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radar,system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, the principle of radar is not confined to any particular frequency range. There are some radar units that operate on frequencies well below 100 megahertz (megacycles) and others that operate in the infrared range and above. The term radar, an acronym for radio detection and ranging, is also used to denote the apparatus for implementing the technique.
Principles of Radar
Radar involves the transmission of pulses of electromagnetic waves by means of a directional antenna; some of the pulses are reflected by objects that intercept them. The reflections are picked up by a receiver, processed electronically, and converted into visible form by means of a display screen (formerly a cathode-ray tube, now typically a liquid crystal display). The range of the object is determined by measuring the time it takes for the radar signal to reach the object and return. The object's location with respect to the radar unit is determined from the direction in which the pulse was received. In most radar units the beam of pulses is continuously rotated at a constant speed, or it is scanned (swung back and forth) over a sector, also at a constant rate. The velocity of the object is measured by applying the Doppler principle: if the object is approaching the radar unit, the frequency of the returned signal is greater than the frequency of the transmitted signal; if the object is receding from the radar unit, the returned frequency is less; and if the object is not moving relative to the radar unit, the return signal will have the same frequency as the transmitted signal.
Applications of Radar
The information secured by radar includes the position and velocity of the object with respect to the radar unit. In some advanced systems the shape of the object may also be determined. Commercial airliners are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. Planes can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot. A ground-based radar system for guiding and landing aircraft by remote control was developed in 1960.
Radar is also used to measure distances and map geographical areas (shoran) and to navigate and fix positions at sea. Meteorologists use radar to monitor precipitation; it has become the primary tool for short-term weather forecasting and is also used to watch for severe weather such as thunderstorms and tornados. Radar can be used to study the planets and the solar ionosphere and to trace solar flares and other moving particles in outer space.
Various radar tracking and surveillance systems are used for scientific study and for defense. For the defense of North America the U.S. government developed (c.1959–63) a radar network known as the Ballistic Missile Early Warning System (BMEWS), with radar installations in Thule, Greenland; Clear, Alaska; and Yorkshire, England. A radar system known as Space Detention and Tracking System (SPADATS), operated collaboratively by the Canada and the United States, is used to track earth-orbiting artificial satellites.
See also stealth technologystealth technology,
designs and materials engineered for the military purpose of avoiding detection by radar or any other electronic system. Stealth, or antidetection, technology is applied to vehicles (e.g.
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Development of Radar
Radar was developed (c.1935–40) independently in several countries as a military instrument for detecting aircraft and ships. One of the earliest practical radar systems was devised (1934–35) by Sir Robert Watson-Watt, a Scots physicist. Although the technology evolved rapidly during World War II, radar improved immensely following the war, the principal advances being higher power outputs, greater receiver sensitivity, and improved timing and signal-processing circuits. In 1946 radar beams from the earth were reflected back from the moon. Radar contact was established with Venus in 1958 and with the sun in 1959, thereby opening a new field of astronomy—radar astronomy.
See G. J. Wheeler, Radar Fundamentals (1967); W. S. Burdic, Radar Signal Analysis (1968); H. Cole, Understanding Radar (1985); M. Skolnik, Radar Handbook (1989).
the scientific technique concerned with the detection, identification, and observation by radio-engineering methods (radar observation) of various objects (targets), as well as the determination of the coordinates of the target, that is, the location, and the time derivatives of these coordinates and the evaluation of other characteristics. When several objects are present, radar must provide the necessary resolution (separate observation) of them. The tasks of radar are handled by both individual radar sets and complex radar systems. Radar navigation is closely associated with radar and employs the same methods and apparatus. Radar is one of the most important areas of modern radio electronics. Radar observation makes use of various principles. Thus active (radiating) radar systems are based on the use of the echo signals created by the reflection of radio waves from the object that is being irradiated by a radar set. In another type of system, that involving active response, the radar beam is reradiated by a transponder located on the object whose position is being determined. In passive radar systems, the target’s own radiation is received. Such radiation can arise from electronic devices located on the target or it can be the target’s own thermal radiation.
