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'''Radar''' is a device that transmits radio waves to detect and/or locate the position and velocity of a distant object such as an aeroplane. The etymology of the name is an acronym of '''radio detection and ranging'''. Radar works by picking up a reflection of the radio waves which it can then use to calculate the position, direction, and speed of the object. In systematic terms, radar is a simple concept which consists of a transmitter and receiver, but it has proved invaluable in wartime, especially during the [[Battle of Britain]] in 1940.
A contraction of '''radio direction and ranging''', '''radar''' is an extremely useful family of techniques not limited to its original role of detecting the location and course of aircraft. It is used for both navigation and collision avoidance by ships and aircraft. It can detect [[artillery|artillery shells]], [[unguided rocket|rockets]], and [[mortar|mortar shells]], and track back to their point of origin so the artillery can be counterattacked. Sometimes annoyingly, it can tell when a car is exceeding the speed limit.


Purpose-built radars characterize weather and the state of a water surface. Certain types can penetrate the ground and detect buried objects. Spinoffs from radar technology led to medical devices for controlled heating of tissue, and then the [[microwave oven]].
Radars have been adapted not only to be able to give range and bearing to a target, but to characterize the size and number of objects in the target, and, with more advanced techniques, actually create pictures of objects. These techniques can work through clouds and at night, where photographic imaging would be useless.
==Article scope and minimal necessary concepts from electronics==
This article is focused on radar applications, with just enough engineering principles to understand the major differences in types of radars used in different applications. Radar signals and processing can become extremely complex, considering not only the transmitted signal type, but transmitted power and the characteristics of the transmitting antenna, of the receiving antenna and receiver, the medium (e.g., air, rain, etc.) through which the signal passes, the nature of the target's surface, and many other factors. This article avoids going into the mathematics of radar engineering, but, to go to the next level, see [[radar equations]].
A few electronic parameters, which are essential inputs to the radar equation, do need to be discussed:
:*'''Carrier frequency'''
:*'''Pulse width'''
:*'''Pulse repetition rate (PRR)''' (also written as '''pulse repetition frequency (PRF)''')
:*'''Transmitter power''' and '''transmitting antenna gain'''
==The basic model==
At the most basic level, a timed, shaped signal signal starts out in the modulator of the transmit side of a radar. At this point, assume, for simplicity, the signal is a single, easily recognized pulse, or [[amplitude modulation|series of amplitude-modulated waves]] rather than a continuous wave. Note that a pulsed radar switches off the carrier frequency when not sending a pulse.
A radar transmitter sends an electromagnetic signal into space, from a directional antenna. If there is a radar target, the target reflects part of the signal back in the direction of a receiver.
Antennas, whether a common antenna used for transmission and reception, or separate transmitting and receiving antennas, have a major role in defining radar system characteristics. In a system with a shared transmit-receive antenna, a '''duplexer''' turns off the sensitive receiver input while transmitting.
A radar system that has one transmitting and one receiving antenna, a significant number of wavelengths apart, is [[bistatic]]. A system of transmitters and receivers, the total being greater than 2, is [[multistatic]].
Synchronized with the transmitted signal, the modulator generates a trigger pulse that tells the receiver when to start listening for the return echo. “Gating” the process of listening, for example, prevents the receiver from becoming confused by echoes that return from close-by rain.
[[Image:IntroShipRadar.png|thumb|Block diagram of basic radar system]]
Within the constraint of gating (and much more complex processing in advanced radars), the receiver detects the reflected signal and the time it arrived. Since the receiver is time-synchronized with the transmitter, it knows the time it took for the round trip of the transmitter's signal. Dividing the round trip time in half, and then dividing by the speed of light, gives the distance from the transmitter to the target.<ref name=NavWSE-7>{{citation
| url = http://www.fas.org/man/dod-101/navy/docs/es310/radarsys/radarsys.htm
| title = ES310: Introduction to Naval Weapons Engineering
| contribution =Chapter 7: Basic Radar Principles}}</ref>
Since the transmitter sent its signal through a directional antenna, and the receiver also uses a directional antenna, sometimes sharing the one used by the transmitter, the direction from which the strongest echo comes is the direction from the antenna to the target.
The receiver processes the signal and converts it to a form suitable for display.
While the original radars, in World War II and still in some specialized applications, had separate displays where the range and bearing showed as a peak along a horizontal line, modern displays are usually '''plan-position indicator (PPI)'''. A PPI, or polar, display logically places the antenna in the center of a circle, and shows the returns around it.
==Frequency bands==
There are several systems for describing the frequency bands used by radar. None is ideal. The [[International Telecommunications Union]] system of [[ITU frequency bands]] is most consistent with more general electronics, including radio.  It has the most extensive coverage of lower-frequency bands that are finding specialized applications, but insufficiently fine-grained detail for the higher radar frequencies. Nevertheless, the ''ITU terminology'' for the lower frequencies, with the higher-frequency terminology using the [[EU-NATO-US frequency bands]] terminology accepted by NATO and EU countries.
