Radar Cross Section Measurements

556 Pages·1993·14.31 MB·English

by  Eugene F. Knott (auth.)

RADAR CROSS
SECTION
MEASUREMENTS
RADAR CROSS
SECTION
MEASUREMENTS
by Eugene F. Knott
Inm5I
VAN NOSTRAND REINHOLD
~ ____ New York
Copyright © 1993 by Van Nostrand Reinhold
Softcover reprint of the hardcover 1s t edition 1993
Library of Congress Catalog Card Number 92-43286
ISBN 0-442-00536-9
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Library of Congress Cataloging-in-Publication Data
Knott, Eugene F.
Radar cross section measurements / Eugene F. Knott.
p. cm.
Includes bibliographical references and index.
ISBN 978-1-4684-9906-3 ISBN 978-1-4684-9904-9 (eBook)
DOI 10.1007/978-1-4684-9904-9
1. Radar cross sections. 2. Radar-Testing. 1. Title.
TK6580.K653 1993
621.3848---dc20 92-43286 CIP
This book is dedicated to the console-bound RCS test range
Radar Operator. Committed by his job to the routine
collection of other people's data, he seldom experiences the
thrill of collecting his own.
Preface
The original campus of the University of Michigan was nearly a perfect
square about a half-mile along a side. A street-sized walk, appropriately
called the Diag, runs diagonally across this square, connecting its southeast
and northwest corners. In 1904 a new engineering building was either started
or finished (I do not remember which) to house classrooms. When another
engineering building was built on the expanded campus across the street
from it many years later, the old building came to be known as West Engine,
to distinguish it from the new East Engine.
Old West Engine is (or maybe by now, was) a four-story, L-shaped
structure that stood at the southeast corner of the original campus. It was
built with an arch in it to straddle the Diag at the apex of the L. You walked
over the Engineering Arch to get from one leg of the L to the other if you
were inside the building, and you walked under it when you entered the
campus from the southeast corner.
Affixed to the masonry wall of the arch was a plaque I often noted in
passing. It bore a quote attributed to Horace Greeley (1811-1872), who I
did not know at the time was the founder, editor, and publisher of the New
York Tribune. It said, simply,
Young man, when theory and practice differ, use your horse sense.
The suggestion seems worthy of an exclamation point instead of a period,
but I do not remember if it had one.
It also seems appropriate advice for young engineers, although Greeley
was neither when he issued this particular admonition. Practice tells us that
Radar Cross Section Measurements is not the first text on ReS measurements,
vii
viii Preface
and theory tells us that it will not be the last. And if there is any horse sense
at all in the book, it will be that the why of RCS testing is probably more
important than the how. If we can understand the why, Mr. Greeley probably
would have admitted, we can always find a way to apply existing technology,
whatever it may be at the time, to the how. Although advances in technology
may change the how, they cannot significantly influence the why.
The original plan for the book listed 16 chapters~three more than this
published version contains~and the first draft actually had all 16. But
without realizing it, Van Nostrand Reinhold editors found it necessary to
apply Greeley's principle to the production of the book. Theory demanded
that it be comprehensive, and therefore fat, but practice dictated that it be
affordable. Horse sense and three chapters comprise the difference. The
missing chapters dealt with bistatic RCS measurements, range preparation
requirements, and test-plan development.
The objective of what remains is to present and defend the facts~and
occasionally the conjecture~governing good RCS measurement practice. It
is intended for people who need to augment their knowledge of testing of
hard (manufactured) targets as measured on or in outdoor and indoor RCS
test ranges. The characteristics of "soft" targets (clutter echoes due to the
natural environment, for example) are considered only in the context of how
they affect the measurement of desired hard-target echoes on a test range.
No description or analysis of clutter signals appears in the book.
Every chapter begins with a brief survey of the chapter's content and ends
with a summary of the presentation. In most cases, the two are within
comfortable agreement. Between them lie the technical detail of fact, test,
and prediction, and sometimes the glitter of conjecture. But wherever
conjecture appears in the text, it is identified as such.
Like all authors, I would like to acknowledge a long list of supporters,
contributors, consultants, reviewers, critics, artists, typists, and editors who
helped me make the book what it is, but my list is short. Although I did
solicit information and data from large aerospace and airframe manufacturers,
and from smaller institutions as well, only one or two responded. I alone
tapped out the text on an obsolete computer with obsolete software, and
built almost all the illustrations. (Ergo, no typists or artists to thank.)
Nevertheless, I can gratefully acknowledge the assistance of one company
and several people.
