PETITIONERS 1015-0001
`
`

`
`INTRODUCTION TO
`RADAR SYSTEMS
`
`Second Edition
`
`Merrill 1. Skolnik
`
`MCGRAW-HILL BOOK COMPANY
`
`Auckland Bogota Guatemala Hamburg Lisbon
`London Madrid Mexico New Delhi Panama Paris
`San Juan S510 Paulo Singapore Sydney Tokyo
`
`PETITIONERS 1015-0002
`
`

`
`INTRODUCTION TO RADAR SYSTEMS
`
`International Edition 1981
`
`Exclusive rights by McGraw-Hill Book Co.-- Singapore for
`manufacture and export. This book cannot be re-exported
`from the country to which it is consigned by McGraw-Hill.
`
`Copyright © 1980, 1962 by McGraw-Hill, Inc.
`
`All rights reserved. tixcept as permitted under the United States Copyright
`
`Act of 1976, no part of this publication may be reproduced or distributed in
`any form or by any means, or stored in a data base or retrieval system,
`without the prior written permission of the publisher.
`
`l234567892OBJE987654
`
`This book was set in Times Roman.
`The editor was Frank J. Cerra.
`
`The production supervisor was Gayle.Angelson.
`
`Library of Congress Cataloging In Publication Data
`
`Skolnik, Merrill Ivan, date
`Introduction to radar systems.
`Includes bibliographical references and index.
`1. Radar.
`1. Title.
`II. Series.
`TK6575.S477
`1980
`621.3848
`79-15354
`
`ISBN 0-07-057909-1
`
`When ordering this title use ISBN 0-07-066572~ 9
`
`L
`
`..f/
`
`Printed in Singapore
`
`PETITIONERS 1015-0003
`
`

`
`CONTENTS
`
`Preface
`
`The Nature of Radar
`
`Introduction
`
`The Simple Form of the Radar Equation
`Radar Block Diagram and Operation
`Radar Frequencies
`Radar Development Prior to World War ll
`Applications of Radar
`References
`
`The Radar Equation
`
`Prediction of Range Performance
`Minimum Detectable Signal
`Receiver Noise
`
`Probability-density Functions
`Signal—to-noise Ratio 4
`Integration of Radar Pulses
`Radar Cross Section of Targets
`Cross-section Fluctuations
`Transmitter Power
`
`.
`
`'
`
`Pulse Repetition Frequency and Range Ambiguities
`Antenna Parameters
`
`System Losses
`Propagation Effects
`Other Considerations
`References
`
`CW and Frequency-Modulated Radar
`
`The Doppler Effect
`CW Radar
`Frequency—modulated CW Radar
`
`.
`
`2.2
`2.3
`2.4
`2.5
`2.6
`2.7
`2.8
`2.9
`2.10
`2.11
`2.12
`2.13
`2.14
`
`3
`
`3.1
`3.2
`3.3
`
`ix
`
`1
`
`1
`
`3
`5
`7
`8
`12
`14
`
`15
`
`15
`16
`18
`
`20
`23
`29
`33
`46
`52
`
`53
`54
`
`56
`62
`62
`65
`
`68
`
`68
`70
`81
`
`PETITIONERS 1015-0004
`
`

