Introduction to radar system


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Introduction to radar system

  1. 1. INTRODUCTION TO SYSTEMS RADAR SYSTEMS Second Edition Second Merrill I. SkolnikMcGRAW-HILL BOOK COMPANYMcGRAW-HILL BOOK COMPANY Auckland Bogotii Guatemala Hamburg Lisbon Auckland Bogota Guatemala Hamburg Lisbon London Madrid Mexico New Delhi Panama Paris London Madrid Mexico New Delhi Panama Paris San Juan S5o Paulo Singapore Sydney Tokyo San Juan Sao Paulo Singapore Sydney Tokyo
  2. 2. INTRODUCTION TO RADAR SYSTEMSINTRODUCTION TO RADAR SYSTEMS International Edition 1981International Edition 1981 Exclusive rights by McGraw-Hill Book Co.-- Singapore forExclusive rights by McGraw-Hili Book Co.-- Singapore for manufacture and export. This book cannot be re-exportedmanufacture and export. This book cannot be re-exported from the country to which it is consigned by McGraw-Hill.from the countlY to which it is consigned by McGraw-Hill.Copyright © 1980, 1962 by McGraw-Hill, Inc. Copyright @ 1980,1962by McGraw-Hill, Inc. All rights reserved. Except as permitted under the United States CopyrightAll rights reserved. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed inAct of 1976, no part of this publication may be reproduced or distributed inany form or by any means, or stored in aadata base or retrieval system, any form or by any means, or stored in data base or retrieval system, without the prior written permission of the publisher.without the prior written permission of the publisher.I 2 3 4 5 6 7 8 9 2 0 BJE 9 8 7 6 5 4 This book was set in Times Roman.This book was set in Times Roman. The editor w s Frank J. Cerra. aThe editor was FrankJ. Cerra. Theproduction supervisor was GayleAngelson.The production supervisor was Gayle Angelson. Library of Congress Cataloging in Pubilcation DataLibrary of Congress Cataloging In Publication Data Skolnik, Merrill Ivan, dateSkolnik, Merrill Ivan, date Introduction to radar systems. Introduction to radar systems. Includes bibliographical references and index. Includes bibliographical references and index. 1. Radar. I. Title. 11. Series. 1. Radar. I. Title. II. Series. TK6575S477 1980 621.3848 79-15354TK6575.s477 1980 621.3848 79-15354 ISBN 0-07-057909-1ISBN 0-07-057909-1When ordering this title use ISBN 0-07-066572-When ordering this title use ISBN 0-07-066572~ 99 Printed in SingaporePrinted in Singapore
  3. 3. CONTENTS Preface IX 1 The Nature of Radar 1 1.1 lntroductiorl Introduction 1 1.2 *llle Sirnple Fortn Kadar Equatiorl The Simple Form of the Radar Equation 3 1.3 1.3 Hlock Radar Block Diagram and Operation 5 1.4 1.4 Radar Frequencies 7 1.5 1.5 Radar Development Prior to World War II Dcvcloprnent I1 8 1.6 1.6 Kadar Applications of Radar 12 References 14 2 The Radar Equation 15 2.1 Range Performance Prediction of Range Performance 15 2.2 Mirlimurn Signal Minimum Detectable Signal 16 2.3 Receiver Noise 18 2.4 Probability-density Functions Functions 20 2.5 Signal-to-noise Ratio Signal-to-noise Ratio 23 2.6 Pulses Integration of Radar Pulses 29 2.7 Radar Cross Sectiorl Targets Radar Cross Section of Targets 33 2.8 Cross-section Fluctuations Cross-section 46 2.9 Transmitter Transmitter Power 522.10 Pulse Repetition and Pulse Repetition Frequency and Range Ambiguities 532.11 ~arameters Antenna Parameters 542.12 System Losses System Losses 562.1 J Propagation Effects Propagation Effects 622.14 Other Consideratiorls Other Considerations 62 Refererlces References 65 3 C W and 3 CW and Frequency-Modulated Radar 68 2.1 3.t Tile Iloppler Effect The Doppler ElTect 68 3.2 3.2 C W Radar CW Radar 70 3.3 3.3 Frequency-modulated C W Radar Frequency-modulated CW Radar 81
  4. 4. J CONTENTS 3.4 Airl>or-neDoppler Navigation Airhorne Doppler Navigation l)~ 3.5 M ultiple-Frequency C W Radar Multiple-Frequency CW Radar LJS References Rekrences l)K 4 MTI and Pulse Doppler Radar MTI and Pulse Doppler Radar 10l 4.1 Introd~iction Introduction 10/ 4.2 Delay-Line Cancelers Delay-Line Cancelers 106 4.3 Multiple, or Staggered, Pulse Repetition Freqiirncics Multiple, or Staggered, Pulse Repetition Frequencies I 14 4.4 Range-Gated Doppler Filters Range-Gated Doppler Filters 117 4.5 Digital Signal Processing Digital Signal Processing I 19 4.6 Other MTI Delay Lines Other MTI Delay Lines 126 4.7 Example of an MTI Radar Processor Example of an MTI Radar Processor 127 4.H Limitations to MTI Performance Limitations to MTI Performance 12L) 4.9 Noncoherent MTI NoncoherentMTI DH 4.10 Pulse Doppler Radar Pulse Doppler Radar DtJ 4.11 MTI from a aMoving Platform MTI from Moving Platform 140 4.12 Ot her Types of MTI Other Typesof MTI 147 References References 14~ Tracking Radar 5 Tracking Radar 152 5.1 Tracking with Radar Tracking with Radar 152 5.2 Sequential Lobing Sequential Lobing 15. 5.3 Conical Scan Conical Scan 155 5.4 Monopulse Tracking Radar Monopulse Tracking Radar 160 5.5 Target-Reflection Characteristics and Angular Accuracy Target-ReflectionCharacteristics and Angular Accuracy 167 5.6 Tracking ininRange Tracking Range 176 5.7 Acquisition Acquisition 177 5.R Other Topics Other Topics In 5.9 Comparison of Trackers Comparison of Trackers IX:2 5.10 Tracking with Surveillance Radar Tracking with Surveillance Radar 1~3 References References IX6 6 Radar Transmitters Radar Transmitters 190 6.1 Introduction Introduction 190 6.2 The Magnetron Oscillator The Magnetron Oscillator 192 6.3 Klystron Amplifier Klystron Amplifier 20() 6.4 Traveling-Wave-Tube Amplifier Traveling-Wave-Tube Amplifier 206 6.5 Hybrid Linear-Beam Amplifier Hybrid Linear-Beam Amplifier lOX 6.6 Crossed-Field Amplifiers Crossed-Field Amplifiers 20X 6.7 Grid-Con trolled Tubes Grid-Controlled Tubes 2.1 (d~ Modulators Modulators 2/·) 6.9 Solid-State Transnlitters Solid-Slale Transmitters 2 III References References no 7 Radar Antennas Radar Antennas 223 7.1 Antenna Parameters Antenna Parameters 221 7.2 Antenna Radiation Pattern and Aperture Distribution Antenna Radiation Pattern and Aperture Distribution 22~ 7.3 Parabolic-Reflector Antennas Parabolic-Reflector Antennas 235 7.4 Scanning-Feed Reflector Antennas Scanning-Feed Reflector Antennas 244 7.5 Lens Antennas Lens Antennas 24~
