Development of a multiple-target CW FM radar is described. This
type of radar has advantages over pulse radars particularly in portable,
battery operated applications.http://archive.org/details/multipletargetcw00hoisN

Hoisington, D. B. (David B.)

Development of a multiple-target CW FM radar is described. This
type of radar has advantages over pulse radars particularly in portable,
battery operated applications.NAhttp://archive.org/details/multipletargetcw00hoi

Calhoun, Institutional Archive of the Naval Postgraduate School

LffiBART'-C'AUICAL tePOK'f SECTIONi'^Jtt ^°STGfiAOUArc SCHOOlHQMESST, CALIFOBNIA 93640NAVAL POSTGRADUATE SCHOOL//Monterey, CaliforniaMULTIPLE-TARGET CW FM RADARDavid B. HOISINGTONJune 1972Technical Report^ for Period July 1971 - June 1972Approved for public release; distribution unlimitedFEDDOCSD 208.1 4/2: NPS-52HS72062NAVAL POSTGRADUATE SCHOOLMonterey, CaliforniaRear Admiral Isham Linder Jack R. BorstingSuperintendent ProvostThe work reported herein was supported by the Naval Postgraduate SchoolReproductions to all or part of this report is authorized.This report was prepared by:SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)REPORT DOCUMENTATION PAGE READ INSTRUCTIONSBEFORE COMPLETING FORM1. REPORT NUMBERNPS-52HS720622. GOVT ACCESSION NO 3. RECIPIENT'S CATALOG NUMBER4. TITLE (and Subtitle)MULTIPLE-TARGET CW FM RADAR5. TYPE OF REPORT & PERIOD COVEREDTechnical ReportJuly 1971 - June 19726. PERFORMING ORG. REPORT NUMBER7. AUTHORCs; B. CONTRACT OR GRANT NUMBERCs;David B. HOISINGTON9. PERFORMING ORGANIZATION NAME AND ADDRESSNaval Postgraduate SchoolMonterey, CA Code 52Hs10. PROGRAM ELEMENT, PROJECT. TASKAREA a WORK UNIT NUMBERS11. CONTROLLING OFFICE NAME AND ADDRESSNaval Postgraduate SchoolMonterey, CA Code 52Hs12. REPORT DATE15 June 197213. NUMBER OF PAGES14 MONITORING AGENCY NAME & ADDHESS(H dllierent from ControlUne Oltlce) IS. SECURITY CLASS, (of thia report)UNCLASSIFIEDISa. DECL ASS! FIC ATI ON/ DOWN GRADINGSCHEDULE16. DISTRIBUTION ST ATEMENT (of this Report)Approved for public release; distribution unlimited17. DISTRIBUTION STATEMENT (ol the abstract entered In Block 20, If different from Report)te. SUPPLEMENTARY NOTES19. KEY WORDS (Continue on reverse aide If neceaaary and Identify by block number)RADARCW radar20. ABSTRACT (Continue on reverse aide If neceaaary and Identify by block number)Development of a multiple-target CW FM radar is described. Thistype of radar has advantages over pulse radars particularly in port-able, battery operated applications.DD 1 JAN 73 1473 EDITION OF 1 NOV 65 IS OBSOLETES/N 0102-014-6601IUNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PAGE (Whan Data Bntmrad)MULTIPLE-TARGET CW m RADARDavid B. HolsingtonNaval Postgraduate SchoolMonterey, CaliforniaAbstractDevelopment of a multiple-target CW FM radar is described. This type of radarhas advantages over pulse radars particularly in portable, battery operatedapplications.INTRODUCTIONAlthougli multiple-target CW FM radars have beensuf;gested [1], little application of FM radar inthis mode of operation seems to have been made.Recent advances in solid-state technology havemade availnble devices that would permit the con-struction of a multiple-target FM radar that wouldbe complctelv solid state except for the cathode-ray tube indicator, and perhaps even a solid-state replacement for the cathode-ray tube willbe available within a few years.Tlie most obvious application for a radar of thistype would be in situations where light weight,low power consumption, and high reliability areof paramount importance. Light weight indicatesoperation at 9 Cllz or liigher to obtain narrow an-tenna beam width wilh reasonable antenna size.It will be shown that doppler frequency shiftscan be of acceptable magnitude at 9 GHz for sur-face targets. Powers in the order of 1 watt canbe