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Monopulse Principles and Techniques, Second Edition, Samuel M

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Monopulse Princi ples and Techniques Sec ond Edi tion For a list ing of re cent ti tles in the Artech House Radar Series, turn to the back of this book. Monopulse Princi ples and Techniques Sec ond Edi tion Sam uel M. Sherman Da vid K. Barton artechhouse.com Li brary of Con gress Cataloging- in- Publication Data A cata log record for this book is avail able from the U.S. Li brary of Con gress. Brit ish Li brary Cat a logu ing in Pub li ca tion Data A cat a logue record for this book is avail able from the British Li brary. ISBN-13 978-1-60807-174-6 Cover de sign by Vicki Kane © 2011 ARTECH HOUSE 685 Can ton Street Norwood, MA 02062 All rights reserved. Printed and bound in the United States of Amer ica. No part of this book may be repro duced or uti lized in any form or by any means, electronic or mechanical, in clud ing pho - to copy ing, re cord ing, or by any in for ma tion stor age and retrieval sys tem, with out per mis sion in writ ing from the pub lisher. All terms men tioned in this book that are known to be trade marks or service marks have been ap pro pri ately cap i tal ized. Artech House can not at test to the accu racy of this in forma tion. Use of a term in this book should not be re garded as affect ing the va lid ity of any trademark or ser vice mark. 10 9 8 7 6 5 4 3 2 1 Contents Preface to the Second Edition . xiii Preface to the First Edition . xv Chapter 1 Introduction . 1 1.1 Review of Radar Principles . 1 1.2 Tracking Radars and the Evolution of Monopulse . 2 1.3 A “Baseline” Monopulse Radar . 6 1.4 Advantages and Disadvantages of Monopulse . 13 1.5 Nonradar Uses of Monopulse . 15 References . 16 Chapter 2 Terminology, Definitions, and Notation . 19 2.1 The Meaning of Monopulse . 20 2.2 Apertures and Illumination Functions . 22 2.3 Patterns, Lobes, and Beams . 22 2.4 Sum and Difference Patterns . 24 2.5 Sum and Difference Notation . 26 2.6 Error Signals . 27 2.7 Complex Signal Representation and Complex Envelopes . 27 2.8 Elevation, Azimuth, and Traverse . 31 References . 38 Chapter 3 The Monopulse Output as a Complex Ratio . 39 3.1 General Principles . 39 3.2 Relative Phase of Difference and Sum . 41 3.3 Some Useful Relationships and Formulas . 41 References . 44 v vi Monopulse Principles and Techniques Chapter 4 Components Used in Monopulse . 45 4.1 Antenna Mounts . 45 4.2 Antennas . 46 4.2.1 Lens Antennas . 47 4.2.2 Single-Reflector Antennas . 50 4.2.3 Cassegrain (Double Reflector) Antennas . 51 4.2.4 Polarization-Twisting Reflector Systems . 52 4.3 Feeds . 54 4.4 Devices for Forming Sums and Differences . 57 4.4.1 Hybrid Junctions . 57 4.4.2 Directional Couplers . 60 4.4.3 Comparators . 63 4.4.4 Other Methods of Obtaining Monopulse Sum (or Reference) and Differences . 65 4.4.5 Graphical Symbols . 66 4.5 Receivers . 68 References . 69 Chapter 5 Amplitude-Comparison and Phase-Comparison Classification . 71 5.1 Definitions and Examples . 72 5.1.1 Definitions . 72 5.1.2 Interpretation . 72 5.1.3 Optimum Monopulse . 74 5.1.4 Amplitude-Comparison Monopulse Antenna Example . 78 5.1.5 Phase-Comparison Monopulse Antenna Example . 80 5.2 Phase Fronts, Phase Centers, and Related Concepts . 82 5.2.1 Transmitting Antennas . 82 5.2.2 Receiving Antennas . 85 5.2.3 Targets . 85 5.2.4 An Example: Phase Fronts of a Two-Element Source . 86 5.2.5 Simplification at Large Distances . 87 5.2.6 Phase Centers of Array Antennas . 90 5.2.7 Phase Centers of Reflector Antennas . 93 5.3 Distinguishing Between Phase- and Amplitude- Comparison Monopulse . 93 5.3.1 Reflector Antenna Examples . 93 5.4 Distinction Based on Relative Phase of Illumination Functions . 98 5.5 Distinction Based on Sum and Difference Patterns . 99 5.6 Apparent Conversion of One Class to the Other . .100 Contents vii 5.7 Summary of Amplitude-Comparison and Phase- Comparison Classification . 102 References . 103 Chapter 6 Optimum Feeds for Space-Fed Amplitude-Comparison Monopulse Antennas . 105 6.1 Nature of Optimization . 105 6.2 f/D Ratio . 107 6.3 Effect of Squint Angle in a Four-Horn Feed . 108 6.4 Optimization of Squint Angle . 111 6.5 Comparison with Measured Patterns . 113 6.6 Beacon Operation . 116 6.7 Comparison of Beam Squint Angle and Feed Offset Angle . 116 6.8 Effect of Squint Angle on Normalized Difference Pattern . 117 6.9 Other Feed Configurations . 118 6.10 Summary of Feed Optimization . 122 References . 