`
`vi CONTENTS
`
`Airborne Doppler Navigation
`3.4
`3.5 Multiple-Frequency CW Radar
`References
`
`4 MT] and Pulse Doppler Radar
`4.1
`Introduction
`
`Delay-Line Cancelers
`4.2
`4.3 Multiple, or Staggered, Pulse Repetition Frequencies
`4.4
`Range—Gated Doppler Filters
`4.5
`Digital Signal Processing
`4.6 Other MTI Delay Lines
`4.7
`Example of an MTI Radar Processor
`4.8
`Limitations to MTI Performance
`4.9 Noncoherent MTI
`
`Pulse Doppler Radar
`4.10
`4.11 MTl from a Moving Platform
`4.12 Other Types of MT]
`References
`
`5 Tracking Radar
`
`5.1
`5.2
`5.3
`
`Tracking with Radar
`Sequential Lobing
`Conical Scan
`
`5.4 Monopulse Tracking Radar
`5.5
`Target-Refiection Characteristics and Angular Accuracy
`5.6
`Tracking in Range
`5.7 Acquisition
`5.8 Other Topics
`5.9
`Comparison of Trackers
`5.10
`Tracking with Surveillance Radar
`References
`
`6 Radar Transmitters
`
`6.1
`
`Introduction
`
`The Magnetron Oscillator
`6.2
`Klystron Amplifier
`6.3
`Traveling—Wave-Tube Amplifier
`6.4
`Hybrid Linear—Beam Amplifier
`6.5
`Crossed-Field Amplifiers
`6.6
`6.7 Grid-Controlled Tubes
`6.8 Modulators
`6.9
`Solid-State Transmitters
`
`References
`
`_
`
`7 Radar Antennas
`
`7.1
`
`Antenna Parameters
`
`7.2 Antenna Radiation Pattern and Aperture Distribution
`7.3
`Parabolic-Refiector Antennas
`
`7.4
`7.5
`
`Scanning-Feed Reflector Antennas
`Lens Antennas
`
`‘)2
`95
`911
`
`101
`101
`
`106
`11-1
`117
`119
`126
`127
`12*)
`1.18
`
`13‘)
`140
`147
`1-18
`
`15:
`
`152
`153
`155
`
`160
`[67
`176
`177
`178
`182
`18}
`186
`
`190
`
`190
`
`192
`200
`206
`208
`208
`213
`21-1
`216
`
`330
`
`223
`
`223
`
`223
`235 .
`
`244
`2-18
`
`«#5-”'
`
`PETITIONERS 1015-0005
`
`

`
`7.6
`7.7
`7.8
`7.9
`7.10
`
`Pattern Synthesis
`Coseeant—Squared Antenna Pattern
`lilfeet of Errors on Radiation Patterns
`Radomes
`Stabilization of Antennas
`References
`
`The Electronically Steered Phased
`Array Antenna in Radar
`lntrodnctinn
`llzisie Concepts
`1’liase Shifters
`
`1
`
`l-'requency—Sean Arrays
`Array lilements
`Feeds for Arrays
`Simultaneous Multiple Beams from Array Antennas
`Random Errors in Arrays
`Computer Control of Phased-Array Radar
`Other Array Topics
`Applications of the Array in Radar
`Advantages and Limitations
`References
`
`Receivers, Displays, and Duplexers
`The Radar Receiver
`
`Noise Figure
`Mixers
`Low-Noise Front-Endse
`
`Displays
`l)uplexers and Receiver Protectors
`References
`
`Detection of Radar Signals in Noise
`Introduction
`
`Matched-Filter Receiver
`Correlation Detection
`Detection Criteria
`Detector Characteristics
`Performance of the Radar Operator
`Automatic Detection
`Constant-Fa1se—Alarm-Rate (CFAR) Receiver
`References
`
`Extraction of Information and Waveform
`Design
`Introduction
`
`Information Available from a Radar
`
`Theoretical Accuracy of Radar Measurements
`Ambiguity Diagram
`
`8.1
`8.2
`8.3
`8.4
`8.5
`8.6
`8.7
`8.8
`
`8.9
`8.10
`8.11
`
`8.12
`
`9.1
`
`9.2
`9.3
`9.4
`9.5
`9.6
`
`10
`
`10.1
`10.2
`10.3
`10.4
`10.5
`10.6
`10.7
`10.8
`
`11
`
`11.1
`11.2
`11.3
`11.4
`
`CONTENTS vii
`
`254
`258
`262
`264
`270
`273
`
`278
`278
`279
`286
`
`298
`305
`306
`310
`318
`322
`328
`334
`335
`337
`
`343
`343
`
`344
`347
`351
`
`353
`359
`366
`
`369
`369
`
`369
`375
`376
`.332
`386
`388
`392
`395
`
`399
`399
`
`399
`
`400
`411
`
`PETITIONERS 1015-0006
`
`

