Spatial Error Analysis: A Unified Application-Oriented Treatment

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SPATIAL ERROR ANALYSIS

IEEE Press 445 Hoes Lane, ~O. Box 1331 Piscataway, NJ 08855-1331

IEEE Press Editorial Board Roger F. Hoyt, Editor in Chief

1. B. Anderson ~ M. Anderson M. Eden M. E. El-Hawary S. Furui

A. H. Haddad R. Herrick S. Kartalopoulos D. Kirk ~ Laplante

M. Padgett W. D. Reeve G. Zobrist

Kenneth Moore, Director ofIEEE Press Karen Hawkins, Executive Editor Linda C. Matarazzo, Assistant Editor Surendra Bhimani, Production Editor Aerospace & Electronic Systems Society, Sponsor AES Liaison to IEEE Press, Myron Greenbaum Cover Design: William T. Donnelly, WT Design

Technical Reviewers Dr. Myron Kayton, Kayton Engineering Company Dr. James R. Huddle, Litton, GCS Dr. Allan 1. Brockstein, Litton Guidance and Control Systems Jack R. Harris, TRACOR Books of Related Interest from IEEE Press . . . THE CALCULUS TUTORING BOOK Carol Ash 1986 Softcover 544 pp IEEE Order No. PP1776

ISBN 0-7803-1044-6

THE PROBABILITY TUTORING BOOK: Intuitive Essentials for Engineers and Scientists and Everyone Else! Carol Ash 1993 Softcover 480 pp IEEE Order No. PP2881 ISBN 0-7803-1051-9 NAVIGATION: Land, Sea, Air, and Space Myron Kayton 1990 Hardcover 480 pp IEEE Order No. PC2469

ISBN 0-87942-257-2

PROBABILISTIC RISK ASSESSMENTAND MANAGEMENT FOR ENGINEERS AND SCIENTISTS Second Edition Kumamoto and Henley 1996 Hardcover 616 pp IEEE Order No. PC3533 ISBN 0-7803-1004-7 INTRODUCTION TO AIRBORNE RADAR George Stimson 1998 Hardcover 592 pp IEEE Order No. PC5744

ISBN 0-7803-3491-4

SPATIAL ERROR ANALYSIS A Unified Application-Oriented Treatment

David Y. Hsu Litton Guidance and Control Systems

IEEE PRESS

Aerospace & Electronic Systems Society, Sponsor The Institute of Electrical and Electronics Engineers, Inc., New York

This book and other books may be purchased at a discount from the publisher when ordered in bulk quantities. Contact: IEEE Press Marketing Attn: Special Sales Piscataway, NJ 08855-1331 Fax: (732) 981-9334 For more information about IEEE PRESS products, visit the IEEE Home Page: http://www.ieee.org/

© 1999 by the Institute of Electrical and Electronics Engineers, Inc. 345 East 47th Street, New York, NY 10017-2394 All rights reserved. No part of this book may be reproduced in any form, nor may it be stored in a retrieval system or transmitted in any form, without written permission from the publisher. Printed in the United States of America 10

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ISBN 0-7803-3453-1 IEEE Order Number PC5705

Library of Congress Cataloging-in-Publication Data Hsu, David Y., 1945Spatial error analysis: a unified, application-oriented treatment / David Y. Hsu. p. em. "Aerospace & Electronic Systems Society, sponsor." Includes bibliographical references and index. ISBN 0-7803-3453-1 1. Electric engineering-Mathematics. 2. Error analysis (Mathematics) 1. IEEE Aerospace and Electronic Systems Society. II. Title. TK153.H78 1998 621.3'01'51-DC21 98-12430 CIP

In Loving Memory of My Mother

Contents

PREFACE xi LIST OF FIGURES xv LIST OF TABLES xvii CHAPTER 1 Introduction 1.1 1.2 1.3 1.4 1.5

1

Notation 2 Direct Problems 2 Inverse Problems 3 Use of Author-Generated M-files 4 SummaryofM-files 5

CHAPTER 2 Parameter Estimation from Samples

7

2.1 Point Estimate of Population Parameters 8 2.2 Sampling Distribution 11 2.3 Interval Estimate of Population Mean 12 2.4 Interval Estimate of Population Standard Deviation 14 2.5 Summary of M-files 19