Another nomenclature system more common for civilian radar applications is the [[IEEE frequency bands]]
{| class="wikitable"
|-
! Frequency range
! (''ITU'') EU/NATO/US ECM band
! Modern application
|-
| 300KHz-3 MHz
| ''MF''
| Experimental navigational aids
|-
| 3-30 MHz
| ''HF''
| Over-the-horizon and surface scanning
|-
| 30-300 MHz
| ''VHF''
| Space surveillance
|-
| 300 MHz -3 GHz
| ''UHF''
| Military early warning and surveillance, wind scanners
|-
| to 250 MHz
| '''A''' band to 250 MHz
| (see above)
|-
| 250-500 MHz
| '''B''' band to 250 MHz
| Military early warning and surveillance, wind scanners
|-
| 500 MHz-1 GHz
| '''C''' band to 500 MHz
| Shipboard long-range surveillance, space research, test range instrumentation, wind profiling
|-
| 1-2 GHz
| '''D''' band
| Air traffic control, military early warning, synthetic aperture radar, ground battlefield sensors, space-based radar
|-
| 2-3 GHz
| '''E''' band
| Planetary exploration, air traffic control, marine navigation, weather
|-
| 3-4 GHz
| '''F''' band
| Air surveillance and tracking,  test range instrumentation
|-
| 4-6 GHz
| '''G''' band
| Radar altimeters, weather radar, airport terminal Doppler weather radar (TDWR, wind shear), military surveillance and air defense
|-
| 6-8 GHz
| '''H''' band
| Air and ship surveillance and navigation, fire control, test range instrumentation, SAR, scatterometer
|-
| 8-10 GHz
| '''I''' band
| Air and ship surveillance and navigation, airborne weather, police speed radar, radar altimeters, air navigation aids, fire control, test range instrumentation
|-
| 10-20 GHz
| '''J''' band
| Doppler navigation, police speed radar, airborne and seaborne search & acquisition Doppler, Airport Surface Detection Equipment (ASDE), atmospheric research
|-
| 20-40 GHz
| '''K''' band
| Airborne navigation, surface mapping, police speed radar, terrain following radar, test range instrumentation, atmospheric & oceanographic research, scatterometers
|-
| 40-60 GHz
| '''L''' band
| --
|-
| 60-100 GHz
| '''M''' band
| Aircraft fire control, radar beacons, missile autonomous guidance, weather, and specialized imaging radars
|}
==Signal variants==
===Simple Pulse Radar===
This type is the most typical radar with a waveform consisting of repetitive short-duration pulses.  While this has been described as a modulated carrier wave, and carrier waves are usually considered continuous, pulsed radars do not send a continuous signal: the carrier is turned on only when it is modulated into pulses
***Insert graphic of carrier-less pulse
===A slightly less simplified model: pulse trains===
In reality, a single pulse with a single return can't really be trusted as a reliable radar detection. Radars usually send multiple pulses in a cell, and want to see responses from several cells, before presenting information. Radar designers want to be sure that they do not assume what they receive are true reflections, not random noise.
***Insert graphic of cell of pulses
In a digital radar, one can record the time characteristics of pulse trains. Radars that use [[analog electronics]] for signal processing are more likely to accumulate signals and, when the cumulative amplitude reaches a level of significance, present a signal. In contrast, processing with [[digital electronics]] remembers the actual pulses and can be thought of overlaying successive received pulses, discarding those that are clearly noise.
Different effects come from the duration of an individual pulse (pulse duration or pulse width), and from the time between pulses (pulse repetition time or PRT). The reciprocal of the PRT is the '''[[pulse repetition frequency]] (PRF)''', a term probably in more general use than PRT.  At the moment, however, PRT is a better way to speak of cells made up of pulses.
The longer the duration of a pulse, the more distant a target can be detected. Remember that the duplexer function disables the receiver while the transmitter is sending, so, if the pulses take too long to return, the receiver not hear it because the duplexer disabled it while the next pulse was outbound. The PRF, therefore, needs to be slower to detect more distant pulses.
If individual pulses are so long that the receiver is blanked when a pulse is returning, this effect imposes a minimum range on the radar system. To detect close-in targets, as, for example, might be needed on an automatic cannon used as the final defense again incoming guided missiles, a short pulse width is necessary. The PRF is fast to get frequent measurements, but each pulse requires very little blanking time.
Since sensing near and distant targets requires quite different signals, the military first used separate radar sets for weapons control and for early warning.  Both the time between (groups) of pulses and the pulse width need to be considered together. These parameters define the PRF, which, historically, has been one of the methods [[electronic intelligence]] systems used to identify radars.  More modern radars, however, can dynamically vary their PRF, combining short- and long-range functions in the same system, although there are specific cases where it remains desirable to separate these functions. Variable PRF also is harder to predict so it can be jammed.
We speak of radar transmission cells, which are time periods in which a group of pulses can be sent, some for short-range and some for long-range detection. A simple [[radar intercept receiver]] may not be able to analyze the fine details of a cell, so the cell repetition rate becomes, to the intercept receiver, the pulse repetition rate.
"the '''maximum unambiguous range''', is just the range corresponding to a time interval equal to the '''pulse repetition time, PRT.''" <ref name=NavWSE-7 />
===The Doppler principle===
To understand the next few examples, at least an intuitive understanding of the [[Doppler effect]] is necessary. Think of a siren on a police car, and how it seems to grow higher in pitch as it comes toward you, while the pitch becomes lower as it moves away from you.  The frequency (i.e., pitch) you are hearing is actually shifting as a result of the source's motion with respect to your position (and your movement, if you are moving).  Frequencies go up (i.e., the pulses are logically closer) when the source -- the radar target -- is getting closer, and go down when it is moving away.  Analyzing this shift can give valuable relative speed information.
===Moving Target Indicator===
By sensing Doppler shifts, a radar with a moving target indicator (MTI) function can distinguish between real stationary objects, real moving objects, and "clutter", or electronic noise giving a false impression of a target.
