Principles of modern radar edited by jerry l eaves, edward k reedy

The subjects of tracking in range, angle, and Doppler frequency are presented in PART 5, "Tracking Ra-dar Techniques and Applications." Finally, PART 6 and PART 7-"Target Discrimination

PRINCIPLES

OF

MODERN RADAR

PRINCIPLES

OF MODERN RADAR

Edited by

and Edward K Reedy

CHAPMAN & HALL

I ® J:f International Thomson Publishing

New York' Albany' Bonn • Boston' Cincinnati Detroit London' Madrid' Melbourne Mexico City • Pacific Grove • Paris • San Francisco • Singapore Tokyo • Toronto • Washington

This edition published by Chapman & Hall, New York

For more information contact:

International Thomson Editores

Campos Eliseos 385, Piso 7

Postfach 100 263

0-69442 Weinheim

Germany International Thomson Publishing - Japan Hirakawaeho-cho Kyowa Building, 3F

1-2-1 Hirakawacho-cho

Chiyoda-ku, 102 Tokyo Japan

All rights reserved No part of this book covered by the copyright hereon may be reproduced or used in any form or by any means graphic,

electronic, or mechanical, including photocopying recording, taping or information storage and retrieval systems without the written

permission of the publisher

7 8 9 XXX 01 00 99 98 97

Library of Congress Cataloging-in-Publication Data

Principles of modem radar

Visit Chapman & Hall on the Internet http://www.chaphaILcomichaphaILhtml

41042 Phone (606) 525-6600 or 1-800-842-3636 Fax: (606) 525-7778 E-mail: order@chaphall.com

PREFACE

This book, Principles of Modern Radar, has as its genesis a Georgia

Tech short course of the same title This short course has been presented nually at Georgia Tech since 1969, and a very comprehensive set of course notes has evolved during that seventeen year period The 1986 edition of these notes ran to 22 chapters, and all of the authors involved, except Mr Barrett, were full time members of the Georgia Tech research faculty

an-After considerable encouragement from various persons at the university and within the radar community, we undertook the task of editing the course notes for formal publication The contents of the book that ensued tend to be practical

in nature, since each contributing author is a practicing engineer or scientist and each was selected to write on a topic embraced by his area(s) of expertise Prime examples are Chaps 2, 5, and 10, which were authored by E F Knott, G W Ewell, and N C Currie, respectively Each of these three researchers is rec-ognized in the radar community as an expert in the technical area that his chap-ter addresses, and each had already authored and published a major book on his subject Several other contributing authors, including Dr Bodnar, Mr Bruder,

Mr Corriher, Dr Reedy, Dr Trebits, and Mr Scheer, also have major book publications to their credit

Principles of Modern Radar is organized into an introductory chapter and

seven parts PART 1 addresses the "Factors External to the Radar," including electromagnetic wave reflectivity and propagation processes and the multipath phenomenon and effects In PART 2, the "Basic Elements of the Radar Sys-tem" are discussed The basic radar task and objective of "Detection in a Con-taminated Environment" of noise and clutter is the subject of PART 3, which includes chapters on noise, clutter, target models, and threshold detection tech-niques PART 4 is titled "Radar Waveforms and Applications," and these four chapters address special techniques that can result in improved radar perfor-mance by introducing waveform conditioning and signal processing trades among the time, frequency, and spatial domains The subjects of tracking in range, angle, and Doppler frequency are presented in PART 5, "Tracking Ra-dar Techniques and Applications." Finally, PART 6 and PART 7-"Target Discrimination and Recognition" and "Radar ECCM", respectively-present important subjects not included in most books on radar Chapters 20 and 21 address polarimetric techniques for target recognition, a very important radar

v

topic of the 1980s, and Chap 22 discusses electronic counter countermeasures (ECCM) that should be an integral part of any radar but often are ignored/ overlooked by radar designers and authors of radar books

Many people have made contributions to the publication of this book, and

we thank them all Among these are those who encouraged us to take on the task of editorship, including all of the contributing authors without whom there would be no book A very special thank-you is extended to Mr Melvin McGee and to his staff, Mr Joseph McKee, and Ms Melanie Luke, for their help in generating and refining the manuscript Also, Ms Shirley Washington is due a special thanks for her support of the short course and the book Finally, we thank Dr H Allen Ecker, formally of Georgia Tech and now with Scientific-Atlanta, for he organized and coordinated the first edition of the Principles of Modem Radar short course back in 1969 and started us all on the course that eventually led to the publication of this book

