Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1981.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Accompanying cassette titled: Radar field test.Bibliography: leaf 18.by Allen Yee.B.S

Yee, Allen

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STUDIES OF POLICE RADARbyAllen YeeSubmitted in Partial Fulfillmentof the Requirements for theDegree of Bachelor of Scienceat theMassachusetts Institute of TechnologyJune, 1981Signature of Author............. .... ... ... .Departmenpo ElectricalEngineeringMay 8, 1981Certified by.. - ..................Thesis SupervisorAccepted by...Chairman, Departmental Committee on ThesesArchivesASSAtHsEJITUAUG 25 1981LIBRARIESI'STUDIES OF POLICE RADARbyAllen YeeSupervised byCharles E. MillerABSTRACTThe intent of this thesis was to do a study of continuouswave radar systems. These are the radars which determine atargets speed. I focused my work on the radar gun, which iscurrently employed by police forces to catch speeders.I conducted tests on the receiver sensivtivity of theradar gun. I built an audio amplifier circuit for the signalsreceived by the radar, so that in addition to seeing thesesignals on an oscilloscope, I was able to hear them. I thentook the radar gun, the audio amplifier and a portable cassettetape recorder onto emorial Drive. There I recorded somesignals of the traffic.From the tests, data, tape recordings and other infor-mation, I have concluded that under ideal conditions, that istarget at zero angle with the radar gun, no other targets inthe radar area and no mechanical or radio interference, theradar will make an accurate reading. However, if all theseconditions are not met, then the radar will not be accurateenough to cast no shadow of doubt.-1-TABLE OF CONTENTSGoal of Thesis ................Basic Principle of Radar......Possible Sources of Error.....Receiver Section of Radar .....Audio Amplifier Circuit ......Field Test ....................Conclusions ...................Bibliography ............................... page............ page.... ........ .page..... ....... page............ page............ page........... .page........ ....page-2-44579151518LIST OF FIGURESFig. 1 Block Diagram of Radar...................2 Multiple Targets ................ .... ....3 Cosine Error .................................4 Receiver Sensitivity Test Setup.........5 Receiver Sensitivity Curve..............6 Audio Amplifier Circuit.................7 Audio Amplifier Frequency Response Curve.8 Tape Recorder Response Curve..............9 Field Test on .Memorial Drive................page....page.. *page...page....page....page....page....page....page-3-688101011141416GOAL OF THESISHave you ever been stopped by a police officer and hesaid to you that you were clocked by the radar gun to begoing at such a speed and that the speed limit was only somuch? If you were indeed going at such a speed, you wouldtake whatever he gave you, without questions. Even if youwere not going at such a speed, you still would not questionhim, because it is almost impossible to fight him. Withthis thesis, I will try to raise some questions, questionsas to whether his radar is accurate all the time, when isit not and under what conditions.BASIC PRINCIPLE OF RADARThe radar unit which I worked with had a transmitterwhich operated on the X-band or a frequency of 1.0525 x 1010hertz, which is 10.525 gigahertz. The transmitter of theradar sends out a sinusoidal signal. When this signal reflectsoff a moving target, the reflected signal's frequency isshifted proportional to the target speed according to theDoppler shift principle.shift in frequency fd = f vCfo: the transmitter frequency - 10.525 gigahertzv: target speedc: speed of light2 (1.0525 x 1010 hertz) vfd = 186000 miles/second2.105 x 1010 hertz(156000 miles/second) (3600 seconds/hour) v-4-31.4 hertz- miles/hour vIf a target was moving at 10 miles/hour, the reflected signalwould have its frequency shifted 31.4 hertz 10 miles/hourimil es/hourwhich equals 314 hertz.Figure 1 shows the internal signal processing of theradar. The radar transmits and receives its signals throughthe same antenna. The transmitted and received signals alsogo through a comparator. Here the two signals are comparedand the difference in frequency is sent to the amplifier.For all subsequent studies, the tests were conducted on thissignal. When the signal is amplified to a certain level,it triggers the digital logic section. Here there is essentiallya counter which counts the frequency and digital logic toconvert the counted frequency to the corresponding milesper hour, which is then displayed on a digital reading.Before the radar becomes operational, the opperatoris supposed to calibrate the radar with a tuning fork. Thetuning fork modulates the transmitted signal so that theradar thinks that it is moving.POSSIBLE SOURCES OF ERRORTypically the antenna of the radar will radiate signalsover a 15 degree angle. If the radar was a quarter of amile away, it would cover the entire width of a four-lane-5-rI,c~oU.0.oelU1d-I .0I -I \" I -\I tI 1luUL-i5-A4s,5oa0j itXI'L%.I -,L20cebIC.I lat2-.2:'4acokxotAII-I I- i m.