CHAPTER 3 One-Dimensional Error Analysis

23

3.1 Normal Distribution 23 3.2 One-Dimensional Error Measures 26 vii

viii

Contents

3.3

3.2.1 3.2.2 3.2.3 3.2.4 Direct

Standard Error 26 Root Mean Square (RMS) Error 26 Mean Absolute Error (MAE) 26 Linear Error Probable (LEP) 27 Problems 28

3.4 3.5

Inverse Problems 29 Summary of M-files 33

CHAPTER 4 Two-Dimensional Error Analysis 35 4.1 4.2

Two-Dimensional Normal Distribution 35 Direct and Inverse Problems 37

4.3

Case 1, A == An P == 0, U E [0, 1] 39 4.3.1 Limiting Situation: u -+ 41 Case 2, A == AC' P == 0, U == 1 41 Two-Dimensional Error Measures 42 4.5.1 Mean Radial Error (MRE) 42 4.5.2 Root Mean Square Radial Error (RMSR) 43 4.5.3 Distance Root Mean Square Error (DRMS) 43 4.5.4 Standard Radial Error (SRE) 43 4.5.5 Circular Error Probable (CEP) 43 4.5.6 CEP Rate and RPE Rate 48

4.4 4.5

4.6 4.7 4.8 4.9 4.10 4.11

°

Case 3, A == A c ' P == 0, U E [0, 1] 49 4.6.1 Limiting Situation: U -+ 50 Case 4, A == Ac ' P E [-1, 1], U E [0,1] 52 4.7.1 Limiting Situation: Ipi -+ 1 55 Case 5, A == A e , P == 0, U E [0, 1] 56 4.8.1 Limiting Situation: a/b == (Jx/ "» 59 Case 6, A == Ae , P E [-1, 1], U E [0, 1] 59 4.9.1 Limiting Situation: Ipi -+ 1 60 PDF of the Angular Position of a Random Point 61 Summary of M-files 64

°

CHAPTER 5 Three-Dimensional Error Analysis 65 5.1 5.2

Three-Dimensional Normal Distribution 65 Direct and Inverse Problems 66

5.3

Case 1, V == Vb, Pxy == Pyz == Pxz == 0,

5.4 5.5

Case 2, V == Vs' Pxy == Pyz == Pxz == 0, U == v == 1 69 Three-Dimensional Error Measures 71 5.5.1 Mean Radial Error (MRE) 71 5.5.2 Root Mean Square Radial Error (RMSR) 71 5.5.3 Distance Root Mean Square Error (DRMS) 71 5.5.4 Standard Radial Error (SRE) 71 5.5.5 Spherical Error Probable (SEP) 72 Case 3, V == Vs' Pxy == Pyz == Pxz == 0, U, V E [0, 1] 76

5.6

U, V E

[0, 1] 68

ix

Contents

Case 4, V == Vs ' Pxy,Pyz,Pxz E [-1, 1], U, v E [0,1] 80 Case 5, V == Ve , Pxy == Pyz == Pxz == 0, U, V E [0, 1] 82 5.8.1 Limiting Situation: al a; == b/(Jy == cla, 83 5.9 Case 6, A == Ae , Pxy' Pyz' Pxz E [-1, 1], U, V E [0, 1] 84 5.10 Summary ofM-files 85 5.7 5.8

CHAPTER 6

Maximum Likelihood Estimation of Radial Error PDF 6.1 6.2 6.3

6.4 6.5 6.6

CHAPTER 7

Basic Assumptions and General Approach 87 Maximum Likelihood Estimation of n, a 88 Dependence of GM/RMS on n 89 6.3.1 ljJ(n/2) for n == Positive Even Integer 90 6.3.2 ljJ(n/2) for n == Positive Odd Integer 90 6.3.3 ljJ(n/2) for 0 < n/2 < 1 90 Dependency of R(P)/RMS on n 92 Relationship Between R(p)/RMS and GM/RMS 93 Summary of M-files 95

Position Location Problems 97 7.1

7.2 7.3

7.4

CHAPTER 8

Single Error-Ellipse Analysis 98 7.1.1 Approach 1. Fictitious Sigma and Cut Angle 101 7.1.2 Approach 2. New Sigmas Along the Major and Minor Axes 104 Consideration of Geometrical Factors 105 Analysis of Multiple Error-Ellipses 109 7.3.1 Mutually Parallel Axes 110 7.3.2 Randomly Oriented Axes 112 Summary of M-files 115