JERRY L EAVES

CONTENTS

Preface \ v

1 Introduction to Radar, J L Eaves \ 1

2 EM Waves and the Reflectivity Process, E F Knott \ 31

3 The Propagation Process, D G Bodnar \ 51

4 Multipath Phenomena and Effects, H A Corriher \ 72

5 Radar Transmiters, G W Ewell \ 107

6 Radar Antennas, D G Bodnar \ 148

7 Radar Receivers, T L Lane \ 182

8 Radar Indicators and Displays, J A Scheer \ 233

9 Detection in Noise, J D Echard \ 253

10 Clutter Characteristics and Effects, N C Currie \ 281

11 Target Models, C R Barrett \ 343

12 Adaptive Threshold and Automatic Detection Techniques,

13 Continuous Wave Radar, W A Holm \ 397

14 MTI and Pulsed Doppler Radar, C R Barrett \ 422

15 Pulse Compression in Radar Systems, M N Cohen \ 465

16 Synthetic Aperture Radar, R N Trebits \ 502

vii

PART 5: TRACKING RADAR TECHNIQUES AND

APPLICATIONS \ 539

PART 6: TARGET DISCRIMINATION AND RECOGNITION \ 619

20 Polarimetric Fundamentals and Techniques, W A

PART 7: RADAR ECCM \ 679

INDEX \ 701

as range, angular coordinates, velocity, and reflectivity signature from the tection

de-Electromagnetic (EM) energy generated within the transmitter unit is routed

to the antenna via the duplexer, a device that permits both transmission and reception of EM waves with a single antenna The antenna serves as a trans-ducer to couple the EM energy into the atmosphere, where it propagates as an

EM wave at the speed of light (approximately 3 X 108 m/s) Generally, the radar antenna will form a beam of EM energy that concentrates the propagating

EM wave in a given direction Thus, the beam can be directed to desired gular coordinates by effectively pointing the antenna in that direction through a combination of mechanical and electrical means

an-An object or target located within the antenna beam will intercept a portion

of the propagating energy The intercepted energy will then be scattered in various directions from the target and, in general, some of it will be backscat-tered in the direction of the radar The time delay between transmission by the radar and reception of the signal reflected by a target located at a range R is found from the relationship

td =

where c is the velocity of light This retroreflected energy is called backscatter,

as opposed to bistatic scatter or EM scatter in other directions

A portion of the backscattered wave is intercepted by the radar antenna, and the collected energy is transduced from the atmosphere or propagation medium into the radar receiver via the transmission lines and the duplexer

ANTENNA TRANSMITTED EM WAVE

S Transmitter

s

REFLECTED EM WAVE

R

Figure 1-1 Radar principle and basic system elements

The receiver amplifies the weak received signals and translates the tion contained at the radio frequency (RF) to video and/or baseband frequen-cies Signals processed by the receiver are then routed to the radar indicator or display, where the data (range, velocity, amplitude, direction, etc.) that were derived within the receiver (signal processor) are presented to the radar opera-tor

informa-In general, the radar derives target information by correlating the received signal with the transmitted signal Target information that can be obtained by radar is given in Table 1-1, along with the correlation process from which it is derived

1.2 BASIC ELEMENTS OF THE RADAR SYSTEM

There are four basic elements in any functional radar: a transmitter, an antenna,

a receiver, and an indicator The basic configuration is illustrated in Figure

Table 1-1 The Derivation of Target Data

Derived by Correlating:

Target

Angular coordinates Antenna beam position with Antenna beam reference Radial velocity (Doppler) Radio frequency with Frequency reference Scattering signature Polarization scattering

INTRODUCTION TO RADAR 3

1-1 The duplexer appears as an element in the diagram; however, it is more correctly a part of the RF transmission line system than a basic element of the radar Figure 1-1 suggests that the signal processor may constitute a fifth basic element of a radar system In many cases, signal processing can be accounted for by some combination of the receiver and indicator operations

wave-form at some required power level The required RF power may be derived directly from a power oscillator such as a magnetron or an extended interaction oscillator (EIO), or it may be derived via an RF amplifier or amplifier chain [traveling-wave tube (TWT) amplifier, crossed-field amplifier, extended inter-action amplifier (EIA) solid-state amplifier, etc.] The waveform is determined

by the particular requirements of the system and can range from an unmodulated continuous wave (CW) for a simple moving target indicator (MTI) radar to a complex frequency, phase, and time code modulated wave for some advanced military radars Radar transmitters are discussed in more detail in Chapter 5

from the radar transmission line into the propagation medium and vice versa

In addition, the antenna provides beam directivity and gain for both sion and reception of the EM energy Various radar antenna concepts and tech-niques are presented and discussed in Chapter 6

target signals, amplify them to a usable level, and translate the information contained therein from RF to baseband Various receiver configurations are em-ployed, including crystal detector, RF amplifier, homodyne, and superhetero-dyne; of these, the superheterodyne receiver is by far the most commonly used configuration in radar receivers Each of these is discussed in Chapter 7 When the receiver's frequency spectral characteristics are optimized to the transmitter waveform, the so-called ideal or matched receiver is produced, which provides

