CHAPTER 2

Basic Radar and Weather Radar Technical Characteristics

WE HAVE ALL heard the expression "blindas a bat" and are aware that a bat's blindnessis only relative. Because of his primarilynocturnal habits and his making his home indark caves, the bat has more highly developed"night vision" than we. You've probablyexperienced the momentary discomfortinvolved in emerging from a dark theater intobright sunlight. No doubt, much the samereaction is experienced by a bat in relation tobright light, which prompts him to avoid it.His "blindness," then, is more a matter ofselective sensitivity than organic defect.

2. The interesting thing about a bat is thathe can flit about at rather high speeds withoutbumping into things, even though he may bein an environment where his vision is seriouslyrestricted. Nature has a way of compensatingfor the impairment or inefficiency of oneorgan of sensory perception by sharpening thesensitivity of others. Research has shown thatthe bat has long since developed a form of"sonar," which is similar to radar, without thehighly technical electronics upon which radardepends. He emits almost continuoushigh-pitched shrieks, which strikeobstructions in his path and echo back. Thebat instantaneously evaluates the "echo data"and alters his course accordingly.

3. The basic notion of radar is similar tothat of a bat's sonar. The exception is thatinstead of "high-pitched shrieks," a radaremits short, intense pulses of radio energythat it directs outward in a narrow beam. If apulse intercepts a target within the path ofthe beam, some of the energy contained inthe pulse is reflected. A small portion of thereflected energy returns to the point oftransmission, where it is received, amplified,and presented on indicators (scopes).

4. Why is the Air Weather Serviceinterested in radar systems? You will find thatradar is the most practical means to detectcertain valuable information, such as distantcumulus cloud formations, for the forecaster.Since AWS is deeply involved in radar storm

detection, it should be obvious that it is toyour advantage to learn all you can about thefundamental principles of radar. If you knowand can apply these principles, you will findthat your work is easier; you will be able totroubleshoot and repair radar and otherelectronic equipment more quickly andaccurately.

5. In this chapter, we begin with adiscussion of basic radar principles. Thisdiscussion explains the way to determine therange and direction of an echo. The discussionalso introduces you to a basic radar system.Following this discussion is a presentation ofthe functional and technical characteristics ofthe AN/FPS-77 and AN/TPQ-ll radar sets.

7. Radar Principles7-1. Radar detects the presence of objects

with radio echoes. Radar indicates thedirection and range of those objects andmakes it possible for you to recognize someof their characteristics. It is a means oflocating aircraft, flying objects, ships, landmasses, cities, rivers, lakes, and suspendedliquid or frozen water particles at short orlong distances. Although radar supplies muchinformation concerning the character of theseobjects (targets), it has limitations when itcomes to locating small targets or individualtargets within the group.

7-2. For example, if a small group ofcampers or a hunting party is lost in theforest, you cannot locate them by radar. Onthe other hand, it is possible to locate a smallboat on the high seas if the boat contains areasonable quantity of metal. Even a floatingoil drum on the surface of the sea can belocated with search radar sets. In searching forsmall objects with radar equipment, thedegree of the success is largely determined bythe skill and experience of the observer.

7-3. Echoes are the sound waves that carryour voices back to us from a reflecting surfacesuch as a cliff. Similarly a radar set can receive

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TIME ELAPSEDo MICROSECONDS

~j ~/ INDICATOR

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TIME ELAPSED100 MICROSECONDS

-3-2,-7 :;::""6::-::'(-;;:AO'R-;-Ds~-:JI-_ - - - - - - - - - -7fI',&

REFLECTED PULSE

Figure 31. Determination of range.

the faint echo of radio waves that return afterit sends out a powerful radio signal. Radiosignals behave in a manner similar to lightwaves. We know that light waves are reflectedby solid materials and are refracted when theypass from one medium to another. Radarradio waves are in the frequency range of theUHF band and above. The higher thefrequency, the more easily the wave isreflected by objects and changing mediums.

7-4. Whether it is a sound or a radio signalthat is transmitted, the original signal must bepowerful enough to produce an echo. In radarequipment, the magnitude of the echo isdetermined by the power output of thetransmitter, the material and size of thetarget, the distance to the target, and thesensitivity of the receiving equipment. Mostradar sets send out signals (pulses) oftremendous energy. These pulses are only afew millionths of a second (microsecond)long. To detect the return signal from thetarget, special antennas and receivers tuned to

the transmitter frequency are used. Unlikeradio, the radar transmitter and receiver are atthe same location.