Risk Analysis

117

8.1 Definition of Notation 119 8.2 Seller's Risk 119 8.3 The Pass/Fail Criterion 121 8.4 Buyer's Risk 122 8.5 A Practical Example 123 8.6 Generalization 125 8.7 Summary of M-files 126

APPENDIX A

87

Probability Density Functions A.l A.2

129

Relationship Between PDF and CDF PDFs Used in This Book 129

129

x

Contents

A.3 A.4

APPENDIX B

Method of Confidence Intervals B.l B.2

APPENDIX C

A.2.1 Normal Distribution 129 A.2.2 Chi-Square Distribution 130 A.2.3 Student-t Distribution 130 A.2.4 Rayleigh Distribution 130 A.2.5 Maxwell Distribution 131 A.2.6 Cauchy Distribution 131 A.2.7 Uniform Distribution 131 Central Limit Theorem 131 Generation of Standard Normal Random Variable 131

133

Confidence Interval and Confidence Limits 133 Determination of y-Confidence Interval 134

Function of N Random Variables 135 C.l

C.2 C.3 C.4

Linear Combination of N Independent Random Variables 135 C.l.l Sum of N Random Variables 136 C.1.2 Average of N Random Variables 136 C.1.3 Difference of Two Random Variables 136 Product of Two Random Variables 136 Sum and Difference of Two Correlated Random Variables 137 Independence and Uncorrelatedness 137

C.5 C.6 C.7

PDF of z PDF of z PDF of z

== x + y 138 == x - y 140 == x/y 140

APPENDIX D

GPS Dilution of Precisions

APPENDIX E

Listing of Author-Generated M-files E.l E.2 E.3 E.4 E.5 E.6 E.7 E.8

M-files M-files M-files M-files M-files M-files M-files M-files

Used Used Used Used Used Used Used Used

BIBLIOGRAPHY 211 INDEX 213 ABOUT THE AUTHOR 217

in in in in in in in in

Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter

1 2 3 4 5 6 7 8

143

145 148 153 158 173 190 200 207

145

Preface

This book is designed to serve two primary purposes. 1. To fill a void in spatial error analysis. The term spatial refers to one-, two-, and three-dimensional spaces. Those interested in obtaining a clear and thorough understanding of the various error measures and their interrelationships, including engineers and scientists in the aerospace industry and the DoD/DoT, as well as developers and users of navigation systems and GPS (Global Positioning System), will find this book a useful source/reference. Particularly significant and unique features of this book include the techniques and solutions used to solve the general, correlated cases in two- and three-dimensional error analyses. 2. To provide a toolbox heretofore not available to the engineering/scientific community for carrying out involved computations associated with spatial error problems. Thus, table look-up and "guestimation" can be avoided. The Spatial Error Analysis Toolbox consists of a set of M-files to be used with MATLAB®. MATLAB, developed by MathWorks, Inc., is a powerful interactive system for scientific/engineering computations and graphic data displays; it must be purchased separately from this book. The M-files and data files of the Spatial Error Analysis Toolbox developed by the author for error analysis, position location, and risk analysis can be obtained from the anonymous FTP site of Mathworks Inc. at ftp.mathworks.com/pub/books/hsu, This book is based on my experiences in (1) searching for scattered, hard-to-find topics in the literature, (2) filtering out truth from typographical or conceptual errors, and (3) using crude approximation-eyeballing a curve or interpolating a tabulated data list to arrive at a solution. It is hoped that by placing the related topics under one cover, by defining important concepts clearly and precisely, and by providing a tool to obtain fast and accurate answers, this will serve as a useful technical source book. Thus, newcomers