`
`viii CONTENTS
`
`11.5
`11.6
`
`12
`
`12.1
`
`12.2
`12.3
`12.4
`12.5
`12.6
`12.7
`12.8
`12.9
`
`13
`
`13.1
`13.2
`13.3
`13.4
`13.5
`13.6
`13.7
`13.8
`13.9
`
`14
`
`14-.1
`14.2
`14.3
`14.4
`
`14.5
`14.6
`14.7
`
`Pulse Compression
`Classification of Targets with Radar
`References
`
`Propagation of Radar Waves
`Introduction
`
`Propagation over a Plane Earth
`The Round Earth
`Refraction
`
`Anomalous Propagation
`Diffraction
`
`Attenuation by Atmospheric Gases
`Environmental Noise
`Microwave-Radiation Hazards
`References
`
`Radar Clutter
`
`Introduction to Radar Clutter
`
`Surface—Clutter Radar Equations
`Sea Clutter
`Detection of Targets in Sea Clutter
`Land Clutter
`
`‘
`
`Detection of Targets in Land Clutter
`Effects of Weather on Radar
`
`Detection of Targets in Precipitation
`Angel Echoes
`References
`
`Other Radar Topics
`
`Synthetic Aperture Radar
`HF Over-the-Horizon Radar
`Air-Surveillance Radar
`
`Height-Finder and 3D Radars
`Electronic Counter-Countermeasures
`Bistatic Radar
`
`Millimeter Waves and Beyond
`References
`
`Index
`
`420
`434
`438
`
`441
`
`441
`442
`446
`447
`450
`456
`459
`461
`465
`466
`
`470
`
`470
`471
`474
`482
`489
`497
`498
`S04
`508
`S 12
`
`517
`
`517
`529
`
`536
`541
`547
`553
`560
`566
`
`571
`
`V
`
`'
`
`_
`
`PETITIONERS 1015-0007
`
`

`
`
`
`PREFACE
`
`Although the fundamentals of radar have changed little since the publication of the first
`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
`revisions and the introduction of topics not found in the original.
`One of the major changes is in the treatment of MTI (moving target indication) radar
`(Chap. 4). Most of the basic MTI concepts that have been added were known at the time ofthe
`first edition, but they had not appeared in the open. literature nor were they widely used in
`practice. Inclusion in the first edition would have been largely academic since the analog
`delay-line technology available at that time did not make it practical to build the sophisticated
`signal processors that were theoretically possible. However, subsequent advances in digital
`technology, originally developed for applications other than radar, have allowed the practical
`implementation of the multiple delay-line cancelers and multiple pulse-repetition-frequency
`MTI radars indicated by the basic MTI theory.
`is another important
`Automatic detection and tracking, or ADT (Secs. 5.10 and 10.7),
`development whose basic theory was known for some time, but whose practical realization
`L had to await advances in digital technology. The principle of ADT was demonstrated in the
`early 1950s. using vacuum-tube technology, as part ‘of the United States Air Force's SAGE
`air-defense system developed by MIT Lincoln Laboratory. In this form ADT was physically
`large, expensive, and difficult to maintain. The commercial availability in the late 1960s ofthe
`solid-state 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.
`Another radar area that has seen much development is that of the electronically steered
`phased-array antenna.
`In the first edition,
`the radar antenna was the subject of a single
`chapter. In this edition, one chapter covers the conventional radar antenna (Chap. 7) and a
`separate chapter covers the phased—array antenna (Chap. 8). Devoting a single chapter to the
`array antenna is more a rellection olinterest rather than recognition of extensive application.
`The chapter on radar clutter (Chap. 13) has been reorganized to include methods for the
`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 fromthose necessary
`for detection in a noise background. Other subjects that are new or which have seen significant
`changes in the current edition include low-angle tracking, “on-axis” tracking, solid-state RF
`sources, the mirrorvscan antenna, antenna stabilization, computer control of phased arrays,
`zolid-state duplexers, CFAR, pulse compression, target classification, synthetic-aperture radar,
`over-the—horizon radar, air-surveillance radar, height-finder and 3D radar, and ECCM. The
`bistatic radar and millimeter-wave radar are also included even though their applications have
`-Iv
`
`PETITIONERS 1015-0008
`
`