7-5. The transmitter sends out pulses ofenergy at precisely timed intervals, then waitsduring a silent period or "listening time"between pulses. This silent period is usuallyonly a few thousandths of a second, ormilliseconds as you are more accustomed tohealing. During the interval betweentransmitted pulses, the receiver is working. Itreceives the return signals (echoes) from thetargets that are in the path of the transmittedsignal.

7-6. Range and Direction Measurement.Echoes from the targets that are near thereceiver are received soon after thetransmitter is turned off. Echoes from targetsthat are at a greater distance are received laterthan those from the nearby objects.Therefore, it follows that when you expectechoes from nearby targets the transmittedpulse must have a short duration. The

29

transmitter must turn off in time for thereceiver to receive the reflected signal fromthe shortest range that is consistent with themaximum range required from the set. We'lldiscuss this further later in the text.

7-7. The time that elapses between thetransmission of the pulse and the return ofthe echo is used to compute the distance ofthe target. Since we know that light and radiowaves travel at the speed of 186,000 miles(300,000,000 meters) per second, we cancompute the distance by multiplying one-halfof the signal travel time by the speed of light.In actual operation, this problem is solvedautomatically and simultaneously by visualindicators such as cathode-ray tubes (CRTs).

7-8. Light or radar pulses travel 327.36yards (982.08 feet) in one-millionth of asecond (1 microsecond). Since the pulsestravel to the target and return, they traveltwice the distance between the radar antennaand the target. Thus, a target 1000 yards fromthe antenna returns a pulse echoapproximately 6 microseconds after the pulseis transmitted. Even though this is anextremely short time, the radar set measuresthe pulse travel time accurately at targetranges of 5 or 10 yards, or to about 0.30microsecond. Thus, radar pulses measurerange or distance.

7-9. A receiver at the same location as thetransmitter intercepts some of this reflectedenergy and produces a visual indication of theobject on the face of a CRT. Figure 31 showsan aircraft that reflects a radar pulse. Thereflected signal is detected by the antenna,and the radar unit converts the time delay toindicate a distance of 32,736 yards. Awell-developed cloud produces a similarindication. To employ the time-rangerelationship, the radar system must measuresmall units of time very accurately. For anA-scan indicator as the one in figure 31, thehorizontal sweep generator makes accuratemeasurement possible. It generates a sawtoothvoltage that causes the sweep dot to moveacross the screen at a uniform speed. In figure31, the face of the scope is calibrated toindicate the range as the electron beamsweeps across the tube.

7-10. Assume that a cathode-ray tube usesa horizontal sweep (A-scan) which produces abeam whose velocity across the screen is 1inch per 100 microseconds. Part of thetransmitted pulse and the echo from theaircraft are applied to the vertical deflectionsystem. They cause vertical deflections on theCRT'indicator. When the pulse is transmitted,a vertical deflection appears at the beginningof the sweep. When the transmitter pulse has

reached the aircraft 100 microseconds later,the beam has moved 1 inch across the face ofthe tube. When the reflected pulse returns tothe receiver after 200 microseconds, thesweep has moved 2 inches. The trace isdeflected vertically at this point. If the widthof the screen is calibrated in yards or miles,you can read the distance to the targetdirectly from the indicator.

7-11. Another way to determine the rangeof an echo is by superimposing range markson the sweep. A range mark oscillatorgenerates the range marks and causes them toappear as either dots or spikes along thesweep. The space between the marksrepresents a given distance. For example, ifthe distance between range marks represents adistance of 5 miles and an echo appears threemarks from the start of the sweep, the echo is15 miles from the transmitter. Range marksare used by both the AN/FPS-77 andAN/TPQ-ll to determine the echo range.

7-12. On some sets. such as theAN/TPQ-ll, a single sweep does not persiston the CRT long enough to be useful.Therefore, it is necessary to repeat the pulsetransmission and the sweep. The twooperations start simultaneously and successiveechoes from the same target are superimposedon each other. The echoes from all targetswithin range are shown on the screen at theirproper sweep time or range.