obtained at 9 (iHz from solid-state devices,and tin's is adequatt- for many applications. Anexperimental program has been initiated to demon-strate the feasibiljl^ of such a radar suitablefor use on small vi-ssi'Js down to the size ofmotor whale boatP.THEORY OF OPERATIONIf the transmitted frequency of a CW radar is sweptlinearly with time as shown in Figure la, the re-ceived echos from fixed targets will have a simi-lar pattern, but will be delayed in time by 2 R/cwhere R is the target range, and c is the velocityof propagation. If now the echo frequency isheterodyned with the transmitted signal to obtainthe beat frequency, f, ,then= 2R d f ^ 2RAfb c dt c T (1)where Af is the transmitter frequency excursionand T is the sweep period. Figure 1 -b shows thatduring the initial portion of the transmittersweep, echos are still being received from the pre-vious sweep, hence the beat frequency is higherthan given by (1). After a time AT correspondingto targets at maximum range tlie difference fre-quencies associated with all targets are as givenby (1). Tlie receiver is gated off during theinitial interval to prevent the passage of theincorrect frequencies. It is convenient to makethe duty cycle, (I -A), equal to 1/2 so that thereceiver is gated on for n time i equal to onehalf of the sweep period, T.Figure 2 shows the boat frfi|urncy spectrum due toa single sweep and a single l.irgct. Tlie spectrumIs rontiiuious and h.is a (sin x)/x envelope.Figure 1-a Radio frequencies vs. timeFigure 1-b Beat frequencies vs. timeTo separate targets at different ranges one ormore filters may be used. The ideal filter formaximum output slgnal-to-noise ratio would bematched to the spectrum. As a practical matterthe filter may be matched as nearly as possibleto the central lobe of the spectrum. The filter6dB (hall amplitude) bandwidth is, then, 1.2/t or2.4/T. This filter has rise and decay timeapproximately equal to t, hence the signal due toone target decays during the receiver dead timeclearing the filter to accept another targetduring the succeeding on time.Figure 3 is a simplified block diagram of thecomplete system. Received echos are convertedto an intermediate frequency for good noise per-formance and convenience in filtering. Thecarrier oscillator is linearly swept over thefrequency band Af. A portion of this signal ismixed with the output of the IF oscillator at thefrequency (f. - f, ) . A bandpass filter passesthe sum frequency (f + f. - f, ) , to the receivermixer. The other input to this mixer is from thereceiving antenna. T\\e received signal at fre-quency (f - f, ) is mixed to form a signal at theIF frequency, f The IF filter with 6dB band-wldtli f = 1.2/i passes this signal to the gatedamplifier, detector and indicator.By sloulv v.iryinc, the frequency of the IF oscil-lator from f to (f - 2R Af/rT), i/icomine1 I max ">;l>'.nals from cich range intervnl from zero to^^o- t>(f + -)O TFigure 2. Beat-frequency spectrum due to aswingle sweep and a single target.CARRIESOSCILLATOR /^ TRANSMITTING<ft>?).<^7-H AWTENNABAKDPASSFILTER» ^IFOSCILLATOR' ^y./<:>^ /" RECEIVING(f, -. f,-vw^ (^ - V V' ANTENNAriLTEKRAMMIDTH f-^GATEDAMPLIFIERtTO DFTECTOR*" AND INDICATORFigure 3. System block diagrammaximum range are successively converted to thefrequency f. and are thus separated to yield tar-get range information. The oscilloscope PPl dis-play is deflected radially in synchronism with theIF oscillator frequency sweep, and deflected inazimuth synchronously with the antenna sweep.The range discrimination of the systems can be de-termined from equation (1) . Targets of equalstrength can be separately detected if their beatfrequencies are separated by f,. It follows thenthe range discrimination, 6R, is given bycTff 1.2c6R =2Af Af (2)Now for TAf>>l it can be shown that the width ofthe transmitted spectrum at half