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AEROSPACE
SENSORS
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Aerospace Sensors
Copyright Momentum Press, LLC, 2013.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,
or transmitted in any form or by any meanselectronic, mechanical, photocopy, recording, or
any otherexcept for brief quotations, not to exceed 400 words, without the prior permission
of the publisher.
First published by Momentum Press, LLC
222 East 46th Street, New York, NY 10017
www.momentumpress.net
ISBN-13: 978-1-60650-059-0 (paperback)
ISBN-10: 1-60650-059-7 (paperback)
ISBN-13: 978-1-60650-061-3 (e-book)
ISBN-10: 1-60650-061-9 (e-book)
DOI: 10.5643/9781606500613
Cover design by Jonathan Pennell
Interior design by Exeter Premedia Services Private Ltd.,
Chennai, India
10 9 8 7 6 5 4 3 2 1
Printed in the United States of America
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v
CONTENTS
SERIES PREFACE xvii
PREFACE xix
ACKNOWLEDGMENTS xxi
ABOUTTHE SERIES EDITOR xxiii
ABOUTTHE EDITOR xxv
1 INTRODUCTION 1
1.1 General Considerations 1
1.1.1 Types of Aerospace Vehicles and Missions 1
1.1.2 The Role of Sensors and Control Systems in Aerospace 3
1.1.3 Specific Design Criteria for Aerospace Vehicles and their Sensors 3
1.1.4 Physical Principles Influencing Primary Aerospace Sensor Design 5
1.1.5 Reference Frames Accepted in Aviation and Astronautics 7
1.2 Characteristics and Challenges of the Atmospheric Environment 10
1.2.1 Components of the Earths Atmosphere 10
1.2.2 Stationary Models of the Atmosphere 11
1.2.3 Anisotropy and Variability in the Atmosphere 11
1.2.4 Electrical Charges in the Atmosphere 12
1.2.5 Electromagnetic Wave Propagation in the Atmosphere 12
1.2.6 Geomagnetism 131.2.7 The Planetary Atmosphere 14
1.3 Characteristics and Challenges of the Space Environment 14
1.3.1 General Considerations 14
1.3.2 Near-Earth Space 15
1.3.3 Circumsolar (Near-Sun) Space 16
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1.3.4 Matter in Space 16
1.3.5 Distances and Time Scales in Deep Space 16
References 17
2 AIRPRESSURE-DEPENDENT SENSORS 19
2.1 Basic Aircraft Instrumentation 19
2.2 Fundamental Physical Properties of Airflow 19
2.2.1 Fundamental Airflow Physical Property Definitions 20
2.2.1.1 Pressure 20
2.2.1.2 Air Density 21
2.2.1.3 Temperature 21
2.2.1.4 Flow Velocity 23
2.2.2 The Equation of State for a Perfect Gas 24
2.2.3 Extension of Definitions: Total, Dynamic, Static, and Stagnation 25
2.2.4 The Speed of Sound and Mach Number 26
2.2.4.1 The Speed of Sound 26
2.2.4.2 Mach Number and Compressibility 27
2.2.5 The Source of Aerodynamic Forces 28
2.3 Altitude Conventions 29
2.4 Barometric Altimeters 30
2.4.1 Theoretical Considerations 32
2.4.1.1 The Troposphere 32
2.4.1.2 The Stratosphere 33
2.4.2 Barometric Altimeter Principles and Construction 34
2.4.3 Barometric Altimeter Errors 37
2.4.3.1 Methodical Errors 37
2.4.3.2 Instrumental Errors 37
2.5 Airspeed Conventions 38
2.6 The Manometric Airspeed Indicator 39
2.6.1 Manometric Airspeed Indicator Principles and Construction 40
2.6.2 Theoretical Considerations 42
2.6.2.1 Subsonic Incompressible Operation 42
2.6.2.2 Subsonic Compressible Operation 42
2.6.2.3 Supersonic Operation 43
2.6.3 Manometric Airspeed Indicator Errors 44
2.6.3.1 Methodical Errors 44
2.6.3.2 Instrumental Errors 45
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CONTENTS vii
2.7 The Vertical Speed Indicator (VSI) 46
2.7.1 VSI Principles and Construction 46
2.7.2 Theoretical Considerations 47
2.7.2.1 Lag Rate (Time Constant) 47
2.7.2.2 Sensitivity to Mach Number 48
2.7.2.3 Sensitivity to Altitude 48
2.7.3 VSI Errors 48
2.8 Angles of Attack and Slip 49
2.8.1 The Pivoted Vane 50
2.8.2 The Differential Pressure Tube 50
2.8.3 The Null-Seeking Pressure Tube 52
References 52
Appendix 53
3 RADARALTIMETERS 55
3.1 Introduction 55
3.1.1 Definitions 55
3.1.2 Altimetry Methods 55
3.1.3 General Principles of Radar Altimetry 56
3.1.4 Classification by Different Features 57
3.1.5 Application and Performance Characteristics 58
3.1.5.1 Aircraft Applications 58
3.1.5.2 Spacecraft Applications 58
3.1.5.3 Military Applications 59
3.1.5.4 Remote Sensing Applications 59
3.1.6 Performance Characteristics 59
3.2 Pulse Radar Altimeters 60
3.2.1 Principle of Operation 60
3.2.2 Pulse Duration 60
3.2.3 Tracking Altimeters 61
3.2.4 Design Principles 63
3.2.5 Features of Altimeters with Pulse Compression 64
3.2.6 Pulse Laser Altimetry 65
3.2.7 Some Examples 66
3.2.8 Validation 66
3.2.9 Future Trends 67
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3.3 Continuous Wave Radar Altimeters 68
3.3.1 Principles of Continuous Wave Radar 68
3.3.2 FMCW Radar Waveforms 69
3.3.3 Design Principles and Structural Features 71
3.3.3.1 Local Oscillator Automatic Tuning 72
3.3.3.2 Single-Sideband Receiver Structure 73
3.3.4 The Doppler Effect 74
3.3.5 Alternative Measuring Devices for FMCW Altimeters 75
3.3.6 Accuracy and Unambiguous Altitude 75
3.3.7 Aviation Applications 77
3.4 Phase Precise Radar Altimeters 783.4.1 The Phase Method of Range Measurement 78
3.4.2 The Two-Frequency Phase Method 78
3.4.3 Ambiguity and Accuracy in the Two-Frequency Method 79
3.4.4 Phase Ambiguity Resolution 80
3.4.5 Waveforms 80
3.4.6 Measuring Devices and Signal Processing 80
3.4.7 Remarks on the Accuracy of CW and Pulse Radar Altimeters 81
3.5 Radioactive Altimeters for Space Application 81
3.5.1 Motivation and History 81
3.5.2 Physical Bases 82
3.5.2.1 Features of Radiation 82
3.5.2.2 Generators of Photon Emission 83
3.5.2.3 Receivers 83
3.5.2.4 Propagation Features 83
3.5.3 Principles of Operation 84
3.5.4 Radiation Dosage 85
3.5.5 Examples of Radioisotope Altimeters 85
References 86
4 AUTONOMOUS RADIO SENSORSFORMOTION PARAMETERS 89
4.1 Introduction 89
4.2 Doppler Sensors for Ground Speed and Crab Angle 90
4.2.1 Physical Basis and Functions 90
4.2.2 Principle of Operation 91
4.2.3 Classification and Features of Sensors for Ground Speed and Crab Angle 92
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CONTENTS ix
4.2.4 Generalized Structural Diagram for the Ground Speed and
Crab Angle Meter 93
4.2.5 Design Principles 94
4.2.6 Sources of Doppler Radar Errors 95
4.2.7 Examples 95
4.3 Airborne Weather Sensors 96
4.3.1 Weather Radar as Mandatory Equipment of Airliners and Transport Aircraft 96
4.3.2 Multifunctionality of Airborne Weather Radar 96
4.3.3 Meteorological Functions of AWR 98
4.3.4 Principles of DWP Detection with AWR 98
4.3.4.1 Developing Methods of DWP Detection 984.3.4.2 Cumulonimbus Clouds and Heavy Rain 100
4.3.4.3 Turbulence Detection 100
4.3.4.4 Wind Shear Detection 102
4.3.4.5 Hail Zone Detection 103
4.3.4.6 Probable Icing-in-flight Zone Detection 104
4.3.5 Surface Mapping 104
4.3.5.1 Comparison of Radar and Visual Orientation 104
4.3.5.2 The Surface-Mapping Principle 105
4.3.5.3 Reflecting Behavior of the Earths Surface 106
4.3.5.4 The Radar Equation and Signal Correction 107
4.3.5.5 Automatic Classification of Navigational Landmarks 107
4.3.6 AWR Design Principles 108
4.3.6.1 The Operating Principle and Typical Structure of AWR 108
4.3.6.2 AWR Structures 109
4.3.6.3 Performance Characteristics: Basic Requirements 110
4.3.7 AWR Examples 111
4.3.8 Lightning Sensor Systems: Stormscopes 114
4.3.9 Optical Radar 114
4.3.9.1 Doppler Lidar 115
4.3.9.2 Infrared Locators and Radiometers 1154.3.10 The Integrated Localization of Dangerous Phenomena 115
4.4 Collision Avoidance Sensors 116
4.4.1 Traffic Alert and Collision Avoidance Systems (TCAS) 116
4.4.1.1 The Purpose 116
4.4.1.2 A Short History 117
4.4.1.3 TCAS Levels of Capability 117
4.4.1.4 TCAS Concepts and Principles of Operation 119
4.4.1.5 Basic Components 122
4.4.1.6 Operation 123
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4.4.1.7 TCAS Logistics 124
4.4.1.8 Cockpit Presentation 126
4.4.1.9 Examples of System Implementation 126
4.4.2 The Ground Proximity Warning System (GPWS) 127
4.4.2.1 Purpose and Necessity 127
4.4.2.2 GPWS History, Principles, and Evolution 127
4.4.2.3 GPWS Modes 128
4.4.2.4 Shortcomings of Classical GPWS 129
4.4.2.5 Enhanced GPWSs 129
4.4.2.6 Look-Ahead Warnings 130
4.4.2.7 Implementation Examples 130
References 131
5 DEVICESAND SENSORSFORLINEARACCELERATION MEASUREMENT 137
5.1 Introduction 137
5.2 Types of Accelerometers 137
5.2.1 Linear and Pendulous Accelerometers 137
5.2.2 Direct Conversion Accelerometers and Compensating Accelerometers 138
5.2.2.1 Direct Conversion Accelerometers 138
5.2.2.2 Compensating Accelerometers 140
5.3 Accelerometer Parameters 143
5.3.1 Acceleration Measurement Range azmax 143
5.3.2 Resolution azmin
143
