Editing Radar (section)

==Engineering==
{{main|Radar engineering details}}
[[File:Radar composantes.svg|thumb|right|Radar components]]
A radar's 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 (radio)|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 [[Radar display|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===
{{main|Antenna (radio)}}
[[File:Electronics Technician - Volume 7 - Figure 2-48.jpg|thumb|AS-3263/SPS-49(V) antenna (US Navy)]]
Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.

Early systems tended to use [[omnidirectional antenna|omnidirectional broadcast antennas]], with directional receiver antennas which were pointed in various directions. For instance, the first system to be deployed, Chain Home, used two straight antennas at [[right angle]]s for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by [[rotation|rotating]] the antenna so one display showed a maximum while the other showed a minimum.
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====
[[File:SPS-10 radar antenna on a Knox class frigate.jpg|thumb|right|Surveillance radar antenna]]
{{main|Parabolic antenna}}
More modern systems use a steerable [[parabola|parabolic]] "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or ''radar lock''.

Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:
Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The [[NEXRAD]] [[Pulse-Doppler]] weather radar uses a symmetric antenna to perform detailed volumetric scans of the atmosphere. Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.

====Types of scan====
* Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan, etc.
* Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching, etc.
* Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.
* [[Conical scanning]]: The radar beam is rotated in a small circle around the "boresight" axis, which is pointed at the target.

====Slotted waveguide====
[[File:Radar antennas on USS Theodore Roosevelt SPS-64.jpg|right|thumb|Slotted waveguide antenna]]
{{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 approach in preference to a parabolic antenna.

====Phased array====
[[File:PAVE PAWS Radar Clear AFS Alaska.jpg|thumb|right|[[Phased array]]: Not all radar antennas must rotate to scan the sky.]]
{{Main|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 centerline perpendicular to the array face. Signals travelling along that beam will be reinforced. Signals offset from that beam will be cancelled. The amount of reinforcement is [[antenna gain]]. The amount of cancellation is side-lobe suppression.<ref>{{cite web|url=http://mit.edu/6.933/www/Fall2000/mode-s/sidelobe.html|title=Side-Lobe Suppression|publisher=MIT|access-date=11 September 2012|archive-date=31 March 2012|archive-url=https://web.archive.org/web/20120331085410/http://mit.edu/6.933/www/Fall2000/mode-s/sidelobe.html|url-status=dead}}</ref>

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 ship-borne [[Aegis Combat System]] and the [[MIM-104 Patriot|Patriot Missile System]]. The massive redundancy associated with having a large number of array elements increases reliability at the expense of gradual performance degradation that occurs as individual phase elements fail. To a lesser extent, phased array radars have been used in [[weather]] [[surveillance]]. As of 2017, NOAA plans to implement a national network of multi-function phased array radars throughout the United States within 10 years, for meteorological studies and flight monitoring.<ref>{{cite web|url=http://www.nssl.noaa.gov/projects/mpar/|title=Multi-function Phased Array Radar (MPAR) Project|author=National Severe Storms Laboratory|publisher=NOAA|access-date=8 February 2017|author-link=National Severe Storms Laboratory|archive-date=2 February 2017|archive-url=https://web.archive.org/web/20170202073609/http://www.nssl.noaa.gov/projects/mpar/|url-status=live}}</ref>

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

As the price of electronics has fallen, phased array radars have become more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance and similar systems.

Phased array radars are valued for use in aircraft since they can track multiple targets. The first aircraft to use a phased array radar was the [[B-1B Lancer]]. The first fighter aircraft to use phased array radar was the [[Mikoyan MiG-31]]. The MiG-31M's SBI-16 [[Zaslon]] [[passive electronically scanned array]] radar was considered to be the world's most powerful fighter radar,{{citation needed|date=January 2023}} until the [[AN/APG-77]] [[active electronically scanned array]] was introduced on the [[Lockheed Martin F-22 Raptor]].