amplitude isAf[2]. It is not surprising that for equal band-width, the range discrimination of tlie CW FM radarIs the same as that of the pulse radar.The radar shown in Figure 3 lias the advantage ofsimplicity, but Information rate is limited sinceonly one range interval can be examined at a time.Information rate could be increased by using amultiplicity of filters. It is significant to de-termine the time required to search all range andazimuth bins. Assume that the maximum range ofinterest is N times tlie range discrimination. Ifsuccessive range intervals examined are separatedby 6R, and 1/T range are intervals examined persecond, then the time required to sweep from tomaximum range, t, ist " NTs(3)If the antenna is rotated one beam width, e„ , inDtime t , then the search tine required to rotates'^the antenna through 360° is360NTeB(4)where 9 is in degrees. For example, if N is 200,BT is 400 usee, and 6 is 3", then t is 9.6seconds. This time is increased if N is increased,or If, as is desirable, some overlap is permittedbetween successive range intervals examined. Thesearch time decreases if a sector less than 360°is scanned.The doppler shift due to target motion is2v f /c where v is the radial component of ve-r o rlocity. If v is 20 yards/sec, representing avery fast surface target, and f is 9 GHz, thedoppler shift is 1100 hertz. Typically f j, wouldbe in the order of 5000 hertz for a marine navi-gation radar, so this doppler shift represents arange error less than one fourth of the rangediscrimination, an entirely acceptable error.COMPARISON WITH PULSE RADAREquation (2) shows for eqoal transmitted band-widths, CW FM and pulse radars have virtually thesame range discrimination. In the pulse radarthe receiver must have a bandwidth comparable tothe transmitted bandwidth. The CW FM radar, onthe other hand, has an IF bandwidth equal tof,, a small fraction of the transmitted bandwidth.Required gain can therefore be obtained with arelatively small nimber of stages. Moreover, therange discrimination may be cliangfd simply bychanging &f, the transmitter fn-qui-ncy deviation.To change range discrimination of a pulsi' radarone generally changes pulse width, receiver band-width, and the pulse repetition frequency.It can be shown that for equal useful averagepovers transmitted, target illumination times,equal receiver noise figures, antenna gains, inte-gration efficiences, integration times, and opti-mized bandwidths, pulse and CW FM radars have thesame maximum range on a given target. Here theOiJ FM radar has a 3dB disadvantage if the receiverduty cycle is 50 percent. Additional loss of upto 3dB may occur if the echo spectrum is not cen-tered in the IF filter. The pulse radar, on theother hand, has a 7dB loss when in a typical case25 pulses from a given target are integrated bymeans of a cathode-ray tube. Typically, then, therange performance of a CW FM radar equals that ofa comparable pulse radar when the average trans-mitted powers are the same.The pulse radar transmits high peak power in shortpulses to obtain a given average power. High volt-ages are therefore required in the relativelycomplicated modulator. In the CW FM transmitterthe power is constant, and only relatively lowvoltages are required in the transmitter. Solidstate oscillators now available can furnish thepower required for a moderate (10 mile) rangemarine navigational radar while operating from thestorage battery voltages available on most smallcraft. A high degree of modulation linearity isrequired in the CW FM transmitter, but the re-quired linearity is not necessarily hard toobtain [3].Pulse radars do not suffer from any range ambi-guity provided echos are not received from targetsat a time greater than the pulse repetition period.CW FM radars may have a range ambiguity problemwhen weak target returns are received at