xi

xii

Preface

in this field will not need to go through the same struggling process as the author has had to do in the past. This book is organized as follows: Chapter 1 defines the notation to be used, classifies the type of problems to be investigated, and describes how to install the author-generated M-files for MATLAB. Chapter 2 discusses two estimation methods of population parameters and quantifies the confidence level about interval estimation. Chapter 3 treats error analysis for the one-dimensional problem. The relationships between root mean square (RMS) error, mean absolute error (MAE), and linear error probable (LEP) are explained. Chapter 4 treats error analysis for the two-dimensional problem. The relationships between mean radial error (MRE), root mean square radial (RMSR) error, distance root mean square (DRMS) error, and circular error probable (CEP) are explained. Chapter 5 treats error analysis for the three-dimensional problem. The relationships between mean radial error (MRE), root mean square radial (RMSR) error, and spherical error probable (SEP) are explained. Chapter 6 shows how to find the maximum likelihood estimate (MLE) for the distribution of radial errors. Chapter 7 discusses solutions for position location problems using error ellipses. Chapter 8 defines the buyer's risk and seller's risk for a precision high-technology product and shows how a reasonable pass/fail criterion can be set so that it will be acceptable to both buyer and seller. Appendices A, B, C provide a brief review of basic concepts in Statistics and Probability. These include (1) various probability density functions (PDFs) and cumulative density functions (CDFs), (2) method of confidence intervals, and (3) linear combination of random variables. They serve as a handy reference for those who have not had formal training in this area. Appendix D shows the basic definitions of the various GPS dilution of precisions in terms of the time and position standard deviations. Appendix E contains the listings for each of the basic author-generated M-files from which other, more involved, M-files are constructed. These programs represent one way to do the job; more clever and efficient approaches may exist. Suggestions for improvement from MATLAB users will be most welcome. These M-file listings may also serve as a guide for the reader who uses computing environments other than MATLAB to develop a personal set of software tools. The collection of M-files used in a chapter is summarized in a table toward the end of that chapter. The name of each M-file as well as its input( s) and output(s) are listed so that the reader will know exactly what to include and what to expect in the MATLAB environment. I would like to express my appreciation to Dr. Allan J. Brockstein, research scientist at Litton Guidance and Control Systems, for his numerous valuable proofreading comments. Thanks are due to Dr. James R. Huddle, chief scientist at Litton Guidance and Control Systems, for his suggestion to include Chapter 8 in this book. I am indebted to Tao Wang from Personal TEX, Inc. for his timely technical support while I was learning PCTEX@ to typeset the book. I would like to acknowledge the many helpful comments from Douglas M. Schwarz concerning the use of his Styled Text Toolbox in making

Preface

xiii

special symbols for the graphs of this book. The Styled Text Toolbox is available to the public at the MathWorks FTP site. I am also indebted to Linda Matarazzo of the IEEE Press for her smooth coordination of the editorial process. Last, but not least, I am grateful to my father, General Si-Yen Hsu, for his love, trust, and constant encouragement. I also wish to thank my family-Charlotte, Henry, and Matthew-whose understanding made the completion of this book possible.

List of Figures

1.1 Results of the MATLAB Command nf2a(l) 5 2.1 Confidence Coefficient for Interval Estimation of Population Mean 14 2.2 Confidence Coefficient for Interval Estimation of Population Mean on a Semilog Plot 15 2.3 Comparison of the Standard Normal PDF with the Student-z, PDF for n == 100 16 2.4 Confidence Coefficient for Interval Estimate of Population Standard Deviation 18 2.5 Confidence Coefficient for Interval Estimate of Population Standard Deviation on a Semilog Plot 19 2.6 Comparison of the Standard Normal PDF with the PDF g(u) for n == 100 20 3.1 Normal Distribution 24 3.2 PDF for the Standard Normal Distribution 25 3.3 PDF and CDF for the Normal Distribution 27 3.4 p==2S~·6h(T:,(J)dtfortheNormalPDF,(J=1 29 3.5 Solution for Example 3.2 31 4.1 Constant PDF Contours of Circular and Elliptical Normal Distributions 36 4.2 Two-Dimensional Elliptical Normal Distribution 37 4.3 Contours of Equal Probability Density 37 4.4 Equal PDF Contour (solid line) and Boundary of Integration Region (dashed line) 39 4.5 PDF and CDF for the Rayleigh Distribution 44 4.6 p == J~ her, (J) dr for the Rayleigh PDF, (J == 1 45 4.7 Probability Versus Ellipticity for p == 0 and R == 1 DRMS and R == 2 DRMS 50 4.8 Normalized Radius Versus Ellipticity for p == 0 and Various p Values 51 4.9 Probability Versus Normalized Radius for u == 0 : 0.2: I and p == 0 51 4.10 CEPt«. versus u: Exact and Two Approximations 52 4.11 Probability Versus Ellipticity for p == 0.3 and R == 1 DRMS and R == 2 DRMS 54 4.12 Geometry for Limiting Situation: p == 1 55 4.13 Probability Versus Correlation Coefficient for R == 1, u == 0 : 0.1 : 1 56