In its encouragement to employees to publish their work, the Boeing
Company allowed me to use company resources, mainly desk-top graphic
work-stations, to construct many of the illustrations that appear in the book.
Had Boeing not granted me access to these facilities, I could not have
accomplished this book-building task.
Eugene Sorensen, a Boeing colleague, assisted in the plotting of several
Preface ix
illustrations in the book. Dr. 1. C. Axtell, Manager of the RCS Technology
Group at Boeing Aerospace for many years, endorsed my request to publish
photographs of the Boeing RCS test range. Dr. E. 1. Nalos, my functional
supervisor at Boeing, displayed persistent interest in my progress on the draft.
Daryl Stone, the engineer tasked to design and monitor the construction
of Boeing's test range in Oregon, kept me honest about how measurements
are made at the site, and on how the instrumentation was designed. Our
discussions were always energetic, accompanied at times by prodigious
arm-flailing and expletives bounced off the walls. I will treasure those
dynamic information-swapping sessions for a long time.
Of the book's 13 chapters, I felt four demanded the scrutiny of people
more knowledgeable than me, so I solicited the judgment of four experts.
Daryl Stone examined Chapter 2 and unearthed several errors that needed
repair. Dr. Robert Dybdal (Aerospace Corporation, El Segundo, California)
carefully reviewed Chapter 4 and offered several suggestions for improvement.
Dr. Dean Mensa (U.S. Navy Pacific Missile Test Center, Pt. Mugu,
California) reviewed Chapter 10; his critique was thorough enough to be
included as a reference cited at the end of that chapter. Mr. Ed Hart, who
formed Fair Play Technical Enterprises, Rutledge, Georgia, after retiring
from Scientific-Atlanta, Inc., reviewed Chapter 11. I am indebted to all four
men for carving time out of their personal lives to review the draft and offer
their comments for improving the book.
Acquisition Editor Steve Chapman was my only contact with Van
Nostrand Reinhold during the 3-year construction and revision of Radar
Cross Section Measurements. I fretfully and aperiodically submitted my
chapter statistics to him, making it clear in each report that "no response
is necessary or desired." Unlike many people who ignore my admonitions,
if not Greeley's, Steve took them to heart: he issued replies only when he
had bad news.
Not least is the support of my wife Ruth, who, at the inception of this
mighty endeavor, encouraged me to "go for it," and once I went for it, urged
me on to completion. She listened to my complaints, tactfully ignored my
techni-babble, and tolerated delays in long-planned home maintenance and
improvement projects. With the construction of Radar Cross Section M easure
ments now behind us, I can be more attentive to her complaints, tackle our
projects, and try to understand Ruthie babble.
EUGENE F. KNOTT
CONTENTS
1 Radar Cross Section Fundamentals 1
1.1 The Need for RCS Measurements
1.2 Electromagnetic Wave Properties 5
1.3 The Radar Range Equation 12
1.4 Radar Cross Section 14
1.5 Polarization Scattering Matrix 17
1.6 Basic Test Range Requirements 22
1.7 Summary 25
2 Instrumentation Systems 27
2.1 The CW Cancellation Radar 28
2.2 The Noncoherent Pulsed Radar 31
2.3 Coherent Radars 45
2.4 Multiband Scattering Matrix Radars 52
2.5 Digital Functions in Multiband Radars 59
2.6 Summary 66
3 Target Support Structures 70
3.1 Foam Columns 70
3.2 String Supports 85
xi
xii Contents
3.3 The Metal Pylon 98
3.4 Other Structures and Techniques 110
3.5 Summary 117
4 Measurement Errors 120
4.1 Instrumentation and Sensitivity 122
4.2 Target Illumination 128
4.3 Background Contributions 143
4.4 Target-Environment Interactions 151
4.5 Summary 159
5 Calibration 163
5.1 Instrumentation Calibration 164
5.2 Calibration Scatterers with Curved Surfaces 173
5.3 Calibration Scatterers with Flat Surfaces 183
5.4 Calibration by Substitution 195
5.5 Summary 205
6 Outdoor Test Ranges 208
6.1 Ground-Plane Configuration 209
6.2 Two-Path Propagation 214
6.3 Effect of Antenna Pattern 223
6.4 Imperfectly Reflecting Ground Planes 232
6.5 Defeating the Ground Plane 241
6.6 Summary 252
7 Indoor Chambers 255
7.1 Wall Reflections 256
7.2 Chamber Configuration 264
7.3 Chamber Absorbing Materials 276
7.4 Test Chamber Evaluation 286
7.5 Summary 296
Contents xiii
8 Compact Ranges 299
8.1 Dielectric Lenses 300
8.2 Large Reflectors 310
8.3 Reflector Edge Configurations 318
8.4 Dual-Reflector Configurations 324
8.5 Chamber Diagnostics 334
8.6 Summary 346
9 Data Processing and Reduction 350
9.1 Statistical Measures 351
9.2 Hard-Target Characteristics 361
9.3 Display Formats 374
9.4 Background Subtraction 377
9.5 Summary 382
10 Radar Imagery 385
10.1 Radar Imaging Principles 386
10.2 The Range Profile 392
10.3 Zero-padding and Windowing 401
10.4 The Cross-Range Profile 408
10.5 Image Formation and Diagnostics 415
10.6 Summary 427
11 Dynamic Test Ranges 430
11.1 The Instrumentation Radar 431
11.2 Dynamic Target Imaging 438
11.3 The Tracking Radar 443
11.4 Metric Data 451
11.5 Some Dynamic RCS Test Ranges 464
11.6 Summary 478
12 Scale-Model Testing 482
12.1 Scaling Laws 483
12.2 Dielectrics and Absorbers 488