5.3.3 Zero Signal (bias) a0
143
5.3.4 Scale FactorKa
145
5.3.5 Biasing Error (Misalignment) 146
5.3.6 Accelerometer Frequency Characteristics 147
5.3.7 Special Accelerometer Parameters 147
5.3.7.1 Magnetic Leakage 148
5.3.7.2 Electromagnetic Noise 148
5.3.7.3 Readiness Time 148
5.3.7.4 Noise Level in the Accelerometer Output 148
5.3.7.5 Sensitivity to External Constant and Variable
Magnetic Fields 148
5.3.7.6 Sensitivity to Changes in Power Supply Voltage 149
5.3.7.7 Sensitivity to External Pressure, Humidity, and Radiation 149
5.4 Float Pendulous Accelerometer (FPA) 149
5.4.1 Basic EMU Design Schemes 150
5.4.1.1 Advantages 151
5.4.1.2 Disadvantages 152
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CONTENTS xi
5.4.2 Hydrostatic Accelerometer Suspensions 154
5.4.3 FPA Float Balancing 155
5.4.4 Hydrodynamic Forces and Moments in the FPA 158
5.4.5 Movement of FPA Float Under Vibration 160
5.5 Micromechanical Accelerometers (MMAs) 161
5.5.1 The Single-Axis MMA 161
5.5.2 The Three-Axis MMA 162
5.5.3 The Compensating Type MMA 163
5.5.4 Solid-State MMA Manufacturing Techniques 164
References 165
6 GYROSCOPIC DEVICESAND SENSORS 167
6.1 Introduction 167
6.1.1 Preliminary Remarks 167
6.1.2 Classification of Gyros 169
6.1.3 Gyroscopic Instruments 169
6.1.4 Positional Gyros 170
6.1.5 The Vertical (or Horizontal) Gyro 171
6.1.6 Orbit Gyro 171
6.1.7 Single Degree of Freedom (SDF) Gyros 171
6.1.8 Gyro Stabilizers 172
6.1.9 Gyroscopic Instruments in Aeronavigation 172
6.1.10 Inertial Navigation Systems (INS) 173
6.1.10.1 Types of INS 173
6.1.10.2 Strapdown INS 174
6.1.11 The Scope of Gyros and Gyro Instruments of Various Types 175
6.2 Single Degree of Freedom (SDF) Gyros 177
6.2.1 The Solid Rotor SDF Gyro 177
6.2.2 The Integrating Gyro 178
6.2.3 Rate of Speed Gauging 178
6.2.3.1 Feedback Contours of the Angular Rate Gauge 179
6.2.3.2 Design Variants 180
6.3 The TDF Gyro in Gimbal Mountings 181
6.3.1 Properties of a Free Gyro 181
6.3.2 Areas of Application, Design Features, and Error Sources 183
6.3.3 Two-Component Angular Speed Measuring Instruments 185
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6.4 The Gyroscopic Integrator for Linear Acceleration (GILA) 186
6.4.1 Principles of GILA Operation 186
6.4.2 Sources of GILA Errors 188
6.5 Contactless Suspension Gyros 189
6.5.1 Introduction 189
6.5.2 The Electrostatic Gyroscope (ESG) 189
6.5.2.1 ESG Accuracy 191
6.5.2.2 The ESG Rotor 192
6.5.2.3 The Rotor Electrostatic Suspension 192
6.5.2.4 Angular Rotor Position Readout 193
6.5.3 Conclusion 195
6.6 The Fiber Optic Gyro (FOG) 195
6.6.1 The Interferometric Fiber Optic Gyro (IFOG) 196
6.6.1.1 The Basic IFOG Scheme and the Sagnac Effect 196
6.6.1.2 Open-Loop Operation 197
6.6.1.3 Closed-Loop Operation 197
6.6.1.4 Fundamental Limitations 198
6.6.1.5 The Multiple-Axis IFOG 199
6.6.1.6 The Depolarized IFOG 199
6.6.1.7 Applications of the IFOG 200
6.6.2 The Resonator Fiber Optic Gyro (RFOG) 200
6.7 The Ring Laser Gyro (RLG) 202
6.7.1 Introduction 202
6.7.2 Principle of Operation 203
6.7.3 Frequency Characteristics and Mode-Locking Counter-Rotating Waves 204
6.7.4 The Elimination of Mode-Locking in Counter-Rotating Waves 205
6.7.5 Errors 206
6.7.6 Performance and application 207
6.7.7 Conclusion 207
6.8 Dynamically Tuned Gyros (DTG) 208
6.8.1 Introduction 208
6.8.2 Key Diagrams and Dynamic Tuning 208
6.8.3 Operating Modes 210
6.8.4 Disturbance Moments Depending on External Factors and
Instrumental Errors 212
6.8.5 Magnetic, Aerodynamic, and Thermal Disturbance Moments 213
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CONTENTS xiii
6.8.6 Design, Application, Technical Characteristics 214
6.8.7 Conclusion 215
6.9 Solid Vibrating Gyros 215
6.9.1 Introduction 215
6.9.2 Dynamic Behavior of the Ideal Solid Vibrating Gyro 217
6.9.3 Operating Modes of the Solid Vibrating Gyro 218
6.9.4 The Nonideal Solid Vibrating Gyro 218
6.9.5 Control of the Solid Vibrating Gyro 221
6.9.6 Axisymmetric-Shell Gyros 221
6.9.7 The HRGHistory and Current Status 222
6.9.8 HRG Design Characteristics 223
6.9.9 Additional HRG References 225
6.10 Micromechanical Gyros 225
6.10.1 Introduction 225
6.10.2 Operating Principles 226
6.10.2.1 Linear-Linear (LL-type) Gyros 226
6.10.2.2 Rotary-Rotary (RR-type) Gyro Principles 228
6.10.2.3 Fork and Rod Gyro Principles 230
6.10.2.4 Ring Gyro Principles 231
6.10.3 Adjustment of Oscillation Modes in Gyros of the LL and RR Types 233
6.10.4 Design, Application, and Performance 235
6.10.4.1 Gyros of the LL and RR-type 235
6.10.4.2 Fork and Rod Gyros 236
6.10.4.3 Ring Gyros 237
6.10.5 Conclusion 239
References 239
7 COMPASSES
245
7.1 Introduction 245
7.2 Magnetic Compasses 247
7.2.1 Brief Historical Sketch 247
7.2.2 The Earths Magnetic Field 249
7.2.3 Magnetic Compass Design Principles and Errors 254
7.2.4 Examples of Magnetic Compasses Structures 257
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7.3 Fluxgate and Gyro-Magnetic Compasses 259
7.3.1 Fluxgate and Gyro-Magnetic Compasses Design Principles 259
7.3.2 Examples of Fluxgate and Gyro-Magnetic Structures 261
7.4 Electronic Compasses 264
References 265
8 PROPULSION SENSORS 267
8.1 Introduction 267
8.2 Fuel Quantity Sensors 267
8.2.1 Mechanical and Electromechanical Methods of Level Sensing 2688.2.1.1 Buoyancy or Float Methods 268
8.2.1.2 Level Sensing Using Pressure Transducers 268
8.2.2 Electronic Methods of Level Sensing 269
8.2.2.1 Conductivity Level Sensing 269
8.2.2.2 Capacitive Level Sensing 269
8.2.2.3 Heat-Transfer Level Sensing 270
8.2.2.4 Ultrasonic Methods 271
8.3 Fuel Consumption Sensors 273
8.3.1 Introduction 273
8.3.2 Flow-Obstruction Methods 273
8.3.2.1 Practical Considerations for Obstruction Meters 275
8.3.3 The Turbine Flow Meter 275
8.3.4 The Vane-Type Flow Meter 277
8.4 Pressure Sensors 278
8.4.1 Basic Concepts 278
8.4.2 Basic Sensing Methods 279
8.4.2.1 The Diaphragm 2798.4.2.2 Capsules 280
8.4.2.3 The Bourdon Tube 280
8.4.3 Signal Acquisition 280
8.4.3.1 Capacitive Deflection Transducers 281
8.4.3.2 Inductive Deflection Transducers 281
8.4.3.3 Potentiometric Deflection Transducers 282
8.4.3.4 Null-Balance Servo Pressure Transducers 282
8.4.4 Operational Requirements 283
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CONTENTS xv
8.5 Engine Temperatures 284
8.5.1 Intermediate Turbine Temperature (ITT) 285
8.5.2 Oil Temperature/Fuel Temperature 287
8.5.3 Fire Sensors 287
8.5.4 Exhaust Gas Temperature (EGT) 288
8.5.5 Nacelle Temperature 288
8.6 Tachometry 289
8.6.1 The Eddy Current Tachometer 289
8.6.2 The AC Generator Tachometer 290
8.6.3 The Variable Reluctance Tachometer 291
8.6.4 The Hall Effect Tachometer 292
8.7 Vibration SensorsEngine and Nacelle 292
8.8 Regulatory Issues 295
References 296
Bibliography 296
9 PRINCIPLESAND EXAMPLESOF SENSORINTEGRATION 297
9.1 Sensor Systems 297
9.1.1 The Sensor System Concept 297
9.1.2 Joint Processing of Readings from Identical Sensors 300
9.1.3 Joint Processing of Readings from Cognate Sensors with
Different Measurement Ranges 301
9.1.4 Joint Processing of Diverse Sensors Readings 302
9.1.5 Linear and Nonlinear Sensor Integration Algorithms 303
9.2 Fundamentals of Integrated Measuring System Synthesis 305
9.2.1 Synthesis Problem Statement 305
9.2.2 Classes of Dynamic System Realization 305
9.2.3 Measurement Accuracy Indices 306
9.2.4 Excitation Properties 307
9.2.5 Objective Functions for Robust System Optimisation 309
9.2.6 Methods of Dynamic System Accuracy Index Analysis Under
Excitation with Given Numerical Characteristics of Derivatives 311
9.2.6.1 Estimation of Error Variance 311
9.2.6.2 Example of Error Variance Analysis 313
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9.2.6.3 Use of Equivalent Harmonic Excitation 314
9.2.6.4 Estimation of Error Maximal Value 315
9.2.7 System Optimization Under Maximum Accuracy Criteria 316
9.2.8 Procedures for the Dimensional Reduction of a Measuring System 318
9.2.8.1 Determination of an Optimal Set of Sensors 318
9.2.8.2 Analysis of the Advantages of Invariant System Construction 319
9.2.8.3 Advantages of the Zeroing of Several System Parameters 320
9.2.9 Realization and Simulation of Integration Algorithms 320
9.3 Examples of Two-Component Integrated Navigation Systems 322
9.3.1 Noninvariant Robust Integrated Speed Meter 322
9.3.2 Integrated Radio-Inertial Measurement 326
9.3.3 Airborne Gravimeter Integration 327
9.3.4 The Orbital Verticant 333
References 336
EPILOGUE 339
INDEX 341
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xvii
SERIES PREFACE
The present series is concerned with sensorsper se, and because the subject matter is so wide-
