Fire Controlman, Volume 2–Fire-Control Radar Fundamentals

Figure 1-1.—Radar surface angular measurements.Table 1-1.—Radar Reference Coordinate TermsTermDefinitionEnergy pulsesThe pulses that are sent out by the radar and are received back from the target.Reflecting targetThe air or surface contact that provides an echo.True northThe direction of the north geographical pole.True bearing/azimuthThe angle measured clockwise from true north in the horizontal plane.Line-of-sight rangeThe length of the line from the radar set directly to the object.Vertical planeAll angles in the up direction, measured in a secondary imaginary plane.Elevation angleThe angle between the horizontal plane and the line of sight.Horizontal planeThe surface of the Earth, represented by an imaginary flat plane which istangent (or parallel) to the Earth’s surface at that location.1-2

RADAR MEASUREMENTSpeak power of the transmitted pulse; pulse-repetitionfrequency (PRF) or pulse-repetition rate (PRR) (PRFand PRR are synonymous terms.); and receiversensitivity, with PRF/PRR as the primary limitingfactor.We stated earlier that radar is used to determine thedistance and direction to and the height of distantobjects. These three pieces of information are known,respectively, by the standard terms range, bearing, andaltitude. The use of these standard terms allows anyoneinterested in a specific target to establish its positionquickly and accurately. Radar operators determine atarget’s range, bearing, and altitude by interpreting itsposition displayed on a specially designed cathode-raytube (CRT) installed in a unit known as a plan positionindicator (PPI).The peak power of a pulse determines how far thepulse can travel to a target and still return a usable echo.A usable echo is the weakest signal that a receiver candetect, process, and present on a display.The PRR determines the rate at which the rangeindicator is reset to zero. As the leading edge of eachpulse is transmitted, the indicator time base used tomeasure the returned echo is reset, and a new sweepappears on the screen.While most radars are used to detect targets, sometypes are used to guide missiles to targets and to directthe firing of gun systems; other types providelong-distance surveillance and navigation information.RANGE ACCURACY.—The shape and width ofthe radio-frequency (RF) pulse influences minimumrange, range accuracy, and maximum range. The idealpulse shape is a square wave that has vertical leadingand trailing edges. The vertical edge provides adefinite point from which to measure elapsed time onthe indicator time base. A sloping trailing edgelengthens the pulsewidth. A sloping leading edgeprovides no definite point from which to measureelapsed time on the indicator time base.Range and bearing (and in the case of aircraft,altitude) are necessary to determine target movement.To be a successful radar operator, you must understandthe capabilities and limitations of your radar system indetermining range, bearing, and altitude.RangeThe radar measurement of range (or distance) ispossible due to the properties of radiatedelectromagnetic energy. This energy normally travelsthrough space in a straight line, at a constant speed, andvaries only slightly due to atmospheric and weatherconditions. The frequency of the radiated energycauses the radar system to have both a minimumeffective range and a maximum effective range.Other factors affecting range are the antenna’sheight, beamwidth, and rotation rate. A higher antennawill create a longer radar horizon, allowing a greaterrange of detection. An antenna with a narrowbeamwidth, provides a greater range capability, since itprovides more concentrated beam with a higher energydensity per unit area. A slower antenna rotation rate,providing more transmitted pulses during the sweep,allows the energy beam to strike each target moretimes, providing stronger echo returns and a greaterdetection range.M I N I M U M R A N G E . — R a d a r d u p l exe r salternately switch the antenna between the transmitterand the receiver so that one antenna can be used forboth functions. The timing of this switching is criticalto the operation of the radar and directly affects theminimum range of the radar system. A reflected pulsewill not be received during the transmit pulse andsubsequent receiver recovery time. The minimumrange of a radar, therefore, is the minimum distancebetween the radar’s antenna and a target at which aradar pulse can be transmitted, reflected from thetarget, and received by the radar receiver. If theantenna is closer to the target than the radar’s minimumrange, any pulse reflected from the target will returnbefore the receiver is connected to the antenna and willnot be detected.From the range information, the operator knowsthe distance to an object. He now needs bearinginformation to determine where the target is inreference to the ship.BearingRadar bearing is determined by the echo’s signalstrength as the radiated energy lobe moves past thet a rg e t . S i n c e s e a r c h r a d a r a n t e n n a s m ovecontinuously, the point of maximum echo return isdetermined either by the detection circuitry as thebeam passes the target or visually by the operator.Weapons control and guidance radar antennas arepositioned to the point of maximum signal return andMAXIMUM RANGE.—The maximum range ofa pulse-radar system depends on carrier frequency;1-3