  5. 5. CONTENTS vii 7.6 7.6 Pattern Synthesis Pattern Sy~~rlicsis 254 7.7 7.7 Cosecant-Squared Antenna Pattern Cosecarit-Squared Arttenna Pattern 258 7.R 7.8 FlTect of Frrors on Radiation Palerns i:fTccl of Errors Radiatiot~Patterns 262 7.9 7.9 Radollles Kadomcs 2647.107.10 Stabilization of Antennas Stabili7ation of Antcnnas 270 References f~cfcrcrlccs 273 8 The ElectronicaJJy Steered Phased 8 llle Electrot~icallySteered Array Antennat in Radar Array A I 278 H.I 11111r od ud ion I r r l otlr~ctior~ 278 H.2 Basic l(or~ccl>ts 1t:tsic onccpts 279 lU Phase S C . sl1irlct.s I I ~ ~ ~ Shifters 286 XA hequellcy-Scan Arrays I,requc~~cy-Scar1 Arritys 298 X.5 Array Illcnierits Array Elemcnts 305 X.6 lcccls for Arrays Feeds for Anays 306 X.7 Sil~lultarlcousMultil>lc 13ea1lisfrom Array Ariterllias Simultaneous Multiple Beams from Array Antennas 310 X.X Random Errors in Arrays Random Errors in Arrays 318 8.9 Computer Control of Phased-Array Radar Computer Control of Phased-Array Radar 3221.10 Otlicr Array Topics Other Array Topics 3288.11 Applications of the Array in Radar Applications of the Array in Radar 334~.12 Advantages arid Limitations Advantages and Limitations 335 Kcfcrcl~ccs Refercnces 337 9 Receivers, Displays, and Duplexers Receivers, Displays, and Duplexers 343 9.1 The Radar Receiver The Radar Receiver 343 9.2 Noise Figure Noise Figure 344 9.3 Mixers Mixers 347 9.4 Low- Noise Front-Ends· Low-Noise Front-Erids 351 9.5 [)is plays Displays 353 9.6 1)uplexers and Receiver Protectors Duplexers and Receiver Protectors 359 References References 366 10 Detectiotl of Radar Signals in Noise Detection or Radar Signals In Noise 36910.1 Introductiot~ Introduction 36910.2 Matched-Filter Receiver Matched-Filter Receiver 36910.3 Correlation Detectiori Correlation Detection 37510.4 Detection Criteria Detection Criteria 37610.5 Detector Cliaracteristics Detector Characteristics 38210.6 Performance of the Radar Operator Performance of the Radar Operator 38610.7 Automatic Detection Automatic Detection 38810.8 Constant-False-Alarm-Rate (CFAR) Receiver Constant-False-Alarm-Rate (CFAR) Receiver 392 References References 395 1 Extractio~~ Information and Waveform of 111 Extraction of Information and Waveform Design Design 399 11.111.1 Introduction Introduction 399 11.211.2 Information Available from aa Radar Information Available from Radar 399 1 1.3 Theoretical Accuracy of Radar Measurements11.3 Theoretical Accuracy of Radar Measurements 400 1 1.4 Ambiguity Diagram11.4 Ambiguity Diagram 411
  6. 6. viiiviii CONTENTS CONTENTS11.511.5 Pulse Compression Pulse 42011.6 Classification of Targets with Radar of with 434 References 438 12 Propagation of Radar Waves of 44112.1 Introduction 4412.2 Propagation over a Plane Earth 44212.3 The Round Earth 44612.4 Refraction 44712.5 Anomalous Propagation 45012.6 Diffraction 45612.7 Attenuation by Atmospheric Gases 45912.8 Environmental Noise Environmental 46112.9 Microwave-Radiation Hazards 465 References 466 13 Radar Clutter 47013.1 Introduction to Radar Clutter 47013.2 Surface-,Clutter Radar Equations Surface-Clutter Equations 47113.3 Sea Clutter 47413.4 Detection of Targets in Sea Clutter 48213.5 Land Clutter 48913.6 Targets Detection of Targets in Land Clutter 49713.7 Effects of Weather on Radar 49813.8 Detection of Targets in Precipitation Precipitation 50413.9 Echoes Angel Echoes 508 References References, 512 14 Other Radar Topics 51714.1 Synthetic Aperture Radar Synthetic Aperture Radar 51714.2 Over-the-Horizon HF Over-the-Horizon Radar 52914.3 Air-Surveillance Air-Surveillance Radar 53614.4 Height-Finder and 3D Radars Height-Finder 3D Radars 54114.5 Electronic Counter-Countermeasures Electronic Counter-Countermeasures 54714.6 Bistatic Radar Bistatic Radar 55314.7 Millimeter Waves and Beyond Millimeter Waves and Beyond 560 References References 566 Index Index 571
  7. 7. PREFACEAlthough the fundamentals of radar have changed little since the publication of the first tlie firstedition,edition. there has been continual development of new radar capabilities and continual im-provements to the technology and practice of radar. This growth has necessitated extensive arid tlierevisions and the introduction of topics not found in the original. (moving One of the major changes is in the treatment of MTI (moving target indication) radar(Chap. 4).(Chap. 4). Most of the basic MTI concepts that have been added were known at the time of the Gat tliefirst edition, but they had not appeared in the open literature nor were they widely used infirst lhey i11practice. Inclusion in the first edition would havebeen largely academic since the analogdelay-line technology available at that time did not make it practical to build the sophisticatedsignal processors that were theoretically possible. However, subsequent advances in digitallechnology, originally developed for applications other than radar, have allowed the practicaltechnology,implementation of the multiple delay-line cancelers and multiple pulse-repetition-frequency pulse-repetition-frequencyMTI radars indicated by the basic MTI theory. (Secs. 5.10 10.7). Automatic detection and tracking, or ADT (Sees. 5)0 and 10.7), is another important evelopment fordevelopment whose basic theory was known for some time, but whose practical realizationilad to await advances in digital technology. The principle of ADT was demonstrated in the ad 1950s. .ofearly 1950s, using vacuum-tube technology, as part of the United States Air Forces SAG E SAGE Laboratory.air-defense system developed by MIT Lincoln Laporatory. In this form ADT was physically expensive, 1960slarge, expensive, and difficult to maintain. The commercial availability in the late 1960s of thesolid-slate minicomputer, however, permitted ADT to be relatively inexpensive, reliable, and of small size so that it can be used with almost any surveillance radar that requires it. sniall it it. Anotlcr radar area that has seen much development is that of the electronically steered Another ptiased-array antenna. In tlie first edition, the radar antenna was the subject of a single phased-array the cliaptcr. I11 tliis chapter. In this edition, one chapter covers the conventional radar antenna (Chap. 7) and a (Chap. 8). separate chapter covers the phased-array antenna (Chap. 8). Devoting a single chapter to the array antenna is more a reflection of interest rather than recognition of extensive application. antenria inore rellection application. The chapter on radar clutter (Chap. 13) has been reorganized to include methods for the o~i (Ctiap. 13) clutter. detection of targets in the presence of clutter. Generally, the design techniques necessary for the detection of targets in a clutter background are considerably different from.those necessary ttie clutter, fromthose for detection in a noise background. Other subjects that are new or which have seen significant cliaiiges " on-axis" changes in the current edition include low-angle tracking, "on-axis" tracking, solid-state RF RF ources, antet~na, stabilization, sources, the mirror-scan antenna, antenna stabilization, computer control of phased arrays, arrays, olid-state CFAR, )Olid-state duplexers, CFAR, pulse compression, target classification, synthetic-aperture radar, synthetic-aperture ver-the-horizon air-surveillance 3D over-the-horizon radar, air-surveillance radar, height-finder and 3D radar, and ECCM. The ECCM. bistatic radar and millimeter-wave radar are also included even though their applications have