ranging in both scope and maturity, this must be reflected within the individual volumes. So,
whereas care has been taken to include a considerable amount of practical material, the pro-
portion of such leavening is inevitably variable. The present volume will be found to includematerial on the basic processes that are addressed by the sensors used in most aspects of aero-
space technology, plus considerable detail on the relevant sensors themselves and their appli-
cations. In the context of aerospace engineering, however, there are many items of complex
equipmentmostly radio and navigationally orientedthat can be considered as sensors in
their own right. This situation has been addressed in a companion volume that is in production
at the time of writing, and will act as an adjunct to the present work.
Sensors cannot, of course, be divorced from their associated instrumentation, and prob-
lems that arise in atmospheric flight (aircraft) are to some considerable extent different from
those relevant to space flight (spacecraft). Nevertheless, many aeronautically-oriented sensors
do qualify for use in spacecraft, especially those like NASAs now-retired shuttle, that need
terrestrial and landing instrumentation. Hence, there is still common ground between the two,and this has been addressed by the inclusion of some basic material on both atmospheric and
space flight.
Much of aerospace engineering is currentlyand happilyan international endeavor,
and the present volume clearly recognizes this by the inclusion of material from both Eastern
and Western countries. In particular, it has benefited greatly from the expertise of the Volume
Editor and contributor, Alexander V. Nebylov, one of Russias most eminent authorities in the
field.
J. Watson,
Series Editor
Swansea, UK, 2012
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xix
PREFACE
This book is devoted to modern sensors and their applications in control systems relevant to
aerospace vehicles.
Two centuries ago, a person who wished to move faster than walking or running would
mount a good horse and enjoy riding using all his or her sensory perceptions. Since that time,many kinds of vehicles have been created for satisfying the desire for fast and elaborate trans-
portation. In the air, and more recently in space, considerable increases in the speed-ranges of
various forms of transport have taken place, but have demanded the creation of a huge variety
of sensing elements for detecting the state and behavior of the relevant vehicles.
A horse-rider does not need to supervise the function of a horses legs, and to turn, it is
enough to give a clear command via the harness. However, all aerospace vehicles demand
the monitoring of many parameters and it is only recently that control systems have become
sufficiently intelligent to relieve the pilot of constantly monitoring the behavior of those
parameters and interpreting their aggregate meaning. Thus, a modern aerospace vehicle may be
compared to a good horse, which itself knows how to operate each leg so as to orient its body
in the direction desired by its rider.Another example is afforded by birds, which may be considered excellent examples of
intelligentflight control. They too can be compared with modern aircraft, which some believe
are actually winning the competition with nature, and which demand near-perfect sensors and
control algorithms for the realization of such phenomenal performance.
The automatic control of aerospace systems with huge numbers of operating parameters is
one of the highest technological achievements of modern civilization, and does indeed compete
functionally with those inherent in natural life-forms, including human beings. However, the
operating principles of the various necessary sensors and automatic systems are often essen-
tially different from those utilized in nature and form the knowledge base of leading designers
and firms in the field of aerospace instrumentation. It should also be noted that the majority of
aerospace sensors differ considerably from those designed for applications in automobile, ship,railway, and other forms of transportation, or those used in industrial, chemical, medicinal, and
other areas. The topic of aerospace sensors therefore merits special treatment, and it is hoped
that this book will to some extent fulfill this requirement.
The intent of the volume is to present the fundamentals of design, construction and appli-
cation of numerous aerospace sensors, a concept born in the International Federation of Auto-
matic Control (IFAC), especially in its Aerospace Technical Committee. An international
team of twelve authors represents four countries from Eastern and Western Europe and North
America, and all of whom have considerable experience in aerospace sensor and systems
design.
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xx PREFACE
The nine chapters in this volume cover the majority of sensors for aircraft and missiles, and
many for spacecraft, satellite, and space probes. Principles of operation, design, and achievable
performance for different sensors are presented along with particulars of their construction. The
introductory Chapter 1 briefly reviews the characteristics of atmospheric and space environ-
ments, this knowledge being essential for understanding the operation of aerospace sensors.
Material on aerospace vehicle classification, specific design criteria, and the requirements of
onboard systems and sensors is presented.
Chapter 2 is devoted to modern achievements in the development of the oldest group of
aircraft sensorsmembranous aneroid and other atmospherically-based instruments. Flight
altitude and components of speed, attack, and slip angle measurements are also considered.
Chapters 3 and 4 introduce radio-altimeters and other autonomous radio sensors for
motional parameters such as ground speed and crab angle. Airborne weather sensors and colli-
sion avoidance devices are also reviewed.
Chapters 5 and 6 cover accelerometers and gyroscopes of various kinds which are broadly
used as basic sensors in the construction of gimbaled and strapdown inertial navigation systems
(INS) and for direct applications in aerospace vehicles.
Chapter 6 was written by six co-authors and became the largest in the book. It recognizes
the particularly important role of INS and separate gyroscopic sensors for aerospace vehicular
navigation and motion control.
Chapter 7 presents the different aspects of magnetic, gyro-magnetic, and electronic com-
pass design and their application to flight.
In Chapter 8, engine parameter information collection systems are considered. Fuel quan-
tity and consumption sensors, pressure pick-ups, tachometers, vibration control, and tempera-
ture sensors are all described.
Finally, Chapter 9 is devoted to principles and examples of sensor integration. The most
important facets of sensor system choice, integrated measuring system optimization, and the
simulation of sensor integration by appropriate algorithms are described. The examples of sen-
sor integration considered include the noninvariant and robust integrated metering of speed,
radio-inertial measurements, airborne gravimetry, and orbital verticality.
The book is written for practicing engineers, designers, and researchers in the area of
control systems for various aerospace vehicles including aircraft, UAVs, missiles, spacecraft,
satellites, and space probes. It may also be used as a study guide for both undergraduate and
graduate students and for postgraduates in aerospace engineering, aeronautics, astronautics,
and various related areas. Moreover, it will be found useful by other people wishing to satisfy
their general interest in the modern aerospace technologies that are so important in shaping our
twenty-first century life-styles.
Alexander V. Nebylov
St.-Petersburg, Russia
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xxi
ACKNOWLEDGMENTS
I would like to thank the contributing authors of this book, all of whom kindly responded posi-
tively to my invitations to participate in an international team tasked with writing reviews of
various aspects of modern aerospace sensor technology. All these authors are distinguished spe-
cialists in their own technologicalfields, and already had heavy design and research workloads.It was not easy for them to find time to write their chapters, and especially to conform to the
required expository style. I am very grateful to all these authors for their responsible approaches
to these constraints, and deeply respect their abiding commitments to their chosen subjects in
science and engineering.