xv

xvi

List of Figures

p-Error-Circle and p-Error-Ellipse, p == 50% 58 Area of 50%-Error-Circle and Area of 50%-Error-Ellipse versus Ellipticity 58 Probability Versus p for a == 2, b == 1.5, and u == 0: 0.1 : 1 60 Geometry for Limiting Situation: p == 1 61 Probability Density Function of Polar Angle 63 Relationships Between Major M-files 64 PDF and CDF for the Maxwell Distribution 72 1 p == fo her, a) dr for the Maxwell PDF, a == 1 73 Probability Versus u with v as Parameter for R == 1 . DRMS 77 Probability Versus u with v as Parameter for R == 2· DRMS 78 Normalized Radius for Various Ellipticities when p == 0.5 79 Normalized Radius for Various Ellipticities when p == 0.95 79 Spherical Volume for Various u and v Values when p == 50% 79 Ratio of 50%-Error Ellipsoidal Volume to 50%-Error Spherical Volume 83 Relationships Between Major M-files 86 GM/RMS Versus n 91 R(P)/RMS Versus n, for Various p 92 R(P)/RMS Versus GM/RMS, withp as Parameter 94 Error-Ellipses 98 Intersection of Two Lines of Position 99 Expanded View at Intersection Point 100 PDF and CDF for the Rayleigh Distribution 101 Old and New Standard Deviations and Cut Angles 101 Sigma Factor Versus Ellipticity u 102 Fictitious Cut Angle Versus Original Cut Angle 103 Probability Versus r, with Cut Angle as Parameter 103 Old a's with Cut Angle and New a's Along Orthogonal Axes 105 Probability Versus r with Ellipticity u as Parameter 106 Radius of Circle with Specified Probability Versus Ellipticity u 106 Probability Versus Cut Angle for Constant Radius 108 Error Factor Versus Cut Angle 111 Input Error-Ellipses and Final Ellipse 113 Geometric Meaning of X~,a and X~,l-b 120 Normalized Pass/Fail Criterion KrJ. as a Function of the Number of Tests N, with Sellar's Risk (X as Parameter 121 8.3 Buyer's Risk versus KrJ.' for N == 8, with A as Parameter 123 8.4 Normalized Pass/Fail Criterion KrJ. as a Function of the Number of Tests N, with Seller's Risk (X as Parameter 124 8.5 Buyer's Risk for KrJ. == 1.25 and N == 8, with A as Parameter 125

4.14 4.15 4.16 4.17 4.18 4.19 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.1 6.2 6.3 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 8.1 8.2

e

List of Tables

1.1 2.1 3.1 3.2 3.3 4.1 4.2 4.3 4.4 5.1 5.2 5.3 5.4 6.1 6.2 6.3 6.4 7.1 7.2 7.3 7.4 7.5 7.6 8.1

M-files Used in Chapter 1 5 M-files Used in Chapter 2 20 One-Dimensional Direct Problem, from R to p 30 One-Dimensional Inverse Problem, from p to R 32 M-files Used in Chapter 3 33 Values of erf(Ro/.J2") 40 Two-Dimensional Direct Problem, from R to p 46 Two-Dimensional Inverse Problem, from p to R 47 M-files Used in Chapter 4 64 Values of erf(Ro/.J2") 68 Three-Dimensional Direct Problem, from R to p 74 Three-Dimensional Inverse Problem, from p to R 75 M-files Used in Chapter 5 85 Observed Radial Errors 93 Errors in x and y Channels 94 Errors in x, y, and z Channels 95 M-files Used in Chapter 6 96 Probability Versus Cut Angle for Constant Radius 107 Significant Parameters of Error-Ellipse, p == 50°A> 109 Significant Parameters of Error-Ellipse, p == 90% 110 Contribution from Each Error Source 112 Parameters of Input Error-Ellipses 113 M-files Used in Chapter 7 115 M-files Used in Chapter 8 126

xvii

Introduction

This book serves as a source/reference book for engineers and scientists working with measurement errors in one-, two-, and three-dimensional space, as well as for people who desire to obtain a clear understanding of the concepts of the various error standards and their interrelationships. Through MATLAB, this book also introduces and provides a convenient tool for computation and comparison so that table look-up and "guestimation" can be avoided. Since the book is application oriented, only the important, relevant results from Probability and Statistics are used. For proofs of theorems and derivations, the reader can refer to excellent textbooks such as [1, 2]. The reader is assumed to have had an introductory course in Probability and Statistics, and to have a fairly good working knowledge of Differential and Integral Calculus. We will be concerned mainly with normally distributed random variables. The normal (Gaussian) distribution is useful because it seems to describe the random observations of most experiments. It also describes the distribution associated with the parameter estimation for most probability distributions. The notation to be used throughout this book is defined in Section 1.1. All problems in error analysis can be cast into two categories: direct problems and inverse problems. In Sections 1.2 and 1.3 we shall define these two types of problems in order to set a pattern for discussion in the chapters to come. Section 1.4 shows how to use the author-generated programs to solve problems in navigation accuracy analysis.