`
`X PREFACE
`
`the chapter on Radar Astronomy since
`been limited. Omitted from this second edition is
`interest in this subject has decreased with the availability ofspace probes that can explore the
`planets at close range. The basic material of the first edition that covers the radar equation,
`the detection of signals in noise, the extraction of information, and the propagation of radar
`waves has not changed significantly. The reader, however, will
`find only a few pages of
`the original edition that have not been modified in some manner.
`One of the features of the first edition which has been continued is the inclusion of
`
`extensive references at the end ofeach chapter. These are provided to acknowledge the sources
`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 cttrrent references or appear in
`publications that are increasingly difficult to find. The references included in the first edition
`represented a large fraction ofthose available at the time. It would have been difficult to add to
`them 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 of-space, only a much
`smaller proportion of what is available could be cited.
`In addition to changes in radar technology, there have been changes also in style and
`nomenclature. For example, db has been changed to dB, and Me is replaced by Ml-la. Also, the
`letter—band nomenclature widely employed by the radar engineer for designating the common
`radar frequency bands (such as L, S, and X) has been officially adopted as a standard by the
`IEEE.
`
`The material in this book has been used as the basis for a graduate course in radar taught
`by the author at the Johns Hopkins "University Evening College and, before that, at several
`other institutions. This course is different from those usually found in most graduate electrical
`engineering programs. Typical EE courses cover topics related to circuits, components, de-
`vices, and techniques that might make up an electrical or electronic system; but seldom is the
`student exposed to the system itself. It is the system application (whether radar, communica-
`tions, navigation, control, information processing, or energy) that is the raison d'étre for the
`electrical engineer. The course on which this book is based is a proven method for introducing
`the student to the subject of electronic systems. It integrates and applies the basic concepts
`found in the student’s other courses and permits the inclusion of material
`important
`to
`the practice of electrical engineering not usually found in the traditional curriculum.
`Instructors of engineering courses like to use texts that contain a variety of problems that
`can be assigned to students. Problems are not included in this book. Although the author
`assigns problems when using this book as a text, they are not considered a major learning a
`technique. Instead, the comprehensive term paper, usually involving a radar design problem or
`a study in depth of some particular radar technology, has been found to be a better means for
`having the student reinforce what is covered in class and in the text. Even more important, it
`allows the student to research the literature and to be a bit more creative than is possible by
`simply solving standard problems.
`_
`A book of this type which covers a wide variety of topics cannot be written in isolation. It
`would not have been possible.without'the many contributions on radar that have appeared in
`the open literature and which have been used here as the basic source-material. A large
`measure of gratitude must be expressed to those radar engineers who have taken the time and
`energy to ensure that
`the results ‘of their work were made available by publication in
`recognized journals.
`,
`.
`On a more personal note, neither edition ofthis book could have been written without the
`complete support and patience of my wife Judith and my entire family who allowed me the
`time necessary to undertake this work.
`
`Merrill 1. Skolnik
`
`L
`
`,
`‘M?
`A
`
`PETITIONERS 1015-0009
`
`