I also wish to express my sincere gratitude to the editor of the series, Dr. Joe Watson, for his
consistent support during the compilation of the book, especially during a difficult period when
a change of publisher occurred. His recommendations and helpful critiques promoted improve-
ments in the structure and quality of the content.
Especial gratitude is due to my colleague from the IFAC Aerospace Technical Committee
(ATC), Prof. Klaus Schilling, who appointed me editor of the book after mooting the idea of
its inclusion within a series at IFAC. Prof. Schilling kindly discussed with me the structure ofthe chapters at the initial preparatory stage. I am also grateful to the members of the ATC and
particularly to its present chairman, Prof. Houria Siguerdidjane.
My numerous colleagues, professors, and researchers at the State University of Aerospace
Instrumentation in St. Petersburg also merit my profound gratitude, as do several generations
of its students, for the fruitful dialogues that have allowed me to realize my own vision of the
problems inherent in developing and perfecting aerospace sensors.
I am also indebted to the editorial and production staff of Momentum Press, especially
Mr. Joel Stein, for their valued suggestions and constructive advice during the preparation of
the manuscript.
Finally, I would like to thank my family, because my work on the book was performed at
the expense of time otherwise available for companionship with my wife Elena and my sonVladimir, both of whom who suffered this with fortitude over several years.
Any criticism regarding the contents and material in the book and the quality of its presen-
tation will be accepted with gratitude.
A.V. Nebylov,
Editor
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xxiii
ABOUTTHE SERIES EDITOR
Dr. Joseph Watson is an Electrical Engineering graduate of the University of Nottingham,
England, and the Massachusetts Institute of Technology, where he held a King George VI
Memorial Fellowship. A Fellow of the IET, Senior Member of the IEEE, and a member of
Sigma Xi (the Scientific Research Society of America), he has published books and papers invarious areas including electronic circuit design, nucleonics, biomedical electronics and gas
sensors; and is a former Associate Editor for the IEEE Sensors Journal. He has also been visit-
ing professor at the University of Calgary, Canada, and the University of California, Davis and
Santa Barbara; and has held various consultancies with firms in the USA, Canada, and Japan.
Since retirement from Swansea University, UK, he has continued as chairman of the UK-based
Gas Analysis and Sensing Group, and as series editor for the present volumes.
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xxv
ABOUTTHE EDITOR
Alexander Nebylov graduated with honors as an Engineer in Missile Guidance from the
Leningrad Institute of Aircraft Instrumentation in 1971. He led many R&D projects in aero-
space instrumentation, motion control systems and avionics, and is a scientific consultant for
various Russian design bureaus and research institutes. He was awarded the Candidate of Sci-ence degree in 1974 and the Doctor of Science degree in 1985, both in Information Processing
and Control Systems, from the State Academy of Aerospace Instrumentation. He achieved the
academic rank of full professor in the Higher Attestation Commission of the USSR in 1986.
For the last two decades, Dr. Nebylov has been with the State University of Aerospace
Instrumentation in St. Petersburg as professor and chairman of Aerospace Devices and
Measuring Complexes, and director of the International Institute for Advanced Aerospace
Technologies. He is an author of fourteen books and numerous scientific papers and has also
been a member of the leadership of the IFAC Aerospace Technical Committee since 2002.
He has also been a member of the Presidium of the International Academy of Navigation and
Motion Control since 1995. In 2006 the title of Honored Scientist of the Russian Federation was
bestowed on Prof. Nebylov.
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1
CHAPTER 1
INTRODUCTION
Alexander V. Nebylov
State University of Aerospace InstrumentationSt. Petersburg, Russia
1.1 GENERALCONSIDERATIONS
1.1.1 TYPES OF AEROSPACE VEHICLES AND MISSIONS
Aircraft are designed to fly within the atmosphere, which makes possible the creation of motion,
lift, and attitude control. Spacecraft are designed for travel in open space; the presence of airis not necessary because the thrust is created by rocket engines. There are three main ways for
an aircraft to gain lift within the atmosphere: the fixed wing for normal airplanes, the rotor (or
rotary wing) for helicopters, and an envelope filled with any gas less dense than the surrounding
air for aerostats such as balloons or airships (dirigibles).
There are also some special aircraft such as surface-effect vehicles that fly at low levels
above land or water. In the case of cushioncrafts (hovercraft), air that is impelled by a propel-
ler and retained by a special skirt forms an air cushion between the hull and the surface. In the
case of wing-in-ground-effect vehicles (WIGs), or ekranoplans intended for use over water, a
dynamic air cushion is produced between the specially configured wing and the surface. In both
cases the close proximity of the ground or water surface requires appropriate approaches in the
measurement of the relevant flight parameters.Taking into account the problems encountered in the provision of appropriate sensors,
aerospace vehicles can be classified on the basis of several main characteristics. The aircraft
or spacecraft may be unmanned, have an obligatory pilot and optional or mandatory other
members, or a crew and passengers. The flight of manned aerospace vehicles has to be smooth
and without significant linear or angular accelerations in order to not injure the crew and
passengers. The maximum tolerance overload for a trained pilot is roughly between four and
eight gravitational units (4g and 8g). This limitation makes the range of some flight parameters
narrower and simplifies the designed functioning of sensors.
Stringent safety requirements for manned vehicles make it necessary that all equipment,
especially sensors, should be highly reliable, and in some cases it is important that the crew
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2 AEROSPACESENSORS
should be able to replace or repair a failed sensor during flight. The acceptable level of failure
probability for navigation and motion control sensors in manned aerospace vehicles should be
no more than 105 to 106. For some unmanned vehiclesfor example, guided missilessuch
requirements can be relaxed, making the sensors cheaper and lighter.
For manned aerospace vehicles, life support systems must be controlled, for which special
sensors may be required, in particular those for the measurement of relative oxygen and carbon
dioxide content, air temperature, and humidity, and for emergency rescue systems. However,
in all cases the mass of the vehicle must be kept in mind when sensors and sensor systems are
being designed. This mass can vary from a few kilograms for microsatellites and space probes,
some short-range guided missiles and pilotless air scouts, to several hundred tons for space
stations, heavy launchers, and great airliners. It is possible to install any number of sensors
on a heavy vehicle without restraint, but each additional sensor can appreciably decrease the
payload of a light vehicle, making it necessary to limit their number and weight. Micromechani-
cal sensors, microelectronics, and a mechatronic approach can often provide solutions to the
weight/reliability problem.
The nature of the vehicles mission can also affect the requirements for onboard sensors
and such missions can include
transport operations with the aim of delivering passengers or cargo to a given area or
port;
travel with no fixed destination (space station, navigation satellite, patrol aircraft, etc.);
the launch of payloads into certain orbits or trajectories;
the rendezvous and docking of two vehicles;
flights to test vehicles performance and characteristics;
sport or tourist flights for pilot or passenger pleasure;
combat missions; and
the exploration of deep space.
Each kind of mission requires its own variety of onboard sensors and criteria for control
algorithm optimization.
Aerospace vehicles also travel at different speeds. The greatest speeds ever achieved by
man-made objects have been by aerospace vehicles, especially spacecraft. As a rule, increas-
ing speed makes the task of accurately positioning the vehicle more difficult. This is especially
obvious when satellite navigation systems are used as the basic means for positioning. Further-
more, not only a vehicles speed, but also its motion dynamics, influences the accuracy of
instruments that measure flight parameters.
Even a single aerospace vehicle can have different modes offlight with different require-
ments for motion measurement. At some stages offlight such demands may be more rigorous,
and the quality and complexity of measurement may vary over time. For example, spacecraft
docking maneuvers have to be very precise, and accurate orientation when correcting orbit and
descent into the atmosphere is mandatory. For guided missiles, the terminal phase of trajectory
control is of front-rank importance; for passenger planes, safe landing requires the greatest
accuracy from the navigation system; and for warplanes, attack maneuvers may put the maxi-
mum load on the relevant sensors. So, the requirements for sensors and sensor systems and the
criteria for system optimization may not be invariant, but may depend on the phase offlight.
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INTRODUCTION 3
1.1.2 THE ROLE OF SENSORS AND CONTROL SYSTEMS IN AEROSPACE
Aerospace vehicles can be considered as representing a class of transportation with unique and
specific features. No other transport vehicles can perform such complex modes of motion along
required trajectories with such great speed and carry such responsibility for mission success.
The specific cost (per unit mass) of aerospace vehicles exceeds that of cars and other means of
transport many times over, and it is well known that some defense tasks may be solved only by
using aerospace technologies. All these features of aerospace vehicles define the specific role
and design of sensors and their control systems. Aerospace vehicles have four other character-
istics, as follows, that are not shared by other vehicles.
Aerospace crafts are the only vehicles permitting motion in 3D space with extreme
dynamics (as compared with submarines, for example). Hence it is always necessary
to control the vehicles 6D motion (linear and angular), for which complex means of
automation plus the direct application of automatic control systems with perfect sensors
are required.
The specific payload cost aboard aerospace vehicles is so great that for economic reasons
it is advisable to decrease the number of crew members. In fact, the creation of pilotless
vehicles may prove economically expedient. Furthermore, for some military applica-
tions, only pilotless vehicles can be used, as is the case of deep space probes which in
principle cannot return to earth. Naturally, pilotless crafts must have full sets of sensors
that provide all the necessary data for automatic control.
