Radar, a word created as an acronym of "radio detection and ranging", 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 shells, rockets, and 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.
The diagram illustrates the bistatic case with separate transmitting and receiving antennas, but the most common situation, which allows considerable simplification, is monostatic, where transmitting and receiving share the same antenna. Multistatic radar uses a system of one or more transmitters and one or more receivers, with more than two antennas in the system..
A few electronic parameters, which are essential inputs to the radar equation, do need to be discussed:
- Transmitter power and transmitting antenna gain
- Effective area of target
- Receiver sensitivity and receiving antenna effective area
Additional parameters include:
The last three parameters may be rapidly varied by a pseudo-random pattern. Such variation can be used to decrease interference (e.g., by a noise source at a certain frequency in the range, or as a form of military electronic counter-countermeasures (ECCM), making it more difficult for an opponent to generate a fake radar signal to counter overall system. Electronic countermeasures (ECM) include brute-force jamming and subtle deception.
In the preceding discussion, a perfect path was assumed for radar propagation. In the real world, when signals pass through air and over ground, a variety of factors can interfere, such as transient atmospheric effects that "bend" the beam, echoes from the ground, etc. The U.S Federal Aviation Administration warns pilots, "The bending of radar pulses, often called anomalous propagation or ducting, may cause many extraneous blips to appear on the radar operator's display if the beam has been bent toward the ground or may decrease the detection range if the wave is bent upward. It is difficult to solve the effects of anomalous propagation, but using beacon radar and electronically eliminating stationary and slow moving targets by a method called moving target indicator (MTI) usually negate the problem."" As will be seen in the MTI discussion, MTI can introduce new problems.
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 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 diplexer (also written 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.
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.
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.
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.
|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|
Another nomenclature system more common for civilian radar applications is that of the IEEE frequency bands.
|Frequency range||Wavelength||IEEE band|
|300KHz-3 MHz||1 km to 100 meters||MF|
|3-30 MHz||100 meters to 10 meters||HF|
|30-300 MHz||10 meters to 1 meter||VHF|
|300 MHz -3 GHz||1 meter to 10 cm||UHF|
|1-2 GHz||30 cm to 15 cm||L band|
|2-4 GHz||15 cm to 5 cm||S band|
|4-8 GHz||5 cm to 3.75 cm||C band|
|8-12 GHz||3.75 cm to 2.5 cm||X band|
|12-18 GHz||2.5 cm to 1.6 cm||Ku band|
|18-26 GHz||1.6 cm to 1.2 cm||K band|
|26-40 GHz||1.6 cm to 750 mm||Ka band|
|40-75 GHz||750 mm to 40 mm||V band|
|75 to 111 GHz||40 mm to 28mm||W band|
|Above 111 GHz||"millimeter wave"|
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.
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 would not hear it because the duplexer disabled it while the next pulse was outbound. The PRT, therefore, needs to be slower to detect more distant pulses. Some very long range radars, to avoid the duplexer problem, have separate, side-by-side, transmitting and receiving antennas. Side-by-side mounting also simplifies experimental work, as with amateur radio weather systems.
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." 
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 directly 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, Df.
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 radargets 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.  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
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.
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 uses 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.
Mechanically scanned phased arrays
While the technique is now completely obsolete, a first step toward phased arrays was to use a rotating mechanical switch to transfer energy to fixed-position elements in an antenna. Soon, however, electronics advanced to do away completely with moving parts as the means of signal distribution.
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 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.
There are some military radars that physically move the PESA array, such as the AN/MPQ-64 ground-based air defense system. Mechanical drive is useful here when the radar is placed on a hill, but periodically drives downward to detect low-flying threats that would otherwise be below its horizon.
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 AN/APG-77 on the F-22 Raptor, the AN/APG-79 on the F-18 Super Hornet and the 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 AN/SPY-3 is the first U.S. shipboard AESA, which will first go into Zumwalt-class destroyers but may be retrofitted; it adds significant capability for littoral operations, making amphibious warfare ships an obvious candidate.
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) cueing the tracking radar (e.g., the 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.
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
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:
* the airplane’s physical geometry and exterior features, * the direction of the illuminating radar, * the radar transmitters frequency, * the used material types.
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 desired by the receiver.
Further, the surface of such an aircraft is selected to absorb as much radar energy as possible.
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 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 before 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.
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 pixels and the relationships among those pixels.
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.
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 transmitting 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 and at the "filter room" of Fighter Command. 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 minelayers.  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 dipoles. 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.  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:
- 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. 
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.
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.
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. 
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. 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 E-band (range 170mi/275km) and FAN SONG (models with different range, frequency, and ECCM).
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-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 radars, active (AESA) or PESA, can shift much faster than any mechanically moving antenna. This helps it "hard" targets such as 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 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.
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 IEEE S-band, also the NATO E/F band, while the AN/SPY-2 provides anti-ballistic missile tracking. In the picture, the AN/SPY-1 is the "billboard" on all four sides of the deckhouse, while the AN/SPY-2 is the hexagonal overlay on each face of the AN/SPY-1.
