (neopentyl polyol) partial ester and at least one linear monocarboxylic acid having from 6 to 12 carbon atoms, and 20 to 50 weight percent of a polyol ester formed by reacting a polyol having 5 to 8 carbon atoms and at least two hydroxyl groups with at least one linear monocarboxylic acid having from 7 to 12 carbon atoms, the weight percents based on the total weight of the composition.
==Radar functions and roles==
===Detection and search radars===
[[Image:e3 sentry zh101 kemble arp.jpg|thumb|right|[[RAF]] Boeing [[E-3 Sentry]] AEW1 ([[AWACS]]) with rotating radar dome. The dome is 30 feet across (9 metres) The E-3 is accompanied by two [[Panavia Tornado ADV|Panavia Tornado F3]].]]
*[[Early warning radar|Early Warning (EW)]] Radar Systems
**Early Warning Radar
**[[Ground-controlled interception|Ground Control Intercept (GCI)]] Radar
**[[Airborne Early Warning|Airborne Early Warning (AEW)]]
**[[Over-the-horizon radar|Over-the-Horizon (OTH)]] Radar
*Target Acquisition (TA) Radar Systems
**[[Surface-to-air missile#Radar-guided SAMs|Surface-to-Air Missile (SAM)]] Systems
**[[Anti-aircraft warfare|Anti-Aircraft Artillery (AAA)]] Systems
*Surface Search (SS) Radar Systems
**Surface Search Radar
**Coastal Surveillance Radar
**Harbour Surveillance Radar
**[[Anti-submarine warfare|Antisubmarine Warfare (ASW)]] Radar
*[[Height finder#Radar|Height Finder (HF) Radar Systems]]
*Gap Filler Radar Systems
===Threat radars===
[[Image:Radar screen.JPG|thumb|Surface search radar display commonly found on ships]]
*Target Tracking (TT) Systems
**[[Anti-aircraft warfare|AAA]] Systems
**[[Surface-to-air missile#Radar-guided SAMs|SAM]] Systems
**[[Precision Approach Radar|Precision Approach Radar (PAR)]] Systems
*Multi-Function Systems
**[[Fire-control system|Fire Control (FC) Systems]]
***Acquisition Mode
***Semiautomatic Tracking Mode
***Manual Tracking Mode
**Airborne Intercept (AI) Radars
***Search Mode
***TA Mode
***TT Mode
***Target Illumination (TI) Mode
***[[Missile guidance|Missile Guidance (MG)]] Mode
===Missile guidance systems===
*[[Air-to-air missile|Air-to-Air Missile (AAM)]]
*[[Air-to-surface missile|Air-to-Surface Missile (ASM)]]
*[[Surface-to-air missile#Radar-guided SAMs|SAM]] Systems
*[[Surface-to-surface missile|Surface-to-Surface Missiles (SSM)]] Systems
===Battlefield and reconnaissance radar===
[[Image:APP-6 Radar.svg|thumb|Military map marking symbol ''Radar'' as of NATO standard [[APP-6a]]]]
*Battlefield Surveillance Systems
**Battlefield Surveillance Radars
*Countermortar/Counterbattery Systems
**Shell Tracking Radars
*Air Mapping Systems
**Side Looking Airborne Radar (SLAR)
**[[Synthetic aperture radar|Synthetic Aperture Radar (SAR)]]
**[[Perimeter Surveillance Radar|Perimeter Surveillance Radar (PSR)]]
**[[Red Color | Red Dawn Radar System]]
===Air Traffic Control and navigation===
[[Image:Heathrow Airport radar tower P1180333.jpg|thumb|Air traffic control radar at [[London Heathrow Airport]]]]
*[[Air traffic control|Air Traffic Control]] Systems
**[[Air traffic control#Radar coverage|Air Traffic Control (ATC) Radars]]
**[[Secondary surveillance radar|Secondary Surveillance Radar (SSR)]] (Airport Surveillance Radar)
**Ground Control Approach (GCA) Radars
**[[Precision Approach Radar|Precision Approach Radar (PAR)]] Systems
*[[Distance Measuring Equipment|Distance Measuring Equipment (DME)]]
*[[Radio beacon|Radio Beacons]]
*[[Radar altimeter|Radar Altimeter (RA)]] Systems
*[[Terrain-following radar|Terrain-Following Radar (TFR)]] Systems
===Space and range instrumentation radar systems===
*Space (SP) Tracking Systems
*Range Instrumentation (RI) Systems
*Video Relay/Downlink Systems
*[[Space-Based Radar]]
===Weather-sensing Radar systems===
*[[Weather radar]]
*[[Wind profiler]]s
<gallery widths="250" heights="100" align="left">
Image:Sturmfront auf Doppler-Radar-Schirm.jpg|Storm front [[Reflectivity|reflectivities]] on a Weather radar screen (NOAA)
Image:Wind_Profiler.jpg|Wind profiling radar
</gallery>
{{clr}}
===Radars for biological research===
* [http://www.sei.org/murrelet.html Bird radar]
* [http://www.pems.adfa.edu.au/~adrake/trews/ww_re_bi.htm Insect radar]
**Surveillance radar (mostly X and S band, i.e. primary [[Air traffic control#Radar Coverage|ATC Radars]])
**Tracking radar (mostly X band, i.e. [[Fire-control system|Fire Control Systems]])
===Through The Wall Radar Systems===
Radar systems which operate using [[Ultra Wideband]] technology can sense a human behind walls. This is possible since the reflective characteristics of humans are generally greater than those of the typical materials used in construction. However, since humans reflect far less radar energy than metal does, these systems require sophisticated technology to isolate human targets and moreover to process any sort of detailed image.