Many kinds of aerospace vehicles cannot principally be serviced or repaired during a long
flight, and consequently it is very important that all systems and sensors be extremely
reliable. The survivability of automatic systems aboard such vehicles can be achieved
only by the application of structural redundancy including backup and interchangeability
of sensors. All this must be considered at the design stage.
Many aerospace vehicles must be able to fly far from, and without communicating
with, any navigational infrastructure, which requires them to be autonomous and self-
maintaining. The role of onboard sensors and systems in this case is therefore more
important than for external data or devices. Thus, criteria for the selection of these sen-
sors must be defined early and the sensors carefully selected.
1.1.3 SPECIFIC DESIGN CRITERIA FOR AEROSPACE VEHICLES
AND THEIR SENSORS
Aerospace vehicles are the most sophisticated, highly developed, and knowledge-intensive
form of transportation permitting people to travel at greater velocities than in any other mode of
transport. Their design and production are the privileges of technologically advanced societies,
requiring great scientific, engineering, industrial, and financial resources and the joint effort of
different enterprises and even countries (Bradshaw and Counsell 1992; Etkin 1982; Fleeman
2001; Grewal, Lawrence, and Andrews 2007; Kayton and Fried 1997; Lawrence 2001; McLeon
1990; Siouris 2004; Wertz and Larson 2000).
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4 AEROSPACESENSORS
Several criteria are especially important in the design of aerospace vehicles and their
sensors.
1. Engines must be reliable, and their operating conditions must be knownany engine
failure in flight greatly endangers any aerospace vehicle. No other means of transporta-
tion places such importance on power plant reliability, which is truly of vital concern.
Accomplishing a mission, and often even surviving, depends totally on engines being in
full working order. Hence, engine reliability has to be verified by employing specialized
sensors.
2. Any aerospace structure and equipment has to be extremely light, because given a fixed
lifting force or engine thrust, each additional kilogram of weight decreases the payload
and the capacity for acceleration. This requirement of light construction does not permit
great rigidity and makes any aerospace vehicle ratherflexible. However, it is essential
to prevent as much flexing and oscillation as possible, and this can be accomplished
both by optimizing the construction and by applying appropriate flight control. Special
sensors forflexing and oscillation distributed along the vehicle body are necessary, and
the design of all onboard sensors must be performed in the light of mass saving. Hence
micromechanical and mechatronic techniques become very important.
3. All aerospace vehicles move in 3D space and require accurate and reliable 3D naviga-
tion and means of motion control. For aircraft, altitude control is of the same importance
as position (latitude and longitude) and airspeed and become critical at landing. In deep
space, altitude is not defined at all and each of the three axes, x, y, and z, is of equal
importance.
4. Accurate attitude control and orientation with reference to relative points in space is
very important for aerospace vehicles. It is necessary to measure the attitude angles with
special sensors of different designs and to have special actuators that are able to influence
these angles.
5. Aerospace vehicles have specific serviceability, reparability, and scheduled and unsched-
uled maintenance requirements. For reasons of safety, and because such vehicles are
very expensive, it is inexpedient to scrimp on servicethis has to be accomplished using
the best equipment and by the best technicians. For airplanes and helicopters the planned
service and deep testing of main power plants and units must be unfailingly regular and
frequent. On the other hand, this service has to be rather quick, again requiring the best
equipment and expertise. For spacecraft, frequent service in stationary service areas is
difficult and often impossible, and this makes careful prelaunch inspection and service
especially important. The main sensors on space vehicles, and particularly on probes for
investigating deep space, have to be backed up. This redundancy makes sure that the
mission does not fail because only one sensor malfunctions, for example.
6. The requirement for clean aerodynamics in aircraft and space carriers places many limits
on body design and sensor distribution, especially for modes of flight where fairings
have to withstand great thermodynamic changes. Fairings and cowls must not prevent
sensors from functioning properly, and those constructed from special materials along
with specially designed sensors can mitigate the problem.
7. Aerospace sensors must be temperature-resistant and able to withstand considerable falls
in temperature. Even if a sensor is installed inside a temperature-controlled module,
any power failure or abrupt depressurization in the module can lead to a precipitous
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INTRODUCTION 5
temperature drop. On some space probes the sensors must remain functional over tem-
perature changes of a thousand degrees or more. The radiation resistance and pressure
resistance of sensors are also important, especially on space vehicles and probes designed
to land on other planets.
1.1.4 PHYSICAL PRINCIPLES INFLUENCING PRIMARY AEROSPACE
SENSOR DESIGN
Aerospace sensors involve a large collection of different units for measuring many physical
parameters that together characterize the state of an aerospace vehicle. Some of them, such as
sensors for temperature or engine operational parameters, use ordinary principles of metrology
and do not differ much from similar sensors on automobiles or ships (see Chapter 8). How-
ever, the most important sensors for aerospace vehicle parameters are essentially different from
sensors for other kinds of vehicles, this being due largely to the great range of speedsand altitudes in addition to the more rapidly changing dynamics faced by aerospace vehicles
(see Chapters 24).
The main parameters relevant to aerospace vehicle position and movement are geographi-
cal coordinates, altitude, velocity (three components), attitude (three angles), three angular
rates, probably linear and angular accelerations, and often parameters relating to motion with
respect to other vehicles or to a given reference point. Several (at least four) methods exist for
measuring the location and velocity of aerospace vehicles:
The simplest method of measuring flight altitude and velocity is to use aerial medium
parameters such as static and dynamic pressure. This can be done cheaply and reliably by
many kinds of onboard devices that have been installed on practically all airplanes since
the beginning of aviation. Altitude as measured by barometric pressure and air speed are
probably the most reliable instrumental data about flight mode and are available aboard
any flying vehicle. Unfortunately, aerial medium parameters are rather unstable and
sensors based on them are inaccurate (see Chapter 2).
One basic method of aerospace vehicle positioning involves the exploitation of natural
physical fields and landmarks. This involves the use of magnetic compasses to navigate
aerospace vehicles using the Earths magnetic field, though magnetic anomalies lessen
the accuracy of such compasses (see Chapter 7). The most stable natural physical field
is the disposition of stars observed in the sky or in space, and star sensors are widely
used for both navigation and attitude control of aircraft and, especially, space vehicles.
Great progress in charge-coupled devices (CCDs) has presented a major opportunity for
creating precise, reliable, and inexpensive star and sun sensors. Existing star sensors
and those under development are highly appropriate tools for broad-spectrum aerospace
vehicles, especially spacecraft and deep space probes.
It should also be stated that during recent decades such exotic and special natu-
ral physical fields as terrain features, gravitational anomalies, ground temperature, and
the ratio of diffusely reflected to incident electromagnetic radiation (terrestrial albedo)
have attracted attention as potential bases for navigation by map-matching. Any natural
physical field can be used to position a vehicle if both a map of this field and sensors for
the current values of the related physical parameters are available on board. The great
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6 AEROSPACESENSORS
advantage of map-matching navigation systems for military applications is that they are
absolutely autonomous and totally secure from artificial jamming. Of course, visual land-
marks also have an important role in vehicle positioning, and for aircraft this method has
been termed pilotage.
The use of artificial physical fields for vehicle positioning became possible when power-
ful radio stations were developed and reliable, complex radio equipment appeared. The
first long-range (e.g., LORAN and CHAYKA) and short-range (e.g., VOR/DME and
TACAN) radio navigation systems were created in the 1940s and 1950s.
Gradually, almost all aircraft took advantage of the availability of such radio navi-
gation systems, but their accuracy remained low due to some fundamental limitations.
Long-range navigation systems with a network of ground stations needed to use radio
waves in the thousand-metre range (about 300 kHz) to service users beyond the horizon.
Because the waves were so long, accurate measurement was impossible, but considerably
improved accuracy came after the development of satellite navigation systems (SNS),
which use radio waves in the ten-metre range (about 30,000 kHz) to shape the special
artificial radio field anywhere on Earth or in terrestrial space where several navigation
satellites are in the zone of direct sight. The SNS is one of the most outstanding engi-
neering achievements in the field of navigation. It is based on the use of both advanced
space and radio technologies, and its importance can be compared with, for example, the
invention of the compass. Further improvements in radio navigation accuracy require the
application of shorter radio wave ranges (especially millimetric radio waves) and this is
already happening in advanced short-range navigation and landing radio systems and
also in homing systems.
The accuracy of any SNS can be approximately doubled if it is used in a differential
regime that permits user coordinates to be corrected by a systematic correlation compo-
nent. This may be achieved by the use of information coming from reference stations
located nearby, that is, again by application of the short-range navigation principle. The
accuracy and reliability of measurements can also be effectively increased by integrating
the SNS with other onboard sensors, notably with the inertial system (see Chapter 6).
The application of inertial navigation principles is the latest and most effective approach
to the autonomous measurement of aerospace vehicle motion parameters. Accelerometers
and gyroscopes are the main primary sensors in the realization of inertial navigation sys-
tems (INS). Three accelerometers with orthogonal sensing axes react to a crafts absolute
linear acceleration along these axes and perform independent measurements of this accel-
eration and of its integral. The subsequent subtraction of a known gravitational compo-
nent allows for the determination of relative acceleration or increments of linear velocity.
Gyroscopes measure the attitude or angular velocity of a vehicle in the inertial refer-
ence frame. By tracking both the current angular velocity and the current linear accel-
eration measured relative to the moving frame, it is possible to determine the linear
acceleration of the system in the inertial reference frame. Integration using the correct
kinematic equations yields the inertial velocities of the vehicle, and integration again
(using the original position as the initial condition) yields the inertial position.