7-13. The direction (bearing) of a targetfrom the radar system is usually given as anangular direction called azimuth. The azimuthis measured from true north if the radarinstallation is permanent. As shown in figure32, the azimuth of the target is measured byusing the direction the radar antenna ispointing.

7-14. The position and shape of theradiating element cause the antenna system tosend out more energy in some directions thanothers. For weather radar, the antenna andreflector produce a single, narrow beam of RFenergy in one direction. This radiation patternconcentrates maximum energy in the

AZIMUTHSCALE

DIRECTIONALANTENNA

Figure 32. Antenna and azimuth scale.

30

'-

•• TRANSMITTER-

~TIMER MODULATOR I

I II

! L ________ POWER _____ ..J ANTENNASUPPLY SYSTEM,------- ------,

I I

INDICATOR RECEIVER I~ ~

,,---...

Figure 33. Block diagram of a basic radar set.

direction the antenna is pointing. Thetransmitting pattern of an antenna system isalso its receiving pattern. Therefore, the sameantenna transmits RF energy and receivesreflected RF energy. The direction theantenna points is the azimuth of the targetand is electrically routed to the directionindicator.

7-15. Basic Radar System. A basic radarsystem consists of six units, as shown in figure33. They are the timer, transmitter-modulator, antenna system, receiver,indicator, and power supply. In some radarsets they may not be included as six separateunits, but the functions of all six arenecessary. For example, the timer may bepart of the transmitter, the indicator may bepart of the receiver, or the power supply maybe included as part of the transmitter andreceiver. On the other hand, there may bemore than six units, but the overall operationof the' system still consists of the same sixfunctions. For example, there may beseparate antennas for the transmitter andreceiver as in the AN/TPQ-ll. However, mostradar sets, such as the AN/FPS-77, use thesame antenna system for both the transmitterand receiver. The following discussion coversthe functions of each unit in the blockdiagram.

7-16. Timer. The timer has several veryimportant functions. As the name implies, ittimes or synchronizes the start of eachtransmitter pulse with the correspondingsweep on the screen of the indicator. This isnecessary for the range indicator to depictaccurately the distance to the target. Thetimer determines the frequency at whichpulses are radiated from the antenna (pulserepetition frequency, PRF). In other words,the timer is a kind of electronic switch thatcauses the transmitter to turn on and off atthe appropriate time. At the same time, thetimer starts the sweep on the cathode-raytube.

7-17. If the time interval between pulses istoo short, a succeeding pulse obscures theCRT while echoes are still returning from thepreceding pulse. If the interval is too long, thechange in the position of the target on theCRT is too great, and the system decreases inusefulness. In addition, too few pulses persecond may allow the image on the CRT tofade between sweeps.

7-18. Transmitter-modula tor. Thetransmitter portion of thetransmitter-modulator contains the circuitsthat generate the carrier frequency of theradar system. The beam directivity and theoperational requirements of the set determinewhat the carrier frequency should be. Thecarrier frequency determines the type of RFgenerator that is used. Both the AN/TPQ-lland AN/FPS-77 use magnetrons as RFgenerators.

7-19. In radar equipment, the magnetron isthe only stage of oscillation at the carrierfrequency. The circuits preceding themagnetron shape the high-voltage pulse thattriggers the magnetron and are called themodulator section. The modulator determinesthe pulse width and pulse amplitude. Themodulator triggers and controls the operationof the magnetron. For accurate rangedetermination, the output pulse must beginand end sharply. Therefore, the output pulseof the modulator, which triggers thetransmitter, must be a square wave with steepleading and trailing edges.

7-20. Antenna system. A radar antennasystem has two functions. It radiates thepulses generated by the transmitter, and itreceives the reflected pulses for the receiver.The most common radar antenna consists of ahalf-wave-length dipole antenna with aparabolic reflector. The dipole antenna is veryshort. At a frequency of 9000 megaHertz(MHz), it is only 17'3centimeters (ern) long.(2.54 cm is 1 inch.) It is mounted at the endof the waveguide (transmission line) leading

31

I I

-, t t t /RANGE MARKS

'RANGE

Figure 34. A-scan indicator.

from the magnetron. The parabolic reflectorconcentrates the radiated energy into anarrow beam that is directed toward a target.The reflector is similar to the parabolicreflector in the headlight of an automobile.