  8. 8. x PREFACEX been limited. Omitted from this second edition is the chapter on Radar Astronomy since interest in this subject has dccrcascti with tltc i~vi~ilithility space prolws tthat cilll explore ttlc sub.ject decreased the availability o f of probes l l i i l call the planets at close range. The basic material of the first edition that covers the radar equation, range.the detection of signals in noise, the extraction of information, and the propagation of radar ofwaves has not changed significantly. The reader, ttowcvcl., wilt find only a fcw pagcs of significantly. however, will few pages the original edition that have not been modified in some manner. One of the features of the first edition which Ilas hcen contintled is the inclt~sionof One features has been continued inclusion ofextensive references at the end of each chapter. These are provided to acknowlcdgc the sources chapter. acknowledge of material used in the preparation of the book, as well as to permit the interested reader to learn more about some particular subject. Some references that appeared in the first edition have been omitted since they have been replaced by more current references or appear in publications that are increasingly difficult to find. The references included in the first edition find. represented a large fraction of those available at the time. I t woilld have been difficult to add to fraction It wouldthem extensively or to include many additional topics. This is not so with the second edition.The current literature is quite large; and, because of the limitations, only a milch The of space, muchsmaller proportion of what is available could be cited. In addition to changes in radar technology, there have been changes also in style and thercnomenclature. For example, db has been changed to dB, and Mc is replaced by M iIlL.~ Also, tthenomenclature. example, d b i AIso, he .letter-band nomenclature widely employed by the radar engineer for designating the commonradar frequency bands (such as L, S, and X ) has been officially adopted as a standard by the frequency (such X)IEEE.IEEE. The material in this book has been used as the basis for a graduate course in radar taughtby the author at the Johns Hopkins University Evening College and, before that, at several .Universityother institutions. This course is different from those usually found in most graduate electrical programs. Typical EE courses cover topics related t o circuits, components, de- tovices, and techniques that might make up an electrical or electronic system; but seldom is the vices, orstudent exposed to the system itself. It is the system application (whether radar, communica- itself.tions, navigation, control, information processing, or energy) that is the raison ditre for the tions, control, dctreelectrical engineer. The course on which this book is based is a proven method for introducing engineer.the student to the subject of electronic systems. It integrates and applies the basic concepts systems.found in the students other courses and permits the inclusion of material important to found of the practice of electrical engineering not usually found in the traditional curriculum. the Instructors of engineering courses like to use texts that contain a variety of problems that th3;t of can be assigned to students. Problems are not included in this book. Althoirgh the author to Although assigns problems when using this book as a text, they are not considered a major learning assigns technique. Instead, the comprehensive term paper, usually involving a radar design problem o r Instead, comprehensive or a study in depth of some particular radar technology, has been found to be a better means for bctter having the student reinforce what is covered in class and in the text. Even more important, it the allows the student to research the literature and to be a bit more creative than is possible by allows the to simply solving simply solving standard problems. a A book of this type which covers a wide variety of topics cannot be written in isolation. I t isolation. It would not have been possiblewithoutthe many contributions on radar that have appeared in possible,withoutthe the open literature and which have been used here as the basic source·-material. A large the source material. measure of gratitude must be expressed to those radar engineers who have taken the time anci to and energy to ensure that the results,:of their work were made available by publication iin energy t o ensure results of puhlication l l recognized journals. . I . On a more personal note, neither edition of this book could have been written without the On th~ complete support complete support and patience of my wife Judith and my entire family who allowed me tllc the to time necessary to undertake this work. Merrill 1. Skolrlik I. Skolnik
  9. 9. CHAPTER ONE THE NATURE OF RADAR1.1 INTRODUCTIONRadar is an electromagnetic system for the detection and location of objects. It operates by is a ntransmitting a particular type of waveform, a pulse-modulated sine wave for example, and pulse-modulateddetects the nature of the echo signal. Radar is used to extend the capability of ones senses fordetects the of signal. to of onesobserving the environment, especially the sense of vision. The value of radar lies not in being a environment,substitute for the eye, but in doing what the eye cannot do...Radar cannot resolve detail as wellsi~hstitute for eye, do-Radar the eye, nor is it capable of recognizing the" color" of objects to the degree of sophisticationas the eye, is the "color" of sophisticationof which the eye is capable. However, radar can be designed to see through those conditions the eye isimpervious to normal human vision, such as darkness, haze, fog, rain, and snow. In addition,irnpervioris t o rairi,radar has the advantage of being able to measure the distance or range t o the object. This is has the to its attribute.probably its most important attribute. An elementary form of radar consists of a transmitting antenna emitting electromagnetic formradiation generated by an oscillator of some sort, a receiving antenna, ~nd an energy-detecting oscilIator anddevice. or receiver. A portion of the transmitted signal is intercepted by a reflecting objectdevice, reRecting(target) and is reradiated in all directions. I.t is the energy reradiated in the back direction that (target) and is all 1.tis of prime interest to the radar. The receiving antenna collects the returned energy and isdelivers it to a receiver, where it is processed to detect the presence of the target and t o extractdelivers t o to toits location and relative velocity. The distance to the target is determined by measuring the its and totime taken for the radar signal to travel to the target and back. The direction, or angular for toposition, of the target may be determined from the direction of arrival of the reflected wave- position, thefront. The usual method of measuring the direction of arrival is with narrow antenna beams. Iffront. The Ifrelative motion exists between target and radar, the shift in the carrier frequency of therelative exists ofreflected wave (doppler elTect) is a measure of the targets relative (radial) velocity and may be (doppler effect)used to distinguish moving targets from stationary objects. In radars which continuously track tothe movement of a target, a continuous indication of the rate of change of target position isthe ofalso available.also available. 1