The growing interest in INSs shown in recent decades has resulted in the develop-
ment of theory and practice essential to their successful implementation. The sensors
themselves are constantly being improved and new types of such devices frequently
appear on the market (see Chapters 5 and 6).
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INTRODUCTION 7
1.1.5 REFERENCE FRAMES ACCEPTED IN AVIATION AND ASTRONAUTICS
Several systems of axes that differ in several respects are employed to investigate aerospace
vehicle motion. The origin of a set of coordinates may be located at the center of a celes-
tial body (including the Earth, the Sun, or the Moon), at the surface of a celestial body, or at
any point on an aerospace vehicle. Classification can also be performed by the position of the
main flat, the orientation of the axes, and the readout of angles (Recommended Practice for
Atmospheric and Space Flight Vehicle Coordinate Systems 1992).
The system of coordinates relative to which the position and attitude of an aerospace vehi-
cle is referred has to be chosen with the vehicle flight program taken into account. It is advisable
to consider which reference frame provides the simplest technical realization of this program,
the simplest equations of vehicular motion, and how these equations can be linearized. Many
systems of axes exist; for example, at the center of mass of the vehicle, at the center of mass of
the Earth, or at any other point. The axes of the reference frame can be fixed in inertial space,
or they can move in this space in a known manner. The following are the main orthogonal-
axis systems used to investigate aerospace vehicle motion and in developing the appropriate
equations:
The body-fixed frame (b-frame) is connected to the vehicle and moves with it. The origin
of this frame is usually at the center of gravity (CG) of the vehicle, and its axes are the
positivexb-axis, or longitudinal axis, which lies fore and aft; the positivey
b-axis, or pitch
axis, which lies in the right wing on the horizontal plane; and the positive zb-axis, or
yaw axis, which is vertical. This axis system is denoted by (xb,y
b,z
b). The Euler angles
(y, q, j) are commonly used to define a missiles attitude: j is roll about thex-axis, q is
pitch about they-axis, and y is yaw about thez-axis (Figure 1.1).
Ro1l
Pitch
Yaw
Xb
ZbYb
CG
Figure 1.1. The body-fixed frame.
Because measuring equipment and transducers are mainly located aboard the
aerospace vehicle, their readouts can be processed most easily in the b-frame. This sim-
plifies the equations of motion, because the vehicles products of inertia become zero.
It is essential also in most aircraft and missiles that thex-z-plane be a plane of symmetry.
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8 AEROSPACESENSORS
For some vehicles, the CG changes during flight and measurements may become
inaccurate if this roaming point coincides with the b-frame center. However, the origin of
the coordinates can be fixed at any stable point in the vehicle. For example, the leading
edge of the root chord, the tip of the nose, or the aerodynamic center of the wings may
serve as reference points.
In the wind frame (w-frame), thex-axis points directly into the relative wind. Thez-axis
remains in the reference plane, but it rotates so that it remains perpendicular to thex-axis.
They-axis completes a right-handed system. The mutual position ofw-axis and b-axis
can be defined by the attack angle a and the sideslip angle b. The transformation from
b-frame to w-frame consists of two rotations. First, the body axes are rotated about the
y-axis through the angle of attacka, then the axes are rotated about thez-axis through the
angle of sideslip b, yielding the wind axes.
The main reason for using the wind axis system is that it makes calculating aero-
dynamic forces more convenient. For instance, lift is by definition perpendicular to the
relative wind, while drag is parallel. So, both lift and drag resolve into a force parallel to
one of the w-frame axes.
Theflight path frame (fp-frame) differs from the w-frame in that its x-axis is aligned
with the velocity vectorVof the vehicle, this being more convenient for navigational
purposes. The relationship between the b-frame and thefp-frame can be defined by the
flight path angle , the bank angle Y, and the aerodynamic angle of roll g.
The distinction between thefp-frame and the w-frame is essential for subsonic air-
craft, whose speed may be commensurate with wind speed. For rockets and other high-
speed vehicles, thefp-frame and w-frame are essentially indistinguishable. In this case,
the attack and sideslip angles may be defined as the b-frame inclination with respect to
thefp-frame.
An inertial frame (i-frame) is fixed in space and is subject to Newtons laws of motion.
Its origin is at the earths center, its z-axis is perpendicular to the equatorial plane, its
x-axis lies in the equatorial plane in a direction that can be specified arbitrarily but as
a rule is directed to the point of the vernal equinox, and its y-axis completes the right-
handed coordinate system.
The parameters of space vehicle orbits are normally given in the i-frame: i is the
inclination of the orbit, w is the argument of the perigee, qa
is the true anomaly, and u
is the argument of the breadth. Otheri-frames are used, for example, when one axis is
directed toward the sun and the other toward any star, or even when two axes are referred
to one star. Such coordinate systems are sometimes calledsighting.
TheEarth-fixed frame (orEarth-surface frame) is used to define location. Itsz-axis is
coincident with the Earths polar axis, while the other two axes are within the equatorial
plane. These three axes are defined as down, north, and east, the origin being located on
the earths surface at a point with zero altitude. This coordinate system is also known as
north-east-down (NED). Any vehicle trajectory in NED can be characterized by its head-
ing angle Y(about thez-axis) and flight path angle qt(about they-axis), which show the
relationship with thefp-frame (Figure 1.2).
TheEarth-centered frame (e-frame) has the same axes, but its origin is located at the
Earths center. Thus the point (0,0,0) denotes the center of the Earth. The e-frame is espe-
cially suitable for controlling the motion of satellites and for global positioning.
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INTRODUCTION 9
The launch-centered inertial frame is inertially fixed and is centered at the launch site at
the instant when a rocket is launched. In this system, thex-axis is commonly taken to be
in the horizontal plane and in the direction of launch, the positivez-axis being vertical,
and they-axis completing the right-handed coordinate system. This frame is suitable for
defining a ballistic missiles position and angular orientation in space.
The orbital base frame (o-frame) is suitable for satellites moving in near-Earth orbits. In
this system, they-axis is a local vertical, that is, a line uniting the center of mass of the
vehicle with the center of mass of the Earth. Thex-axis is transverse to the direction of
motion of the satellite and so is perpendicular to the local verticals and to the surface of
the orbit. Thez-axis is a vector normal to the surface of the orbit such that the system
becomes right-handed.
It is advisable to provide overlapping of one or two axes of the o-frame with the
b-axis beginning at the satellite CG. In this case, simplified equations of angular motion
exist, and the influence of moving parts inside the satellite during changes in the posi-
tions of the axes is minimal. During small angles of deviation of the satellite from the
position of the given orientation in the o-frame, aviation terminology is used, and the
Euler angles may be called roll angle g, yaw angle Y, and pitch angle q.
The geodetic frame (g-frame orgeodetic data) is used in navigation, geodesy, survey-
ing by cartographers, and satellite navigation systems to translate positions indicated on
their devices to their real position on the Earth. This is needed because the Earth is not a
perfect sphere. The difference in coordinates between two particular data can vary from
one place to another and can be up to hundreds of metres or even several kilometres.
Different data use different estimates for the precise shape and size of the Earth. The
most commonly used geodetic data corresponds to WGS-84 (the World Geodetic Sys-
tem) adopted by the International Civil Aviation Organization (ICAO; Office of GEOINT
Sciences 1984) and F. Krasovskys ellipsoid, which is popular in Russia.
Figure 1.2. The earth-fixed frame.
CG
Zfp
Yfp
Xfp Ye
Xe
Ze
t
.
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10 AEROSPACESENSORS
All the above-named coordinate systems are rectangular and right-handed, where three
Cartesian axes are perpendicular to each other. The relevant coordinates are of the form (x,y,z).
Thexy-,yz-, andxz-planes divide the three-dimensional space into eight subdivisions known as
octants. Only the first octant of three-dimensional space is labeled, mostly because it contains
all the points whosex,y, andzcoordinates are positive.
Besides Cartesian frames, some types of spherical and polar coordinate systems may be
considered. They are especially suitable for the investigation of radar sensors and systems.
Any coordinates may be easily converted from one frame to another on the basis of the
formulae of recalculation.
1.2 CHARACTERISTICS AND CHALLENGESOF THEATMOSPHERICENVIRONMENT
1.2.1 COMPONENTS OF THE EARTHS ATMOSPHERE
The atmosphere fills the expanse above the surfaces of land and water. Its density decreases
gradually as the distance from the surface increases, and becomes negligible at a great altitude
that may be considered the conditional boundary between the atmosphere and outer space.
Earths atmosphere consists of air that has a density of 1.23 kg m3 and pressure of 760 mm
of mercury (or 29.9 in. of mercury; other equivalents being 1.014 105 N m2, 101.4 kPa,
and 1,033 gf cm2) at sea level at latitude 45. The air pressure at a height of 80 km is 105 of
that at sea level; and at 400 km (the outer boundary of the Earths atmosphere) it decreases by
1010. The chemical constituents of the air are nitrogen (N2) 78%; oxygen (O
2) 21%; argon (Ar)
0.93%; carbon dioxide (CO2
) 0.03%; and other rare gases (Ne, He, Kr, Xe), ozone (O3
), and
hydrogen (H2) in very small quantities. The total mass of air is estimated to be 5.136 1015 tons
(Allen 1973).