--m..,..----~ liaIm.m.m--.mm!IIm--.mlmm4il.mmmmlmm~04%* ;!b 1 'l \x. .._0_rImmmm | , " Ihighway. In this case the radar will not be able to pickone particular car (figure 2). The radar will usually lockonto the car with the strongest signal or with the highestspeed. However the operator does not know which car theradar has locked onto. He can only guess by making a visualsighting.Another source of error is cosine error. Figure 3 showsthat only if the angle between the radar and the line oftravel of the traffic is zero degrees, the radar will givean accurate speed reading. Any other angle will produce adiscrepancy. The displayed speed is equal to the cosineof that angle times the actual speed.RECEIVER SECTION OF RADARThe percentage of transmitted signal which reflectsoff a target is very small, and the percentage of reflectedsignal- which is received by the radar is even smaller. There-fore the radar has to be very sensitive to the reflected signaland block out all other signals.Figure 4 shows the setup I used to test the sensitivityof the radar's receiver section. A microwave oscillatorwhich is modulated by an audio oscillator set at 1570 hertzor 50 miles per hour, transmits a sinusoidal signal througha coaxial cable placed in front of the radar gun. With an-7-2. tVL P fDJs PINY6-b Z. cOS 9 CtAcTV*L.Co S/Al!SPffbI %-1. r ro  A 4 6' rS P FIFb )r-1 4VAcE 3.oscilloscope, I looked at the signal received by the radar.I took the signal at the point in the radar gun before itwas amplified, so that I got the received signal before anyprocessing.Figure 5 is the curve that was obtained from this test.The vertical axis is the peak to peak voltage amplitude ofthe signal. The horizontal axis is the frequency swept bythe microwave oscillator. The receiver is most sensitiveat frequency 10.485 gigahertz (8 volts peak to peak). Thisis not the frequency the transmitter is rated at (10.525gigahertz). This was probably due to the fact that the scaleof the microwave oscillator from which the frequency wasread was not correct.AUDIO PAPLIFIER CIRCUITHaving looked at the received signal through an oscil-loscope, I could not determine the different frequenciesin the signal. There were many signals of different frequenciessuperimposed on each other.Another way to determine some of the different frequenciesinvolved was to be able to hear the signal. This could bedone because the frequencies of the signal are in the audiorange (10 miles/hour = 314 hertz, 100 miles/hour = 3140 hertz).A circuit diagram of the audio amplifier circuit ison figure 6. The main component is a National SemiconductortC.RO W 9VIAU I OS0/.L, 4TORArf#LEtNNP/I cRE174J j02". RACir IFeRE.yq 7 El.V9Sws, I rl VryIa orEsr rvp/lostA RE QUA u'GY - CI 1r * /+Fotr S S /sr u/ ry c uR VfOSClLLA ToR--- -- ---------------_ ! I I I I I IREC E vf4 12.vLa4Qo AMP( I FtP CQ t.u rI_ F160 I.sl1LM 386 audio amlifi er chin. All the capacitors and resistorswere noncritical except for the 150 microfarad capacitor,which determines the low frequency -cutoff of the circuit.At half power output, the impedance of the low frequencycutoff capacitor equals the impedance of the speaker:1c = 22 ohmsC Sif I want a low frequency cutoff at 300 hertz orabout 10 miles per hour1c 300 hertz = 22 ohmsc (00 O hertz) (22 ohms)c = 151.5 microfaradsThe 10 kiloohm potentiometer is the volume control forthe circuit.The signal from the radar gun has a maximum peak topeak voltage of 8 volts, while the LM 386 audio amplifierchip has a maximum input voltage rated at .4 volts. Thereforethe 1 megaohm potentiometer is used to reduce the signal.I could have used two resistors to act as a voltage dividerinstead of the potentiometer to restrict the signal, howeverthere might be instances when the signal from the radar gunis very weak and I would like to have the capability to beable to hear a strong signal by not deamplifying the weaksignal.