  10. 10. 2 INTRODUCTION TO RADAR SYSTEMS SYSTEMS The name radar reflects the emphasis placed by the early experimenters on a device todetect the presence of a target and measure its range. Radar is a contraction of the words radio ofdetection and ranging. It was first developed as a detection device to warn of the approach of of approach ofhostile aircraft and for directing antiaircraft weapons. Although a well-designed modern radar modern radarcan usually extract more information from the target signal than merely range, the measure- thement of range is still one of radars most important functions. There seem to be no other of radarscompetitive techniques which can measure range as well or as rapidly as can a radar. The most common radar waveform is a train of narrow, rectangular-shape pulses modu- modu-lating a sinewave carrier. The distance, or range, to the target is determined by measuring the TRtime TR taken by the pulse to travel to the target and return. Since electromagnetic energy energy 8 of e= 10 mis,propagates at the speed of light c = 3 x 10 m/s, the range R isR R = eTR (1.1 ) 2The factor 2 appears in the denominator because of the two-way propagation of radar. With propagation of radar. Withthe range in kilometers or nautical miles, and TR in microseconds, E q . (1.1) becomes o m e s TR Eq. bec orEach microsecond of round-trip travel time corresponds to a distance of 0.081 nautical mile, of of 0.081 nautical mile,0.093 statute mile, 150 meters, 164 yards, or 492. feet. 492· feet. Once the transmitted pulse is emitted by the radar, a sufficient length of time must elapse of must elapseto allow any echo signals t o return and be detected before the next pulse may be transmitted. to next be transmitted.Therefore the rate at which the pulses may be transmitted is determined by the longest range at determined by longest range at Ifwhich targets are expected. If the pulse repetition frequency is too high, echo signals from some repetition echo sometargets might arrive after the transmission of the next pulse, and ambiguities in measuring of measuring I I! . . .III~E0u 1,000:;:::l0c:.u0> c:0<-III::l0::l0>.0 100E0c::J 10 L-_l...-..I..-I-1...J....1-U-L_-.J..---L--1-....L....L..l...L.J---_...1.--4-~ ................... 10 100 1,000 Pulse repetition frequency, Hz frequency, HzFigure 1.1 Plot of maximum unambiguous range as a function of the pulse repetition frequency. of of the pulse repetition frequency.
  11. 11. THE NATURE OF RADAR 3 range might result. Echoes that arrive after the transmission of the next pulse are called range result. second-time-arOlmd (or multiple-time-around) echoes. Such an echo would appear to be at a secorrd-tinte-arotrrtd (or much shorter range than the actual and could be misleading if it were not known to be a second-time-around echo. The range beyond which targets appear as second-time-around second-time-around echo. second-time-around echoes is called the maximum unambiguous range and is echoes is rna.uintttrn trr~arnhigtrous rattge c Runamb = 2fp (1.2) where fp = pulse repetition frequency, in Hz. A plot of the maximum unambiguous range as a where./, = Hz. function of pulse repetition frequency is shown in Fig. 1.1. Although the typical radar transmits a simple pulse-modulated waveform, there are a number of other suitable modulations that might be used. The pulse carrier might be frequency- or phase-modulated to permit the echo signals to be compressed in time after frequency- reception. reception. This achieves the benefits of high range-resolution without the need t o resort to a to. short pulse. The technique of using a long, modulated pulse to obtain the resolution of a short of pulse, but with the energy of a long pulse, is known as pulse compression. Continuous pulse, pulse waveforms (CW)) also can be used by taking advantage of the doppler frequency shift to (CW separate the received echo from the transmitted signal and the echoes from stationary clutter. separate from Unmodulated CW waveforms do not measure range, but a range measurement can be made CW do measurement by applying either frequency- or phase-modulation. 1.2 THE SIMPLE FORM OF THE RADAR EQUATION 1.2 THE SIMPLE The radar equation relates the range of a radar to the characteristics of the transmitter, The of receiver. antenna, target, and environment. It is useful not just as a means for determining the receiver, antenna, just maximum distance from the radar to the target, but it can serve both as a tool for under- from standing radar operation and as a basis for radar design. In this section, the simple form of the is the radar equation is derived. If the power of the radar transmitter is denoted by P,, and if an isotropic antenna If P" if is used (one which radiates uniformly in all directions), the power density (watts per unit area) is (one power at a distance R from the radar is equal to the transmitter power divided by the surface area distance from 4nR2 rapius 4n:R 2 of an imaginary sphere of radius R, or I pt Power density from isotropic antenna = - = 4 P, 2 (1.3) 4nR2 n:R Radars employ directive antennas to channel, or direct, the radiated power Pt into some P, particular direction. The gain G of an antenna is a measure of the increased power radiated in direction. gain of of the direction of the target as compared with the power that would have been radiated from an the direction isotropic antenna. It may be defined as the ratio of the maximum radiation intensity from the isotropic antenna. from a subject antenna to the radiation intensity from a lossless, isotropic antenna with the same antenna power input. (The radiation intensity is the power radiated per unit solid angle in a given input. (The direction.) The power density at the target from an antenna with a transmitting gain G is direction.) from Power density from directive antenna = :::~2 Power density from directive antenna = - Pt G (1.4) 4nR2 The target intercepts a portion of the incident power and reradiates it in vqrious directions. The target intercepts a portion of the incident power and reradiates it in v~rious directions.