rangessomewhat smaller or larger than the return from avery strong target. Figure 2 shows the IF spec-trum of a given target. If the IF filter is at afrequency 2/t higher than the center of thlaspectrum in an effort to locate a target at arange difforing by 2r/Af, the filter will respondto the side bands of the strong-target spectrum.The filter output in this case will be In theorder of 26 dB less than if the strong target re-turn were centered In the filter, but thisspurious response could make it difficult to de-tect the return from a navigational buoy with arelatively large object such as a ship nearby.This range ambiguity can be reduced, for example,by shaping the IF spectrum of targets to reducethe amplitude of the side bands. This could beaccomplished by modulating the receiver gainbefore the IF filter with a smooth weighting func-tion rather than by employing square-wave gatingas has been assumed. Further work needs to bedone in this area.EXPERIMENTAL PROGRAMAs the first step in demonstrating the feasibilityof a solid-state CW FM radar, a hybrid unit wasconstructed from laboratory components. Thebasic systiiii block diagram was as indicated inFigure 3. Operation was at a frequency of about9 GHz with a power output of 2 watts. The trans-mitter frequency deviation could be set at anyvalue up to 12 Mllz. The receiver intermediatefrequency was 30 MHz, and the IF filter was alinear phase (Gaussian) filter with a 6 dB band-width of 5.1 kHz. Receiver noise figure was 13.8dB. Circular parabolic reflectors with a 3 de-gree beam width were used for both transmittingand receiving antennas. These antennas could notconveniently be scanned, so a type A (amplitudevs range) presentation was used.Figure U shows a typical display obtained withthe antenna oriented as shown in Figure 5.Figure 6 shows a high resolution display obtainedwith the antenna oriented as shown in Figure 7.Note that objects separated by AO yards are com-pletely resolved. This display was obtained witha Af of 12 MHz for a theoretical range resolutionfrom equation (2) of 33 yards.Figure 4. Long-range display3° BEAMWIDTHSTACKKMBY TOWERPOINT CABRILLO4000 YDSPOINT ALONESPACIFICOCEANMONTEREYCOAST GUARD PIERMONTEREY WHARFSPANAGEL HALLFigure 5. Kadar field of view (long-range)Figure 6. Short-range display of campusBEAM WIDTHINGERSOLLHALUGANBULLARDROOTj343 YDS305YDS4-RADARrj- ii , 173YDS133YDSLi_LSPANAGEL HALLFigure 7. Radar field of view (short-range)CONCLUSIONSThe feasibility of a CW FM radar with a rangeresolution of about 30 yards has been demonstra-ted. It remains to be shown that such a radarcan be built with all solid-state components in asmall, lightweight package. Development of thenecessary solid-state circuitry is proceeding astime and limited funding pemit.ACKN()WLtD(TMENTAppreciation is expressed to Lieutenant Ronald P.Lewis, tISN , who performed much of the experimentalwork reported in this paper as part of the require-ments for a Master of Science tliesis.REFERENCES1. Luck, D. G. C. , FM Radar, McGraw-Hill BookCompany, Inc., New York, 19A92. Berkowitz, R. S., Modern Radar, P. 225, JohnWiley & Sons, Inc., New York, 1965.3. Lewis, R. P., A Developmental CW FM Multi-Target Radar, M.S. Thesis, Naval PostgraduateSchool, Monterey, California, 1971.INITIAL DISTRIBUTION LISTNo. Copies1. Defense Documentation Center 12Cameron StationAlexandria, VA 223142. Library, Code 0212 2Naval Postgraduate SchoolMonterey, CA 939403. Dean of Research 1Code 023Naval Postgraduate SchoolMonterey, CA 939404. Professor David B. Hoisington 10Electrical Engineering Dept.Code 52Naval Postgraduate SchoolMonterey, CA 93940U 16^731DUDLEY KNOX LIBRARY - RESEARCH REPORTS5 6853 01060286 5

Multiple-target CW FM radar

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Multiple-target CW FM radar

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