The atmosphere is conventionally divided into several layers, each having different proper-
ties. Four main zones exist: the troposphere (up to 11 km where the temperature falls as altitude
increases), the stratosphere (from 11 to 50 km where the temperature rises with altitude), the
mesosphere (5085 km where the temperature again falls), and the thermosphere (85800 km
where the temperature again rises). The tropopause, mesopause, and thermopause are rather
thin layers that share a mixture of the characteristics of the main layers that surround them
(Prolls 2004).
Ultraviolet radiation converts ordinary oxygen (O2) into ozone (O
3), which is located
in the stratosphere with maximal concentration at 2528 km. This is why the stratosphere is
sometimes called the ozonosphere, and it is in this layer that most ultraviolet solar radiation is
absorbed.
The oceans provide great areas over which aircraft may fly, but where the characteristics
of the atmosphere need to be measured by the numerous sensors installed on such aircraft. The
main domain for aircraft flight is of course the troposphere and occasionally the stratosphere.
The mesosphere can be considered the portal to space through which spacecraft must pass at
launch and re-entry.
The ionosphere is markedly affected by solar activity (Warneck 1999). The great influence
of the ionosphere on radio wave propagation means that considerable attention must be given
to this upper layer in the atmosphere in the design of relevant sensors.
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INTRODUCTION 11
1.2.2 STATIONARY MODELS OF THE ATMOSPHERE
Several models of atmospheric density and pressure are used around the world. Such models
relate atmospheric pressure to height and allow altitude to be measured with rather simple
barometric devices. Airspeed can also be measured via the dynamic pressure resulting from
the motion of the aircraft. The accuracy of such simple measuring instruments based on the
barometric approach depends directly on the quality of the relevant model, the simplest being
the so-called isothermal model described by the following formula:
pH
=p0exp(gH/R
aT
a) (1.1)
wherepH
andp0
are the air pressures at heightHand at sea level (H= 0);gis the gravitational
acceleration at altitudeH;Ra
and Ta
are the ideal gas constant of air and its absolute temperature
for the isothermal atmosphere.
Isothermal conditions are not uniform, and air temperature generally varies as height
increases. As the vertical gradient of temperature is not constant, especially in the stratosphere,
the conditional standard atmosphere has been adopted. According to this international agree-
ment, the temperature at sea level is taken as constant and equal to 15 C; a constant vertical
temperature gradient of6.5C for each 1 km of height is adopted for the range 011 km; and
a constant temperature of56.5C is assumed for any height above 11 km. It is considered
that r0
= 760 mm of mercury, and that the relative humidity is 0%. Unfortunately, neither this
nor any other stationary model can accurately predict atmospheric parameters at any point in a
flight path.
1.2.3 ANISOTROPY AND VARIABILITY IN THE ATMOSPHERE
Atmospheric anisotropy is affected not only by the dependence of pressure and other param-
eters on height, but also by geographical coordinates that play a part in determining the aver-
age parameters of atmospheric and wind fluctuations. Average air temperature depends on the
latitude, and at low (tropical) latitudes the average temperature may be about 40C higher
than at high (polar) latitudes. Differences in temperature cause differences in air pressure and
winds. Heat exchange between the oceans and the atmosphere is irregularly distributed over
the Earths surface. All these effects bring anisotropy and instability to atmospheric processes.
Oceans and other waters release water vapor into the atmosphere and the average relative
humidity may reach 9095% in some places on the Earths surface, depending on air tempera-
ture and pressure. Clouds of various types are the visible displays of vapor concentration in the
atmosphere. The humidity index strongly influences the way the suns rays heat land and water.
All these and many other factors make the task of accurate weather prognostication very com-
plex, and the relevant problems become almost insoluble. The real condition of the atmosphere
is always a stochastic phenomenon to a greater or lesser degree, and mathematical models pro-
vide only approximations based on average conditions.
Gusts of wind are important characteristics of the atmosphere and numerical models exist
that describe the strength of the wind, the dynamics of wind change, and the structure of the
space in which they occur. The speed of wind gusts may exceed the average speed of the wind
by 1.5 to 2.5 times and sometimes even more.
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12 AEROSPACESENSORS
By analyzing weather statistics over many years and over different areas of the Earths
surface, it is possible to take into account the following limiting values:
Lowest temperature: 89C (Antarctica, 1983)
Highest temperature: 58C (Libya, 1922)
Lowest atmospheric pressure in the Northern Hemisphere (not taking into account tropical
cyclones and tornados): 741 mm of mercury (Aleut cyclone)
Highest atmospheric pressure: 804 mm of mercury (Siberian anticyclone) (Gresswell 1967).
Wind speeds at sea level can exceed 60 m s1 in massive storms, and at high altitudes may
be even greater. In particular, the jet stream, at altitudes of 718 km, can cover great areas of
hundreds or thousands of square kilometres. The highest recorded speed of the jet stream is
100150 m s1. The distribution of average wind speeds and directions, taking into account the
height, area, and season, has been thoroughly investigated and described in numerous reference
books (CIRA 1965).
The normal speed of sound propagation at sea level in dry air at 0C is 331 m s1. At a
temperature of 15C its value at sea level is 340 m s1, at 10 km it becomes 294 m s1, at 50 km
it returns to 338 m s1, and at 80 km it falls to 258 m s1. The speed of sound depends also on
the humidity of the air.
1.2.4 ELECTRICAL CHARGES IN THE ATMOSPHERE
Electrical charges appear in the atmosphere as the result of the relative motion of air masses
(static electricity), solar radiation, and other causes. Two thousand thunderstorms disturb the
Earths atmosphere at any moment, which means there are around 50,000 storms on Earth everyday. Lightning is a high-energy electrical discharge partly in the form of electromagnetic radia-
tion over a wide frequency range. Such a burst of energy can disturb electronics that incorporate
no special protective features. Local ionization of atmospheric air may be artificially produced
to measure airflow velocity.
1.2.5 ELECTROMAGNETIC WAVE PROPAGATION IN THE ATMOSPHERE
The main source of energy reaching the Earth is in the form of solar radiation and this includes
the entire light spectrum. Energy density at the upper bound of the atmosphere is characterized
by the solar constant and is equal to 2 cal cm2 min1 (8.4 j cm2 min1 or 1.4 103 W m2).
The planetary albedo of the Earth, the amount of energy reflected back to space, is 3040%.
Fortunately, atmospheric air is an excellent medium for radio wave transmission. However,
in some frequency bands the absorption of electromagnetic energy increases, and this has to be
taken into account in radar design. The loss of radio wave energy in the ionosphere is almost
complete only at l > 3 m but is negligible for ten-metre waves. However, the troposphere
allows the passage of long radio waves (l 3 cm and l 8 mm) and essentially absorbs waves
ofl < 3 cm, where molecules of water vapor and oxygen have resonances. In hard rain and fog
the conductivity falls, especially at l < 5 cm.
Radio noise invariably reaches an antenna along with any useful signal, and may be atmo-
spheric or galactic. At l < 10 cm atmospheric noise is more powerful than galactic noise.
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INTRODUCTION 13
Two varieties of atmospheric noise are especially intense at l = 1.35 cm and l = 0.5 cm, where
the resonance radiation of vapor and oxygen molecules takes place.
The ionosphere reflects short radio waves, which is why this band can be used for long-
distance radio communication on Earth. Unfortunately, the top of the atmosphere acts as a lens
for radio waves and forces them to spread when they are deflected. This affects all signals sent
from satellites and other space vehicles to aircraft.
1.2.6 GEOMAGNETISM
The Earths magnetic field can be thought of as a simple dipole located at the center of Earth
and at 11 to the axis of rotation. This inclination accounts for the displacement of the magnetic
pole from the geographical pole. However, some small-scale and large-scale anomalies are
superimposed on this main geomagnetic field. The most significant large-scale anomalies are
located in the areas above Siberia (increasing) and the Atlantic side of South America (decreas-
ing). They strongly influence the motion of charged particles in the belts of radiation up to
heights of thousands of kilometres above the Earth.
Detailed maps of the geomagnetic field have been obtained through experimentation, but
this field is unstable for many reasons. Electric currents in the upper atmosphere (ionosphere
and higher) cause the values of some components of the field to vary, and become especially
large during auroras, when geomagnetic storms take place. Slow geological processes also
cause the values of the main components of the field to vary.
The magnetic field of the Earth is always compressed on the side toward the Sun due to the
stream of solar plasma (solar wind). The area where the geomagnetic field is deformed by this
compression but generally preserves the direction of the lines of force is called the magneto-
sphere. Upon entering Earths magnetosphere the ions (protons) of the solar wind curve toward
the West, electrons toward the East.
These flows of separated, charged particles cause a strong current at the upper bound of the
sunward side of the magnetosphere. In this area the solar wind decelerates, and its great kinetic
energy converts into thermal energy, which causes the plasma temperature to shoot up to more
than ten million degrees. This happens at a distance of tens of thousands of kilometres from the
Earths surface. At the opposite side (removed from the sun), the tail of the geomagnetic field
extends millions of kilometres beyond the Earth.
The geomagnetic field, together with the ionosphere, performs the important function
of smoothing the harmful influence of radiation from the Sun on humans and many kinds of
man-made hardware, especially sensors. When the Sun flares, the flow of plasma and radiation
increases manifold. Short wave radiation arrives at the Earth in 810 minutes and is mainly
absorbed by the atmosphere, whereas the plasma (solar wind), which moves at a velocity of
4001,000 km sec1, appears in 23 days, most of it being blocked by the magnetosphere. This
natural defense helps aircraft more than it does spacecraft, which need artificial protection. The
delay in the arrival of the gust of solar wind after a solar flare may therefore be predicted by
the reception of a pulse of electromagnetic waves and so used to forecast geomagnetic storms.