Radar systems measure such quantities as the distance to a target (range-finding), the direction from which the signals arrive (direction finding), and the radial and angular velocities of the target’s motion. Radar observation can also reveal many characteristic features of a particular target. For example, it can measure the parameters of the ice cover of the surface of water, the moisture content of the atmosphere, or the dimensions and configuration of an object. The data may be obtained either continuously or just once within a certain time interval. The objects may be single or multiple, or they may constitute a continuous formation. Track-while-scan systems provide for the tracking of targets that have been detected while simultaneously scanning a volume of space in search of new targets.
The basis for the most common type of radar—the active (radiating) system—is the phenomenon of radio-wave reflection. The simplest characteristic of a target’s reflective properties (in the direction of the radar set’s receiving antenna with a given direction of the field of the scanning radiation) is the radar cross section (RCS)σ. This cross section makes it possible to determine the power density of the field P2 at the receiving antenna of the radar set in terms of the power density P1 of the radiation reflected from the target by means of the formula
P1σ = P2 · 4πR2
where R is the distance from the target to the radar set. Depending on the nature of the reflection or emission of radio waves, radar targets are first of all classified as either concentrated, that is, discrete targets with dimensions that are small in comparison with the dimensions of the volume resolved by the radar set, or distributed. Distributed targets, in turn, may be surface or three-dimensional. Surface targets include the earth’s surface with fields, bushes, and snow, the surface of the ocean, and the surface of the moon, and three-dimensional targets include the various inhomogeneities in the atmosphere such as clouds, rain, snow, and man-made dipole interference. Smooth surfaces on which the dimensions of the irregularities are a small fraction of the irradiating wavelength, as for instance, a calm water surface or a concrete roadway, act as specular reflectors; that is, definite phase relations are maintained between the irradiating wave and the reflected wave. When the irregularities are commensurate with or larger than the irradiating wavelength, scattered reflection occurs; that is, the waves reflected from different elements of the surface are superimposed with random phases. In general, real surfaces produce reflections that contain both specular and scattered components.
When the dimensions of a single target are compared with the radar wavelength and the volume of space resolved by the radar set, three cases can be distinguished. In the first, the dimensions of the target are many times greater than the wavelength, and the result is optical scattering, both surface and boundary. The second case arises when the dimensions of the target do not differ markedly from the wavelength; here, the result is resonance scattering. The third case is encountered when the wavelength is much greater than the dimensions of the target, a situation giving rise to Rayleigh scattering. These cases differ not only in the intensity of the reflection but also in the nature of the reflected signal’s dependence on the wavelength and the polarization of the scanning signal.
Of special practical interest is the case in which the target’s dimensions are large in comparison with the wavelength. This case is often encountered because the most commonly used radar wavelengths are in the centimetric range and most of the targets—aircraft, ships, rockets, and spacecraft—have surface and boundary dimensions that are many times greater than the wavelength. It is typical of this kind of scattering (optical) that the RCS does not depend on the polarization of the scanning signal and that it is possible to divide a large object into separate, practically independent parts. As in optics, a large role is played here by scintillation, that is, the phenomenon of intensive reflection of the waves from the protuberances on the object, and by specular reflection from the smooth portions of the target. The calculation scattering of waves by a surface is based on the application of optical methods, mainly the Huygens-Kirchhoff principle, according to which the reflected field is the summation of the fields from the individual parts of the “illuminated” surface.
In resonance scattering, the value of the RCS depends markedly on the wavelength and is a maximum. This phenomenon is used in radar jamming with the release from aircraft of chaff—metallized strips with a length equal to one-half the wavelength. In Rayleigh scattering, the target’s RCS is inversely proportional to the fourth power of the wavelength, directly proportional to the square of the target’s volume, and independent of the target’s shape. Such relationships explain the advantages of using relatively short wavelengths, such as those in the centimetric range, to detect small objects like artillery shells or drops of rain.
Development of radar. The phenomenon of radio-wave reflection was observed by H. Hertz as far back as the years 1886–89. In 1897, A. S. Popov recorded the effect on signal strength made by a ship intersecting the path of radio waves. The idea of detecting a ship by the reflection of radio waves from it was clearly formulated in a patent application by the German engineer C. Hülsmeyer in 1904. His application also contained a detailed description of the necessary equipment.