1

Chapter 1 • Introduction

2

1.1 NOTATION We will write random variables symbolically in boldface type as, for example, the random variable x. Often, it is necessary to find the probability that the value of a random variable x is less than or equal to some real number x; this we write as P{x:S x}

The notation shown in the following list will be used throughout this book, and any deviation from it will be noted immediately to avoid any confusion. Let ~ stand for "equal by definition." ] ~ row vector ]' ~ column vector • ~ == [x]' ~ position vector of one component • ~ == [x, y]' ~ position vector of two components • ~ == [x, y, z]' ~ position vector of three components • get) ~ probability density function of a random variable t • G(t) ~ cumulative distribution function of get), G(t) == J~oog(u) du • P{V E A} ~ probability that the random vector Vfalls into region A • Ntp; (1"2) ~ normal distribution with mean fl and variance (1"2 • [ • [

1.2 DIRECT PROBLEMS Let the vectors ~ == [x]', ~ == [x, y]', and ~ == [x, y, z]' represent one-, two-, and three-dimensional random vectors, with corresponding probability density functions, 11 (x), h.(x, Y), and hex, y, z), respectively. The direct problem consists of finding the probability p == P{ V E A} when the region A is specified. Variables flx' fly' flz' and R are defined in Section 1.3. For one-dimensional problems, A could be an interval specified as IX-flxl:S

(1.1)

R

For two-dimensional problems, A could be a circular region: (x -

flx)2

+ (y -

fl y)2

:S R 2

(1.2)

or A could be an elliptical region with elliptical scale k (x -

flxi + (y - fli b2

a2

< k2

(1.3)

-

For three-dimensional problems, A could be a spherical region: (x -

flx)2

+ (y -

fl y)2

+ (z -

flz)2

:S R 2

(1.4)

or A could be an ellipsoidal region with ellipsoidal scale k (x -

flxi + (y - flyi + (z - flzi

a2

b2

c2

< k2 -

(1.5)

Section 1.3 • Inverse Problems

3

== P{ ~

Thus, the relationship betweenfi (x) and p

P == P{f}

E

A} == P{x

E

(,u - R,,u

+ R)} ==

Furthermore, the relationship betweenjjfx, y) and p

p = P{Vz

E

A} = P{(x, y)

E

E

A} =

A} is

I

Ji+R

fi(x) dx

Ji-R

== P{t2

H

E

A} is

h(x, y) dxdy

A

and the relationship between hex,y, z) and p

p = P{tJ

E

A} = P{(x, y, z)

E

== P{~

A} =

HI

E

A} is

!3(X, y, z) dxdydz

A

The direct problem becomes "given the dimension of A, find p == P{V E A}," where the dimension of A is specified in terms of R or k in Equations (1.1) through (1.5). If exact integration is possible, p can be expressed as a closed-form formula; otherwise p can be obtained through numerical integration of a single, double, or triple integral.

1.3 INVERSE PROBLEMS

For the inverse problem, the probability p is given, and one is to find, depending on dimensionality, the half-length R of the interval centered at the mean llx for the onedimensional problem; the radius R or the scale k of the circle or ellipse centered at the mean (llx' ,uy) for the two-dimensional problem; and the radius R or the scale k of the sphere or ellipsoid cente!ed at the mean (,ux' ,uy' ,uz) for the three-dimensional problem, such thatp == P{V E A}. That is, given p, find the dimension of A in terms of R or k in Equations (1.1) through (1.5) such that Ji+ R

JJi-R or

H

fl(X)dx

== p

h(x, y) dx dy

=P

A

or

HI

!3(x,y,z)dxdydz=p

A

Two approaches are used in this book to solve the general inverse problem: given p, find R such that F(R) == p.

