Introduction to Airborne Radar

570 Pages·1998·13.84 MB·English

by  George W. Stimson

INTRODUCTION TO
A
IRBORNE
R
ADAR
SECOND EDITION
GEORGE W. STIMSON
MENDHAM, NEW JERSEY
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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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ii
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
(Page numbers are in blue print.)
I.Overview VI. Air-to-Air Operation
1. Basic Concepts (3) 25. The Crucial Choice of PRF (325)
2. Approaches to Implementation (15) 26. Low PRF Operation (335)
3. Representative Applications (35) 27. Medium PRF Operation (355)
28. High PRF Operation (369)
II.Essential Background Information
29. Automatic Tracking (383)
4. Radio Waves & Alternating Current Signals (49)
VII. High Resolution Ground Mapping
5. Nonmathematical Understanding of Radar (59)
6. The Ubiquitous Decibel (71) 30. Meeting Resolution Requirements (393)
31. Synthetic Array Radar (SAR) Principles (403)
III.Radar Fundamentals 32. SAR Design Considerations (425)
7. Choice of Radio Frequency (83) 33. SAR Operating Modes (431)
8. Directivity and the Antenna Beam (91)
VIII. Radar In Electronic Warfare (EW)
9. Pulsed Operation (107)
34. Electronic Countermeasures (ECM) (439)
10. Detection Range (115)
35. Electronic Countercountermeasures (ECCM) (457)
11. Range Equation (135)
36. EW Intelligence Functions (469)
12. Pulse Delay Ranging (151)
13. Pulse Compression (163)
IX. Advanced Concepts
14. FM Ranging (177)
37. Electronically Steered Array Antennas (ESAs) (473)
IV. Pulse Doppler Radar 38. ESA Design (481)
39. Antenna RCS Reduction (493)
15. Doppler Effect (189)
40. Advanced Radar Techniques (499)
16. Spectrum of a Pulsed Signal (199)
• Approaches to Multi-frequency Operation (500)
17. Mysteries of Pulsed Spectrum Unveiled (209)
• Small Target Detection (504)
18. Sensing Doppler Frequencies (235)
• Bistatic Target Detection (507)
19. How Digital Filters Work (253)
• Space Time Adaptive Processing (509)
20. Digital Filter Bank and The FFT (267)
• True Time Delay (TTD) Beam Steering (511)
21. Measuring Range Rate (281)
• Three-Dimensional SAR (515)
41. Advanced Waveforms & Mode Control (519)
V. The Problem of Ground Clutter
42. Low Probability of Intercept (LPI) (525)
22. Sources & Spectra of Ground Return (293)
43. Advanced Processor Architecture (535)
23. Effect of Ambiguities on Ground Clutter (309)
24. Ground Moving Target Detection (317) X. Representative Radar Systems (545)
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
It is hoped that you will find this book as interesting bearing aircraft, spanning the history of airborne radar
and enjoyable to read as it was to write. from the Bristol Beaufighter of 1940 to the B-2 Bomber
and F-22 fighter of today.
Key Features
What’s New
As you will undoubtedly find, the book is unique in
several respects. If you’re familiar with the first edition, you may be
First, beginning from scratch, it presents the wide wondering what’s new in the second?
range of airborne radar techniques in the form of an Prompted by the advent of “stealth,” the daunting
unfolding saga, not of individuals, but of radar con- prospect of ever more sophisticated radar countermea-
cepts and principles. Each chapter tells a story, and the sures, and the explosive growth of digital-processing
story flows naturally on from chapter to chapter. throughput, which has made practical many radar tech-
Second, the book is designed to fulfill the needs of all niques long considered “blue sky,” 12 new chapters
who want to learn about radar, regardless of their tech- have been added. Briefly, they cover the following:
nical backgrounds. It has sufficient technical depth
• Electronically steered array antennas (ESAs)—
and mathematical rigor to satisfy the instructor, the
besides providing extreme beam agility, they’re a
engineer, the professor. Yet, as long as a reader has a
“must” for stealth
basic understanding of algebra and knows a little
• Antenna RCS reduction—also a crucial require-
trigonometry and physics, the text painlessly takes the
ment of stealth
reader in bite-sized increments to the point of being
able to talk on a sound footing with the radar experts. • Low-probability of intercept techniques (LPI) —
Third, every technical concept is illustrated with a sim- besides greatly reducing vulnerability to counter-
ple diagram immediately next to the text it relates to. measures, they amazingly enable a radar to detect
Every illustration has a concise caption, which enables targets without its signals being usefully detected
it to stand alone. by an enemy
Fourth, to keep the text simple, where additional detail
• Electronic countermeasures, counter countermea-
may be desired by some readers but not all, it is conve-
sures, and intelligence functions
niently placed in a blue “panel” which one may skip,
on a first reading, and come back to later on and exam- • Multi-frequency operation and small-signal target
ine at leisure. Exceptions, caveats, and reviewers com- detection—also essential in the era of stealth—
ments are presented without detracting from the sim- plus space-time adaptive processing, true-time-
plicity of the text in brief “side notes.” delay beam steering, and 3-D SAR
These features lead to the perhaps most unique aspect
• New modes and approaches to mode control that
of the book. One can follow the development of each take advantage of the ESA’s versatility
chapter by reading just the text, or just the illustrations
• Advanced airborne digital processing architec-
and captions, or by seamlessly moving along between
tures—key to most of the above capabilities
text and illustrations.