The interference of continuous radio waves arriving at a receiver by way of two paths—from a transmitter and, after reflection, from a moving ship—was first observed by the American engineers A. H. Taylor and L. C. Young in 1922. In 1932 the interference due to reflection from an aircraft was first observed by the American engineers B. Trevor and P. Carter. In 1924 the British scientist E. Appleton measured the height of the Kennelly-Heaviside layer, or E layer of the ionosphere, by observing the alternating intensification and weakening of the signal produced by varying the oscillation frequency of the transmitter, thus causing (as in the motion of a reflecting object) a variation in the phase difference between wave trains that have traversed two paths. In 1925 the British scientists G. Breit and M. Tuve published the results of their work on the determination of the height of the Kennelly-Heaviside layer. The determination involved a measurement of the time delay of a pulsed signal reflected from the layer relative to a signal arriving from a path along the earth’s surface.
In the USSR, work on radar was expanded in 1933 on the initiative of M. M. Lobanov and under the direction of Iu. K. Ko-rovin and P. K. Oshchepkov. The first radar set put into practical use, which operated on the basis of beats that appeared when an aircraft intersected a beam traveling from a transmitter to a receiver, was developed under the direction of D. S. Sto-gov in 1938. A pulse radar method was developed in 1937 at the Leningrad Physicotechnical Institute under the direction of Iu. B. Kobzarev.
The subsequent development of radar and its military and economic applications was associated with the mastering of microwaves, the improvement of radar methods, the introduction of computer technology, and the use of advances in the related sciences. The development of radar measuring apparatus for antiaircraft and naval artillery was of special significance. The advent and application, almost simultaneously with radar, of radar countermeasures, such as passive and active interference and protective coatings, led to the development of new radar methods. Radar methods are now used to deal with a variety of problems in the national economy in such areas as navigation, meteorology, aerial photography, and mineral prospecting.
The advent in the 1950’s and 1960’s of space technology complicated and broadened the tasks of radar. The development of rockets and spacecraft required accurate measurement of the trajectories and parameters of their motion for control and for the prediction of the trajectory necessary for an accurate landing of a spacecraft on the earth and other planets. Accurate measurement is also necessary in order to correlate results of scientific measurements, meteorological data, and photographs with the coordinates of a spacecraft and to determine the spacecraft’s relative position. Radar enables two spacecraft to locate one another, rendezvous, and automatically dock. In many of the applications of radar in outer space, a close relationship exists between the radar system and the systems that transmit information, namely, the radio telemetry, space television, and radio communication systems. The radar system is also often linked to the command system and to the computers of the spacecraft’s control system. Often these systems have a common communication channel (common antennas and circuits in the transmitting and receiving apparatus), and in a number of cases they operate with a common signal.
Radar astronomy is another important field of application. By receiving radio signals that have been reflected from planets, radar can measure the distances to planets with great precision and thereby help reduce the error in the determinations of basic astronomical units. It can provide more precise orbital parameters, establish (from the spectral broadening of the reflected radio signal) the periods of rotation of the planets (of Venus, in particular), and examine the topography of planet surfaces. From 1961 to 1963, a group of scientists in the USSR headed by V. A. Kotel’nikov made observations of Venus, Mercury, Mars, and Jupiter.
With the creation of antiballistic-missile defense systems (ABM), radar is now called upon to deal with the complex problems involved in the destruction of enemy rockets. These problems include the detection and tracking of enemy rockets and the guidance of defensive missiles launched against the rockets.
Fundamental principles and methods of radar. Among the numerous principles and methods of radar, the most important are associated with the range of action, the measurement of distance, direction finding, protection against passive interference (the method of selecting moving targets), and resolution (the sidewise-scanning method).
The range of a radar set having its transmitter and receiver positioned at the same site and receiving reflected signals in the absence of passive interference is given by the basic radar equation
where R is the operating range, P is the average power of the scanning signals, T is the time within which the target must be detected or located, Se is the effective area of the receiving antenna, Ω is the solid angle within which the observation is made, Ep is the energy of the reflected signal that is needed to detect the target with a specified reliability or to establish the target’s location with a specified accuracy, and L is a loss factor arising from the difference between real and ideal systems.