The waveform of a basic MTI radar uses a low-PRF train of pulses, which minimizes the error caused by echoes and other artifacts that interfere with range measurement.  Range measurement at the low PRF is good while speed measurement is less accurate than at a high PRG's, so, to measure both, the radar may have to transmit a mixture of a high-PRF signal for speed measurement and a low-PRF signal for range measurement.
Most surveillance radars use MTI, often in a combination with other signals, so they can concentrate on the moving targets of interest, not distracted by stationary objects or random echoes. An airborne MTI (AMTI) radar  must compensate for the more complex relative speed relationships introduced by the motion of the aircraft carrying the radar.
When the Doppler shift is relatively slight, the method of comparison is based on phase comparison.  A phase comparator, in general terms, compares the difference in time of the leading edge of the waveform of two signals. When the signals are "out of phase" at a constant frequency, the low and high amplitudes of the signal are significantly different in time. When two signals are in phase, they could be directy overlaid and still present the same waveform.
In order to measure the phase, a sample of the transmitter pulse is fed into a phase comparator, which also samples the return signal. The output of the phase comparator is used to modulate the display information. A typical application is to suppress the display of stationary objects, which allows evaluation of the moving elements of a tactical display. See [[Radar MASINT#Moving Target Indicator]].
Stationary targets are those returns with constant range over a series of pulses. For moving transmitters, of course, returns from fixed objects on the ground will be changing in range and therefore displayed. MTI systems for moving transmitters must provide a modified input to the phase comparator, which includes the phase advance associated with the motion of the transmitter.
MTI and pulse Doppler radar systems cannot measure velocities above a certain value, known as the first blind speed or maximum unambiguous speed. . For MTI systems, the first blind speed occurs when the change in range between pulses is exactly one-half of the wavelength. This changes the phase by 3600 which is the same as 00, or no phase shift at all. The target moving at the first blind speed will appear to be stationary and be canceled from the display. Since this condition will only be temporary, it is of no concern.
===Pulse Doppler Radar===
As with the MTI system, pulse Doppler radar uses the Doppler effect to reject error and track moving targets. To a pulse doppler radar, speed is more important than range, so it uses a faster PRF than the MTI radar. The PRF of a pulse Doppler radar, for example, might be 300 kHz, while the PRF of an MTI radar may be 300 Hz. The good news here is that the pulse trains, for the two purpose, are sufficiently different that a single radar set can send and process both types.  There are applications, however, where only accurate range or accurate speed is needed.
Take a conventional pulsed radar, and split the transmitted signal (i.e., before the power amplifier). Send this signal, along with a split of the received signal. The output of the mixer is the Doppler shift, '''D<sub>f</sub>'''.
The most common application is to color code the return information on the PPI display.  For navigation and search applications, it can be convenient to display, in different colors, the targets that are moving toward the receiver, are not moving, or are moving away from it.
Pulsed Doppler is also the standard weather radar. The pulsed Doppler radar can detect and graphically display information about the relative motion of winds inside of storm cells and has proved useful in detecting tornadoes. A Doppler velocity display of a tornado will show the two colors which correspond to opposite directions of motion side-by-side.
'''High range resolution radar''' is a specialized form of pulse Doppler. It must use a very short pulse width to pick out specific targets from background noise. An intelligent receiver and display can be configured to display targets of interest, based on their physical size measured by this radar. Basic high range resolution radar, unfortunately, only works at short ranges.
An improvement on pulse compression radar, '''pulse compression radar'''gets around the range restriction by modulating the frequency or phase of a pulse that has very high power but very narrow width. The modulation is another source of information that allows an even shorter pulse to give the same information. The shorter the pulse, the lower the average power, which is the limitation of power supplies, and the more energy that can go into a single pulse.
===Continuous-Wave (CW) Radar===
When pulses are not used, but rather either a continuous receive ''and'' transmitted signal, or a non-pulsed signal switched back and forth from transmit and receive, a basic '''continuous wave''' radar can only give information on azimuth and target speed, but not on range. Until additional features add to the CW paradigm, an individual radar cannot determine range.
CW radars at multiple locations, however, can exchange azimuth information and triangulate on the azimuth, to get an approximation of range.
Police radars are commonly CW, since the speed is the only measurement of interest. With these relatively simple radars, there will be an error in the speed measurement, and the radar will always determine a speed slightly lower than the actual speed. It is unwise, therefore, to argue to a traffic judge, without strong evidence, that the radar was wrong and you were not speeding.
CW radar systems are used in military applications where the measuring the range rate is desired, while keeping continuous contact with the target. Continuous contact, for example, is the ideal for a weapons system illuminator for a [[semi-active radar homing]] missile. 
CW can be enhanced further by modulating the CW signal: '''Frequency-modulated Continuous-wave radar (FM-CW)''' If the frequency of a CW radar is continually changed with time, the frequency of the echo signal will differ from that transmitted and the difference will be proportional to the range of the target. Accordingly, measuring the difference between the transmitted and received frequencies gives the range to the target.
Adding [[frequency modulation]] (FM) to the CW signal is one of the first steps in getting more information. Such radars typically have separate receiving and transmitting antennas, which are preferably separated by some number of wavelengths of the signal.