a maximum output signal-to-noise ratio In most cases, the radar designer will choose to trade off a little signal-to-noise performance for some other consid-eration, such as design simplicity or range measurement accuracy Thus, the ideal matched receiver is rarely achieved, but it does serve as a commonly used reference for performance comparisons

con-vey target information to the user The indicator configuration and information format are dependent upon the particular radar application and the needs of the user Two common indicators are (1) the plan position indicator (PPI), where target range and angle data are displayed on a cathode ray tube for surveillance radar applications and (2) an audio speaker or earphones, where the presence

of a moving object is signaled by a Doppler frequency, as in a perimeter alarm radar Various types of radar indicators and their applications are presented in Chapter 8

Table 1-2 Pre~World War II Highlights and Milestones in Radar

Hyland (NRL) detected aircraft with CW radar

Sir Watson-Watt (Britain) and Page (NRL) demonstrated pulsed radar Radar received more attention at the NRL and the Signal Corps in the United States, and at various laboratories in Britain, due to military buildups prior to World War II

British scientists visited the United States and demonstrated magnetron They suggested that the United States develop microwave aircraft-intercept and antiaircraft fire-control radars

November 1940 The Radiation Laboratory was established at the Massachusetts Institute of

Technology It was staffed primarily by physicists, as suggested by the British, and the initial staff of 40 expanded to about 4000 by mid-1945 The research and development activities of this laboratory were documented in many reports, and after World War II a 28-volume set of books (Radiation Laboratory Series) was published to make available to all engineers and scientists the great body

of information and new techniques in radar and related fields

1.3 HISTORICAL DEVELOPMENTS

Although radar did not come into its own until its widespread development and application in World War II, the principle was known and advocated by many now famous scientists of the early 1900s Table 1-2 provides a chronological list of some highlights and milestones that led to the giant step achieved during World War II

Since World War II, the use of radar has expanded phenomenally It has been

applied not only to numerous military problems but also to many private and commercial uses Some of the major postwar developments that stimulated the widespread application and use of radar are as follows:

• High-power klystrons

• Low-noise traveling-wave tube (TWT)

• Parametric amplifiers and masers

• EIO and EIA

• Monopulse

• Over-the-horizon (OTH) radar

• Short pulse techniques

• Synthetic aperture radar

Very-high-speed integrated circuits (VHSIC)

1.4 THE RADAR EQUATION

The radar range equation, or simply the radar equation, is the single most scriptive and useful mathematical relationship available to radar designers and researchers In its most complete form, the radar equation accounts for not only the effects of each major parameter of the radar system but also those of the target, target background, and the propagation path and medium Thus, one can use the radar equation to conduct performance and cost studies based on various parameter and scenario trade-offs or conditions for the radar, target, and envi-ronment

de-A thorough understanding of the radar equation by a practicing radar engineer

is essential The relationship is developed in most radar texts of note and is also included here due to its central importance in radar

antenna that radiates the EM energy isotropically (omnidirectionally), as shown

in Figure 1-2(a) Since the EM energy radiates in all directions with equal strength, the power density is constant over the surface of an imaginary sphere

prop-agation medium is assumed) due to the conservation of energy principle

There-fore, the power density per unit area at a distance R from the radar is found by

WATTS/m 2

Figure 1-2d Power density, PD at radar due to backscatter from target at range R with ReS a

/'

, / ,,/

Now if the unity gain omnidirectional antenna is replaced with a directional

shown in Figure 1-2(b), the power density within the beam at range R is now

given by

Next, assume that a target is located within the radiated beam at a range R from

the radar, as shown in Figure 1-2c The propagating EM wave will strike the target and, as a result, the incident energy will be scattered in various directions Some of the energy will be reflected (backscattered) in the direction of the radar

The difference between a target's RCS and its geometric cross section is often

a point of confusion for beginners, since both are defined and expressed as an area Although there is always a definite relationship between an object's geo-metric area and its RCS area, it is generally very difficult and not practical to state this relationship explicity Some insight into the relationship is gained by expressing a target's RCS in terms of the target's equivalent flat plate area

Suppose that a target's geometric area normal to the radar line of sight is An;

then the power intercepted and collected by that area is

That amount of power, Pc, will be reradiated by the target, and since we have defined Pc and An for the case of backscatter, the energy reradiated or reflected

by An will be directed toward the illuminating radar Thus, the backscattering

process provides power gain relative to an isotropic reradiation process cordingly,