Yet another unique feature. Recognizing that people • Detection and tracking of low-speed moving tar-
interested in airborne radar love airplanes, dispersed gets on the ground—an important topic missed in
through the book are photos and renderings of radar- the first edition.
iv
To illustrate the application of the basic radar princi- For the new chapters: Doug Benedict, John Griffith,
ples, the book ends by briefly describing a dozen or so Don Parker, Steve Panaretos, Howard Nussbaum, Robert
airborne radars currently in service in applications rang- Rosen, Bill Posey, John Wittmond, Dave Sjolund, Lee Tower,
ing from long-range surveillance to environmental moni- Larry Petracelli, Robert Frankot,and Irwin Newberg.
toring. I am extremely grateful to Merrill Skolnik and Russell
Also warranting mention, the first three chapters Lefevre (who reviewed an early draft of the second edi-
have been extensively modified to provide a complete tion for the IEEE) for their encouragement and helpful
overview of virtually all of the basic principles and suggestions.
advanced features presented in the body of the book. Also, thanks are due to Hugh Griffiths of University
These chapters may be useful in providing a “stand- College London and his colleagues, Dr. David Belcher
alone” briefing on modern radar for students wanting a and Prof. Chris Oliver of DERA Malvern, for the excel-
quick introduction to the subject. lent SAR maps they provided; and to Gerald Kaiser,
then professor at the University of Massachusetts-
Acknowledgements
Lowell, who on his own initiative in anticipation of the
Needless to say, I’m deeply grateful to the following second edition combed through the first from cover to
engineers of the Hughes Aircraft Company (now a part cover to spot overlooked typos and other errors.
of Raytheon) past and present, who have reviewed vari- In addition, abundant thanks go to Hughes’ ever
ous sections of the book and contributed valuable sug- helpful Al Peña for securing the negatives of the first
gestions, technical information, and insights. edition for reuse in this edition.
For the first edition: Eddie Phillips, Ben DeWaldt, Finally, special thanks to Shyam Reyes, for his invalu-
Nate Greenblatt, Dave Goltzman, Kurt Harrison, Scott able aid with page composition and artwork, and to
Fairchild, Verde Pieroni, Morris Swiger, Jeff Hoffner, John Dudley Kay and Denise May of SciTech, without whom
Wittmond, Fred Williams, Pete Demopolis, Denny Riggs, the publication of this edition would not have been
and Hugh Washburn. possible.
G.W.S., San Marino, California
v
Contents
Part I Overview of Airborne Radar Combining Signals of Different Phase 61
Combining Signals of Different Frequency 62
Chapter 1 Basic Concepts 3
Resolving Signals into I and Q Components 67
Radio Detection 4
Determining Target Position 6 Chapter 6 The Ubiquitous Decibel 71
Exploiting the Doppler Effect 10 What Decibels Are 71
Ground Mapping 11 Converting from Power Ratios to dB 74
Converting from dB to Power Ratios 75
Chapter 2 Approaches to Implementation 15
Representing Power Ratios Less Than One 75
Generic “Pulsed” Radar 15
Using Decibels 76
Generic Pulse-Doppler Radar 25
Power Gain in Terms of Voltage 77
Generic Radar for Stealth 30
Decibels as Absolute Units 77
Chapter 3 Representative Applications 35
Hazardous-Weather Detection 36
Part III Radar Fundamentals
Navigational Aid 36
Ground Mapping 39 Chapter 7 Choice of Radio Frequency 83
Reconnaissance and Surveillance 40 Frequencies Used for Radar 83
Fighter/Interceptor Mission Support 41 Frequency Bands 84
Air-to-Ground Weapon Delivery 43 Influence of Frequency on Radar Performance 85
Short-Range Air-to-Sea Search 45 Selecting the Optimum Frequency 88
Proximity Fuses 45
Chapter 8 Directivity and the Antenna Beam 91
Distribution of Radiated Energy in Angle 91
Part II Essential Groundwork Characteristics of the Radiation Pattern 96
Electronic Beam Steering 100
Chapter 4 Radio Waves and Alternating
Angular Resolution 101
Current Signals 49
Angle Measurement 102
Nature of Radio Waves 49
Antenna Beams for Ground Mapping 106
Characteristics of Radio Waves 52
Chapter 9 Pulsed Operation 107