`
`CHAPTER
`
`ONE
`
`THE NATURE OF RADAR
`
`1.1 INTRODUCTION
`
`Radar is an electromagnetic system for the detection and location of objects. It operates by
`transmitting a particular type of waveform, a pulse—modulated sine wave for example, and
`detects the nature of the echo signal. Radar is used to extend the capability of one's senses for
`observing the environment, especially the sense ofvision. The value of radar lies not in being a
`substitute for the eye, but in doing what the eye cannot do,Radar cannot resolve detail as well
`as the eye, nor is it capable of recognizing the “ color” of objects to the degree of sophistication
`“of which the eye is capable. However, radar can be designed to see through those conditions
`impervious to normal human vision, such as darkness, haze, fog, rain, and snow. In addition,
`radar has the advantage of being able to measure the distance or range to the object. This is
`probably its most important attribute.
`An elementary form of radar consists of a transmitting antenna emitting electromagnetic
`radiation generated by an oscillator ofsome sort, a receiving antenna, and an energy-detecting
`device, or receiver. A portion of the transmitted signal is intercepted by a reflecting object
`(target) and is reradiated in all directions. l.t is the energy reradiated in the back direction that
`is of prime interest
`to the radar. The receiving antenna collects the returned energy and
`delivers it to a receiver, where it is processed to detect the presence ofthe target and to extract
`its location and relative velocity. The distance to the target is determined by measuring the
`time taken for the radar signal to travel to the target and back. The direction, or angular
`position, of the target may be determined from the direction of arrival ofthe reflected wave-
`front. The usual method of measuring the direction ofarrival is with narrow antenna beams. If
`relative motion exists between target and radar, the shift
`in the carrier frequency of the
`T reflected wave (doppler effect) is a measure of the target's relative (radial) velocity and may be
`used to distinguish moving targets from stationary objects. In radars which continuously track
`the movement of a target, a continuous indication of the rate of change of target position is
`also available.
`
`I
`
`PETITIONERS 1015-0010
`
`

`
`2 INTRODUCTION TO RADAR SYSTEMS
`
`The name radar reflects the emphasis placed by the early experimenters on a device to
`detect the presence of a target and measure its range. Radar is a contraction ofthe words radio
`detection and ranging. It was first developed as a detection device to warn of the approach of
`hostile aircraft and for directing antiaircraft weapons. Although a well-designed modern radar
`can usually extract more information from the target signal than merely range, the measure-
`ment of range is still one of radar’s most important functions. There seem to be no other
`competitive techniques which can measure range as well or as rapidly as can a radar.
`The most common radar waveform is a train of narrow, rectangu|ar~shape pulses modu-
`lating a sinewave carrier. The distance, or range, to the target is determined by measuring the
`time TR taken by the pulse to travel to the target and return. Since electromagnetic energy
`propagates at the speed of light c = 3 x 108 m/s, the range R is
`
`_LTR
`R— 2
`
`(1.1)
`
`The factor 2 appears in the denominator because of the two-way propagation of radar. With
`the range in kilometers or nautical miles, and TR in microseconds, Eq. (1.1) becomes
`
`R(km) = 0.l5TR(ps)
`
`or
`
`R(nmi) = 0.08lTR(;zs)
`
`Each microsecond of round-trip travel time corresponds to a distance of 0.081 nautical mile,
`0.093 statute mile, 150 meters, 164 yards, or 492- feet.
`Once the transmitted pulse is emitted by the radar, a sufficient length of time must elapse
`to allow any echo signals to return and be detected before the next pulse may be transmitted.
`Therefore the rate at which the pulses may be transmitted is determined by the longest range at
`which targets are expected. If the pulse repetition frequency is too high, echo signals from some
`targets might arrive after the transmission of the next pulse, and ambiguities in measuring
`ll.
`
`10,000
`
`K
`
`n
`
`T'
`
`5
`I!
`.3 spec
`
`OC u
`
`’Ch
`
`C 2 32
`
`1.000
`100
`Pulse repetition’ frequency, Hz
`
`s
`10.000
`
`100
`
`-3‘
`EO
`
`CD
`
`10
`10
`
`Figure 1.1 Plot of maximum unambiguous range as a function of the pulse repetition frequency.
`
`PETITIONERS 1015-0011
`
`