PI

where G b is the power gain of the equivalent flat plate area, An' over an isotropic

source As is shown in Chapter 6, antenna power gain and effective receiving

aperture (area) are related as follows:

(1-8)

Substituting for G b , we have

Thus, the backscatter RCS of a target with an equivalent flat plate area, An'

normal to the beam is

in phase, if none of the backscattered energy resulted from multiple reflections, and if there were no depolarization effects Examination of the relationship be-tween RCS and the equivalent flat plate area shows that a target's RCS area is proportional to the square of an equivalent geometric area and also that RCS is inversely proportional to the wavelength of the illuminating wave The rela-tionships between RCS and geometric area for several simple geometric bodies are discussed in Chapter 2

Referring again to Figures 1-2(b) and 1-2(c), we see that the power density

INTRODUCTION TO RADAR 11

radar, as shown in Figure 1-2(d), is

The power received by the radar, P" is determined by the effective capture area,

A e , of the receiving antenna, as shown in Figure 1-2e, and is given by

I

Power density at target

Equivalent power reradiated toward radar

w

(1-14)

Power received by radar - - - '

As discussed in Chapter 6, the effective capture area and gain of an antenna are related by

(1-15) where A is the wavelength of the EM wave, Substituting for A" we have

1 The propagation medium and path

2 Atmospheric noise

3 System losses (nonideal components)

4 Thermal noise introduced within the radar

5 Signal processing losses (nonideal)

Many excellent texts address radar performance and the effect of these

Figure 1-3 depicts a more realistic operational scenario than that shown in

L, accounts for all system, medium, and propagation losses Also assume that,

and output, respectively Thus, the signal-to-noise (power) ratio at the radar receiver output terminals is given by the signal-to-noise ratio at the radar re-

bandwidth Noise factors are defined in terms of a reference temperature, i.e.,

the system noise factor is expressed in decibels (logarithmic form), it is

gen-erally called the system noise figure

internal to the radar, respectively, and let A represent the radar receiver signal

and that the output signal power is

sig-SIGNAL-TO-NOISE RATIO FOR NON-IDEAL CASE

= A"L' WHEREL= LELj

Figure 1-4 Radar equation and output signal-to-noise ratio for a realistic scenario

(1-24)

minimi-zation of losses both inside and outside the radar

1.4.1 Maximum Detection Range

radar performance measure After rearranging the radar equation as follows

the maximum detection range corresponds to unity (SNR)o' Under that tion,

This is a useful form of the radar equation from which one can assess radar performance as a function of radar, target, and environment parameters

1.4.2 (SNR)o and Rmax Expressed in Decibel Form

Radar engineers often prefer to convert the radar equation to the logarithmic form, wherein the various components are expressed in decibels (dB), and then

to compute the result using the simple arithmetic operations of addition and subtraction

If the radar equation components are quantified as power in watts, length in centimeters, radar cross section in square meters, range in meters, and bandwidth in megahertz, then the logarithmic forms are

extra-2 A target's ReS can rarely be characterized as a constant

Thus, the radar engineer must use statistical techniques to assess detection formance To illustrate this, consider the simple experiment described in Figure 1-5 Assume that a radar transmits EM pulses at some pulse repetition fre-quency (prf) and that initially no backscatter energy is received due to an ab-sence of targets; thus, the voltage at the detector output terminals is due solely

MAKE 100 MEASUREMENTS OF V,

ONCE AFTER EACH TRANSMISSION

VOLTAGE INCREM ENTS OF LI v = 0.1 VOLT

FOR ILLUSTRATION PURPOSES ONLY (FABRICATED DATA, NOT TO SCALE)

to radar system noise The first step in the experiment is performed by making

100 separate measurements of the detector output voltage at a fixed time delay after each radar transmission Next, the number of measurements that were within the intervals of 0 to 0.1 V, 0.1 to 0.2 V, 0.2 to 0.3 V, and so on are determined, and a probability histogram is constructed as shown in Figure 1-6 The histogram is a discrete function due to the finite voltage interval (0.1 V) and the limited number of measurements The discrete probability histogram would approach a continuous probability density function if the voltage incre-ment and the number of measurements were made to approach zero and infinity, respectively, as shown in Figure 1-7 Thus, the curve shown in Figure 1-7 is the probability density function of the detector output voltage for (SNR)o = 0, that is, when no target signal is present

Next in the experiment, assume that the voltmeter is replaced by a threshold detector and indicator, as shown in Figure 1-8 The operating characteristics of the threshold detector are defined as follows:

PROBABILITY HISTOGRAM FOR (S/Nb = 0

FOR ILLUSTRATION PURPOSES ONLY (FABRICATED DATA, NOT TO SCALE)

p(v)