This equation is modified according to the particular conditions under which a radar set is used. Thus for ground radar sets designed to detect aerial targets at a certain height, the power radiated from the antenna is utilized most efficiently by choosing a scanning pattern for the antenna that will ensure constancy of the received signals over the entire operating sector regardless of distance. The radar range equations for a radar set receiving signals that are retransmitted from a radar beacon are written separately for two identical distances. One equation is used for the transmission from the radar set to the beacon; the other deals with the retransmission back to the radar set. Both equations that represent the dependency of distance on the channel’s energy potential, that is, on the power of the transmitter and the sensitivity of the receiver, contain R2 instead of R4.
The radar range at microwave frequencies is limited by the curvature of the earth and is equal (in km) to
where h1 and h2 are, respectively, the heights of the target and the radar set above the earth’s surface (in km). The range increases substantially at decametric (short) wavelengths owing to the propagation and consequent reflection of the waves from both the ionosphere (at an average height of 300 km) and the earth’s surface.
The discovery in 1947 by the Soviet scientist N. I. Kabanov of the phenomenon of long-range scattering of decametric waves from the earth’s surface and of the return of these waves to the transmission source after reflection from the ionosphere suggested the theoretical possibility of creating a system that would extend the maximum range of radar. Such a system can be arranged in two ways. In the first, the transmitter and receiver are widely separated and targets are observed between them. The other arrangement employs oblique-return scanning, in which the received signals travel back to the site from which they were radiated (Figure 1).
Two methods are used in the measurement of distance by means of reflected signals. The first, or pulse, method involves the radiation of a pulse and the measurement of the time delay necessary for reception of the reflected or retransmitted pulse from the target. Measurements are facilitated if the reflected signal is not superposed on the scanning signal, that is, if the object is far enough away from the radar set. This method is implemented in the simplest case (Figure 2) by using a pulse transmitter, a receiver (usually of the superheterodyne type), a synchronizer that triggers the transmitter and provides a time scale, and a cathode-ray tube display with a scale for distance readings. Versions of this design include multiple-scale designs using the vernier principle and tracking systems that automatically measure the distance.
The second method of distance measurement depends on observing the interference between two continuous waves, the scanning radiation and the radiation reflected or retransmitted from the target. This method is realized by changing the frequency of the scanning radiation in linear fashion. A mixer attached to both the transmitter and receiver mixes direct and echo signals. The signals entering the mixer, because of their different frequencies, produce beats at a frequency that is proportional to the distance being measured (Figure 3, a and b). After detection, amplification, and limiting, the signals pass on to a frequency meter, which measures the frequency of the beats and has a scale that can be calibrated directly into distance units.
The radial velocity of a target is generally determined with a high degree of accuracy by measuring the Doppler frequency. Signals of long duration must be used here in order to obtain an accurate resolving capability for the velocity and high accuracy
in its measurement. However, an accurate resolving capability for distance involves the use of signals with a large bandwidth. It is therefore advisable in radar to employ complex signals with a large bandwidth having a large base, which is the product formed by multiplying the spectral bandwidth by the duration. In the case of simple signals, such as single monochromatic pulses, the broadening of the signal spectrum in order to obtain better distance resolution is accompanied by a worsening of the velocity resolution.
The direction of a target can be determined by observation from a single, stationary point or by spaced reception. The direction-finding method that has been commonly used in apparatus located at a single point is that wherein the amplitudes of signals are compared. Known as the amplitude method, this method can achieve high accuracy together with automatic directional tracking and a high signal-to-noise ratio. In the simplest case, a comparison is made of the amplitudes of the target echoes corresponding to two different positions of the antenna scanning pattern (Figure 4). From the sign and value of the difference between these signals (the error signal), it is possible to judge the value and sign of the difference between the direction to the target and the equisignal direction (where the error signal is zero). After amplification, the error signal is fed to a tracking system that rotates the antenna to follow the movement of the object; that is, the antenna “tracks” the equisignal direction.