  12. 12. 4 INTRODUCTION TO RADAR SYSTEMS R A D A R SYSTEMSThe measure of the amount of incident power intercepted by the target and reradiated back inthe direction of the radar is denoted as the radar cross section (J, and is defined by the relation a, P,G a = P, G2 ~ Power density of echo signal at radar = ---- - (1.5) 4nR2 4nR2 4rrR 4n:RThe radar cross section (J has units of area. It is a characteristic of the particular target and is a ameasure of its size as seen by the radar. The radar antenna captures a portion of the echopower. If the effective area of the receiving antenna is denoted A., the power P, received by the A",radar is (1.6) RmaxThe maximum radar range R max is the distance beyond which the target cannot be detected. Itoccurs when the received echo signal power P, just equals the minimum detectable signal Smin. P,just S,,,Therefore 1 P GA (J ]1/ 4 R max = [ (4~)2S:in (1. 7)This is the fundamental form of the radar equation. Note that the important par~ antenna par-ameters are the transmitting gain and the receiving effective area. Antenna theory gives the relationship between the transmitting gain and the receivingeffectiveeffective area of an antenna as (1.8)Since radars generally use the same antenna for both transmission and reception, Eq. (1.8) can (1.8)be substituted into Eq. (1.7), first for A,e then for G, to give two other forms of the radar (1.7), Aequation R max = [P G:A I 2 (4n:) Smin (J r /4 (1.9) Rmax = [ P, A;O ] 1/4 ( 1.10) 4rrA. 2Smln These three forms (Eqs. 1.7, 1.9, and 1.10) illustrate the need to be careful in the inter- forms (Eqs.·1.7, 1.9, 1.10)pretation of the radar equation. For example, from Eq. (1.9) it might be thought that the range A. 1/2 , A. - 1/2of a radar varies as All2, but Eq. (1.10) indicates a 1-12 relationship, and Eq. (1.7) shows therange to be independent of 1.The correct relationship depends on whether it is assumed the A.. introduc~gain is constant or the effective area is constant with wavelength. Furthermore, the introduc- istion of other constraints, such as the requirement to scan a specified volume in a given time,can yield a different wavelength dependence. These simplified versions of the .radar equation do not adequately describe the perfor- of radar domance of practical radar. Many important factors that affect range are not explicitly included.In practice, the observed maximum radar ranges are usually much smaller than what would bepredicted by the above equations, sometimes by as much as a factor of two. There are manyreasons for the failure of the simple radar equation to correlate with actual performance, as the failure performance,discussed in Chap. 2. . 2. _ , 9 , . . ...... · . 1 1
  13. 13. THE N A T U R E OF RADAR T H E NATURE RADAR 5 S1.31.3 RADAR BLOCK DIAGRAM AND OPERATIONThe operation of a typical pulse radar may be described with the aid of the block diagramTtleshown in Fig. 1.2. The transmitter may be an oscillator. such as a magnetron. that is "pulsed" i n Fig. 1.2. Tlle transtnitter oscillator, magnetron, " pulsed"(turned on and off) by the modulator to generate a repetitive train of pulses. The magnetron on) rnodulator prohnhly beenhas prohahly heen the most widely used of the various microwave generators for radar. A radar.typical radar for the detection of aircraft at ranges of 100 or 200 nmi might employ a peaktypicrtl tile dctcctionpower of the order of a megawatt. an average power of several kilowatts, a pulse width of megawatt,several microseconds. and a pulse repetition frequency of several hundred pulses per second. microseconds, second.The waveform generated by the transmitter travels via a transmission line to the antenna. antenna, space.where it is radiated into space. A single antenna is generally used for both transmitting and pro~ectedreceiving. The receiver must be protected from damage caused by the high power of the duplexer. totransmitter. This is the function of the duplexer. The duplexer also serves to channel thereturned echo signals to the receiver and not to the transmitter. The duplexer might consist of devices,two gas-discharge devices. one known as a TR (transmit-receive) and the other an ATR(anti-transmit-receive). The TR protects the receiver during transmission and the ATR directs(anti-transmit-receive). ferritethe echo signal to the receiver during reception. Solid-state ferrite circulators and receiverprotectors with gas-plasma TR devices and/or diode limiters are also employed as duplexers. duplexers. The receiver is usually of the superheterodyne type. The first stage might be a low-noiseRF amplifier. such as a parametric amplifier or a low-noise transistor. However. it is not amplifier, However,always desirable to employ a low-noise first stage in radar. The receiver input can simply be stage,the mixer stage. especially in military radars that must operate in a noisy environment.Although a receiver with a low-noise front-end will be more sensitive, the mixer input can range,have greater dynamic range. less susceptibility to overload, and less vulnerability to electronicinterference.interference. (LO) RF The mixer and local oscillator (LO) convert the RF signal to an intermediate frequency(IF). A " typical" I F(IF). / "typical" IF amplifier for an air-surveillance radar might have a center frequency of 30 MHzor 60 MHz and a bandwidth of the order of one megahertz. The IF amplifier should bedesigned as a matc/ted filter; i.e., its frequency-response function H ( f ) should maximize the n~atclted filter; i.e., frequency-response H(f)peak-sigtial-to-mean-noise-powerratio at the output. This occurs when the magnitude of thepeak-signal-to-mean-noise-power 1frequency-response function [H(f) I is equal to the magnitude of the echo signal spectrum H(f)(I S(.f) 1, IS(f) I. and the phase spectrum of the matched filter is the negative of the phase spectrum of (Sec. 10.2).the echo signal (Sec. 10.2). In a radar whose signal waveform approximates a rectangularpulse, the conventional IF filter bandpass characteristic approximates a matched filter whenpulse. IF r a After maximizing the signal-to-noise ratio in the IF amplifier, the pulse modulation is amplifier,extracted by the second detector and amplified by the video amplifier to a level where it can be acd -the product of the IF bandwidth B and the pulse width t is of the order of unity, that is, Bt ~ 1. 1. Tronsrnitler Pulse Dupleller modulalorAntenna 4 - Low - noise RF RF Mixer IF amplifier amplifier (matched filter) . . IFigure 1.2 Block diagram of a pulse radar. diagram radar.
