George W. Stimson introduction to Airborne Radar (Se)
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INTRODUCTION TO
AIRBORNE
RADAR
GEORGE W. STIMSON
SECOND EDITION
MENDHAM, NEW JERSEY
ii
Acquisition and Product Development: Dudley R. Kay
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Illustrations and Layout: George Stimson and Shyam Reyes
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©1998 by George Stimson III. All rights reserved. No part of this book may be reproduced or used in any
form whatsoever without written permission from the publisher except in the case of brief quotations
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This book is dedicated to Meade A. Livesay (left), veteran engineer, technical man-
ager, and past President of the Hughes Radar Systems Group, who envisioned and
commissioned the original writing of the book. He is seen here examining an
advance copy of the first edition, with the author.
Brief Outline
I. Overview
1. Basic Concepts (3)
2. Approaches to Implementation (15)
3. Representative Applications (35)
II. Essential Background Information
4. Radio Waves & Alternating Current Signals (49)
5. Nonmathematical Understanding of Radar (59)
6. The Ubiquitous Decibel (71)
III. Radar Fundamentals
7. Choice of Radio Frequency (83)
8. Directivity and the Antenna Beam (91)
9. Pulsed Operation (107)
10. Detection Range (115)
11. Range Equation (135)
12. Pulse Delay Ranging (151)
13. Pulse Compression (163)
14. FM Ranging (177)
IV. Pulse Doppler Radar
15. Doppler Effect (189)
16. Spectrum of a Pulsed Signal (199)
17. Mysteries of Pulsed Spectrum Unveiled (209)
18. Sensing Doppler Frequencies (235)
19. How Digital Filters Work (253)
20. Digital Filter Bank and The FFT (267)
21. Measuring Range Rate (281)
V. The Problem of Ground Clutter
22. Sources & Spectra of Ground Return (293)
23. Effect of Ambiguities on Ground Clutter (309)
24. Ground Moving Target Detection (317)
VI. Air-to-Air Operation
25. The Crucial Choice of PRF (325)
26. Low PRF Operation (335)
27. Medium PRF Operation (355)
28. High PRF Operation (369)
29. Automatic Tracking (383)
VII. High Resolution Ground Mapping
30. Meeting Resolution Requirements (393)
31. Synthetic Array Radar (SAR) Principles (403)
32. SAR Design Considerations (425)
33. SAR Operating Modes (431)
VIII. Radar In Electronic Warfare (EW)
34. Electronic Countermeasures (ECM) (439)
35. Electronic Countercountermeasures (ECCM) (457)
36. EW Intelligence Functions (469)
IX. Advanced Concepts
37. Electronically Steered Array Antennas (ESAs) (473)
38. ESA Design (481)
39. Antenna RCS Reduction (493)
40. Advanced Radar Techniques (499)
• Approaches to Multi-frequency Operation (500)
• Small Target Detection (504)
• Bistatic Target Detection (507)
• Space Time Adaptive Processing (509)
• True Time Delay (TTD) Beam Steering (511)
• Three-Dimensional SAR (515)
41. Advanced Waveforms & Mode Control (519)
42. Low Probability of Intercept (LPI) (525)
43. Advanced Processor Architecture (535)
X. Representative Radar Systems (545)
(Page numbers are in blue print.)
About the Author
G
eorge Stimson became fascinated with radio
waves as a teenage amateur radio enthusiast,
designing and building transmitters and
receivers.
His first brush with radar, which came in the early years
of World War II, was bouncing echoes off Navy blimps in
between experiments outside the ultra-high frequency lab at
Stanford University. Upon receiving his bachelor’s degree in
electrical engineering, he did some additional course work at
Caltech, went through the Navy’s radar schools at Bowdoin
and MIT, and wound up as an electronics officer on an
attack transport.
Following the war, he served as an engineer on Southern
California Edison’s frequency-change project and at its com-
pletion joined Northrop’s Snark Missile project. There quite
by chance he became involved in technical publications and
motion pictures.
