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Introduction to airborne radar
INTRODUCTION TO AIRBORNE RADAR GEORGE W. STIMSON SECOND EDITION MENDHAM, NEW JERSEY ii Acquisition and Product Development: Dudley R. Kay Production and Manufacturing Services: Denise G. May Illustrations and Layout: George Stimson and Shyam Reyes Cover Design: Carolyn Allen - IntelliSource Publishing and elaine kilcullen Page Composition by Lehigh Press Colortronics Printed by World Color Book Services ©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 embodied in critical articles and reviews. For information, contact the publisher, SciTech Publishing, Inc., 89 Dean Road, Mendham, NJ 07945. Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 ISBN 1-891121-01-4 SciTech Publishing, Inc. 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Six Hills Way, Stevenage, SGI 2AY, UK PO Box 10, Bellingham, PO Box 1331, 445 Hoes Lane Phone: +44 (0) 1438 313311 WA 98227-0010 USA Piscataway, NJ 08855-1331 USA Fax: +44 (0) 1438 313465 Phone: (360) 676-3290 Phone: (800) 678-IEEE E-mail: books@iee.org.uk Fax: (360) 647-1445 Fax: (732) 981-9667 http://www.iee.org.uk E-mail: book orders@spie.org E-mail: customer.service@ieee.org IEE Order No.: RA 101 http://www.spie.org http://www.ieee.org SPIE Order No.: PM56 IEEE Order No.: PC5744 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 [...]... but powerful radar enables the pilot to home in on an intruder hidden behind or in a cloud bank a hundred and fifty miles away 1 Radar = Radio Detection And Ranging Click for high-quality image 2 Rather than rejecting echoes from the ground, as when searching for airborne targets, the radar may use them to produce real-time high-resolution maps of the terrain PART I Overview of Airborne Radar Radio... takes to scan the complete frame, the frame time (Fig 7) Incidentally, in the world of radar the term target is broadly used to refer to almost anything one wishes to detect: an aircraft, a ship, a vehicle, a man-made structure on the ground, a specific point in the terrain, rain (weather radars), aerosols, even free electrons Like light, radio waves of the frequencies used by most airborne radars... generic type is a pulse-doppler radar tailored to meet the special requirements of stealth aircraft Click for high-quality image 1 Generic “Pulsed” Radar PULSED RADAR Duplexer Modulator Transmitter Synchronizer lay sp Di This radar (Fig 1) is capable of automatic searching, single-target tracking, and real-beam ground mapping In the previous chapter, we learned that a pulsed radar consists of four basic... Protection Device Indicator Servo Synchronizer This unit synchronizes the operation of the transmitter and the indicator by generating a continuous stream of very short, evenly spaced pulses They designate the times at which successive radar pulses are to be transmitted and are supplied to the modulator and indicator 15 Controls Power Supply 2 Elements outlined in blue must be added to the transmitter,... simple pulsed radar consists of a single feed and a parabolic “dish” reflector, which forms the transmitted beam and reflects the returned echoes into the feed From Antenna From Antenna (a) (b) To Receiver 7 To Receiver Receiver protection device: (a) allows the weak echoes to pass from the duplexer to the receiver with negligible attenuation; but, (b) blocks any signals strong enough to damage the... voltage that causes the beam to trace a vertical path from the bottom of the display to the top Since the start of each trace is thus synchronized with the transmission of a radar pulse, if a target echo is received, the distance from the start of the trace to the point at which the target blip appears will correspond to the round-trip transit time for the echo, hence to the target’s range For this... the target he wishes to track To lock onto a target, the operator takes control of the antenna with the hand control, aligns the antenna in azimuth so as to center the range trace on the target blip, adjusts the elevation of the antenna to maximize the brightness of the blip, runs the marker up the trace until it is just under the blip, and presses the lock-on button In the indicator, the circuit that... closed) just long enough to allow the target echo to pass through and into the automatic tracking circuit When the lock-on switch is depressed, control of the range gate is transferred to an automatic range tracking circuit (see panel below) which keeps the gate continuously centered on the target Click for high-quality image 13 Hand control for a simple pulsed radar Operator gains control of antenna... three generic radars First is a radar of the sort used by the all-weather interceptors of the 1950s and 1960s, called simply a “pulsed” radar In different configurations, it still is used today The second generic type is a far more capable one, called a “pulse-doppler” radar It is the kind used in the current generation of conventional fighter and attack aircraft In various forms, it too has a variety... fighter/attack aircraft Radar display is in upper right side of instrument panel Combining glass for head-up display is in center of windscreen Stored map for navigation is projected on display at lower center Indicator The indicator contains all of the circuitry needed to: (a) display the received echoes in a format that will satisfy the operator’s requirements; (b) control the automatic searching and . 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. 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. 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