  14. 14. 6 INTRODUCTION TO RADAR SYSTEMS , -~ (a) (b) modulation); ( h A-scop~ pr~senta­Figure 1.3 (a) PPI presentation displaying range vs. angle (intensity modulation); (0)) A-scope presenta- (a)tion displaying amplitude vs. range (deflection modulation). modulation). "4 (CRT).properly displayed, usually on a cathode-ray tube (CRT). Timing signals are also supplied to signals suppliedthe indicator to provide the range zero. Angle information is obtained from the pointing zero. fromdirection of the antenna. The most common form of cathode-ray tube display is the plan formposition indicator, or PPI (Fig. 1.3a), which maps in polar coordinates the location of the (Fig. 1.3a),target in azimuth and range. This is an intensity-modulated display in which the amplitude ofthe receiver output modulates the electron-beam intensity (z axis) as the electron beam is made axis)to sweep outward from the center of the tube. The beam rotates in angle in response to the Aantenna position. A B-scope display is similar to the PPI except that it utilizes rectangular,rather than polar, coordinates to display range vs. angle. Both the B-scope and the PPI, being angle.intensity modulated, have limited dynamic range. Another form of display is the A-scope, range. formshown in Fig. 1.3b, which plots target .amplitude (y axis) vs. range (x axis), for some fixed Fig. amplitude axis) axis), fixeddirection. This is a deftectiort:inodulated display. It is more suited for tracking-radar applica- deflection-modulated tion than for surveillance radar. The block diagram of Fig. 1.2 is a simplified version that omits many details. I t does not include several devices often found in radar, such as means for automatically compensating the (AFC) receiver for changes in frequency (AFC) or gain (AGe), receiver circuits for reducing interfer- (AGC), interfer- ence from other radars and from unwanted signals, rotary joints in the transmission lines to signals, allow movement of the antenna, circuitry for discriminating between moving targets and unwanted stationary objects (MTn and pulse compression for achieving the resolution benefits (MTI), achieving of a short pulse but with the energy of along pulse. If the radar is used for tracking, some a long If means are necessary for sensing the angular location of a moving target and allowing the antenna automatically to lock-on and to track the target. Monitoring devices are usually devices included to ensure that the transmitter is delivering the proper shape pulse at the proper delivering power level and that the receiver sensitivity has not degraded. Provisions may also be in- sensitivity corporated in the radar for locating equipment failures so that faulty circuits can be easily for failures faulty found and replaced. the " raw-video" Instead of displaying the" raw·video" output of a surveillance radar directly on the CRT, CRT, it might first be processed by an automatic detection and tracking (ADT) device that quantizes first anautornaticdetection (ADT) device the radar coverage into range-azimuth resolution cells, adds (or integrates) all the echo pulses cells, received within each cell, establishes a threshold (on the basis of these integrated pulses) that cell, establishes (on pulses) permits only the strong outputs due to target echoes to pass while rejecting noise, establishes rejecting noise, and maintains the tracks (trajectories) of each target, and displays the processed information
  15. 15. THE NATURE OF RADAR 7 NATURE OF to the operator. These operations of an ADT are usually implemented with digital computer operator. technology. techriology. A common form of radar antenna is a reflector with a parabolic shape, fed (illuminated) A from a point source at its focus. The parabolic reflector focuses the energy into a narrow beam, from focus. just as does a searchlight or an automobile headlamp. The beam may be scanned in space by mechanical pointing of the antenna. Phased-array antennas have also been used for radar. In a phased array. the bcam is scanned by electronically varying the phase of the currents across pllascd array, tllc beam of the aperture. aperture. 1.4 RADAR FREQUENCIES R A D A R FREQUENCIES Conventional radars generally have been operated at frequencies extending from about 220 M Hz to 35 G Hz, a spread of more than seven octaves. These are not necessarily the limits, MHz 35 GHz,., since radars can be, and have been, operated at frequencies outside either end of this range. since of Skywave HF over-the-horizon (OTH) radar might be at frequencies as low as 4 or 5 MHz, and HF (OTH) frequencies groundwave HF radars as low as 2 MHz. At the other end of the spectrum, millimeter radars HF have operated at 94 GHz. Laser radars operate at even higher frequencies. GHz. The place of radar frequencies in the electromagnetic spectrum is shown in Fig. 1.4. Some frequencies 1.4. of the nomenclature employed to designate the various frequency regions is also shown. Early in the development of radar, a letter code such as S, X, L, etc., was employed to designate radar frequency bands. Although its original purpose was to guard military secrecy, desigr~ate the designations were maintained, probably out of habit as well as the need for some conven- ient short nomenclature. This usage has continued and is now an accepted practice of radar of 1.1 lists engineers. Table 1.1 lists the radar-frequency letter-band nomenclature adopted by the engineers. IEEE. 15 These are related to the specific bands assigned by the International Telecommunica- IEEE. These are to tions Union for radar. For example, although the nominal frequency range for L band is 1000 to 2000 MHz, an L-band radar is thought of as being confined within the region from 1215 to to 2000 L-band 1215 1400 MHz since 1400 MHz since that is the extent of the assigned band. Letter-band nomenclature is not a Wovelenath Wavelength 10 km fOkm Ikm l km 100 m 100m 10m 10m Im 1m lOcm !Oem tcm 1cm 1mm lmm O.lnm OIMm I I --VlF -LF-+-MF -VLF -+HF-t-VHF-+ LF--------+ ---MF- - H F - -VHF- fo- UHF-I-SHF- ..... E H F --+ UHF---c+SHF-+k EHF- Very low Very low Low Low Medium Medium High Very high Ultrohigh high Ultrohigh Super Extremely frequency frequency frequency frequency frequency frequency frequency high hrgh hiqh frequency frequency frequency I Myriometric Myriometric Kilometric Hectometric Kilometric Hecrometric Decometric Metric Decimetric Centimetric Millimetric Decimilli- Decimilli- woves woves woves woves woves waves woves woves wovcs woves woves waves woves metric woves Bond Bond 4 Bond 5 Bond 5 Bond 6 Bond 6 Bond Bond 7 Bond 8 Bond 8 I Bond 9 Bond Bond 1 0 Bond 10 Bond l l II 12 Bond 12 mg!~e9~~ry~i~s r~ d ::i~~~OiA f w Submillimeler I --- For Fo r ~roodcost OTH OTH infra redm in f m >------l bond • rodor II• t - - - - - - * I designotions L Letter designotions L J C X Ku Ka S C X Ku Ka. I I .. 4 Audio frequencies Audio frequencies • c , Microwove region Microwave rtQion I -- 1 I Video frequencies Video frequencies I I I I I * • 30 Hzz 30H 300Hz 300Hz 3kHz 3kHz 30kHz 30kHz 300kHz 300kHz 3MHz 3MHz 30MHz 30MHz 300 M H z 3 0 0 MHz 3GHz 3GHz 30GHz 30GHz 300GHz 300GHz 3 , 0 0 0 GHr 3,000 GHz Frequency Frequency Figure 1.4 Radar frequencies and electromagnetic spectrum. Figure 1.4 Radar frequencies and the electromagnetic spectrum.