<ref name=NavWSE-10>{{citation
| url = http://www.fas.org/man/dod-101/navy/docs/es310/cwradar/cwradar.htm
| title = ES310: Introduction to Naval Weapons Engineering
| contribution =Chapter 10: CW Radar / Doppler}}</ref>  What this does in effect is to put a unique "time stamp" on the transmitted wave at every instant.  By measuring the frequency of the return signal, the time delay between transmission and reception can be measure and therefore the range determined as before. Measurements are usually simplest when the modulating frequency changes at a constant rate.
The CW/FM radar can look at the returned frequency, compare it with what it is currently transmitting, and determine how long ago the reflected signal was transmitted. That information can give a good indication of range.
CW/FM is useful in applications such as radar altimeters, where the transmitting and receiving antennas can be located on the top and bottom of the aircraft, and the aircraft's motion substitutes for a need to scan the radar over multiple azimuths. Scanning antennas, as is necessary for a function such as long-distance air search, requires the beam to move in azimuth. That motion traditionally required spinning the antennas, although there are newer [[phased array]] techniques that allow the equivalent of scanning through all-electronic management of a large number of small antenna elements.
===High Frequency Over-the-Horizon (HF OTH) Radar===
This radar operates in the high frequency (HF) portion of the electromagnetic spectrum (3-30 MHz), where the electromagnetic waves "bounce" off the ionospheric layer of the upper atmosphere. OTH radar of this type can give coarse early warning of targets up to 2,000 nmi away.
==Antenna technology variants==
===3-D Radar===
Conventional air surveillance radar measures the location of a target in two dimensions-range and azimuth. The elevation angle, from which target height can be derived, also can be determined. "3-D" radar  measures range in a conventional manner but, but uses a second conceptual antenna to measure elevation. The elevation-measuring component has a beam  that is mechanically or electronically rotated about a vertical axis to obtain the azimuth angle of a target, plus either a scanning narrow beam or multiple fixed beams to measure the elevation angle.  Since ground and sea targets stay on a surface, 3-D radar is used in air surveillance application.
This type of radar, with two antennas moving in different conceptual paths -- which may be implemented either with two mechanical antennas or one phased array -- measures both azimuth and elevation.
===Phased-Array Radar===
An electronically scanned phased-array antenna can position its beam rapidly from one direction to another without mechanical movement of large antenna structures. Agile, rapid beam switching permits the radar to track many targets simultaneously and to perform other functions as required.  Where imaging radar use a similar basic concept, adding together multiple received signals, phased array radars add together many outgoing signals, creating far more complex beams than could be created by any one antenna.
====Passive electronically scanned array====
In PESA, which represented a major advance over mechanically scanned antennas, there is a single source of radar energy, which is then shifted among large numbers (often thousands) of transmitting elements, usually that can receive as well.  The elements are passive in that they do not have independent energy sources.
Phased arrays not only are faster and more maintenance-free, but can do things that radars with moving antennas cannot. For example, it was assumed that a [[stealth]] aircraft could not use radar, since its reduced radar return would be useless if it was transmitting a signal. No low-probability-of-intercept radar was available for the first-generation [[F-117 Nighthawk]] aircraft, but, the [[B-2 Spirit]] has a a [[APQ-181 | AN/APQ-181]] is an all-weather, low probability of intercept (LPI) radar system for aiming its weapons, and for following and avoiding ground terrain.
====Active electronically scanned array====
As much as an advance over PESA as PESA was over mechanically steered antennas, every transmit/receive element in an AESA has its own source of radar energy. This allows the construction of extremely complex, low-probability-of-intercept active beams. There is no longer a single point of failure.
Different elements can be forming a complex [[low probability of intercept]] (LPI) transmitted signal, receiving signals reflected back to the AESA, or jamming hostile radars. Further, the AESA can receive either the reflections of its own transmitters, or listen for the reflections from another radar source, such as a large airborne radar aircraft. An AESA-equipped fighter can close to engagement range with a target its pilot sees clearly on the radar display, yet the fighter may be emitting no signals that can be tracked. The first warning received by the target is likely to be an incoming air-to-air missile.
AESA moves a generation ahead in [[low-probability-of-intercept]], as it can spread its signal among a pseudo-random set of transmitting antennas, creating a beam that is very hard to visualize. An AESA radar, such as the [[APG-77|AN/APG-77]] on the [[F-22 Raptor]], the [[APG-79|AN/APG-79]] on the [[F-18 Super Hornet]] and the [[APG-81|AN/APG-81]] on all models of the [[F-35 Joint Strike Fighter]] can also be used for [[electronic attack]] with the antennas transmitting to interfere with an enemy radar, or using them to receive hostile signals and collect [[electronic intelligence]].
The [[SPY-3|AN/SPY-3]] is the first U.S. shipboard AESA, which will first go into [[Zumwalt-class]] [[destroyer]]s but may be retrofitted; it adds significant capability for [[littoral (military)|littoral operations]], making [[amphibious warfare]] ships an obvious candidate.
===Tracking Radar===
Conceptually, a tracking radar continuously follows a single target in angle (azimuth and elevation) and range to determine its current position and projected path. The radar return may, in addition to the main radar, come to the seeker of a [[guided missile]] using [[semi-active radar homing]]. The single-target tracking radar provides target location almost continuously.
While tracking radars traditionally stayed with a single target, limiting the capacity of a weapons system, modern tracking systems -- also called fire control -- have methods for handling multiple targets with a single tracker. A phased array antenna can generate multiple simultaneous beams that otherwise would require multiple physical antennas. Alternatively, a physical antenna, used in an air defense system, may be "time-shared", with the main search radar (e.g., the AN/SPY-1 on AEGIS air defense ships) [[measurement and signature intelligence#cueing|cueing]] the tracking radar (e.g., the [[SPG-62|AN/SPG-62]] on AEGIS ships) to illuminate the target, only when the missile needs continuous information for the final stage of its attack.