In 1951, he was hired by Hughes Aircraft Company to
write a widely circulated technical periodical called the
Radar Interceptor. Working closely with the Company’s top
designers, in the ensuing years he observed at first hand the
fascinating evolution of airborne radar from the simple sys-
tems for the first all-weather interceptors to the advanced
pulsed doppler systems of today. He witnessed the develop-
ment of the first radar-guided air-to-air missiles, the first
incorporation of digital computers in small airborne radars,
the birth of laser radar, SAR, and the programmable digital
signal processor; and he saw the extension of airborne radar
technology to space applications.
Following his retirement in 1990, he has remained active
in the field, teaching a short course in modern radar at the
National Test Pilots School in Mojave, writing a technical
brochure on Hughes antenna radiation-pattern and RCS
measurement facilities, producing a fully narrated interactive
multimedia presentation on the new HYSAR radar, and writ-
ing the article on radar for the 1998 edition of the
Encyclopedia Americana.
Preface
iv
It is hoped that you will find this book as interesting
and enjoyable to read as it was to write.
Key Features
As you will undoubtedly find, the book is unique in
several respects.
First, beginning from scratch, it presents the wide
range of airborne radar techniques in the form of an
unfolding saga, not of individuals, but of radar con-
cepts and principles. Each chapter tells a story, and the
story flows naturally on from chapter to chapter.
Second, the book is designed to fulfill the needs of all
who want to learn about radar, regardless of their tech-
nical backgrounds. It has sufficient technical depth
and mathematical rigor to satisfy the instructor, the
engineer, the professor. Yet, as long as a reader has a
basic understanding of algebra and knows a little
trigonometry and physics, the text painlessly takes the
reader in bite-sized increments to the point of being
able to talk on a sound footing with the radar experts.
Third, every technical concept is illustrated with a sim-
ple diagram immediately next to the text it relates to.
Every illustration has a concise caption, which enables
it to stand alone.
Fourth, to keep the text simple, where additional detail
may be desired by some readers but not all, it is conve-
niently placed in a blue “panel” which one may skip,
on a first reading, and come back to later on and exam-
ine at leisure. Exceptions, caveats, and reviewers com-
ments are presented without detracting from the sim-
plicity of the text in brief “side notes.”
These features lead to the perhaps most unique aspect
of the book. One can follow the development of each
chapter by reading just the text, or just the illustrations
and captions, or by seamlessly moving along between
text and illustrations.
Yet another unique feature. Recognizing that people
interested in airborne radar love airplanes, dispersed
through the book are photos and renderings of radar-
bearing aircraft, spanning the history of airborne radar
from the Bristol Beaufighter of 1940 to the B-2 Bomber
and F-22 fighter of today.
What’s New
If you’re familiar with the first edition, you may be
wondering what’s new in the second?
Prompted by the advent of “stealth,” the daunting
prospect of ever more sophisticated radar countermea-
sures, and the explosive growth of digital-processing
throughput, which has made practical many radar tech-
niques long considered “blue sky,” 12 new chapters
have been added. Briefly, they cover the following:
• Electronically steered array antennas (ESAs)—
besides providing extreme beam agility, they’re a
“must” for stealth
• Antenna RCS reduction—also a crucial require-
ment of stealth
• Low-probability of intercept techniques (LPI) —
besides greatly reducing vulnerability to counter-
measures, they amazingly enable a radar to detect
targets without its signals being usefully detected
by an enemy
• Electronic countermeasures, counter countermea-
sures, and intelligence functions
• Multi-frequency operation and small-signal target
detection—also essential in the era of stealth—
plus space-time adaptive processing, true-time-
delay beam steering, and 3-D SAR
• New modes and approaches to mode control that
take advantage of the ESA’s versatility
• Advanced airborne digital processing architec-
tures—key to most of the above capabilities
• Detection and tracking of low-speed moving tar-
gets on the ground—an important topic missed in
the first edition.
v
To illustrate the application of the basic radar princi-
ples, the book ends by briefly describing a dozen or so
airborne radars currently in service in applications rang-
ing from long-range surveillance to environmental moni-
toring.
Also warranting mention, the first three chapters
have been extensively modified to provide a complete
overview of virtually all of the basic principles and
advanced features presented in the body of the book.
These chapters may be useful in providing a “stand-
alone” briefing on modern radar for students wanting a
quick introduction to the subject.