N~co AV~O

P DENSITY = p(V)

co P(V<co) = J p(V)dv = I

19

is the single pulse probability of false alarm, Pfa; no target signal is present, that is, (SNR)o = 0; and no integration is employed

It should now be clear that the value of Pfa is dependent upon V T and that one could set V T to produce a desired (allowable or tolerable) Pfa Assume for this experiment that V T was set so that the resulting Pfa is O.l

Next, assume that the experiment is repeated, but now with a target present

at a range corresponding to the exact same time delay after transmission as used before, and that (SNR)o = 1, as calculated from the radar equation given pre-viously Also assume that the experiment is repeated for (SNR)o = 2 and (SNR)o

= 4 and that the resulting probability density functions are as shown in Figure 1-9 For a detection threshold voltage V T1 and the resulting Pfa = 0.1, the probability of detection can be determined for (SNR)o = 1, 2, and 4 Further-more, the process could be repeated for other values of Pfa by resetting V T as required Thus, families of curves can be generated to show the relationship between the probability of detection, P d , and the signal-to-noise ratio, (SNR)o, versus the allowed probability of false alarm, Pfa Figure 1-10 illustrates the results of the simple experiment just described

It is important to understand that this example predicts the probability of detecting a steady (nonftuctuating) target for a single pulse (single sample, ob-

servation, look, etc.} and that several other factors which affect the probability

of detection were not considered Some of these factors are listed in Table

propagation medium and path, the receiver frequency response relative to a matched filter, the number of pulses included in the decision process, and whether coherent or noncoherent signal processing is employed These factors are discussed in greater detail in Parts 1,2, and 3 ofthis book The remainder

of the book, which includes Parts 4 through 6 and Chapter 22, introduces and discusses other important topics related to radar, such as waveforms, signal processing, tracking, target recognition, and electronic counter countermea-sures

Table 1-3 Other Considerations in the Determination of Pd'

• Effects of unmatched receiver

• Effects of target signal (steady or fluctuating)

• Effects of antenna pattern (scan modulation multipath)

• Interference other than noise (clutter, electromagnetic interference, electronic countermeasures

• Number of samples included in decision

Coherent or noncoherent?

Independent or dependent?

1.5 SOME MAJOR FUNCTIONS PERFORMED BY RADAR·

Radars generally are called upon to perform many functions If the functions are not simultaneous, they are called modes For example, an antiaircraft fire control radar usually has at least two modes: (1) an acquisition mode to enable the radar to locate the target with sufficient accuracy to commence tracking and (2) a tracking mode to furnish position coordinates for aiming guns Some major functions of radar with brief definitions are as follows:

tracking

thing or event For example, special-purpose detection radars might be used to detect an intrusion (unidentified object entering a guarded area or airspace), a nuclear blast, or a rocket launching

height-finder radar usually scans a horizontal fan beam in elevation to determine the elevation angle of the target

radar provides the relative location coordinates of the target

display of a representation of a portion of the earth's surface Mapping radars are concerned primarily with terrain features and major cultural targets

fea-tures, navigational aids (buoys, beacons, etc), and other objects of interest (nearby ships, aircraft, etc.) The acquired information is useful when steering, maneuvering, or controlling a vehicle

secure information regarding the terrain, the location of objects of interest, or any other desired information regarding the situation Reconnaissance usually implies the observation of unfamiliar territory or territory not accessible to con-tinual observation (surveillance)

radar ordinarily determines the range and azimuth of objects within its area of detection

of interest, usually by means of the Doppler effect For example, police radars usually operate solely in this mode

usually implies the observation of familiar territory

*Extracted from reference material provided by R C Johnson

INTRODUCTION TO RADAR 23

so as to fly a path which closely follows the terrain along a generally mined course A terrain-avoidance radar normally scans a solid forward sector

predeter-in order to sample the upcompredeter-ing three-dimensional profile For example, with

a terrain-avoidance capability, an aircraft might change course to fly between two mountains rather than fly directly over one of them

a path which closely follows the terrain profile along a predetermined course

A terrain-following radar normally scans ahead in elevation to determine the upcoming terrain profile

of a target A tracking radar usually "locks onto" the return signal and matically tracks in angular coordinates and in range

a target while continuing to scan in a search or acquisition mode New position coordinates are obtained with each scan