A typical tracking radar might measure the target location at a rate of 10 times per second. Range instrumentation radars are typical tracking radars.
Military tracking radars employ sophisticated signal processing to estimate target size or identify specific characteristics before a weapon system is activated against them. These radars are sometimes referred to as fire-control radars.
====Track-While-Scan (TWS) Radar====
There are two different TWS radars.
One is more or less the conventional air surveillance radar with a mechanically rotating antenna. Target tracking is done from observations made from one rotation to another. The other TWS radar is a radar whose antenna rapidly scans a small angular sector to extract the angular location of a target.
====Track-via-missile====
In a track-via-missile system, there are cooperating radar sets both on the ground and in the missile. By downlinking its radar picture to the ground control station, the missile control system can be extremely accurate by correlating the (physically closer) missile view with the (more powerful) ground radar view.
===The signal reflected by the target===
With more and more computer control, the more complex a signal can be. Remember that radar assumes a directional antenna, which sends out the signal in a narrow beam.
====Polarization====
Particular transmitting antenna arrangements can '''polarize''' the signal, so a receiver, or multiple receivers, could listen for horizontally, vertically, or, when using helical antennas, circularly polarized signals. Long and short pulses could go out simultaneously, with different polarizations.
The target can impose polarization effects as it reflects the signal. If the receiving antenna is separate from that of the transmitter, the receiver can further discriminate by looking for the most likely target return polarization.  Rain imposes a circular polarization, which can degrade signals being used to track objects, but is also a valuable tool for weathermen.
For example, circular polarization is used to minimize the interference caused by rain, or, in weather radar, to measure the rain itself. Horizontal or vertical polarization returns usually indicate metal surfaces, such as those aircraft.
Random polarization can come from an irregular surface such as earth with embedded stone, so navigation radars must be able to cope with many polarization variants in the received signal
====Radar cross section====
If all the energy directed on to the target were returned to the receiver, the strength of the return would be a function of the power arriving at the target and the target's '''radar cross section (RCS)'''. This is a vastly simplified assumption, in that targets almost always absorb some energy, and they are also not perfect reflectors. RCS is most commonly measured rather than calculated. Some of the factors that affect it are:<ref name=EU-RCS>{{Citation
| url = http://www.radartutorial.eu/18.explanations/ex09.en.html
| contribution = Radar Cross-Section
| title = Radar Basics
| publisher = European Union}}</ref>
    * the airplane’s physical geometry and exterior features,
    * the direction of the illuminating radar,
    * the radar transmitters frequency,
    * the used material types.
====Stealth====
Not only do conventional aircraft have metal reflecting surface, the aircraft is primarily parallel to the earth, so the reflected signal tends to be horizontally polarized. [[Stealth]] aircraft have "faceted" surfaces so that any signals that do reflect from their [[radar-absorbing material]] outer skin will bounce in a direction away from that of the transmitting antenna.
Further, the surface of such an aircraft is selected to absorb as much radar energy as possible.
==Imaging radars==
Conventional radars show a point, perhaps on a projected line, that displays the position, course, and speed of a target. Several techniques, however, can form a photograph-like image of a scanned area. Imaging radars cannot resolve moving objects as well as they can do for fixed or slowly moving targets.  For example, inverse synthetic aperture radar (ISAR) can image the ground, slow-moving targets such as ships and tanks, but not jet aircraft. Synthetic aperture, inverse synthetic aperture, and side-looking airborne radar techniques as a group, are referred to as imaging radars.
Imaging radar is completely dependent both on powerful computers and on advanced radar theory. The "synthetic" concept comes from "synthesizing" the equivalent of a very large physical antenna, either from small physical
''receiving'' antennas, or, more commonly, using the radar signals returned to a single antenna aboard an aircraft moving along a precise path. The individual radars, whether multiple units or the single aircraft antenna, have their signals combined by computers to create a combined image with better resolution than any individual radar. As with optical telescopes, the larger the effective aperture size, the greater the resolution. Other astronomical techniques, such as [[Very Long Baseline Interferometry]] in radio astronomy, "add" the signals in like manner.
In the most basic form, the receivers look only at the [[amplitude]] of the return, and, when more processing power is available, also considers the [[phase]] and the [[polarization]]. With a moving antenna, as on an airplane, [[Doppler shift]] can also be added to the image reconstruction.
The two modern types, SAR and ISAR, use different principles. SAR takes multiple images along the flight path of the SAR platform, while ISAR takes Doppler measurements perpendicular to the flight path.
===Side-Looking Airborne Radar===
The first type of imaging radar to be invented, '''Side-Looking Airborne Radar (SLAR)''', this uses a a large side-looking antenna (i.e., one whose beam is perpendicular to the aircraft's line of flight) and is capable of high-range resolution. It is not as accurate as synthetic aperture radar, but is much simpler to build; it was operational as early as the Vietnam War.
===Synthetic Aperture Radar===
'''Synthetic aperture radar (SAR)''' is employed on an aircraft or satellite and generally its antenna beam is oriented perpendicular to its direction of travel. The SAR achieves high resolution in angle (cross range) by storing the sequentially received signals in memory over a period of time and then adding them as if they were from a large array antenna.