`
`THE NATURE or RADAR 3
`
`range might result. Echoes that arrive after the transmission of the next pulse are called
`second-time—around (or multiple—time—around) echoes. Such an echo would appear to be at a
`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
`echoes is called the maximum unambiguous range and is
`
`C
`Runamb = if:
`
`wheref, = pulse repetition frequency, in Hz. A plot of the maximum unambiguous range as a
`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
`reception. This achieves the benefits of high range-resolution without the need to resort to a
`short pulse. The technique of using a long, modulated pulse to obtain the resolution ofa short
`pulse, but with the energy of a long pulse,
`is known as pulse compression. Continuous
`waveforms (CW) also can be used by taking advantage of the doppler frequency shift to
`separate the received echo from the transmitted signal and the echoes from stationary clutter.
`Unmodulated CW waveforms do not measure range, but a range measurement can be made
`by applying either frequency— or phase—modulation.
`
`1.2 THE SIMPLE FORM OF THE RADAR EQUATION
`
`The radar equation relates the range of a radar to the characteristics of the transmitter,
`receiver, antenna, target, and environment. It is useful not just as a means for determining the
`maximum distance from the radar to the target, but it can serve both as a tool for under-
`standing radar operation and as a basis for radar design. In this section, the simple form of
`the radar equation is derived.
`If the power of the radar transmitter is denoted by P,, and if an isotropic antenna
`is used (one which radiates uniformly in all directions), the power density (watts per unit area)
`at a distance R from the radar is equal to the transmitter power divided by the surface area
`4rrR2 of an imaginary sphere of radius R, or
`
`Power density from isotropic antenna =
`
`P
`41rR2
`
`(13)
`
`Radars employ directive antennas to channel, or direct, the radiated power P, into some
`particular direction. The gain G of an antenna is a measure of the increased power radiated in
`the direction of the target as compared with the power that would have been radiated from an
`isotropic antenna. It may be defined as the ratio of the maximum radiation intensity from the
`subject antenna to the radiation intensity from a lossless, isotropic antenna with the same
`power input. (The radiation intensity is the power radiated per unit solid angle in a given
`direction.) The power density at the target from an antenna with a transmitting gain G is
`
`Power density from-directive antenna =
`
`41rR2
`
`(1.4)
`
`The target intercepts a portion of the incident power and reradiates it in various directions.
`
`PETITIONERS 1015-0012
`
`

`
`4 INTRODUCTION TO RADAR SYSTEMS
`
`The measure of the amount of incident power intercepted by the target and reradiated back in
`the direction of the radar is denoted as the radar cross section 0, and is defined by the relation
`
`Power density of echo signal at radar =2
`
`P,G 0
`
`41rR2 41rR2
`
`(1.5)
`
`The radar cross section a has units of area. It is a characteristic ofthe particular target and is a
`measure of its size as seen by the radar. The radar antenna captures a portion of the echo
`power. If the effective area ofthe receiving antenna is denoted A,., the power P, received by the
`radar is
`
`P,G 0
`P,GA,.o'
`P'=41rR241rR2 "e=(”4n)Tia
`
`(1.6)
`
`The maximum radar range Rm, is the distance beyond which the target cannot be detected. lt
`occurs when the received echo signal power P,just equals the minimum detectable signal S,,,,,,.
`Therefore
`
`Rmax =
`
`
`
`P,GAea ‘/4
`(4TC)2Smin
`
`
`
`(1.7)
`
`_ This is the fundamental form of the radar equation. Note that the important antenna par—
`ameters are the transmitting gain and the receiving effective area.
`Antenna theory gives the relationship between the transmitting gain and the receiving
`effective area of an antenna as
`
`41cAe
`12
`
`G ——
`
`(1.8)
`
`( 1.8) can
`Since radars generally use the same antenna for both transmission and reception, Eq.
`be substituted into Eq. (1.7),'first for‘Ae then for G, to give two other forms of the radar
`equation
`
`PG2l2<r
`R,,,,,= —‘———
`<4n)3s....
`
`P,A§cr
`= ——-
`4nA2Sm‘n
`
`Rmax
`
`
`
`
`‘/4
`
`1/‘
`
`
`
`
`I
`
`5
`.:3
`
`1.9
`
`1
`
`1.10
`
`(
`
`1
`
`)
`
`These three forms (Eqs.' 1.7, 1.9, and 1.10) illustrate the need to be careful in the inter—
`pretation of the radar equation. For example, from Eq. (1.9) it might be thought that the range
`of a radar varies as 1”’, but Eq. (1.10) indicates a ,1.“ "2 relationship, and Eq. (1.7) shows the
`range to be independent of ,1. The correct relationship depends on whether it is assumed the
`gain is constant or the effective area is constant with wavelength. Furthermore, the introduc-
`tion 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-
`mance 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 be
`predicted by the aboveequations, sometimes by as much as a factor of two. There are many
`reasons for the failure of the simple radar equation to correlate with actual performance, as
`discussed in Chap. 2.
`'
`" '
`'
`'
`
`PETITIONERS 1015-0013
`
`