7-21. When the transmitter signal strikes a~get, only a part of the energy is reflected.Since the reflected signal is nondirectional,only a small part of the transmitted energyreturns to the reflector. The reflector focusesthe return on the dipole antenna. The dipolereturns the RF energy to the waveguide andon to the receiver. Electronic switching tubes(duplexers) in the waveguide connect theantenn~ .system to the transmitter duringtransmitting and to the receiver duringreceiving so that the same antenna is used forboth transmitting and receiving.

7-22. Receiver. All radar receivers are ofthe superheterodyne type. The mixing(heterodyning) action takes place in thereceiver waveguide. The local oscillatoroutput is coupled into the waveguide by aprobe or a loop. A crystal detector, circuitresonant at the IF frequency, is located in themixing waveguide. The output signal from thecrystal mixer is applied to the first IFamplifier circuit. The IF is usually in therange of 20 to 60 MHz. Several wide-bandstages amplify the IF signals.

7-23. The output signal from the IFamplifier section is applied to the videodetector. The video detector applies the signalto the video amplifiers. The video amplifiersare ~i?e-band, resistance-capacitance-coupledamplifiers. The output of the video amplifiersIS a series of rectified pulses, which areapplied to the CRT .

. 7-2~. I!",di~ator. The indicator producesVISUalindications to depict the position of thetargets, such as showers or clouds. The maincomponent in the indicator is the cathode-ray

tube. The rectified video pulses from thereceiver are applied to the grid or cathode ofsome types of indicator displays or to thevertical deflection plates of other displays.

7-25. The A-scan, shown in figure 34,furnishes an indication of range andamplitude (intensity). This type of scanconsists of a single horizontal line across theface of the CRT. The left end of thehorizontal trace shows part of the radiatedp~lse and is called the main bang. The pulses(pips) that appear to the right of this pulserepresent the targets and are at a distancecorresponding to their range or distance fromthe transmitter.

7-26. All targets along the sweep of theA-scan appear as upward vertical deflectionsof the sweep since the video is applied to theCRT vertical deflection plates. The amplifierhas a gain control that the operator uses tovary the amplitude of the echoes. You set thegain control so that electronic disturbances(noise) in the amplifiers appear as innumerablesmall echoes across the whole trace. Thisnoise is called grass.. ~-27. ~he Pp'I-scan (plan positionindicator) ISa maphke presentation of the areacovered by the radar with the antennalocation as the center of the presentation asshown in figure 35. Targets appear as brightspots on the scope. You determine theazimuth of a target from the compass rosearound the scope. The compass rose iscalibrated in degrees. You determine therange of the targets by calibrated range marks(which appear as circles) along the sweep as itmoves from the center to the edge of theCRT.

7-28. The PPI-scan is developed by shiftingthe start of the sweep (time base) to thecenter of the CRT and by apparently rotating

Figure 35. PPI presentation.

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A MAGNETIC FIELD FORCES THEBEAM AT A RIGHT ANGLE TO THE

LINES OF FORCE

DEFLECTIONCOILS

TIME BASE A-8 IS FOBMEDBY SWEEP CURRENT IN

DEFLECTION COILS.

A

AS THE COILS ARE REVOLVEDIN SYNCHRONISM WITH THEROTATION OF THE ANTENNATHE TIME BASE LINE SCANSTHE FACE OF THE SCREEN.

Figure 36. Relation of PPI deflection coil position to CRT presentation.

the time base. The rotation is not caused byrotating a single time base sweep but bygenerating multiple sweeps and slightlychanging the position of each succeedingsweep. This makes it appear that a singlesweep is rotating. The sweep is rotated by

rotating the deflection coils, as shown infigure 36, or by rotating their magnetic field.The video is applied to the grid or cathode ofthe PPI type indicator. This causes the echoesto appear as bright spots on the face of theCRT. The bright spots correspond to the

RADOME

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Figure 37. Radome, AN/FPS-77

33

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Figure 38. Antenna, AN/FPS-77.

34

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Figure 39. Control amplifier, AN/FPS'77.

35

location of the targets when the direction theantenna points is synchronized with theposition of the CRT sweep.