Acknowledgements
Needless to say, I’m deeply grateful to the following
engineers of the Hughes Aircraft Company (now a part
of Raytheon) past and present, who have reviewed vari-
ous sections of the book and contributed valuable sug-
gestions, technical information, and insights.
For the first edition: Eddie Phillips, Ben DeWaldt,
Nate Greenblatt, Dave Goltzman, Kurt Harrison, Scott
Fairchild, Verde Pieroni, Morris Swiger, Jeff Hoffner, John
Wittmond, Fred Williams, Pete Demopolis, Denny Riggs,
and Hugh Washburn.
For the new chapters: Doug Benedict, John Griffith,
Don Parker, Steve Panaretos, Howard Nussbaum, Robert
Rosen, Bill Posey, John Wittmond, Dave Sjolund, Lee Tower,
Larry Petracelli, Robert Frankot, and Irwin Newberg.
I am extremely grateful to Merrill Skolnik and Russell
Lefevre (who reviewed an early draft of the second edi-
tion for the IEEE) for their encouragement and helpful
suggestions.
Also, thanks are due to Hugh Griffiths of University
College London and his colleagues, Dr. David Belcher
and Prof. Chris Oliver of DERA Malvern, for the excel-
lent SAR maps they provided; and to Gerald Kaiser,
then professor at the University of Massachusetts-
Lowell, who on his own initiative in anticipation of the
second edition combed through the first from cover to
cover to spot overlooked typos and other errors.
In addition, abundant thanks go to Hughes’ ever
helpful Al Peña for securing the negatives of the first
edition for reuse in this edition.
Finally, special thanks to Shyam Reyes, for his invalu-
able aid with page composition and artwork, and to
Dudley Kay and Denise May of SciTech, without whom
the publication of this edition would not have been
possible.
G.W. S., San Marino, California
vii
Contents
Part I Overview of Airborne Radar
Chapter 1 Basic Concepts 3
Radio Detection 4
Determining Target Position 6
Exploiting the Doppler Effect 10
Ground Mapping 11
Chapter 2 Approaches to Implementation 15
Generic “Pulsed” Radar 15
Generic Pulse-Doppler Radar 25
Generic Radar for Stealth 30
Chapter 3 Representative Applications 35
Hazardous-Weather Detection 36
Navigational Aid 36
Ground Mapping 39
Reconnaissance and Surveillance 40
Fighter/Interceptor Mission Support 41
Air-to-Ground Weapon Delivery 43
Short-Range Air-to-Sea Search 45
Proximity Fuses 45
Part II Essential Groundwork
Chapter 4 Radio Waves and Alternating
Current Signals 49
Nature of Radio Waves 49
Characteristics of Radio Waves 52
Chapter 5 Key to a Nonmathematical
Understanding of Radar 59
How a Phasor Represents a Signal 59
Combining Signals of Different Phase 61
Combining Signals of Different Frequency 62
Resolving Signals into I and Q Components 67
Chapter 6 The Ubiquitous Decibel 71
What Decibels Are 71
Converting from Power Ratios to dB 74
Converting from dB to Power Ratios 75
Representing Power Ratios Less Than One 75
Using Decibels 76
Power Gain in Terms of Voltage 77
Decibels as Absolute Units 77
Part III Radar Fundamentals
Chapter 7 Choice of Radio Frequency 83
Frequencies Used for Radar 83
Frequency Bands 84
Influence of Frequency on Radar Performance 85
Selecting the Optimum Frequency 88
Chapter 8 Directivity and the Antenna Beam 91
Distribution of Radiated Energy in Angle 91
Characteristics of the Radiation Pattern 96
Electronic Beam Steering 100
Angular Resolution 101
Angle Measurement 102
Antenna Beams for Ground Mapping 106
Chapter 9 Pulsed Operation 107
Advantages of Pulsed Transmission 107
Pulsed Waveform 108
viii
Output Power and Transmitted Energy 111
Chapter 10 Detection Range 115
What Determines Detection Range 115
Electrical Background Noise 116
Energy of the Target Signal 122
Detection Process 125
Integration and Its Leverage