==See also==
{{portalpar|Electronics|Nuvola_apps_ksim.png}}
{{nautical portal}}
* [[Crossed-field amplifier]]
* Definitions
** [[Amplitude monopulse]]
** [[Bistatic Doppler]]
** [[Bistatic range]]
** [[Constant false alarm rate]]
* [[Gallium arsenide]]
* [[Klystron tube]]
* [[List of radars]]
* [[Cavity magnetron]]
* [[Over-the-horizon radar]]
* [[Radio]]
* Radar History
** [[Secrets of Radar Museum]]
* Similar detection and ranging methods
** [[LIDAR]]
** [[LORAN]]
** [[Sonar]]
* [[Traveling wave tube|Traveling wave tube (TWT)]]
* [[Types and uses of radar]]
** [[3D radar]]
** [[Active Electronically Scanned Array]] (AESA)
** [[Automatic Radar Plotting Aid]]
** [[Bistatic radar]]
** [[Continuous-wave radar]]
** [[Doppler radar]]
** [[Fm-cw radar]]
** [[Imaging radar]]
** [[Incoherent scatter]]
** [[Low probability of intercept]]
** [[Millimetre cloud radar]]
** [[Monopulse radar]]
** [[Passive radar]]
** [[Planar array radar]]
** [[Precision Approach Radar]]
** [[Pulse-doppler]]
** [[Radar gun]], for traffic policing and as used in some sports
** [[SCR-270 radar]]
** [[X-band radar]]
** [[H2S radar]]
** [[Chain Home]]
** [[Man portable radar]]
==Notes==
{{reflist}}
==References==
* Barrett, Dick, "''[http://www.radarpages.co.uk/index.htm All you ever wanted to know about British air defence radar]''". The Radar Pages. (History and details of various British radar systems)
* Buderi, "''[http://www.privateline.com/TelephoneHistory3/radarhistorybuderi.html Telephone History: Radar History]''". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.)
* Ekco Radar [http://www.ekco-radar.co.uk/ WW2 Shadow Factory] The secret development of British Radar.
* ES310 "''[http://www.fas.org/man/dod-101/navy/docs/es310/syllabus.htm Introduction to Naval Weapons Engineering.''". (Radar fundamentals section)]
* Hollmann, Martin, "''[http://www.radarworld.org/index.html Radar Family Tree]''". [http://www.radarworld.org/ Radar World].
* Penley, Bill, and Jonathan Penley, "''[http://www.penleyradararchives.org.uk/history/introduction.htm Early Radar History] - an Introduction''". 2002.
==Further reading==
* Buderi, Robert, ''The invention that changed the world: the story of radar from war to peace'', Simon & Schuster, 1996. ISBN 0-349-11068-9 ISBN 0-316-90715-4
* Hall, P.S., T.K. Garland-Collins, R.S. Picton and R.G. Lee, ''Radar'', Brassey's (UK) Ltd., 1991, Land Warfare Series: Vol 9, ISBN 0-08-037711-4.
* Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994.
* Jones, R.V., ''Most Secret War'', ISBN 1-85326-699-X. R.V. Jones' account of his part in British Scientific Intelligence between 1939 and 1945, working to anticipate the German's radar, radio navigation and V1/V2 developments.
* Le Chevalier, François, ''Principles of Radar and Sonar Signal Processing'', Artech House, Boston, London, 2002. ISBN 1-58053-338-8.
* [[Merrill Skolnik|Skolnik, Merrill I.]], ''Introduction to Radar Systems'', McGraw-Hill (1st ed., 1962; 2nd ed., 1980; 3rd ed., 2001), ISBN 0-07-066572-9. The de-facto radar introduction bible.