There are two variants of this method. In the first, which is simpler, it is necessary to have only one receiving channel coupled to one antenna. Through either a mechanical or electronic commutation of the appropriate circuits, two positions are assumed by the antenna in its scanning pattern, and an error signal is produced that controls the tracking system. The signals being compared are generated in sequence. In the second variant, the monopulse method, there are two receiving channels coupled to two antennas and the first and second signals are generated simultaneously. The monopulse method is free of the errors caused by signal fluctuations, which are unavoidable in the first variant.
For radar sets operating in the centimetric wavelength range, the first direction-finding variant is accomplished by conical scanning, that is, by the whirling of a radio beam that is inclined with respect to the axis of the antenna’s reflector (the equisignal direction). In synchronism with the rotation of the beam, two orthogonal voltages are developed that are used at the output of the signal channel in the commutation of phase-modulation detectors to obtain an error signal. In the second variant, four beams and two error signals exist simultaneously (from each of the orthogonal pairs of beams).
In addition to the comparison method, the amplitude method is used in the analysis of the envelope of the received signals. This method can provide approximately the same direction-finding accuracy while at the same time scanning with a narrow beam a sector in which there might be several targets.
The use of spaced receivers makes it possible to achieve highly accurate direction finding by measuring the difference in the time of arrival of the signals. Depending on the form of the signals to be received, this measurement can be made by using pulse, correlation, and phase methods.
There has been great progress in the phase method, which is based on the measurement of the phase difference of the high-frequency oscillations received by antennas that are separated by a certain distance called the baseline. This method has the merit of high accuracy, which is attained chiefly by increasing as necessary the baseline. The method is free of errors caused by fluctuations in the signal, which has the same amplitude in the channels of the phase system. When the radio frequency is converted to an intermediate (lower) frequency in a superheterodyne receiver, the phase difference remains unchanged, and its measurement to an accuracy of about 1 ° presents no technical difficulties. In implementing this method, it is important both to preserve the identity and stability of the phase characteristics of the separate receiving channels that admit the oscillations whose phase difference is being measured and to keep the frequency of the received waves and the baseline constant (or else to exercise special control over their change).
The phase method is very convenient also for accurate measurement of the angular velocity of a target emitting its own radiation. By lengthening the baseline, it is possible to increase the sensitivity of the system to a change in the angular coordinates by a large factor, thereby obtaining measurable phase differences for even the slightest angular displacement of the target. The complexity of measuring angular coordinates and their derivatives with these systems is due to the multiplicity of the channels in the design, the stringent requirements on the phase characteristics of the channels, and the need to use a high-performance digital computer for processing the data.
The progress in the phase methods of measuring angular coordinates and their derivatives with radar has been used in radio astronomy, where interferometers with very long baselines (of the order of several thousand km) are used. With these devices, an angular resolution of the order of a thousandth of a second of arc can be achieved.
Of great importance in radar is the method of picking out moving targets, that is, the detection of signals reflected from targets when the signals are masked by radio waves reflected from nearby objects. In the observation of low-flying aircraft, projectiles, or targets moving on the ground, the echoes can be camouflaged by radio waves reflected from buildings, hills, and forests. Choppy seas can reflect waves and thus hamper the detection of submarine periscopes. In the observation of aerial targets, waves can be reflected “clouds” of passive dipole interference. With the coherent-pulse method, the phase of the radiated radio waves is remembered so that when a signal reflected by a target is received, the target’s motion will show as a change in the phase difference between the received and transmitted signals. The periodic trains of pulses that are received from stationary or slow-moving background objects will not differ considerably in phase from the transmitted signals. With the use of filters, these pulses are suppressed and only the signals from moving objects are passed on to the output display. Two designs are used in implementing this method. The first utilizes a transmitter that produces oscillations whose phase can be controlled by, for example, a klystron, as in Figure 5. The other design has a transmitter in which the phase of the oscillations from pulse to pulse is random. Here, a magnetron can be used, as in Figure 6, and the phase of the superhigh-frequency magnetron oscillations is remembered by locking the phase of the coherent local oscillator with each pulse of the scanning signal.