The ordinary duration of a solarflare is only 12 hours, but a large amount of energy is
released at this time, which causes geomagnetic storms to last much longer. Severe geomag-
netic storms cause communication problems, greatly increase drag on spacecraft, cause elec-
tronic circuits to malfunction, and cause some people to become depressed.
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14 AEROSPACESENSORS
Solarflares are not the only source of hard radiation in near-Earth space. The Earth has two
radiation belts where high energy particles, mainly protons, are located. The internal radiation
belt begins at an altitude of 500600 km in the Western Hemisphere and from about 1,500 km
in the Eastern Hemisphere, up to 5,00010,000 km. Above the southern part of the Atlantic
Ocean, the lower edge of this belt is as low as 300 km due to the magnetic anomaly. This inter-
nal radiation belt does not cover the whole Earth, but only about as far as latitudes 45 north
and south (Beregovoy et al. 1989; Connerney 1993). The external radiation belt covers the
equatorial zone at an altitude of more than 104 km and down to 300 km at latitudes 5570.
The separation of radiation belts into internal and external belts is a convention; actually, these
belts overlap.
1.2.7 THE PLANETARY ATMOSPHERE
The atmosphere of a planet is its covering of gas, and is defined by its chemical constituents and
by its mass, density, and temperature distributions. On the basis of most of these parameters,
the planets in our solar system can be separated into two groups. The four comparatively small
planets closest to the Sun form the Earth group. The next four great planets belong to the
Jupiter group. The last planet, Pluto, has very specific features and cannot be associated with
any groupwhether it is a planet at all has become a matter for discussion.
The planets in the Earth group have, on average, thinner atmospheres than do the larger
planets. Mercury, with its small mass (5% of the mass of the Earth) and the high temperature of
its sunward side, has an extremely thin atmosphere, with a surface pressure of only 2 1010 Pa.
The atmospheric pressure at the surface of Mars is about 0.6 kPa (1701 that of Earth), and on
the surface of Venus the pressure is 9 Mpa (100 times greater than that of Earth). The main
component of both atmospheres is carbon dioxide. A powerful hothouse effect acts to main-
tain the high temperature of Venus lower atmosphere at around 740 K. A major feature of the
weaker Mars atmosphere is the great fluctuation of temperature between day and night and
between equator and winter pole, sometimes exceeding 100 K. The average high temperature
may be 22C and the low 70C (Cadle 1966; Davis 2005; Pikelner 1976).
The giant planets, Jupiter, Saturn, Uranus, and Neptune, have massive atmospheres of gases
with small molecular weights, the main components being hydrogen and helium for Jupiter and
Saturn, and methane and ammonia for Uranus and Neptune. The atmospheric pressure at the
surface may exceed 100 gPa.
1.3 CHARACTERISTICS AND CHALLENGESOF THESPACEENVIRONMENT
1.3.1 GENERAL CONSIDERATIONS
All space beyond the Earth and its atmosphere is considered as cosmic or outer space. It can be
divided into near-Earth space, circumsolar space, and deep space, this last category including
interstellar space, our galaxy, and even other galaxies. Space boundaries are more a matter of
rhetoric than of reality. The planetary atmosphere fades into interplanetary space, but this can
be considered the continuation of the Suns atmosphere, which fades into deep space.
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INTRODUCTION 15
1.3.2 NEAR-EARTH SPACE
Currently, near-Earth space is the most explored area of the cosmos. Thousands of artificial satel-
lites circle the Earth in orbits of different altitudes, ellipticities, and inclinations. Launching into
a low-earth orbit (LEO), an altitude of 150300 km, requires minimal energy, but such orbits
cannot be maintained due to the presence of residual atmosphere in near space. Geostationary
orbits (GEOs) at an altitude of about 36,000 km are also thickly settled, but higher orbits are
not popular. Some satellites with essentially elliptical orbits come into this zone only in apogee.
Actually, such altitudes correspond with the notional upper bound of near-Earth space.
Space is essentially a vacuum. The remnants of the atmosphere disappear quickly at alti-
tudes greater than 100 km, as is shown in Table 1.1 (Beregovoy et al. 1989).
Table 1.1. Atmospheric densities
Altitude, km 100 200 300 400 600 800 30,000
Density,
kg m35 107 3 1010 2 1011 3 1012 1013 1014
less than
1021
The vacuum of space affects a spacecrafts equipment by changing the conditions of heat
exchange, by the appearance of corona discharges, by volatilizing lubricants, and by sublimat-
ing materials and varying their mechanical qualities.
Among the numerous tangible objects in near-Earth space, approximately 5% are operat-
ing satellites, 12% are dead satellites, 18% are the last stages of carrier rockets and their
fragments, and 65% are the wreckage of burst rockets, satellites exploded by design, and other
debris (Reynolds, Fisher, and Rice 1983).Space garbage is the wrongful consequence of human activity in the space age and it poses
considerable danger to any vehicle moving in near-Earth space. The relative speed of colli-
sion with such numerous particles of former artificial artifacts and waste may be more than
10 km s1, which makes collision with even a 1 cm particle very dangerous. Such objects num-
ber more 300,000, whilst the number of objects with dimensions less than 1 cm is more than
106. The number of objects with dimensions of more than 10 cm is about 8,000 and their total
mass is more than 3,000 tons. These fragments are described in catalogs such as that published
by NORAD. The garbage distribution is rather smooth at different altitudes from 400 km to
1,500 km, having a density in the order of magnitude ofFg
= 1010 m2 h1. The probability of
a space vehicle collision with a fragment of space garbage is given by the formula
1 exp( )c g v f g v f F A T F A Tr = (1.2)
ifrc
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1.3.3 CIRCUMSOLAR (NEAR-SUN) SPACE
The solar system includes nine primary planets, six of which have their own natural satellites,
and many meteoroids with circumsolar orbits. Also, some transient space bodies come into the
area of solar gravity, are not absorbed, and move away. Meteoroids can be classified by mass,
constituents, structure, and orbits. One of these classes encompasses the comets, which have
rather small nuclei and extended tails of sparse matter.
Within the comparatively great expanse between the orbits of Mars and Jupiter lies the
asteroid belt. There are a total of more than 50,000 asteroids, the three largest being Ceres
(770 km in diameter), Pallada (490 km), and Vesta (384 km). The large asteroids, comets, and
other meteoroids with consistent orbits may be interesting bodies for investigation, but small
meteoroids and micrometeoroids present the main danger to space vehicles.
1.3.4 MATTER IN SPACE
Only part of the matter in the cosmos is concentrated in the stars and their planets, which occupy
not more than 1030 percent of the total volume of the observable universe. The remaining mat-
ter, at least several percent and probably more than half, is distributed sparsely throughout
space. Typical values of this density are several atoms per m3 in space between galaxies, about
one atom per cm3 in anabranches of our galaxy, and several atoms per cm3 in interplanetary
space.
At such low densities, matter behaves differently from solid substances on Earth. Because
almost all atoms in space are ionized (especially near the bright stars), electrical and magnetic
forces become essentially more important than short-range forces. Most atoms and ions
move chaotically in local frames of reference, and at velocities ranging from several hundred
kilometres per second to almost the velocity of light (Sellers 2000).
The average kinetic energy of atoms chaotic motion has values from 102 to more than
102 electron-volts. In addition, there are small numbers of particles like electrons, protons,
and atomic nuclei of heavy elements that have energies up to 1020 electron-volts. During solar
flares, the sun emits particles with energies of hundreds of millions of electron-volts. Some
high-energy particles, of 1 GeV and more, have galactic or extragalactic origin, these forming
the cosmic rays. The exact mechanism of particle acceleration in cosmic rays is still unknown,
but it is certain that this acceleration occurs not in stars but in outer space. The existence of such
acceleration processes shows that humans still have much to learn about the nature of space.
1.3.5 DISTANCES AND TIME SCALES IN DEEP SPACE
The average distance from the Earth to the Sun, one astronomical unit (AU) of distance, is
149.6 106 km.
Light travels 1 AU in 499 sec = 8.32 min.
The distance from the Earth to the Moon is 0.3560.407 106 km, to Venus 40260 106 km,
and to Mars is 80380 106 km.
The average distance from the Sun to Mercury is 0.39 AU, to Venus 0.72 AU, to the
Earth 1 AU, to Mars 1.52 AU, and to Jupiter 5.20 AU.
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INTRODUCTION 17
The average distance from the Sun to the farthest planet, Pluto, is 39.5 AU.
A light-year is equal to 9.46 1013 km or 63,239 AU.
The distance from the Sun to Alpha Centauri is 4.3 light-years; to Alpha Canis Major (Sirius)
9 light-years; and to Alpha Gemini (Castor) 45 light-years.
One parsec (pc) is 206,265 AU or 3.086 1014 km or 3.26 light-years.
An important unit of length used in astronomy is parsec (pc). One parsec is 206,265 AU or
3.086 1014 km or 3.26 light-years.
The diameter of our galaxy is 25 kiloparsecs (kpc) or 81,500 light-years.
The Sun is 10 kpc from the galactic center.
REFERENCES
Allen, C. W. 1973.Astrophysical Quantities. London: Athlone.
Beregovoy, G. T., V. I. Yaropolov, I. I. B