are maintained at that position either manually or byautomatic tracking circuits.method of transmitting energy. The most commonmethod, used for applications from navigation to firecontrol, is the pulse-modulation method. The othermethod of transmitting is continuous-wave (CW).CW radars are used almost exclusively for missileguidance.You need to be familiar with two types of bearing:true and relative.TRUE BEARING.—True bearing is the anglebetween true north and a line pointed directly at thetarget. This angle is measured in the horizontal planeand in a clockwise direction from true north.Pulse ModulationIn the pulse method, the radar transmits the RF in ashort, powerful pulse and then stops and waits for thereturn echo. By measuring the elapsed time betweenthe end of the transmitted pulse and the received echo,the radar can calculate a range. Pulse radars use oneantenna for both transmitting and receiving. While thetransmitter is sending out its high-power RF pulse, theantenna is connected to the transmitter through aspecial switch called a duplexer. As soon as thetransmitted pulse stops, the duplexer switches theantenna to the receiver. The time interval betweentransmission and reception is computed and convertedinto a visual indication of range in miles or yards.Pulse-radar systems can also be modified to use theDoppler effect to detect a moving object. The Navyuses pulse radars to a great extent.RELATIVE BEARING.—Relative bearing is theangle between the centerline of the ship and a linepointed directly at the target. This angle is measured ina clockwise direction from the bow. Mostsurface-search radars provide only range and bearinginformation. Both true and relative bearing angles areillustrated in figure 1-2.AltitudeAltitude or height-finding radars use a very narrowbeam in the vertical plane. This beam is scanned inelevation, either mechanically or electronically, topinpoint targets. Tracking and weapons-control radarsystems in current use scan the beam by moving theantenna mechanically or the radiation sourceelectronically.Continuous WaveMost air-search radars use electronic elevationscanning techniques. Some older air-search radarsystems use a mechanical elevation scanning device;but these are being replaced by electronically scanningradar systems.In a CW radar the transmitter sends out a“continuous wave” of RF energy. Since this beam ofRF energy is “always on”, the receiver requires aseparate antenna. One disadvantage of this method isthat an accurate range measurement is impossiblebecause there is no specific “stop time”. This can beovercome, however, by modulating the frequency. Afrequency-modulated continuous wave (FM-CW)radar can detect range by measuring the differencebetween the transmitted frequency and the receivedfrequency. This is known as the “Doppler effect”. Thecontinuous-wave method is usually used byfire-control systems to illuminate targets for missilesystems.RADAR TRANSMISSION METHODSRadar systems are normally divided into twooperational categories (purposes) based on theirRADAR SYSTEM ACCURACYTo be effective, a radar system must provideaccurate indications. That is, it must be able todetermine and present the correct range, bearing, and,in some cases, altitude of an object. The degree ofaccuracy is primarily determined by two factors: theresolution of the radar system and existingatmospheric conditions.Figure 1-2.—True and relative bearings.1-4

Range ResolutionRange resolution is the ability of a radar todistinguish between two targets on the same bearing,but at slightly different ranges. The degree of rangeresolution depends on the width of the transmittedpulse, the types and sizes of the targets, and theefficiency of the receiver and the indicator.Bearing ResolutionFigure 1-3.—Ducting effect on the radar wave.Bearing, or azimuth, resolution is the ability of aradar system to separate objects at the same range, butat slightly different bearings. The degree of bearingresolution depends on the radar’s beamwidth and therange of the targets. The physical size and shape of theantenna determines beamwidth. Two targets at thesame range must be separated by at least onebeamwidth to be distinguished as two objects.usable range. Usable range varies widely with suchweather conditions. The higher the frequency of theradar system, the more it is affected by weatherconditions, such as rain or clouds.Other FactorsSome other factors that affect radar performanceare operator skill; size, composition, angle, andaltitude of the target; possible Electronic Attack (EA)activity; readiness of equipment (completed plannedmaintenance system requirements); and weatherconditions.Atmospheric ConditionsSeveral conditions within the atmosphere can havean adverse effect on radar performance. A few of theseare temperature inversion, moisture lapse, waterdroplets, and dust particles.The temperature and moisture content of theatmosphere normally decrease uniformly with anincrease in altitude. However, under certain conditionsthe temperature may first increase with height and thenbegin to decrease. Such a situation is called atemperature inversion. An even more importantdeviation from normal may exist over the ocean. Sincethe atmosphere close to the surface over large bodies ofwater may contain more than a normal amount ofmoisture, the moisture content may decrease morerapidly at heights just above the sea. This effect isreferred to as moisture lapse.Q1. For radar surface angular measurements, whatis considered to be at the center of the coordinatesystem?Q2. What determines radar bearing?Q3. What is the most common method of radartransmission?Q4. What two factors determine radar accuracy?BASIC RADAR SYSTEMSRadar systems, like other complex electronicssystems, are composed of several major subsystemsand many individual circuits. Although modern radarsystems are quite complicated, you can easilyunderstand their operation by using a basic blockdiagram of a pulse-radar system.Either temperature inversion or moisture lapse,alone or in combination, can cause a large change in therefraction index of the lowest few-hundred feet of theatmosphere. The result is a greater bending of the radarwaves passing through the abnormal condition. Thisincrease in bending, referred to as ducting, may greatlyaffect radar performance. The radar horizon may beextended or reduced, depending on the direction inwhich the radar waves are bent. The effect of ducting isillustrated in figure 1-3.FUNDAMENTAL (PULSE) RADAR SYSTEMSince most radars used today are some variation ofthe pulse-radar system, this section discussescomponents used in a pulse radar. All other types ofradars use some variation of these units. Refer to theblock diagram in figure 1-4.Water droplets and dust particles diffuse radarenergy through absorption, reflection, and scattering.This leaves less energy to strike the target, so the returnecho is smaller. The overall effect is a reduction in1-5