===Inverse Synthetic Aperture Radar===
'''Inverse Synthetic Aperture Radar (ISAR)''' is similar to SAR, except that it obtains cross-range resolution by using Doppler frequency shift that results from target movements relative to the radar.
===Fusion===
New techniques combine SAR and ISAR to improve resolution, to produce extremely accurate images that can be used for imagery interpretation and for [[Radar MASINT#noncooperative target recognition |noncooperative target recognition]] <ref name=PAPSON>{{citation
| contribution = Multiple location SAR/ISAR image fusion for enhanced characterization of targets (abstract)
| last1 = Papson | first1 = Scott | last2 = Narayanan | first2 =Ram M.
| url = http://cat.inist.fr/?aModele=afficheN&cpsidt=17644772
| title = Proceedings of SPIE, the International Society for Optical Engineering  , Radar sensor technology conference 9
|date = 31 March 2005
| volume = 5788
| pages =  128-139, }}</ref>
The main focus of this paper is the development of fusion strategies for multiple location synthetic aperture radar (SAR), and inverse synthetic aperture radar (ISAR) images. The techniques being developed are to be used in conjunction with super-resolution and target identification strategies for non-cooperative target recognition (NCTR).
Using SAR and ISAR imaging of the same scanned area provides  different aspects or looks can provide additional clues about the shape, dimensions, and special features of a target. Traditional SAR or ISAR methods use techniques to filter out instantaneous noise, but do not devote as much processing to increasing the overall accuracy of the mapping/image forming product. Many traditional SAR/ISAR processing techniques seek to maximize the instantaneous [[signal-to-noise ratio]] for a signal in the presence of additive noise. Just as more powerful image compression techniques are not instantaneous, but work with multiple parts of the image, the overall map solution improves when multiple sources of information are available both for individual [[pixel]]s and the relationships among those pixels.
==Applications==
Since the original application of radar was military, as a decisive component of the  British defense in the [[Battle of Britain]] was its system of radars and command posts, it is only historically fair to present military applications first. The British "Chain Home" and "Chain Home Low" radars, built in the 1930s, have very little similarities, in the details of their implementation, to modern systems.
===Military===
Radar originally was developed to meet the needs of the military services, and it continues to have critical applications for national defense purposes. For instance, radars are used to detect aircraft, missiles, artillery and mortar projectiles, ships, land vehicles, and satellites. In addition, radar controls and guides weapons; allows one class of target to be distinguished from another; aids in the navigation of aircraft and ships; and assists in reconnaissance and damage assessment.
Military radar systems can be divided into three main classes based on platform: land-based, shipborne, and airborne. Within these broad classes, there are several other categories based mainly on the operational use of the radar system. For the purposes of this report, the categories of military radars will be as described below, although there are some "gray" areas where some systems tend to cover more than one category. There is also a trend to develop multimode radar systems. In these cases, the radar category is based on the primary use of the radar.
====Land-Based Air Defense Radars====
These radars cover all fixed, mobile, and transportable 2-D and 3-D systems used in the air defense mission.
=====Historic: Battle of Britain Chain Home=====
Based on experiments done in 1935, the British Chain Home system, in some respects, shows that technology sometimes goes in cycles. With separate, nonmoving tranmitting and receiving antennas, in groups of four per site, Chain Home, in modern terms, was a [[multistatic]] system.  It required a great deal of manual operation and operator skill, both at the individual radar stations<ref name=Neale1985 >{{citation
| title = The GEC Journal of Research
| Volume = 3
| issue = 2
| year =  1985
| pages = 73-83
| title = CH - The First Operational Radar
| first = B.T. | last = Neale
| url = http://www.radarpages.co.uk/mob/ch/chainhome.htm}}</ref> and at the "filter room" of Fighter Command.<ref name=Clark1997>{{citation
| id = ADA397960
| title = Deflating British Radar Myths of World War II
| publisher = Air Command and Staff College
| author = Clark, Gregory C.
| url = http://handle.dtic.mil/100.2/ADA397960 
| date = March 1997}}</ref>
These limitations and design choices must be considered in the state of theory and technology at the time, and with the perspective that while British radar might have been technically inferior than its German counterparts, the British had developed an [[integrated air defense system]] (IADS), a key part of which was the filter room, at a time when they would have been defeated without an IADS.  Obviously, the designers did not deliberately select a multistatic design to detect stealth aircraft that were more than half a century in the future. Their design was a matter of constraints:
:*The available electronic technology, which could not generate high-power microwaves or even signals shorter than HF,
:*The requirements of antennas, using the available wavelengths, that could provide the needed transmitting and receiving patterns
:*Very early ideas of what frequencies would be most effective for aircraft detection.
The original plan was for each station to have the choice of operating on any of four allocated spot frequencies in the band 20 to 55 MHz as a counter-measure to possible jamming and as alternative frequencies should interference or propagation effects cause operational problems.
In its first operational version, Chain Home had an effective range of 80 miles, and up to 200 miles under favorable conditions. Radar stations, spaced approximately 30 miles along the coast, could not detect aircraft flying below 1000 feet. An additional system, Chain Home Low, was installed to detect low-flying aircraft, especially [[minelayer]]s. <ref name=Dowding2>{{citation
|  title = Battle of Britain in the Words of Air Chief Marshal Hugh Dowding, Air Officer Commanding-in-Chief, Fighter Command, Royal Air Force
| contribution = Part 2 - Organisation of British Defences
| first = Hugh C. T.  | last Dowding, G.C.B., G.C.V.O., C.M.G., A.D.Cs,
| date = 20 August 1941
| url = http://www.spitfiresite.com/history/articles/2007/11/battle-of-britain-by-hugh-dowding-02.htm
}}</ref> Chain Home Low had a 30 mile range.