Phased-array [[interferometry]] or [[aperture synthesis]] techniques, using an array of separate dishes that are phased into a single effective aperture, are not typical for radar applications, although they are widely used in [[radio astronomy]]. Because of the [[thinned array curse]], such multiple aperture arrays, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques could increase spatial resolution, but the lower power means that this is generally not effective.

[[Synthetic aperture radar|Aperture synthesis]] by post-processing motion data from a single moving source, on the other hand, is widely used in space and [[airborne radar system]]s.

===Frequency bands===
{{Main|Radio spectrum#IEEE}}

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

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

The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world. They have been adopted in the United States by 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.

{| class="wikitable"
|+ '''Radar frequency bands'''
|- style="background:#ccc;"
!Band name!!Frequency range!!Wavelength range!!Notes
|-
|[[High frequency|HF]]||3–30 [[Hertz|MHz]]||10–100 [[metre|m]]||Coastal radar systems, [[over-the-horizon]] (OTH) radars; 'high frequency'
|-
|[[VHF]]||30–300&nbsp;MHz||1–10 m||Very long range, ground penetrating; 'very high frequency'. Early radar systems generally operated in VHF as suitable electronics had already been developed for broadcast radio. Today this band is heavily congested and no longer suitable for radar due to interference.
|-
|P||<&nbsp;300&nbsp;MHz||>&nbsp;1&nbsp;m||'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF. Often used for remote sensing because of good vegetation penetration.
|-
|[[UHF]]||300–1000&nbsp;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'. Efficiently produced and received at very high energy levels, and also reduces the effects of [[nuclear blackout]], making them useful in the missile detection role.
|-
|[[L band|L]]||1–2 [[Hertz|GHz]]||15–30 [[centimetre|cm]]||Long range air traffic control and [[surveillance]]; 'L' for 'long'. Widely used for long range [[early warning radar]]s as they combine good reception qualities with reasonable resolution.
|-
|[[S band|S]]||2–4&nbsp;GHz||7.5–15&nbsp;cm||Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'sentimetric', its code-name during WWII. Less efficient than L, but offering higher resolution, making them especially suitable for long-range [[ground controlled interception]] tasks.
|-
|[[C band (IEEE)|C]]||4–8&nbsp;GHz||3.75–7.5&nbsp;cm||Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
|-
|[[X band|X]]||8–12&nbsp;GHz||2.5–3.75&nbsp;cm||[[Missile]] guidance, [[marine radar]], weather, medium-resolution mapping and ground surveillance; in the United States the narrow range 10.525&nbsp;GHz ±25&nbsp;MHz is used for [[airport]] radar; short-range tracking. Named X band because the frequency was a secret during WW2. Diffraction off raindrops during heavy rain limits the range in the detection role and makes this suitable only for short-range roles or those that deliberately detect rain.
|-
||[[Ku band|K<sub>u</sub>]]||12–18&nbsp;GHz||1.67–2.5&nbsp;cm||High-resolution, also used for satellite transponders, frequency under K band (hence 'u')
|-
|[[K band (IEEE)|K]]||18–24&nbsp;GHz||1.11–1.67&nbsp;cm||From [[German language|German]] ''kurz'', meaning 'short'. Limited use due to absorption by [[water vapor]] at 22&nbsp;GHz, so K<sub>u</sub> and K<sub>a</sub> on either side used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band operates at 24.150 ± 0.100&nbsp;GHz.
|-
|[[Ka band|K<sub>a</sub>]]||24–40&nbsp;GHz||0.75–1.11&nbsp;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, and by police for detecting speeding motorists. Operates at 34.300 ± 0.100&nbsp;GHz.
|-
|mm||40–300&nbsp;GHz||1.0–7.5&nbsp;[[millimetre|mm]] ||[[Millimetre band]], subdivided as below. Oxygen in the air is an extremely effective attenuator around 60&nbsp;GHz, as are other molecules at other frequencies, leading to the so-called propagation window at 94&nbsp;GHz. Even in this window the attenuation is higher than that due to water at 22.2&nbsp;GHz. This makes these frequencies generally useful only for short-range highly specific radars, like [[power line]] avoidance systems for [[helicopter]]s or use in space where attenuation is not a problem. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
|-
|[[V band|V]]||40–75&nbsp;GHz||4.0–7.5&nbsp;mm || Very strongly absorbed by atmospheric oxygen, which resonates at 60&nbsp;GHz.
|-
|[[W band|W]]||75–110&nbsp;GHz||2.7–4.0&nbsp;mm||Used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
|}