  16. 16. 8 INTRODUCTION TO RADAR SYSTEMS SYSTEMSTable 1.1 Standard radar-frequency letter-band nomenclature 1.1 letter-band Specific radiolocatio~l Specific radio location Band Nominal (radar) bands based on designation frequency range ITU assignments for region 2 2 HF HF 3-30 MHz VHF VHF 30-300 MHz 138-144 MHz 138-144 216-225 2 16-225 UHF UHF 300-1000 MHz 420-450 420-450 MHz 890-942 890-942 L 1000-2000 MHz 1215-1400 MHz 1215-1400 S 2000-4000 MHz 2300-2500 MHz 2300-2500 2700-3700 2 700-3 700 C 4000-8000 MHz 5250-5925 MHz 5250-5925 X 8000-12,000 MHz 8500-10,680 MHz 8500- 10,680 Ku 12.0-18 GHz 13.4-14.0 GHz 13.4-14.0 15.7-17.7 15.7- 17.7 , K 18-27 GHz 24.05-24.25 G H z 24.05-24.25 GHz .. _,; Kg K, 27-40 GHz 33.4-36.0 G H z 33.4-36.0 GHz rnrn mm 40-300 GHzsubstitute for the actual numerical frequency limits of radars. The specific numerical frequency for frequencylimits should be used whenever appropriate, but the letter designations of Table 1.1 may belimits should appropriate, of desired.used whenever a short notation is desired.1.5 RADAR DEVELOPMENT PRIOR TO WORLD WAR I11.5 DEVELOPMENT IIAlthough the development of radar as a full-fledged technology did not occur until World WarAlthough full-fledgedII, the basic principle of radar detection is almost as old as the subject of electromagnetism11,itself. Heinrich Hertz, in 1886, experimentally tested the theories of Maxwell and demonstrateditself. 1886, Lthe similarity between radio and light waves. Hertz showed that radio waves could be reflectedthe ,Iby metallic and dielectric bodies. It is interesting to note that although Hertzs experimentswere performed with relatively short wavelength radiation (66 cm), later work in radio engin-were (66 cm),eering was almost entirely at longer wavelengths. The shorter wavelengths were not activelyeering was wavelengths. toused to any great extent until the late thirties. 1903 a In 1903 a German engineer by the name of Hiilsmeyer experimented with the detection of engineer of radio waves from ships. 1904radio waves reflected from ships. He obtained a patent in 1904 in several countries for a n anobstacle detector and ship navigational device. 2e His methods were demonstrated before the ship de~ic .~German Navy, but generated little interest. The state of technology at that time was not Germansufficiently adequate to obtain ranges of more than about a mile, and his detection technique sufficiently adequate rangeswas dismissed on the grounds that it was little better than a visual observer. was on the grounds Marconi recognized the potentialities of short waves for radio detection and strongly Marconi recognized use in 1922 for urged their use in 1922 for this application. In a speech delivered before the Institute of Radio Engineers, he said: 3 Engineers, he said: As was first shown by Hertz, electric waves can be completely reflected by conducting bodies. In As was first shown Hertz, electric reflected bodies. some my tests I have noticed the effects reflection some of iny tests J have noticed the effects of reflection and detection of these waves by metallic objects miles away. objects miles away. It ~eems to me that it should be possible to design apparatus by means of which a ship could !eems to me that should design apparatus means
  17. 17. THE NATURE RADAR T H E N A T U R E OF R A D A R 9 radiate or project a divergent beam of these rays in any desired direction, which rays, if coming if across a metallic object, such as another steamer or ship, would be reflected back to a receiver screened reflected back receiver from the local transmitter on the sending ship, and thereby, immediately reveal the presence and immediately presence bearing of the other ship in fog or thick weather. in Although Marconi predicted and successfully demonstrated radio communication be- successfullytween continents, he was apparently not successful in gaining support for some of his other ofideas involving very short waves. One was the radar detection mentioned above; the other was waves.the suggestion that very short waves are capable of propagation well beyond the optical line of ofsight-a phenomenon now know11 as tropospheric scatter. He also suggested that radio wavessight-a phenometlon knownbe used for the transfer of power from one point to the other without the use of wire or other oftrat~smissiot~transmission lines.lit~cs. In the autumn of 1922 A. ti. Iaylor arid L. C. Young of tile Naval Research Laboratory aututnrl H. Taylor and thedetected a wooden ship using a CW wave-interference radar with separated receiver andtransmitter. The wavelength was 5 m. A proposal was submitted for further work but was not wavelerlgthaccepted.accepted. The first application of the pulse technique to the measurement of distance was in the ofbasic scientific investigation by Breit and Tuve in 1925 for measuring the height of the 1925 ofionosphere. 4 .1 6 ~ . ~i ~ n o s p h e r e . However, more than a decade was to elapse before the detection of aircraft by detection of bypulse radar was demonstrated. The first experimental radar systems operated with CW and depended for detection upon firstthe interference produced between the direct signal received from the transmitter and the interference transmitter thedoppler-frequency-shifted signal reflected by a moving target. This effect is the same as the therhythmic flickering, or flutter, observed in an ordinary television receiver, especially on weak flickering, weakstations, when an aircraft passes overhead. This type of radar originally was called C Wstations, of CWwal}e-inte~rerelceradar. Today, such a radar is called a bistatic C W radar. The first experimen-wqaoe-irtfer-erence bistatic CWtal detections of aircraft used this radar principle rather than a monostatic (single-site) pulse pulseradar because CW equipment was readily available. Successful pulse radar had to await the thedevelopment of suitable components, especially high-peak-power tubes, and a better under- high-peak-power betterstanding of pulse receivers. The first detection of aircraft using the wave-interference effect was made in June, 1930, by first wave-interference byL. 1... tlyland of the Naval Research Laboratory. It was made accidentally while he was A Hyland Laboratory.l was working with a direction-finding apparatus located in an aircraft on the ground. The transmit- ter at a frequency of 33 MHz was located 2 miles away, and the beam crossed an air lane from