Optimal processing methods for the signals received in coherent radar have made it possible to obtain a high capability for angular resolution in radar sets that are in motion relative to their targets. Such resolution is possible even when the antenna dimensions are comparatively small, that is, when the radio beam is broad. Thus, for the purpose of mapping a locality, a sidewise-scanning method has been developed, which has a synthesized antenna aperture. In radar sets that use this method, the antenna extends in the direction of travel of the aircraft and picks up from each elementary area of a locality echoes that differ in their time lags (due to the motion of the aircraft) and their Doppler frequencies. Since with optimal processing the signals are remembered and summed with the appropriate phase shifts, it becomes possible to obtain the effect of inphase addition of signals. This effect is similar to the effect that would occur with a fixed inphase antenna having an equivalent size D— a size along the flight path determined by the displacement of the aircraft during the period T of the coherent accumulation of the signal:
D = v·T
where v is the speed of the aircraft. Because of the Doppler effect, the frequency change of the oscillations Δf for the surface elements dispersed over the width of a radio beam θ = λ/d, where λ is the wavelength and d is the diameter or side of the an tenna aperture, is equal to
Consequently, after optimal signal processing, the duration of the compressed pulse τ will be equal to
which corresponds to the maximum achievable longitudinal resolution along the flight path. This resolution is given by the relation d= τv and will be equal to ½d if the same aircraft antenna is used not only for receiving but also for radiating, thereby doubling the phase shifts of the reflected oscillations.
REFERENCESTeoreticheskie osnovy radiolokatsii. Edited by V. E. Dulevich. Moscow, 1964.
Sovremennaia radiolokaisiia. Moscow, 1969. (Translated from English.)
Teoreticheskie osnovy radiolokatsii. Edited by Ia. D. Shirman. Moscow, 1970.
Voprosy statislicheskoi teorii radiolokatsii, vols. 1–2. Edited by G. P. Tartakovskii. Moscow, 1973–74.
A. F. BOGOMOLOV
What does it mean when you dream about radar?
If one monitors a radar screen in a dream, it can represent one’s sense of intuition; being receptive to the signals other people are giving out.
An acronym for radio detection and ranging, the original and still principal application of radar. The name is applied to both the technique and the equipment used.
Radar is a sensor; its purpose is to provide estimates of certain characteristics of its surroundings of interest to a user, most commonly the presence, position, and motion of such objects as aircraft, ships, or other vehicles in its vicinity. In other uses, radars provide information about the Earth's surface (or that of other astronomical bodies) or about meteorological conditions. To provide the user with a full range of sensor capability, radars are often used in combinations or with other elements of more complete systems.
Radar operates by transmitting electromagnetic energy into the surroundings and detecting energy reflected by objects. If a narrow beam of this energy is transmitted by the directive antenna, the direction from which reflections come and hence the bearing of the object may be estimated. The distance to the reflecting object is estimated by measuring the period between the transmission of the radar pulse and reception of the echo. In most radar applications this period will be very short since electromagnetic energy travels with the velocity of light.
Kinds of radar
The physical nature of radars varies greatly. Several radars are available for use on small boats as a safety and navigation aid, some so small as to be carried by an operator. Another radar seen in a hand-held form is that used by police to measure the speed of automobiles.
Perhaps the largest radars are those covering acres of land, long arrays of antennas all operating together to monitor the flight of space vehicles or astronomical bodies. Other very large radars are designed to monitor flight activity at substantial distances. These are large mainly because they must use longer-than-usual radio wavelengths associated with ionospheric containment of the signal for over-the-horizon operations.
More common in size are those radars seen at airports, with rotating antennas 20– 40 ft (6–12 m) wide. Radars intended for mobile use, particularly airborne radars, are quite compact.