Even so, the AEGIS ships use an AN/SPG-62, operating in the IEEE X-band (NATO I/J band), for final attack guidance for the semi-active radar homing of their RIM-156 Standard SM-2 surface-to-air missiles. 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.
Note that S-band has superior range to X-band, but X-band is more precise in positioning. In the illustration, note that a Ticonderoga-class cruiser supplements the AN/SPY-1 with a longer-range, L-band, AN/SPS-49. It is the mechanically scanned antenna on the right of the center mast, below the AN/SPS-64 navigational radar.
There is also a separate AN/SPQ-9 radar for gunnery fire control.
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 (ATC), 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 transponders 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.
ATC radar, as opposed to airborne radar, makes use of its fixed location. The ASR-11 shown in the picture actually has two rotating antennas, one for true radar reflection, and mounted above it. The lower primary radar distinguishes between object reflections, and specific levels of rainfall intensity at known points. It can determine range and azimuth, but not altitude. Altitude information normally comes from the transponder, although additional height-finding radars may be installed in the immediate vicinity of airports.
Airborne aviation radar
Airborne radar has a wide range of applications in commercial and military aircraft. Especially for forward-looking radars, physical constraints of the aircraft's construction, on the size and shape of its antenna, are major determinants of its performance. The physical design of many high-performance combat aircraft, such as the F-15 Eagle, often starts with the antenna requirements, which determine the shape and size of the rest of the aircraft.
In addition to collision avoidance, 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. 
Radar is widely used for collision avoidance at sea, and, when in range of land, navigating and piloting. It complements the Automatic Identification System, a transponder-based approach. The navigator can either watch a traditional PPI screen to see potential sources of collisions, or have the Automatic Radar Plotting Aid send messages to the master navigational display on a chartplotter. Such messages contain the coordinates of specific radar targets within a user-specified range.
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, also used on larger commercial vessels, being the AN/SPS-64, which is a pulsed radar operating in the E/F and I/J military bands or IEEE S or X bands
Both passive and active radar beacons are deployed to assist radars. Passive beacons are the reverse of stealth technology: reflectors angled to send back a strong signal in the direction of the transmitter. Active radar beacons, or RACONs, as used the United States Coast Guard, responds to a received radar pulse by transmitting an identifiable mark back to the radar set.
Navigational RACONs produce targets on radar displays, encoded as a character beginning with a dash for identification. The inherent delay in the racon causes the displayed response to appear behind the echo from the structure on which the racon is mounted. Racons are used in the U.S. for the following purposes:
- to identify aids to navigation, both seaborne (e.g. buoys) and land-based (e.g. lighthouses)
- to identify landfall or positions on inconspicuous coastlines
- to indicate navigable spans under bridges
- to identify offshore oil platforms and similar structures
- to identify and warn of environmentally-sensitive areas (such as coral reefs)
While navigational RACONs ar placed at fixed, precisely surveyed locations, search and rescue transponders, carried in lifeboats, 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. . Like air traffic control, it does not depend on radar alone, but AIS transponders, and voice communications between controllers and ships.
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. The Terminal Doppler Weather Radar shown, however, is short-range and much more precise in locating air movement in the immediate area of airports, such as microbursts and wind shear.
Other weather radars measure precipitation and categorize clouds. This has been done from land, sea, air, and space. Reflections from precipitation tend to be circularly polarized, so purpose-built measurement systems will usually use a helical or other antenna optimized for receiving circularly polarized signals.
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.
Radar is used extensively to track aircraft, unmanned aircraft, spacecraft, and missiles to measure and quantify the actions that take place during test flights. Tracking radars may cue separate telemetry receiving antennas.
- Federal Aviation Administration (26 August 2010), 4: Air Traffic Control, Section 5, Surveillance Systems, Aeronautical Information Manual
- , Radar Cross-Section, Radar Basics, European Union
- Charles A. "Bert" Fowler (11 March 1998), The Standoff Observation of Enemy Ground Forces: From Project PEEK to JointSTARS, Massachusetts Institute of Technology
- Papson, Scott & Ram M. Narayanan (31 March 2005), Multiple location SAR/ISAR image fusion for enhanced characterization of targets (abstract), Proceedings of SPIE, the International Society for Optical Engineering , Radar sensor technology conference 9, vol. 5788, at 128-139,
- Neale, B.T. (1985), CH - The First Operational Radar, at 73-83
- Clark, Gregory C. (March 1997), Deflating British Radar Myths of World War II, Air Command and Staff College, ADA397960
- , Part 2 - Organisation of British Defences, Battle of Britain in the Words of Air Chief Marshal Hugh Dowding, Air Officer Commanding-in-Chief, Fighter Command, Royal Air Force, 20 August 1941
- Camacho, Joseph P. (May 2000), Federal Radar Spectrum Requirements Report, National Telecommunications and Information Administration, U.S. Department of Commerce, NTIA Special Publication 00-40
- National Imagery and Mapping Agency (now National Geospatial Intelligence Agency), original version by Nathaniel Bowditch (1995), Chapter 13: Radar Navigation, American Practical Navigator
- Navigation Center, Vessel Traffic System, U.S. Coast Guard