4

Chapter 1 • Introduction

The first approach is via numerical trial-and-error. We make an initial guess R o of the true solution and compute Po == F(R o). If Po > P, decrease R o; if Po < p, increase Ri; The process is repeated until adequate resolution is attained. The second approach uses Newton-Raphson's method [3] to find the root of the equivalent problem G(R) == F(R) - P == 0

We start with an initial guess Ro, and we iterate according to R i+ 1

== R, - G(Ri)f G'(R i),

for i == 0, 1, 2, ...

(1.6)

where G' is the derivative of G, until a certain accuracy criterion is satisfied. This process is implemented in the M-file newton.m. 1.4 USE OF AUTHOR-GENERATED M-FILES

All the programs used in this book are MATLAB M-files. Each has been tested thoroughly with MATLAB Version 4.2c and Version 5.2 on various PC platforms (Pentium [166 MHz, 66 MHz], 486-60 MHz computers). In order to install and use these M-files, the reader should follow these steps: 1. Create a subdirectory c:\mfile md c.xrnfile 2. Copy all files from a.vmfile to c.vmfile copy a:\mfile \ *.* c:\mfile 3. (for Version 5.2) Start MATLAB. Click path browser to include c.vmfile in the MATLAB path. (for Version 4.2c) Include c.xmfile in the path of MATLAB by adding the line 'c.vmfile' in the file c.unatlabxmatlabrc.m through an editor. Start MATLAB. 4. Enter nf2a(I). The user should see the numerical result

ans == 0.6827 produced with an accompanying graph (see Figure 1.1) which describes the geometric meaning of the number 0.6827. This indicates a successful installation of the M-files. The names and functions of these author-generated M-files are listed in the last section of each chapter. Those who wish to jump right in for a hands-on experience are encouraged to do so by referring to these sections in Chapters 1 through 8.

Section 1.5 • Summary of M-files

0.4

5

Area under Normal PDF f(x), for x in [-1, 1] is 0.6827

-

r-----...----...----r-----..,.~--,.__--_r__"'I"--

__

0.35 0.3 0.25

~ 0.2 0.15 0.1 0.05 0-4

-3 Figure 1.1

-2

o

-1

2

x

3

4

Results of the MATLAB Command nf2a(l).

1.5 SUMMARY OF M-FILES The M-files used or generated in this chapter are summarized in Table 1.1. TABLE 1.1

M-files Used in Chapter 1

File Name nf2a.m nfl.m nf2.m pf3.m newton.m

MA TLAB Command p == nf2a(r) == nfl(x) p == nf2(r) pf3('fname') x == newton(xo,' fun',' dfun', tol)

y

The three files nfLm, nC2.m, and pC3.m are called within the test program nC2a.m.

Parameter Estimation from Samples

All possible outcomes of a physical experiment constitute the population space of a random variable being observed; a finite set of observed outcomes of this experiment is called a sample. The frequency with which an outcome occurs depends on the corresponding probability density function (PDF) associated with the population space. Parameters of the population space such as mean and standard deviation are defined through the PDF. A random variable t with a PDF J(t) will be denoted as t ro..I J(t), and a random variable z with a standard normal probability density function j(z)

1 p [Z2] = ,J21tex -2 £ N(O, 1)

(2.1)

will be abbreviated as z ro..I N(O, 1). In general, if Xro..Ig(x), then its mean value or expected value is defined as JI x

= E[x] = J~oo x g(x) dx

and the standard deviation is defined as

The ratio of o, to /lx expressed in units of percentage (%), IOO(J xl/lx' is called the coefficient of variation of the random variable x. We may drop the subscript and simply write u or (J when the context makes it clear as to which random variable is being discussed. The term variance is used to refer to the square of the standard deviation 7

8

Chapter 2 • Parameter Estimation from Samples

or var[x] == (J~. Variance is also called the second central moment from the general definition of the kth central moment E[(x - ,ux)k]. Since it is sometimes impractical, if not impossible, to construct the entire population space, often a finite-sized sample is used to estimate the parameters of the population space. In this book, Greek letters are used to denote population parameters; for example, ,u is used for population mean and (J for population standard deviation. Roman letters are used to denote the corresponding estimates from a finite sample such as m for sample mean and 8 for sample standard deviation. There are two types of estimation for the population parameters: point estimation and interval estimation. Both estimates are expressed in terms of the observed values of the random variable. Section 2.1 introduces point estimation for both population mean and population standard deviation. Section 2.2 presents two important theorems concerning sampling distributions. Sections 2.3 and 2.4 discuss the interval estimation of population mean and population standard deviation, respectively.