a nearby airfield. When aircraft passed through the beam, Hyland noted an increase in the the received signal. This stimulated a more deliberate investigation by the NRL personnel, but the but the work continued at a slow pace, lacking official encouragement and funds from the govern- ment. although it was fully supported by the NRL administration. By 1932 the equipment was nrent. was demonstrated to detect aircraft at distances as great as 50 miles from the transmitter. The NRL NRL work on aircraft detection with CW wave interference was kept classified until 1933, when several Bell Telephone Laboratories engineers reported the detection of aircraft during the of the course of other experiments.5 The N R L work was disclosed in a patent filed and granted to experiments. NRL patent granted to Taylor, Young, and Hyland6 on a "System for Detecting Objects by Radio." The type of radar Hyland 6 a" System Radio." type of radar described in this patent was a CW wave-interference radar. Early in 1934, a 60-MHz CW wave-interference 6O-MHz CW wave-interference radar was demonstrated by NRL. The early CW wave-interference radars were useful only for detecting the preserrce of the presence of the target. The problem of extracting target-position information from such radars was a difficult was a difficult one and could not be readily solved with the techniques existing at that time. A proposal was that A proposal was made by N R L in 1933 to errlploy a chain of transmitting and receiving stations along a line to RL 1933 employ of a line to be guarded. for the purpose of obtaining some knowledge of distance and velocity. This was of of and velocity. This was
  18. 18. to INTRODUCTION TO RADAR SYSTEMS10 SYSTEMS never carried out, however. The limited ability of C W wave-interference radar to be anything however. CW wave-interference more than a trip wire undoubtedly tempered what little official enthusiasm existed for radar. It was recognized that the limitations to obtaining adequate position information coiild could be overcome with pulse transmission. Strange as it may now seem, in the early days pulse transmission:radar encountered much skepticism. Nevertheless, an effort was started at NRL in the spring Nevertheless, N RLof 1934 to develop a pulse radar. The work received low priority and was carried out prin- 1934cipally by R. M. Page, but he was not allowed to devote his full time to the effort. The first attempt with pulse radar at NRL was at a frequency of60 MHz. According to first frequency of 60Guerlac,t the first tests of the 6O-MHz pulse radar were carried out in late December, 1934,Guerlac, 60-MHzand early January, 1935. These tests were "hopelessly unsuccessful and a grievous disappoint- 1935. were" hopelesslyment." No pulse echoes were observed on the cathode-ray tube. The chief reason for this chieffailure was attributed to the receivers being designed for CW communications rather than forfailurepulse reception. The shortcomings were corrected, and the first radar echoes obtained at reception.NRL using pulses occurred on April 28, 1936, with a radar operating at a frequency of 1936, of28.3 MHz and a pulse width of 5 ,US. The range was only 24 miles. By early June the range was28.3 IlS. 2! miles.25 miles. It was realized by the NRL experimenters that higher radar frequencies were desired, frequenciesespecially for shipboard application, where large antennas could not be tolerated. However,the necessary components did not exist. The success of the experiments at 28 MHz encouraged ofthe NRL experimenters to develop a 200-MHz equipment. The first echoes at 200 MHz werereceived July 22, 1936, less than three months after the start of the project. This radar was also 1936, ofthe first to employ a duplexing system with a common antenna for both transmitting and first transmitting receiving. The range was only 10 to 12 miles. In the spring of 1937 it was installed and tested onreceiving. 12 of 1937the destroyer Leary. The range of the 200-MHz radar was limited by the transmitter. The ofdevelopment of higher-powered tubes by the Eitel-McCullough Corporation allowed animproved design of the 200-MHz radar known as XAF. This occurred in January, 1938. Although the power delivered to the antenna was only 6 kW, a range of 50 miles-the limit of of miles-the of the sweep-was obtained by February. The XAF was tested aboard the battleship New York, sweep-was in maneuvers held during January and February of 1939, and met with considerable success. of considerable Ranges of 20 to 24 kiloyards were obtained on battleships and cruisers. By October, 1939, wereorders were placed for a manufactured version called the CXAM. Nineteen of these radarsof were installed on major ships of the fleet by 1941. of The United States Army Signal Corps also maintained an interest in radar during the early 1930s. 7 The beginning of serious Signal Corps work in pulse radar apparently resulted 1930s. of apparentlyfrom a visit to NRL in January, 1936. By December of that year the Army tested its first pulse 1936. of pulse radar, obtaining a range of 7 miles. The first operational radar used for antiaircraft fire control antiaircraft was the SCR-268, available in 1938, The SCR-268 was used in conjunction with searchlights SCR-268, 1938!8 for radar fire control. This was necessary because of its poor angular accuracy. However, its fire of its range accuracy was superior to that obtained with optical methods. The SCR-268 remained the standard fire-control equipment until January, 1944, when it was replaced by the SCR-584 replaced microwave radar. The SCR-584 could control an antiaircraft battery without the necessity for antiaircraft battery searchlights or optical angle tracking, . tracking.. In 1939 the Army developed the SCR-270, a long-range radar for early warning. The attack (n 1939 attackon Pearl Harbor in December, 1941, was detected by an SCR-270, one of six in Hawaii at the of thetime. (There were also 16 SCR-268s assigned to units in Honolulu.) But unfortunately, thetime.! 16 unfortunately, thetrue significance of the blips on the scope was not realized until after the bombs had fallen. A significance Amodified SCR-270 was also the first radar to detect echoesfrom the moon in 1946. SCR-270 echoes! The early developments of pulse radar were primarily concerned with military applica- oftions. Although it was not recognized as being a radar at the time, the frequency-modulated