Airborne and spaceborne radars have been developed to perform ground mapping with extraordinary resolution by special Doppler-sensitive processing while the radar is moved over a substantial distance. Such radars are called synthetic-aperture radars (SARS) because of the very large virtual antenna formed by the path covered while the processing is performed. Interferometry can provide topological information (3D SAR), and polarimetry and other signal analysis can provide more information on the nature of the surface (type of vegetation, for example). See Synthetic aperture radar (SAR)
Radars intended principally to determine the presence and position of reflecting targets in a region around the radar are called search radars. Other radars examine further the targets detected: examples are height finders with antennas that scan vertically in the direction of an assigned target, and tracking radars that are aimed continuously at an assigned target to obtain great accuracy in estimating target motion. In some modern radars, these search and track functions are combined, usually with some computer control. Surveillance radar connotes operation of this sort, somewhat more than just search alone. There are also very complex and versatile radars with considerable computer control, with which many functions are performed and which are therefore called multifunction radars. Very accurate tracking radars intended for use at missile test sites or similar test ranges are called instrumentation radars. Radars designed to detect clouds and precipitation are called meteorological or weather radars.
Some radars have separate transmit and receive antennas sometimes located miles apart. These are called bistatic radars, the more conventional single-antenna radar being monostatic. Some useful systems have no transmitter at all and are equipped to measure, for radarlike purposes, signals from the targets themselves. Such systems are often called passive radars, but the terms radiometers or signal intercept systems are generally more appropriate.
The terms primary and secondary are used to describe, respectively, radars in which the signal received is reflected by the target and radars in which the transmission causes a transponder (transmitter-responder) carried aboard the target to transmit a signal back to the radar.
It is convenient to consider radars composed of four principal parts: the transmitter, antenna, receiver, and display (see illustration).
The transmitter provides the rf signal in sufficient strength (power) for the radar sensitivity desired and sends it to the antenna, which causes the signal to be radiated into space in a desired direction. The signal propagates (radiates) in space, and some of it is intercepted by reflecting bodies. These reflections, in part at least, are radiated back to the antenna. The antenna collects them and routes all such received signals to the receiver, where they are amplified and detected. The presence of an echo of the transmitted signal in the received signal reveals the presence of a target. The echo is indicated by a sudden rise in the output of the detector, which produces a voltage (video) proportional to the sum of the rf signals being received and the rf noise inherent in the receiver itself. The time between the transmission and the receipt of the echo discloses the range to the target. The direction or bearing of the target is disclosed by the direction the antenna is pointing when an echo is received.
A duplexer permits the same antenna to be used on both transmit and receive, and is equipped with protective devices to block the very strong transmit signal from going to the sensitive receiver and damaging it. The antenna forms a beam, usually quite directive, and, in the search example, rotates throughout the region to be searched. See Antenna (electromagnetism)
The radar reflections are among the signals received by the antenna in the period between transmissions. Most search radars have a pulse repetition frequency (prf), antenna beam-width, and rotation rate such that several pulses are transmitted (perhaps 20 to 40) while the antenna scans past a target. This allows a buildup of the echo being received. Most radars are equipped with low-noise rf preamplifiers to improve sensitivity. The signal is then “mixed” with (multiplied by) a local oscillator signal to produce a convenient intermediate-frequency (i-f) signal, commonly at 30 or 60 MHz; the same principle is used in all heterodyne radio receivers. The local oscillator signal, kept offset from the transmit frequency by precisely this intermediate frequency, is supplied by the transmitter oscillators during reception. After other significant signal processing in the i-f circuitry (of a digital nature in many newer radars), a detector produces a video signal, a voltage proportional to the strength of the processed i-f signal. This video can be applied to a cathode-ray-tube (CRT) display so as to form a proportionately bright spot (a blip), which could be judged to originate from a target echo. However, increasingly radars use artificial computerlike displays based on computer analysis of the video. Automatic detection and automatic tracking (based on a sequence of dwells) are typical of such data processing, reports being displayed for radar operator management and also made instantly available to the user system. See Cathode-ray tube, Mixer, Preamplifier
Radar carrier frequencies are broadly identified by a nomenclature that originated in wartime secrecy and has since been found very convenient and widely accepted. The spectrum is divided into bands, the frequencies and wavelengths of which are given in the table. The charged layers of the ionosphere present a highly refractive shell at radio frequencies well below the microwave frequencies of most radars. Consequently, over-the-horizon radars have been built in the 10-MHz area to exploit this skip path.