7-29. Power supply. Although the powersupplies for the various components in a radarsystem are not located in a single unit, theyare usually represented by a single block in ablock diagram. In most larger radar sets, thereceiver, transmitter, timer, and indicatoreach has a separate power supply. Each powersupply is furnished AC power from a centralsource. Since the aquadag voltage of acathode-ray tube is higher than most othervoltages of a radar system, an additionalhigh-voltage power supply is required for theindicator. The transmitter magnetron requiresan even higher voltage with a large amount ofpower. Therefore, the modulator, whichsupplies the magnetron pulse, must have ahigh-voltage power supply.

8. Unit Functions8-1. Both the AN/FPS-77 and AN/TPQ-ll

radar systems are composed of major units.This section gives you an idea of the physicalappearance of each major unit and presents abrief description of the function of each unitin the system.

8-2. AN/FPS-77. The purpose of this radarset is to detect weather phenomena, such asstorms and precipitation, and to display andrecord a vertical and horizontal cross sectionof the weather. Clouds and other weatherphenomena reflect radiated RF energy invarying amounts, depending on thecomposition of the phenomena. The radarprocesses the return signals and displays themon cathode-ray tubes to provide intensity,range, height, and azimuth information aboutthe weather "target." Keep the basic radarblock diagram and the purpose of the set inmind as you read the following discussion ofthe major units, starting with the antenna.

8-3. Antenna. The antenna is protectedfrom the weather by a radome, as shown infigure 37. The radome consists of fiberglasssections that are bolted together. The radomeis bolted to the platform that supports theantenna. The antenna assembly, shown infigure 38, includes an 8-foot parabolicreflector (C). The RF pulse from themagnetron is coupled through a waveguide(H) to the J-type feed horn (A). The horn ismounted at the focal point of the reflector bysupporting struts (B).

8-4. The pedestal (G) contains the servocomponents that operate the azimuth motor(F) . and provide continuous azimuthinformation for the indicators. A yoke

assembly (D) is mounted on top of thepedestal and is capable of continuous rotationthrough 3600

• It carries the antenna dish andall the servo components that drive theantenna in elevation and provides acontinuous indication of the antennaelevation angle. To permit elevationmovement of the antenna dish, the dish ispivoted on the yoke assembly. It iscounterbalanced by an adjustable weight. Acrank and mechanical linkage converts therotary motion of the elevation motor (E) intooscillatory motion of the antenna dish. Amotor-driven jackscrew determines the anglethat the dish moves through. The jackscrew isremotely controlled from the console, whichadjusts the mechanical linkage to provide therequired scanning arc. An azimuth scale onthe pedestal and an elevation scale on theyoke assembly permit initial installation andalinement of the antenna assembly. You levelthe antenna during initial installation bymeans of leveling bolts on the pedestal andtwo built-in spirit levels.

8-5. The antenna positioning andmovement is controlled either automaticallyor manually. In the automatic mode, theazimuth servomotor drives the reflector,clockwise, through 360 ° at 5 rpm. In themanual mode, you can rotate the reflectorclockwise or counterclockwise at speeds up to8 rpm. In automatic mode, the elevationservomotor raises or lowers the reflector from_2° to +60° at a minimum rate of 10° persecond. There are three automatic elevationsector scans: _2° to +15°, _2° to +30°, and _2°to +60°. The sector scans are controlled bycam-operated switches and are selected by aswitch at the main console. In the manualmode, you can move the reflector from _2° to+60° in 4 seconds.

8-6. Servoamplifiers are required togenerate the power that operates the antennamotors. The antenna servoamplifiers, asshown in figure 39, are housed in a cabinet inthe shelter located at the base of the antennatower. The azimuth and elevationservoamplifiers receive two types of signals.They are either constant signals that areamplified to drive the antenna motorscontinuously in the automatic mode or errorsignals from the manual antenna positioningcontrols on the console. The manual errorsignals are "nulled" when the antenna reacheswithin ±0.5° of the position "called for" bythe manual controls. The antennaservoamplifiers are associated solely with theantenna drive motors. The range and heightindicator (RHI) and PPI deflection yokes aredriven by separate followup servosystems.

36

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Figure 40. Receiver transmitter (RTM), ANjFPS-77 .

37

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Figure 41. Console, ANjFPS-77.

38