123 Chapter 7 Monopulse in Array Antennas . 125 7.1 Principles of Operation . 125 7.2 Array Coordinates . 128 7.3 Arrays with Space Feeds . 128 7.4 Arrays with Constrained Feeds . 132 7.4.1 Constrained-Fed Arrays Divided into Quadrants or Subarrays . 132 7.4.2 Constrained-Fed Arrays with Independent Illuminations . 134 7.4.3 Efficient Illumination Tapers for Phased Arrays . 136 7.5 Classification as Amplitude or Phase Comparison . 138 7.6 Special Types of Arrays . 141 References . 141 Chapter 8 Monopulse Processors . 143 8.1 Functions and Properties of Monopulse Processors 144 8.2 Range Gating . 146 8.3 Angular Coordinates for Monopulse Calibration . 147 8.4 Exact Monopulse Processor . 148 8.5 Processor Using Phases and Linear Amplitudes of s and d . 150 8.6 Processor Using I and Q . 152 viii Monopulse Principles and Techniques 8.7 Processor Using Phases and Logarithmic Amplitudes of s and d . 153 8.8 Processor Using Dot-Product Detector with AGC . 155 8.9 An Approximate Dot-Product Detector . 158 8.10 Noncoherent Processor Using Sum and Difference of |v1| and |v2| . 162 8.11 Processor Using s + d and s − d . 165 8.12 Processor Using log |v1| and |v2| . 166 8.13 Processor Using s ± jd . 170 8.14 Two-Channel Monopulse Using s + d and s − d . 174 8.15 Phase-Amplitude Monopulse . 177 8.16 Multiplexed Monopulse Receivers . 178 8.17 Conopulse . 181 8.18 Summary of Monopulse Processors . 182 References . 185 Chapter 9 Response to Unresolved Targets . 187 9.1 Review of Monopulse Response to a Point Target . 190 9.2 The Meaning of Unresolved Targets . 191 9.3 Superposition as an Approximation . 191 9.4 The Two-Target Problem . 193 9.5 The Complex Indicated Angle . 195 9.6 Physical Interpretation . 200 9.7 Measurement of the Imaginary Part (Quadrature- Phase) Component . 201 9.8 Effect of Local-Oscillator Frequency . 203 9.9 Detection of Presence of Unresolved Targets . 203 9.10 Mean and Variance of Indicated Angle . 204 9.11 Weighted Mean of Indicated Angle . 207 9.12 Possibility of Determining Angles of Unresolved Targets . 209 9.13 Information Derivable from Real Part . 214 9.14 Removal of Initial Assumptions . 216 9.15 Extensions of Monopulse Techniques and Fundamental Limitations . 217 9.16 Closed-Loop Tracking . 218 9.17 More Than Two Targets . 219 9.18 Nonindependent Targets . 221 References . 222 Chapter 10 Monopulse Angle Errors . 225 10.1 Error Due to Noise . 226 10.1.1 Analytical Model . 227 Contents ix 10.1.2 Noise Statistics . 227 10.1.3 Conversion of Voltages to Powers . 229 10.1.4 Conversion of d/s Error to Angular Error . 229 10.1.5 Bias in Monopulse Ratio . 231 10.1.6 Exact Solution for Probability Density Function of d/s . 233 10.1.7 First-Order Approximation for S/N >> 1 235 10.1.8 Higher-Order Approximation for S/N > 1 238 10.1.9 Multiple-Pulse Estimates . 241 10.1.10 Fluctuating Targets . 242 10.1.11 “Inexact” Monopulse Processors . 247 10.1.12 Closed-Loop Versus Open-Loop Operation 247 10.2 Errors Due to Clutter . 247 10.2.1 Random Error from Clutter Distributed Across the Beam . 248 10.2.2 Number of Independent Clutter Samples 249 10.2.3 Random Error from Clutter at a Specific Angle in the Beam . 250 10.2.4 Bias Error Due to Clutter . 252 10.3 Dynamic Lag Error . 252 10.3.1 Tracking-Loop Error Coefficients . 252 10.3.2 Pass-Course Problem . 254 10.4 Radar-Dependent Errors . 256 10.4.1 Monopulse Network Effects on the Boresight Axis . 256 10.4.2 Boresight Axis Shift with Radar Frequency 259 10.4.3 Polarization Effects . 261 10.5 Target-Dependent Noise Errors . 263 10.5.1 Glint Error . 263 10.5.2 Scintillation Error . 266 References . 269 Chapter 11 Multipath . 271 11.1 Flat-Earth Specular Model . 272 11.2 Effect on Detection . 276 11.3 Effect on Closed-Loop Elevation Tracking . 277 11.4 Types of Multipath Remedies . 280 11.5 Beam Pattern Design . 281 11.6 Range and Doppler Resolution . 282 11.7 Smoothing and Averaging . 285 11.8 “Low-E” Modes . 286 11.9 Offset-Null Tracking . 288 x Monopulse Principles and Techniques 11.10 Elevation Patterns Having a Symmetrical Ratio . 288 11.11 Double-Null Tracker . 293 11.12 Use of the Complex Indicated Angle . 294 11.13 Independent-Target Methods . 301 11.14 Diffuse Multipath Effects on Monopulse . 302 11.14.1 Power of Specular and Diffuse Multipath Reflections . 303 11.14.2 Distribution of Multipath in Elevation Angle . 305 11.14.3 Multipath Error for Closed-Loop Tracking 305 11.14.4 Effect of Range or Doppler Resolution and Smoothing .
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