1.6 RADAR NOMENCLATURE

Military electronic equipment and systems are designated by the AN clature system as prescribed in Military Specification MIL-N-18307C (AS G) and its amendments Military radars (and other military electronic systems) are designated by the letters AN followed by a slash, three letters, a dash, and a numeral Table 1-4 provides a summary of the AN nomenclature designation systems The system indicator, AN, does not mean that the Army, Navy, and Air Force use the equipment; it simply means that the radar is assigned in the military AN designation systems The three letters following the dash indicate installation (Fixed, Mobile, Aircraft, etc.), type (radar, radio, sonar, etc.), and purpose (detection, communications, identification, etc.), respectively, of a particular electronic system The numeral following the dash indicates the po-sition of a particular system in the chronological sequence Subsequent models

nomen-of a given AN system are designated by a letter following the numeral For

example, AN/TPS-ID designates the fourth model of the first ground

trans-portable search radar that was described under the AN system Some common

AN (and other) designations for various radar applications are given in Table

Table 1-4 The AN Nomenclature Designation System

C-Air transportable

(inacti-vated, do not use)

D-Pilotless carrier

F-Fixed

G-Ground, general ground

use (includes two or more

ground-type installations)

K -Amphibious

M-Ground, mobile

(in-stalled as operating unit in

a vehicle which has no

function other than

trans-porting the equipment)

P-Pack or portable (animal

or man)

S-Water surface craft

T -Ground, transportable

V-General utility (includes

two or more general

in-stallation classes,

air-borne, shipboard, and

ground)

V-Ground, vehicular

(in-stalled in vehicle designed

for functions other than

carrying electronic

equip-ment, etc., such as tanks)

W-Water surface and

un-derwater

Second Letter

Type of equipment A-Infrared

B-Pigeon C-Carrier (wire) D-Radiac E-Nupac F-Photographic G-Telegraph or teletype I-Interphone and public address

J-Electromechanical (not otherwise covered) K-Telemetering L-Countermeasures

M -Meteorological N-Sound in air P-Radar Q-Sonar and underwater sound

R-Radio S-Special types, magnetics, etc., or combinations of types

T -Telephone (wire) V-Visual and visible light W-Armament (peculiar to armament, not otherwise covered)

X-Facsimile or television

Y -Data processing

Third Letter

Purpose A-Auxiliary assemblies (not complete operating sets used with or part of two or more sets or sets series)

B-Bombing C-Communications (receiving and transmitting) D-Direction finder and/or reconnaissance E-Ejection and/or release G-Fire control or search- light directing H-Recording and/or reproducing (graphic meteorological and sound) L-Searchlight control (inactivated, use G) M-Maintenance and test assemblies (including tools)

N-Navigational aids (including altimeters, beacons, compasses, racons, depth sounding, approach, and landing) P-Reproducing (inactivated, do not use) Q-Special, or combination

of purposes R-Receiving, passive detecting

S-Detecting and/or range and bearing

T -Transmitting W-Control X-Identification and recognition

INTRODUCTION TO RADAR 25

Table 1.5 Some Common Radar Types and

Nomenclatures

Surveillance

Air defense-land based (FPS, MPS), shipboard (SPS, SPY)

Airborne early warning (APS, A W AC)

Air traffic control-en route (ARSR), terminal (ASR)

Mortar and artillery location (TPS)

Speed measurement-traffic radars

Intrusion-jungle, burglar alarm (PPS)

Harbor monitoring-collision avoidance

Height finders (AHSR)

Tracking, guidance, and navigation-precision approach (TPM)

Land and shipboard trackers (FPS, FPQ, MPS)

Airborne interceptor (AI)

Wind field (Doppler radars)

Clear air turbulence

Hydrology

nient for generally describing the frequency limits of operation without being specific (which may still be prohibited for security reasons) Furthermore, the band designations are such that the characteristics of radar power sources, prop-agation, target reflectivity, and so on are generally similar within a band, but these characteristics may differ from band to band Consequently, letter bands are still useful in describing the operating frequencies of radar Although the original band designations were unofficial, standardized usage has been defined

by organizations such as the IEEE The designations from IEEE Standard

521-1976 are shown in Table 1-6, along with some notes on other fairly common usages Additional information on common usage and applications of the radar frequency bands are given in Table 1-7.8

The Department of Defense recently issued a directive on designating quencies used for performing ECM operations This directive has been imple-mented in AFR55-44 (identical to AR 105-86, OPNAVIST 3480.9B, and MCO 3480.1), which governs ECM operations in the United States and Canada The

fre-13 ECM bands are listed in alphabetical order In practice, each band is divided

Standard Radar Bands' ECM Bands 2

'Prom IEEE Standard 521-1976, November 30, 1976

'Prom APR 55-44 (AR105-86, OPNAVINST 3430.9B, MeO 3430.1), October 27, 1964

4The following approximate lower frequency ranges are sometimes given letter designations: P-band (225-390

MHz), G-band (150-225 MHz), and I -band (100-150 MHz)

'The following approximate higher frequency ranges are sometimes given letter designations: Q-band (36-46

GHz), V-band (46-56 GHz), and W-band (56-100 GHz)