* Skolnik, Merrill I., ''Radar Handbook''. ISBN 0-07-057913-X widely used in the [[United States|US]] since the 1970s. New 3rd Edition, February 2008, ISBN 0-07-148547-3; 978-0-07-148547-0
* Stimson, George W., ''Introduction to Airborne Radar'', SciTech Publishing (2nd edition, 1998), ISBN 1-891121-01-4. Written for the non-specialist. The first half of the book on radar fundamentals is also applicable to ground- and sea-based radar.
* Bragg, Michael., ''RDF1 The Location of Aircraft by Radio Methods 1935–1945'', Hawkhead Publishing, Paisley 1988 ISBN 0-9531544-0-8 The history of ground radar in the UK during World War II
* Latham, Colin & Stobbs, Anne., ''Radar A Wartime Miracle'', Sutton Publishing Ltd, Stroud 1996 ISBN 0-7509-1643-5 A history of radar in the UK during World War II told by the men and women who worked on it.
* Pritchard, David., ''The Radar War Germany's Pioneering Achievement 1904–1945'' Patrick Stephens Ltd, Wellingborough 1989., ISBN 1-85260-246-5
* Zimmerman, David., ''Britain's Shield Radar and the Defeat of the Luftwaffe'', Sutton Publishing Ltd, Stroud, 2001., ISBN 0-7509-1799-7
* Brown, Louis., ''A Radar History of World War II'', Institute of Physics Publishing, Bristol, 1999., ISBN 0-7503-0659-9
* Bowen, E.G., ''Radar Days'', Institute of Physics Publishing, Bristol, 1987., ISBN 0-7503-0586-X
* Howse, Derek, ''Radar At Sea The Royal Navy in World War 2'', Naval Institute Press, Annapolis, Maryland, USA, 1993, ISBN 1-55750-704-X
==External links==
{{commons|Radar}}
* [http://www.xs4all.nl/~aobauer/Huelspart1def.pdf Christian Hülsmeyer and about the early days of radar inventions]
* [http://www.warmuseum.ca/cwm/radar/index_e.html Radar: The Canadian History of Radar - Canadian War Museum]
* [http://www.radartutorial.eu/index.en.html Radar technology principles]
* [http://www.radar-france.net The first operational radar in France 1934]
* [http://mysite.wanadoo-members.co.uk/historicalradar/index.html Historic Radar Archive]
* [http://www.rfzone.org/free-rf-ebooks/ Radar and RF related eBooks]
* [http://math.la.asu.edu/~kuang/LM/030902-Radar_History10.pdf History of Radar]
* [http://www.metamaterials.net Radar Invisibility with Metamaterials]
* [http://crr.sesm.it Radar Research Center-Italy]
* [http://www.purbeckradar.org.uk/ Early radar development in the UK]
* [http://ourworld.compuserve.com/homepages/edperry/ewtutor1.htm Principles of radar target acquisition and weapon guidance systems]
* [http://www.radartechnology.eu/ Cloaking and radar invisibility]
* [http://www.secretsofradar.com The Secrets of Radar Museum]
* [http://www.rades.hill.af.mil/ 84th Radar Evaluation Squadron]
* [http://www.skybrary.aero/index.php/Radar Radar on Skybrary]
* [http://www.ekco-radar.co.uk/ASV19/asv.php EKCO WW II ASV radar units]
[[Category:Radar]]
[[Category:Microwave technology]]
[[Category:Measuring instruments]]
[[Category:Navigational equipment]]
[[Category:Air traffic control]]
[[Category:Acronyms]]
{{Link FA|ar}}
[[ar:رادار]]
[[bn:রাডার]]
[[bg:Радиолокатор]]
[[ca:Radar]]
[[cs:Radar]]
[[da:Radar]]
[[de:Radar]]
[[el:Ραντάρ]]
[[es:Radar]]
[[eo:Radaro]]
[[fa:رادار]]
[[fr:Radar]]
[[gl:Radar]]
[[ko:레이더]]
[[hr:Radar]]
[[id:Radar]]
[[it:Radar]]
[[he:מכ"ם]]
[[lv:Jūras radars]]
[[lt:Radaras]]
[[hu:Rádiólokátor]]
[[ml:റഡാര്]]
[[ms:Radar]]
[[nl:Radar]]
[[ja:レーダー]]
[[no:Radar]]
[[nn:Radar]]
[[pl:Radar]]
[[pt:Radar]]
[[ro:RADAR]]
[[ru:Радиолокационная станция]]
[[simple:Radar]]
[[sk:Radar]]
[[sl:Radar]]
[[sr:Радар]]
[[fi:Tutka]]
[[sv:Radar]]
[[vi:Radar]]
[[tr:Radar]]
[[uk:Радар]]
[[ur:Radar]]
[[zh:雷达]]