DuplexerDUPLEXERThe duplexer is basically an electronic switch thatpermits a radar system to use a single antenna totransmit and receive. The duplexer disconnects theantenna from the receiver and connects it to thetransmitter for the duration of the transmitted pulse.The switching time is called receiver recovery time,and must be very fast if close-in targets are to bedetected.RECEIVERReceiverTRANSMITTERThe receiver accepts the weak RF echoes from theantenna system and routes amplified pulses to thedisplay as discernible video signals. Because the radarfrequencies are very high and difficult to amplify, asuperheterodyne receiver is used to convert the echoesto a lower frequency, called the intermediate frequency(IF), which is easier to RPOWERCONTROLGROUPDisplaysMost of the radars that FCs operate and maintainhave a display, or multiple displays, to provide theoperator with information about the area the radar issearching or the target, or targets, being tracked. Theusual display is a cathode-ray tube (CRT) that providesa combination of range, bearing (azimuth), and (insome cases) elevation data. Some displays provide rawdata in the form of the signal from the radar receiver,while others provide processed information in the formof symbology and alphanumerics.Figure 1-4.—Basic radar block diagram.SynchronizerThe heart of the radar system is the synchronizer. Itgenerates all the necessary timing pulses (triggers) thatstart the transmitter, indicator sweep circuits, andranging circuits. The synchronizer may be classifiedas either self-synchronized or externally synchronized. In a self-synchronized system, pulses aregenerated within the transmitter. Externallysynchronized system pulses are generated by sometype of master oscillator external to the transmitter,such as a modulator or a thyratron.Figure 1-5 shows four basic types of displays.There are other variations, but these are the major typesencountered in fire control and 3-D search radars.TYPE A.—The type A sweep, or range sweep,display shows targets as pulses, with the distance fromthe left side of the trace representing range. Variationsin target amplitude cause corresponding changes in thedisplayed pulse amplitude. The display may be bipolarvideo when used with Moving Target Indicator (MTI)or pulse Doppler radars.TransmitterThe transmitter generates powerful pulses ofelectromagnetic energy at precise intervals. It createsthe power required for each pulse by using ahigh-power microwave oscillator (such as a magnetron) or a microwave amplifier (such as a klystron)supplied by a low power RF source.TYPE B.—The type B sweep, or bearing sweep, ismostly found with gunfire control radars and is usedwith surface gunfire to spot the fall of shot. The rangemay be full range or an interval either side of the rangegate.For further information on the construction andoperation of microwave components, review NEETSModule 11, Microwave Principles, NAVEDTRA172-11-00-87.TYPE E.—Two variations of type E are shown.Both provide range and elevation or height of a target.These are associated with height-finding radars and are1-6

Figure 1-5.—Types of radar displays.Antenna Systemgenerally used to determine the height or elevationangle only. Range is determined from processing or atype P display.The antenna system routes the pulse from thetransmitter, radiates it in a directional beam, picks upthe returning echo, and passes it to the receiver with aminimum of loss. The antenna system includes theantenna; transmission lines and waveguide from thetransmitter to the antenna; and transmission lines andwaveguide from the antenna to the receiver.TYPE P.—This display is commonly called a PPI(plan position indicator). Own ship is usually thecenter. Range is measured radially from the center.The range display can be selected, and the radar sourceis usually selectable. The PPI can display raw video orsymbology and alphanumerics, or both. The type Pdisplay is most commonly found in the CombatInformation Center (CIC) and in weapons controlstations.Before we discuss some types of antennas used infire control, we need to review the basic principles ofelectromagnetic wave radiation and reflectors.The radar energy that forms the target-trackingand illumination beams is transmitted by an antennaat the control point. Radiated energy tends to spreadAdditional information on how individual displaysare produced is available in NEETS modules 6, 9, and18.1-7

out equally in all directions, as shown in figure 1-6.Figure 1-6 compares the radiation from a radio antennawith that from a lamp. Both light waves and radiowaves are electromagnetic radiation; the two arebelieved to be identical, except in frequency ofvibration. From both sources, energy spreads out inspherical waves. Unless they meet some obstruction,these waves will travel outward indefinitely at thespeed of light.that radio wave energy must be concentrated to beuseful. We can concentrate this energy by mounting asuitable reflector behind the antenna, to form a largepart of the radiated energy into a relatively narrowbeam. The following paragraphs discuss the morecommonly used reflectors.PARABOLIC REFLECTORS.—You should befamiliar with the use of polis

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