Table 1-7 Radar Frequency Bands and General Usages

3-30 MHz 30-300 MHz 300-1000 MHz 1-2 GHz 2-4 GHz 4-8 GHz 9-12 GHz

12-18 GHz 18-27 GHz 27-40 GHz 40-100+ GHz

Usage

OTH surveillance Very-long-range surveillance Very-long-range surveillance Long-range surveillance

En route traffic control Moderate-range surveillance Terminal traffic control Long-range weather Long-range tracking Airborne weather detection Short-range tracking Missile guidance Mapping, marine radar Airborne intercept High-resolution mapping Satellite altimetry Little use (water vapor) Very-high-resolution mapping Airport surveillance Experimental

INTRODUCTION TO RADAR 27

into 10 channels (e.g., A-7) whose width increases with the band frequency width; that is, each channel for Band A is 25 MHz wide, while each channel for Band M is 4000 MHz wide A key feature of AFR 55-44 is contained in paragraph 1 of Attachment 1 thereto: "The phonetic alphabet will be used to identify the frequency of ECM operations to identify an exact frequency, the frequency will be specified as Band-Channel base (lowest frequency in any channel) plus frequency in megacycles [sic] above the base frequency Exam-ple: 1315 mcfs would be 'Delta 4 plus 15'."

Two widespread misuses of the ECM letter-band designations have oped First, they have sometimes been used for radars As stated in IEEE Stan-dard 521-1976: "The letter designations for Electronic Countermeasure oper-ations as described in Air Force Regulation No 55-44, Army Regulation No 105-86, and Navy OPNAV Instruction 4530.9B are not consistent with radar practice and shall not be used to describe radar frequency bands " Second, even when used to describe ECM equipment, the band is commonly designated by its alphabetic rather than its phonetic equivalent (e.g., an I-band rather than India-band jammer) Thus, speaking of "a J-band radar" is incorrect on two counts

devel-Leaders of the radar community are making a vigorous effort to stop the use

of the ECM letter designations for radar bands However, this misuse is ing somewhat pervasive and may be impossible to correct

becom-1.8 REFERENCES

1 L N Ridenour, Radar System Engineering, MIT Radiation Laboratory Series, Vol I,

McGraw-Hill Book Co., New York, 1947

2 D J Povejsil, R S Raven, and P Waterman, Airborne Radar, D Van Nostrand Company,

Princeton, N.J., 1961

3 M I Skolnik, Introduction to Radar Systems, 2nd ed., McGraw-Hill Book Co., New York,

1980

4 D K Barton, Radar Systems Analysis, Artech House, Dedham Mass., 1976

5 R S Berkowitz (Ed.), Modern Radar, John Wiley & Sons, Inc., New York, 1965

6 F Nathanson, Radar Design Principles, McGraw-Hili Book Co., New York, 1969

7 M I Skolnik (Ed.), Radar Handbook, McGraw-Hill Book Co., New York, 1970

8 F E Nathanson, Radar Range Calculation, 1979 Short Course Notes, Technology Service

Corporation, Silver Springs, Md., 1979

PART 1

FACTORS EXTERNAL TO THE

RADAR

• EM Waves and the Reflectivity Process-Chapter 2

• The Propagation Process-Chapter 3

• The Multipath Phenomena and Effects-Chapter 4

Although a radar itself is an entity, its operational performance is affected by phenomena that are external to it Thus, the radar designer or analyst needs an understanding not only of the radar proper but also of the principles and effects

of those external factors that affect radar performance Part 1 of Principles of

Obviously, a radar's performance is dependent on the reflectivity properties

of the objects it illuminates After first introducing and describing the principles

of EM waves, Chapter 2 addresses the reflectivity process, including sions of the polarization scattering matrix and radar cross section Methods of calculating radar cross section are illustrated by two examples: one using geo-metric optics techniques and one using physical optics techniques

discus-The EM waves radiated by the radar must propagate to (and from) the target through a medium that may range from nearly lossless to very lossy Further-more, anomalies or gradients in the propagation medium can cause the EM wave to bend (refract) from a straight-line path or sometimes to be trapped in

a propagation duct Particles such as falling hydrometeors, fog, dust, and smoke

in the atmospheric propagation medium also affect radar performance by uating and backscattering the propagating EM wave These topics and others are addressed in Chapter 3

atten-In Chapter 4, the multipath phenomenon and its effects are described and discussed It is shown that the multipath phenomenon is generally a degrading factor in radar performance due to resulting target signal fading, target range ambiguities, and target angular position ambiguities; however, the multipath phenomenon can also be used to advantage in some applications, such as ground plane reflecting ranges and target height determination

29

2

EM WAVES AND THE REFLECTIVITY

PROCESS

Eugene F Knott

2.1 THE EM WAVE PHENOMENON

Everyone is familiar with waves in one form or another A very common ample is the way waves propagate over the quiet surface of a pond when a stone

ex-is thrown into the water Concentric rings expand away from the center of the disturbance, becoming weaker the farther they travel The wave on the surface

of the water is a transverse wave, signifying that the actual motion of the water particles (up and down in this case) is at right angles to the direction of prop-agation EM waves are also transverse, although no particle motion may be involved Instead, it is the intensities of the electric and magnetic field strengths that vary in planes transverse to the direction of propagation.)