on Detection Range 127
Postdetection Integration 131
Chapter 11 The Range Equation, What It
Does and Doesn’t Tell Us 135
General Range Equation 135
What the Equation Tells Us 138
Equation for Volume Search 140
Fluctuations in Radar Cross Section 142
Detection Probability 142
Cumulative Detection Probability 147
Chapter 12 Pulse Delay Ranging 151
Basic Technique 151
Range Ambiguities 153
Eliminating Ambiguous Return 155
Resolving Ambiguities 156
Eliminating Ghosts 157
How Many PRFs? 159
Single-Target Tracking 161
Chapter 13 Pulse Compressions 163
Linear Frequency Modulation (Chirp) 163
Binary Phase Modulation 169
Chapter 14 FM Ranging 177
Basic Principle 177
Accounting for the Doppler Shift 179
Eliminating Ghosts 180
Performance 185
Part IV Pulse Doppler Radar
Chapter 15 Doppler Effect 189
Doppler Effect and Its Causes 189
Where and How the Doppler Shift Takes Place 190
Magnitude of the Doppler Frequency 192
Doppler Frequency of an Aircraft 195
Doppler Frequency of Ground Return 196
Doppler Frequency Seen by a Semiactive
Missile 197
Chapter 16 Spectrum of Pulsed Signal 199
Illustrative Experiments 200
Bandwidth 200
Coherence 202
Line Width versus Duration of Pulse Train 204
Spectral Sidelobes 206
Chapter 17 Mysteries of Pulsed Spectrum
Unveiled 209
Crux of the Matter 209
Fourier Series 213
Spectrum Explained from a Filter’s
Point of View 222
Mathematical Explanation of the Pulsed
Spectrum 225
Chapter 18 Sensing Doppler Frequencies 235
Doppler Filter Bank 235
Analog Filters 238
Digital Filtering 240
Providing Adequate Dynamic Range 248
Chapter 19 How Digital Filters Work 253
Inputs to the Filter 253
What the Filter Does 256
Discrete Fourier Transform 259
Implementing the DFT 260
Sidelobe Reduction 263
CONTENTS
CONTENTS
ix
Filtering Actual Signals 264
Chapter 20 The Digital Filter Bank
and the FFT 267
Basic Concept 268
A Representative FFT 268
FFTs for Filter Banks of Any Size 274
Rules of Thumb for Estimating Number
of Computations 277
Chapter 21 Measuring Range Rate 281
Range Differentiation 281
Doppler Method 283
Potential Doppler Ambiguities 284
Resolving Doppler Ambiguities 286
Part V Return from the Ground
Chapter 22 Sources and Spectra of
Ground Return 293
What Determines the Amplitude of the
Ground Return 294
Mainlobe Return 296
Sidelobe Clutter 299
Altitude Return 302
Relation of Clutter Spectrum to Target
Frequencies 303
Return from Objects on the Terrain 306
Chapter 23 Effect of Range and Doppler
Ambiguities on Ground Clutter 309
Dispersed Nature of the Clutter 310
Range Ambiguities 311
Doppler Profile 314
Doppler Ambiguities 314
Chapter 24 Separating Ground-Moving
Targets from Clutter 317
Problem of Detecting “Slow” Moving Targets 317
Classical DPCA 318
Notching Technique 320
Combined Notching and Classical DPCA 321
Precise Angle Measurement 322
Part VI Air-to-Air Operation
Chapter 25 The Crucial Choice of PRF 325
Primary Consideration: Ambiguities 325
The Three Basic Categories of PRF 329
Low PRF Operation 330
Chapter 26 Low PRF Operation 335
Differentiating Between Targets and Clutter 335
Signal Processing 340
Less Sophisticated Signal Processing 346
Advantages and Limitations 346
Getting Around Limitations 347
Chapter 27 Medium PRF Operation 355
Differentiating Between Targets and Clutter 355
Signal Processing 359
Rejecting Ground Moving Targets (GMTs) 360
Eliminating Blind Zones 361
Minimizing Sidelobe Clutter 364
Sidelobe Return from Targets of Large RCS 365
Chapter 28 High PRF Operation 369
High PRF Waveform 370
Isolating the Target Returns 370
Mechanization 373
Ranging 375
Problem of Eclipsing 376
Improving Tail Aspect Performance 378
Chapter 29 Automatic Tracking 383
Single-Target Tracking 383