-12-On figsure 7 is the frequency responce of the audioamplifier circuit. It turns out that its low frequency cutoffis not at 300 hertz, the calculated point, but at approximately200 hertz. The calculation is only a crude method. Thiscircuit will accurately reproduce a signal down to 200 hertzor about 7 miles per hour.I wanted some way of recording the signals I heard throughthe audio amplifier. I decided to record the results usinga portable cassette tape recorder. In order to make a oodrecording, I had to match the output of the audio amplifiercircuit to the input of the tape recorder. I first had toconduct a test to obtain the response curve of the tape re-corder (figure 8). I used a signal generator set at 1000hertz as the input to the recorder. At certain voltage levels,from 0 to 1000 millivolts, I recorded these signals. I thenplayed the tape to the corresponding input and with a oscil-loscope, I read the output voltages.To get the most accurate reproduction, I would wantto stay in the linear range, from 0 to 200 millivolts.The output of the audio amplifier circuit has a maximumoutput of 15 volts (supply voltage). I used a 100 ohm resistorand a 1 kiloohm potentiometer between the output of the audioamplifier circuit and the input of the tape recorder. Withthe potentiometer, I would be able to control the volLumelevel to the tape recorder.-13-24.0 )0oo4vbl AAPLi t"erOuTpo' - Oa LT S32IIa1-0 4 2.4 3 6I 1 -C ..· I~~ -I-'1Ob 6d 70t tr6· tld t 0 4!L4./ VOI TSFJ6T)S E k. SPOAIS- 1-too/Of,5;,h1I/doO7.,Sooop&eue-Al (vyII_1T----- · · - - -  1 - -- -^C IFRe:&uow cY -IIII ·E. TisekFIELD TESTThe field test was conducted on Memorial Drive in frontof the sailing pavilion across from Walker Dining Hall atabout 4 p.m. The tests were conducted with the radar gunin four different positions with respect to Memorial Driveand the traffic.Figure 9 shows the different positions. A cassettetape recording was made at this time. At zero degrees andthe traffic approaching, the signals on the tape had a constanttone and the volume increased with time. This would indicatethat the cars were moving at a constant speed, and as thecars approached, the signal became stronger. At 4 dereeswith the traffic approaching, the signal tarted very abruptly,and the duration was much shorter. This is due to the factthat the radar area is much smaller than in the zero degreecase. At zero degrees and the traffic departing, the signalswere constant and decreasing in volume. At 45 degrees withthe traffic departing, the signals sounded very much likethey did in the case of the traffic approaching, except thatthey too had a decreasing volume. In none of the cases werethere only one clear signal for a long period of time. Therewere always other signals introduced.CONCLUS IONSFrom listening to the tape recorded signals, it isdifficult for me to distinguish many of the signals. When-15-III 4I 1IIILJ'=paeA*:z00Lu4c,10.lCZ.IC)LI0031IqVAmJO43ILIIIIIIIN3,IIIIItwo cars are very close to each other and moving at aboutthe same speed, the radar gun sees them as one target.Although if the two cars were more distinguishable, the radargun is supposed to lock onto either the car giving the strongestsignal or the car moving faster. However the operator doesnot lKnow to which car the radar gun is reading.Even though cosine error will help the motorist by alwaysgiving a lower speed reading, it is still an inaccuracy inthe system.There is always the possibility of interference in thearea. Any transmissions at the radar's receiver frequencywill mask the weak reflected signal. One of the simplestforms of mechanical interference is if the operator has hisengine fan running. This could act like a tuning fork andgive the radar a false reading.Only if there is one car in the radar area, the carand the radar at a zero degree angle and no radio ormechanical interference, then the radar will make an accuratereading. Anything less than all these conditions, the radarwill perform with inaccuracies.-17-BIB3LIOGRAPHYIntroduction to Radar Systems,by Merril I. Skolnik, 1980, MffcGraw-Hill Book CompanyHandbook of Operational Amplifier Circuit Design,by David F. Stout, 1976, McGraw-Hill Book CompanyShortcomings of Radar Speed Measurement,by P. David Fisher, IEEE Spectrum December 1980Tell It to the Judge - the Shaky Reliability of Police Radar,by Erik H. Arctander, Popular Science August 1980-18-

Studies of police radar

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