CH operated by sending pulse trains, from multiple transmitters, into the area to be covered. Reflected energy from all illuminated targets were received by a set of crossed [[dipole]]s. The pulse displayed, on a two-dimensional display, as a vertical deflection proportional to the signal strength, and the horizontal deflection being the time of arrival of the pulse at the receiver. <ref name=Neale1985 /> This is simple in practice, and indeed gained from the long wavelength and lack of fine resolution -- a formation of bombers would show, more or less, as a single return. Since that return would have different times of arrival at different receivers, it was possible to triangulate the exact position. This took considerable operator skill and human coordination, to deal with issues such as multipath echoes.
Another simplification was the use of horizontal polarization, for three reasons:<ref name=Neale1985 />
#Since an approaching aircraft's wings produce a predominantly horizontal reflector, the strongest return will be horizontally polarized.
#The signal phase change of π radians on reflection from the ground is constant for all relevant angles of elevation; this is particularly important for the formation of the vertical polar diagram required for height finding.
#Since a horizontally polarized antenna is symmetrical with respect to ground, the cabling can use simple balanced, open-wire transmission capable of withstanding very high voltage peaks. Other polarizations, at the long wavelengths involved, would need coaxial cable that might well have been beyond the manufacturing technologies of the time.
While the antennas themselves were fixed, the operator could manually simulate a receiving array by varying the ratio, fed to the display, of the east-west dipole on the Y axis, and the north-south dipole to the other axis. As in loop direction finding, the operator would sweep for a minimum deflection, which was sharper and more precise than a maximum as long as the signal-to-noise ratio was  adequate. For weak signals or high noise, the lesser precision of the maximum was accepted.  <ref name=Neale1985 />
These were the steps for determining target azimuth. For heightfinding (i.e., elevation), a similar principle was used, but with dipoles at two different heights, nominally 215 and 95 feet. While there was no electronic computation, an electromechanical computer called the "fruit machine" corrected for curvature of the earth and gave the target elevation with reference to the receiver.
<blockquote>Counting of aircraft in close formation (raid strength) relied on the skill of the operator who, when experienced, was able to make an assessment by observing the 'beat' rate of the composite echoes. To assist in this assessment, the transmitted pulse could be momentarily shortened to 6 microseconds (from 20 microseconds) by a push button on the console, the shorter pulse improving the range resolution by about 3:1.</blockquote>
In practice, every CH system was calibrated against a cooperative target, and was extremely dependent on operator skill developed with long experience.  WAAFS (Women's Auxiliary Air Force) operators had the best results. Signals at extreme ranges, well below 'noise' level, were detected and tracked. Human pattern recognition clearly played a part, although the cognitive mechanisms involved are still not understood.
Unlike scanning (searchlight) radars, CH, being a 'floodlit' system, provided up-date at the pulse repetition rate with a corresponding integration gain when using a CRT with a long persistence phosphor. <ref name=Neale1985 />
German radar, in many respects, was more advanced, but German intelligence also had a failure of imagination in understanding the British system. PWhile Germany had sent out electronic reconnaissance using Zeppelins, they did not consider that a radar system could work at HF and thus could not understand the purpose of the CH towers.<ref name=Clark1997 />  Germany had actually developed working radars several years before Britain, using VHF and UHF, but created an [[integrated air defense system]] later than Britain.
=====First-generation SAM: Soviet S-75/SA-2=====
The first [[surface-to-air missile]] (SAM) system widely deployed by the Soviet Union, the [[S-75 Dvina]] (NATO identifier [[SA-2]], designation name GUIDELINE) had, in its original versions, two primary radars at the firing battery level, SPOON REST long-range search [[EU-NATO-US frequency bands|E-band]] (range 170mi/275km) and [[FAN SONG (radar)|FAN SONG]] (models with different range, frequency, and ECCM).
SA-2 regimental headquarters have an additional [[EU-NATO-US frequency bands|C-band]] FLAT FACE long-range radar as well a [[EU-NATO-US frequency bands|E-band]] SIDE NET height-finder; the regimental headquarters coordinates the multiple batteries.
=====Advanced: MIM-104 Patriot=====
The [[MIM-104 Patriot]] air defense system combines several kinds of radar functions into one physical system, designated AN/MPQ-65 in the latest version. Unlike most air defense radar, this system manages all features from "detection to kill". The main radar does both search and fire control, with fire control supplemented with a Track Via Missile (TVM) radar sensor in the missile itself. Exceptionally accurate positioning comes from synchronized ground and missile radar, linked by the launching battery's computers. The AN/MPQ-53/65 proper is a [[passive electronically scanned array]] (PESA) radar which is equipped with IFF, electronic counter-countermeasure (ECCM) and track-via-missile (TVM) guidance subsystems.
In the PESA are in excess of 5,000 individual transmit-receive elements, which do not move, but are switched electrically. Other functions in the array include [[identification friend or foe]] to detect friendly aircraft, additional antennas to reduce interference, and the receiver for missile-generated tracking.  The radar is frequency-agile.
[[Phased-array radar]]s, active (AESA) or PESA, can shift much faster than any mechanically moving antenna. This lets it detect "hard" targets such as stealth aircraft, ballistic missile reentry vehicles, and cruise missiles. The radar is also highly resistant to [[electronic countermeasures]].