Chapter 5 Key to a Nonmathematical
Understanding of Radar 59 Advantages of Pulsed Transmission 107
How a Phasor Represents a Signal 59 Pulsed Waveform 108
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CONTENTS
Output Power and Transmitted Energy 111 Part IV Pulse Doppler Radar
Chapter 10 Detection Range 115 Chapter 15 Doppler Effect 189
What Determines Detection Range 115 Doppler Effect and Its Causes 189
Electrical Background Noise 116 Where and How the Doppler Shift Takes Place190
Energy of the Target Signal 122 Magnitude of the Doppler Frequency 192
Detection Process 125 Doppler Frequency of an Aircraft 195
Integration and Its Leverage Doppler Frequency of Ground Return 196
on Detection Range 127
Doppler Frequency Seen by a Semiactive
Postdetection Integration 131 Missile 197
Chapter 11 The Range Equation, What It Chapter 16 Spectrum of Pulsed Signal 199
Does and Doesn’t Tell Us 135 Illustrative Experiments 200
General Range Equation 135 Bandwidth 200
What the Equation Tells Us 138 Coherence 202
Equation for Volume Search 140 Line Width versus Duration of Pulse Train 204
Fluctuations in Radar Cross Section 142 Spectral Sidelobes 206
Detection Probability 142
Chapter 17 Mysteries of Pulsed Spectrum
Cumulative Detection Probability 147
Unveiled 209
Crux of the Matter 209
Chapter 12 Pulse Delay Ranging 151
Fourier Series 213
Basic Technique 151
Spectrum Explained from a Filter’s
Range Ambiguities 153
Point of View 222
Eliminating Ambiguous Return 155
Mathematical Explanation of the Pulsed
Resolving Ambiguities 156
Spectrum 225
Eliminating Ghosts 157
Chapter 18 Sensing Doppler Frequencies 235
How Many PRFs? 159
Doppler Filter Bank 235
Single-Target Tracking 161
Analog Filters 238
Chapter 13 Pulse Compressions 163 Digital Filtering 240
Linear Frequency Modulation (Chirp) 163 Providing Adequate Dynamic Range 248
Binary Phase Modulation 169
Chapter 19 How Digital Filters Work 253
Chapter 14 FM Ranging 177 Inputs to the Filter 253
Basic Principle 177 What the Filter Does 256
Accounting for the Doppler Shift 179 Discrete Fourier Transform 259
Eliminating Ghosts 180 Implementing the DFT 260
Performance 185 Sidelobe Reduction 263
viii
CONTENTS
Filtering Actual Signals 264 Classical DPCA 318
Notching Technique 320
Chapter 20 The Digital Filter Bank
and the FFT 267 Combined Notching and Classical DPCA 321
Basic Concept 268 Precise Angle Measurement 322
A Representative FFT 268
FFTs for Filter Banks of Any Size 274
Part VI Air-to-Air Operation
Rules of Thumb for Estimating Number
of Computations 277 Chapter 25 The Crucial Choice of PRF 325
Primary Consideration: Ambiguities 325
Chapter 21 Measuring Range Rate 281
The Three Basic Categories of PRF 329
Range Differentiation 281
Low PRF Operation 330
Doppler Method 283
Potential Doppler Ambiguities 284 Chapter 26 Low PRF Operation 335
Resolving Doppler Ambiguities 286 Differentiating Between Targets and Clutter 335
Signal Processing 340
Less Sophisticated Signal Processing 346
Part V Return from the Ground
Advantages and Limitations 346
Chapter 22 Sources and Spectra of
Getting Around Limitations 347
Ground Return 293
What Determines the Amplitude of the Chapter 27 Medium PRF Operation 355
Ground Return 294 Differentiating Between Targets and Clutter 355
Mainlobe Return 296
Signal Processing 359
Sidelobe Clutter 299
Rejecting Ground Moving Targets (GMTs) 360
Altitude Return 302
Eliminating Blind Zones 361
Relation of Clutter Spectrum to Target
Minimizing Sidelobe Clutter 364
Frequencies 303
Sidelobe Return from Targets of Large RCS 365
Return from Objects on the Terrain 306
Chapter 28 High PRF Operation 369
Chapter 23 Effect of Range and Doppler
High PRF Waveform 370
Ambiguities on Ground Clutter 309
Isolating the Target Returns 370
Dispersed Nature of the Clutter 310
Mechanization 373
Range Ambiguities 311
Ranging 375
Doppler Profile 314
Problem of Eclipsing 376
Doppler Ambiguities 314
Improving Tail Aspect Performance 378
Chapter 24 Separating Ground-Moving
Targets from Clutter 317 Chapter 29 Automatic Tracking 383
Problem of Detecting “Slow” Moving Targets 317 Single-Target Tracking 383
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