The EM waves emitted by radars are harmonic in time This phenomenon could be simulated in the still pond if a vibrating plunger replaced the single stone A continuous stream of expanding wavefronts would flow away from the plunger as long as the plunger was activated If the wavelength from crest to crest or trough to trough was measured, it would be found to vary inversely with the frequency of the vibrating plunger The product of the wavelength and frequency would be found to be a constant, and this constant is the velocity of propagation For EM waves propagating through free space, the velocity of propagation is the familiar speed of light, or 0.2997925 mlns, very nearly 1 fil

ns (A nanosecond is a billionth of a second.) The electromagnetic spectrum includes X-rays as well as very low frequencies, as shown in Figure 2-1 The electric and magnetic field strengths of an EM wave vary sinusoidally with time (t) and distance; ifthe frequency of the source is denotedj, they have the forms

Ex = Eo cos (wt - kR)

Hy = Ho cos (wt - kR)

(2-1)

31

Ex = electric field in volts per meter

Hy = magnetic field in amperes per meter

Eo = maximum amplitude of the electric field

R = distance measured from some origin

SHORT RADIO WAVES

is required in many EM problems Generally, elect~ field strength E is

de-scribed in volts per meter and magnetic field strength H is dede-scribed in amperes

per meter The ratio of transverse electric to transverse magnetic field strength

is an impedance that is characteristic of the medium in which the wave is agating For free space, this ratio is about 377 ohms; for Teflon, it is about 545 ohms

prop-EM WAVES AND THE REFLECTIVITY PROCESS 33

y

z Figure 2-2 A "snapshot" of the electric and magnetic field intensities at a particular moment in time

The Poynting vector S is the vector cross product

and is the power density, say in watts per square meter, ofthe wave in a plane perpendicular to the direction of propagation Since field strengths vary sinu-soidally with time, so does power density; however, the P~ynting vector can

be averaged over time to give the complex Poynting vector S*

where fI is the complex conjugate of H Thus, S* is a measure of the actual power transfer across a surface, whether that surface is real or imaginary

phase are families of parallel planes The plane wave is nonexistent in the real world but can be closely approximated by a spherical wave at very large dis-tances from a source Unless the medium is lossy, the plane wave does not decay in strength with increasing distance Nevertheless, the concept of a plane wave is extraordinarily useful because the phase fronts are fiat The field struc-ture in some uniform transmission lines is essentially that of a plane wave

The spherical wave is probably the most common one found in nature, and as the name implies, surfaces of constant phase are concentric spheres centered on the source At great distances from the source, the spherical phase fronts can

be approximated locally as planar phase fronts The electric and magnetic field strengths of a spherical wave decay with increasing distance from the source; hence, the power density decays with the square of the distance As will be shown in a later section, radar echoes are received by means of a double traverse

of the same path; hence, the received power as given by the radar range tion decays with the fourth power of distance

of constant phase are concentric cylinders centered on a line source Cylindrical waves, like plane waves, are relatively rare in nature, but they are very useful

in the analysis of some near-field problems The field intensities decay inversely with the square root of the distance; hence, the power density decays with in-creasing distance In this respect, the cylindrical wave seems to be a hybrid combination of a plane wave and a spherical wave

A simple plane wave is linearly polarized because the electric and magnetic field strengths remain in their respective planes independently of time and dis-tance For a given direction of propagation, this is also true of spherical wave fronts Linear polarization is commonly described in terms ofthe orientation of the electric field vector with respect to the local horizontal or vertical planes and is directly associated with the antenna used to launch or receive a wave Most radars are vertically or horizontally polarized, but some have the capa-bility to transmit and receive both polarizations

A pair of plane waves with the same frequency and direction of propagation can be added together to create an elliptically polarized wave.2 This occurs if the two plane waves are of different polarizations and if there is a phase shift between them A simple example is the addition of a plane wave of amplitude

E, polarized in the x-direction and one of amplitude E2 polarized in the

y-di-rection, but advanced in phase by an angle 1/;:

a special case of elliptical polarization Another special case is circular

plus or minus phase shift is used, the wave will have right or left circular larization, and the field will rotate clockwise or counterclockwise Circularly