`
`THE NATURE or RADAR 5
`
`,...~;-
`
`1.3 RADAR BLOCK DIAGRAM AND OPERATION
`
`The operation of a typical pulse radar may be described with the aid of the block diagram
`shown in Fig.
`l.2. The transmitter may be an oscillator. such as a magnetron, that is “ pulsed”
`(turned on and off) by the modulator to generate a repetitive train of pulses. The magnetron
`has probably been the most widely used of the various microwave generators for radar. A
`typical radar for the detection of aircraft at ranges of 100 or 200 nmi might employ a peak
`power of the order of a megawatt, an average power of several kilowatts, a pulse width of
`several microseconds, and a pulse repetition frequency of several hundred pulses per second.
`The waveform generated by the transmitter travels via a transmission line to the antenna,
`where it
`is radiated into space. A single antenna is generally used for both transmitting and
`receiving. The receiver must be protected from damage caused by the high power of the
`transmitter. This is the function of the duplexer. The duplexer also serves to channel the
`returned echo signals to the receiver and not to the transmitter. The duplexer might consist of
`pr 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
`the echo signal to the receiver during reception. Solid-state ferrite circulators and receiver
`protectors with gas-plasma TR devices and/or diode limiters are also employed as duplexers.
`The receiver is usually of the superheterodyne type. The first stage might be a low-noise
`RF amplifier, such as a parametric amplifier or a low-noise transistor. However,
`it
`is not
`always desirable to employ a low-noise first stage in radar. The receiver input can simply be
`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
`have greater dynamic range, less susceptibility to overload, and less vulnerability to electronic
`interference.
`
`;
`
`The mixer and local oscillator (LO) convert the RF signal to an intermediate frequency
`(IF). A “ typical " IF amplifier for an air-surveillance radar might have a center frequency of 30
`or 60 MHZ and a bandwidth of the order of one megahertz. The IF amplifier should be
`designed as a matched filter; i.e., its frequency-response function H(f ) should maximize the
`peak-signal—to—mean~noise—power ratio at the output. This occurs when the magnitude ofthe
`frequency-response function [H( f )|
`is equal to the magnitude of the echo signal spectrum
`|S(f)
`and the phase spectrum of the matched filter is the negative of the phase spectrum of
`the echo signal (Sec. 10.2). In a radar whose signal waveform approximates a rectangular
`pulse, the conventional IF filter bandpass characteristic approximates a matched filter when
`the product of the IF bandwidth B and the pulse width 1? is of the order of unity, that is, Br 2 1.
`After maximizing the signal-to-noise ratio in the IF amplifier, the pulse modulation is
`extracted by the second detector and amplified by the video amplifier to a level where it can be
`
`Transmitter
`
`Anlenno
`
`modulator
`
`“'59
`
`
`
`
`
`
`
`Low — noise
`RF
`ompmie,
`
`.
`.
`IF amplifier
`(matched filter)
`
`2d
`detector
`
`_
`video
`amplifier
`
`
`
`Figure 1.2 Block diagram of a pulse radar.
`
`PETITIONERS 1015-0014
`
`