===Modulators===
[[Modulation|Modulators]] act to provide the waveform of the RF-pulse. There are two different radar modulator designs:
* High voltage switch for non-coherent keyed power-oscillators.<ref>{{cite web|url=http://www.radartutorial.eu//08.transmitters/Radar%20Modulator.en.html|title=Radar Modulator|work=radartutorial.eu|access-date=29 November 2015|archive-date=7 December 2015|archive-url=https://web.archive.org/web/20151207181518/http://www.radartutorial.eu/08.transmitters/Radar%20Modulator.en.html|url-status=live}}</ref> These modulators consist of a high voltage pulse generator formed from a high voltage supply, a [[pulse forming network]], and a high voltage switch such as a [[thyratron]]. They generate short pulses of power to feed, e.g., the [[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,<ref>{{cite web|url=http://www.radartutorial.eu//08.transmitters/Fully%20Coherent%20Radar.en.html|title=Fully Coherent Radar|work=radartutorial.eu|access-date=29 November 2015|archive-date=8 December 2015|archive-url=https://web.archive.org/web/20151208042044/http://www.radartutorial.eu//08.transmitters/Fully%20Coherent%20Radar.en.html|url-status=live}}</ref> fed by a waveform generator and an exciter for a complex but [[Coherence (physics)|coherent]] waveform. This waveform can be generated by low power/low-voltage input signals. In this case the radar transmitter must be a power-amplifier, e.g., a [[klystron]] or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use [[pulse compression]] techniques.

===Coolant===
Coherent microwave amplifiers operating above 1,000 watts microwave output, like [[traveling wave tube|travelling wave tubes]] and [[klystron]]s, require [[Coolant#Liquids|liquid coolant]]. The electron beam must contain 5 to 10 times more power than the microwave output, which can produce enough heat to generate plasma. This plasma flows from the collector toward the cathode. The same magnetic focusing that guides the electron beam forces the plasma into the path of the electron beam but flowing in the opposite direction. This introduces FM modulation which degrades Doppler performance. To prevent this, liquid coolant with minimum pressure and flow rate is required, and deionized water is normally used in most high power surface radar systems that use Doppler processing.<ref>{{cite web | url = http://www.cientificosaficionados.com/libros/CERN/vacio9-CERN.pdf | author = J.L. de Segovia | title = Physics of Outgassing | publisher = Instituto de Física Aplicada, CETEF "L. Torres Quevedo", CSIC | location = Madrid, Spain | access-date = 12 August 2012 | archive-date = 5 January 2012 | archive-url = https://web.archive.org/web/20120105205714/http://cientificosaficionados.com/libros/CERN/vacio9-CERN.pdf | url-status = live }}</ref>

[[Coolanol]] ([[silicate]] [[ester]]) was used in several military radars in the 1970s. However, it is [[hygroscopic]], leading to [[hydrolysis]] and formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire.<ref>{{cite web | url = http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA250517&Location=U2&doc=GetTRDoc.pdf | author = Stropki, Michael A. | year = 1992 | title = Polyalphaolefins: A New Improved Cost Effective Aircraft Radar Coolant | publisher = Aeronautical Research Laboratory, Defense Science and Technology Organisation, Department of Defense | location = Melbourne, Australia | access-date = 18 March 2010 | archive-date = 5 June 2011 | archive-url = https://web.archive.org/web/20110605100528/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA250517&Location=U2&doc=GetTRDoc.pdf | url-status = dead }}</ref> Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named [[Pollution prevention in the US|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.