====Radars for Land Warfare====
Various radars are used for warning of incoming [[mortar]] shells, free-flight rockets, and [[howitzer]] shells. The U.S. has [[Radar MASINT#Counterartillery Radar|three layers of radars]] of different ranges, which can track the incoming fire's trajectory back to its source, and send targeting information to retaliatory weapons before the incoming fire lands.
====Sea Navigation Radars====
Navigation radars need to high immunity to rain and waves, and still see small targets such as buoys as well as nearby ships and land masses. Military and commercial radars for this function are quite similar, a representative military model being the AN/SPS-67, which is a pulsed radar operating in the G-band (4-6 GHz).
====Warship combat radars====
While there is a strong trend to multifunction radars, warships, especially with an area air defense mission, often have supplementary radars both for backing up the main radar, and for final guidance. On AEGIS air defense ships, the main multifunction radar started out as the AN/SPY-1 phased array radar operating in the E/F band. The AN/SPY-2 has upgrades for better [[littoral warfare]] performance, while the AN/SPY-3 provides [[anti-ballistic missile]] tracking.
Even so, the AEGIS ships use an AN/SPG-62, operating in the I/J band, for final attack guidance for the [[semi-active radar homing]] of their [[Standard SM-2]] [[surface-to-air missile]]s. Since the AN/SPG-62 is required to illuminate the target for only the last few seconds of the missile's flight, the combination of an AN/SPY-1 and multiple AN/SPG-62's can handle a large number of missiles and targets.
====Airborne radars====
With the exception of high-performance search radars on dedicated radar surveillance aircraft (e.g., [[E-3 Sentry]] for air surveillance and [[E-8 Joint STARS]] for ground surveillance), the strong tendency in new design is to merge all radar functions into one system. Automatically electrically scanned arrays are a major breakthrough in such integration. See [[EF-18 Growler]] for an example of how the AN/APG-79 radar can be used for air and surface search, fire control, and even electronic warfare.
===Air Traffic Control===
Radar is a vital part of air traffic control, but even though the display presented to an air traffic controller may look like a regular PPI display, a good deal of the information is not coming from true radar reflections, but from [[transponder]]s aboard the aircraft. Transponders both act as a beacon, and transmit encoded information on the aircraft's identity, course, speed, and altitude. True radar is definitely secondary to transponders in general ATC, although it still has important applications in managing the immediate vicinity of the airport.
===Aviation===
In aviation, radar-based systems are used to determine height over ground (i.e., radar altimeter), weather conditions, and vector velocity from a navigational reference point. Terrain-avoidance radars allow very low flight appropriate for military operations. <ref name=NTIA-Spectrum>{{citation
| url = http://www.ntia.doc.gov/osmhome/reports/ntia00-40/index.htm
| title = Federal Radar Spectrum Requirements Report
| publisher = National Telecommunications and Information Administration, U.S. Department of Commerce
| id =  NTIA Special Publication 00-40
| date = May 2000
| first = Joseph P.| last = Camacho
}}</ref>
===Safety at Sea===
Radar is widely used for collision avoidance at sea, and, when in range of land, navigating and piloting.<ref name=Bowditch>{{citation
| url = http://www.irbs.com/bowditch/pdf/chapt13.pdf
| author = National Imagery and Mapping Agency (now National Geospatial Intelligence Agency), original version by Nathaniel Bowditch
| year = 1995
| title = American Practical Navigator
| contribution = Chapter 13: Radar Navigation
}}</ref>
A [[search and rescue transponder]] is an active radar beacon/repeater that will help the final localizing, by local radio, of broad position information from the satellites of the [[Global Maritime Distress and Safety System]].
Radars are also used on shore for harbor surveillance supporting the [[vessel traffic system]] (VTS), which is the ship equivalent of air traffic control, used in busy ports. <ref name=USCG-VTS>{{citation
| url = http://www.navcen.uscg.gov/mwv/vts/vts_home.htm
| title = Vessel Traffic System
| author = Navigation Center
| publisher = U.S. Coast Guard}}</ref>. Like air traffic control, it does not depend on radar alone, but both a transponder-like capability in early deployment, the [[automatic identification system]] (AIS) and voice communications between controllers and ships.
===Weather===
Weather radar is a key tool for [[meteorology]], with displays of precipitation and wind speeds types being quite familiar to the general public. More specialized meteorological radars detect and measure extreme wind phenomena, such as tornadoes, wind shear at airports, and hurricanes.  Due to the large areas covered by cyclones, hurricanes, tropical storms and typhoons, radars for that application are likely to be in a space satellite.
Other weather radars measure precipitation and categorize clouds. This has been done from land, sea, air, and spafe.
Airborne or spaceborne '''scatterometer radar''' measures the combination of return echo power variation with aspect angle. It allows measurement of the direction and speed of winds, and of water surfaces.
===Instrumentation===
Radar is used extensively  to track aircraft, unmanned aircraft, spacecraft, and missiles to measure and quantify the actions that take place during test flights.
==References==
==References==
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Radar is a device that transmits radio waves to detect and/or locate the position and velocity of a distant object such as an aeroplane. The etymology of the name is an acronym of radio detection and ranging. Radar works by picking up a reflection of the radio waves which it can then use to calculate the position, direction, and speed of the object. In systematic terms, radar is a simple concept which consists of a transmitter and receiver, but it has proved invaluable in wartime, especially during the Battle of Britain in 1940.

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