`
`6 INTRODUCTION TO RADAR SYSTEMS
`
`0)
`‘O
`
`
`
`:3
`‘a
`
`E4
`
`la]
`
`(bl
`
`Figure 1.3 (a) PP] presentation displaying range vs. angle (intensity modulation); (b) A—scope presenta-
`tion displaying amplitude vs. range (deflection modulation).
`
`properly displayed, usually on a cathode-ray tube (CRT). Timing signals are also supplied to
`the indicator to provide the range zero. Angle information is obtained from the pointing
`direction of the antenna. The most common form of cathode-ray tube display is the plan
`position indicator, or PP! (Fig. 1.3a), which maps in polar coordinates the location of the
`target in azimuth and range. This is an intensity-modulated display in which the amplitude of
`the receiver output modulates the electron-beam intensity (2 axis) as the electron beam is made
`to sweep outward from the center of the tube. The beam rotates in angle in response to the
`antenna position. A B-scope display is similar to the PP] except that it utilizes rectangular,
`rather than polar, coordinates to display range vs. angle. Both the B~scope and the PH, being
`intensity modulated, have limited dynamic range. Another form of display is the A-scope,
`shown in Fig. l.3b, which plots target ‘amplitude (y axis) vs. range (x axis), for some fixed
`direction. This is a de’flectio‘n-"modulated display. It is more suited for tracking-radar applica~
`tion than for surveillance radar.
`
`The block diagram of Fig. 1.2 is a simplified version that omits many details. It does not
`include several devices often found in radar, such as means for automatically compensating the
`receiver for changes in frequency (AFC) or gain (AGC), receiver circuits for reducing interfer-
`ence from other radars and from unwanted signals, rotary joints in the transmission lines to
`allow movement of the antenna,‘ circuitry for discriminating between moving targets and
`unwanted stationary objects (MTI), and pulse compression for achieving the resolution benefits
`of a short pulse but with the energy of a ‘long pulse. If the radar is used for tracking, some
`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
`included to ensure that the transmitter is delivering the proper shape pulse at the proper
`power level and that the receiver sensitivity has not degraded. Provisions may also be in-
`corporated in the radar for locating equipment failures so that faulty circuits can be easily
`found and replaced.
`Instead of displaying the “ raw‘-video ” output of a surveillance radar directly on the CRT,
`it might first be processed by an‘autornatic'detection and tracking (ADT) device that quantizes
`the radar coverage into range-azimuth resolution cells, adds (or integrates) all the echo pulses
`received within each cell, establishes a threshold (on the basis of these integrated pulses) that
`permits only the strong outputs due to target echoes to pass while rejecting noise, establishes
`and maintains the tracks (trajectories) of each target, and displays the processed information
`
`,
`W}
`
`PETITIONERS 1015-0015
`
`

`
`THE NATURE or RADAR 7
`
`to the operator. These operations of an ADT are usually implemented with digital computer
`technology.
`A common form of radar antenna is a reflector with a parabolic shape, fed (illuminated)
`from a point source at its focus. The parabolic reflector focuses the energy into a narrow beam,
`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 beam is scanned by electronically varying the phase of the currents across
`the aperture.
`
`1.4 RADAR FREQUENCIES
`
`Conventional radars generally have been operated at frequencies extending from about
`A 220 MHz to 35 GHz, a spread of more than seven octaves. These are not necessarily the limits,
`‘since radars can be, and have been, operated at frequencies outside either end of this range.
`Skywave HF over-the-horizon (OTH) radar might be at frequencies as low as 4 or 5 MHZ, and
`groundwave HF radars as low as 2 MHZ. At the other end of the spectrum, millimeter radars
`have operated at 94 GHZ. Laser radars operate at even higher frequencies.
`The place of radar frequencies in the electromagnetic spectrum is shown in Fig. 1.4. Some
`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,
`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
`engineers. Table l.l
`lists the radar-frequency letter-band nomenclature adopted by the
`IEEE.‘ 5 These are related to the specific bands assigned by the International Telecommunica-
`tions Union for radar. For example, although the nominal frequency range for L band is 1000
`to 2000 MHZ, an L-band radaris thought of as being confined within the region from 1215 to
`1400 MHz since that is the extent of the assigned band. Letter-band nomenclature is not a
`
`lOkm
`
`lkm
`
`Wavelength
`lOOm
`10m
`
`lm
`
`
` VHF
`
`frequency
`
`frequency
`
`High
`frequency
`
`Veryhiqh
`frequency
`
`M