2.1 POINT ESTIMATE OF POPULATION PARAMETERS

When a set of N observations {Xl, X2, ... , XN} of a random variable x is obtained from an experiment, the sample mean m and sample standard deviation 8 are computed via the point estimation formulas N

m == LXi/N

(2.2)

i=l

8

==

N

L(Xi - m)2/(N - 1)

(2.3)

i=l

These two formulas indicate the dependence of the point estimates on the observations and are implemented in the built-in MATLAB M-files mean.m and std.m, respectively. The term? is also called an unbiased estimate of the variance. We will define "unbiased estimate" and show why? is an unbiased estimate of (J2 next'. A statistic y is called an unbiased estimate of the parameter 11 if the expected value of y equals 11; that is, E[y] == 11. Thus, to verify that 8 2 is an unbiased estimate of (J2, we need to show that

First, we know that if the random variable x is normally distributed with N(,u, (J2) PDF, then the sample mean m == L~=l x.] N is also a normally distributed

'u can be shown that E [s] = {r(N/2)/r[(N - 1)/2]}J2/(N - 2) 0"#0", hence, s is a biased estimate of the standard deviation 0".

9

Section 2.1 • Point Estimate of Population Parameters

random variable with N(Il, (J2 / N) PDF. Now proceeding, we obtain

E[?]

= E[N ~ 1 ==

E

(x, - m)2]

E{_I_ f.

N - l i=l

I

[(x; - Ji) - (m - Ji)f}

N

== E {- - L [(Xi - 11) - 2(Xi - Il)(m - 11) + (m - 11) ] N - l i=l

1 E{ == - -

N -1

2

LN [(Xi -

i=l

= ~1 {E[f. (x, N

l=l

= N 1-

1

2 }

11)2 - 2(Xi - Il)(m - 11) + (m - 11) 2 ]} Ji)2] - 2E[f. (x, - Ji)(m - Ji)] l=l

+ E[f. (m l=l

Jii]}

t1 E[(Xi - Jii] - 2NE [Nt1 (x,'N- 11) (m - Ji)] + t1N E[(m - Jii] }

{N

1 == --1 {N(J2 - 2NE[(m - 11)2] + NE[(m - 1l)2]}

N-

=N

1 2 _ 1 {No- - NE[(m -

Jiil}

1 { N(J2_N(J2} ==-N-l N

== -1- [N(J2 - (J2] == (J2 N -1

We have employed the fact

f.

(Xi -

i=l

N

Ji)

= m-

u

to arrive at the conclusion. An example will illustrate how these two files (mean.m, std.m) are used. EXAMPLE 2.1

From the set of 100 observed values of a random variable x contained in the data file eg2.1, find its sample mean and sample standard deviation.

The four MATLAB commands

load c e vmf i Le ve qz L v

x

= eg2

mean(x) std(x) show that m

== 0.1318 and s == 5.3140.

10


Equation (2.3) shows that one needs to find the value of the sample mean m before s can be determined. The following equation provides a more straightforward approach: N

N

i=1

i=1

NL:xf - (L: X i) 2 s==

N(N - 1)

The mean deviation (MD) for the sample

{Xl,

X2, ... , XN}

IS

defined as

N

MD

L:IXi-ml

i=1

N

If we construct another sample {YI, Y2, ... , YN} with Yi == Xi + d for i == 1,2, ... , N, d being a constant, then the two means differ by d. Nevertheless, the two mean deviations are identical, as seen in Example 2.2. EXAMPLE 2.2

Given a sample of twelve elements

{102, 115, 110, 109, 112, 121, 103, 113, 106, 114, 113, 123} find the mean and the mean deviation. Repeat the same process but first deduct 100 from each member of the sample. Dse the six MATLAB commands

x=[102, 115, 110, 109, 112, 121, 103,113,106,114,113,123];

N

= length(x);

xml = mean (x ) mdl

= sum (abs (x-xml) ) /N

xm2 = mean (x-lOa) md2

= sum(abs(x-IOO-xm2)

)/N

to obtain m, == 111.75, m: == 11.75, and MD l == MDz == 4.7917.

In the CONTINUOUS case, the root mean square (RMS == JE[x 2 ]) and standard deviation (0) of a random variable x with mean J1 == E[x] are related by (RMS)2 == E[x2] == E[(x - J1 + J1)2] == E[(x - J1)2] == (J2

+ J12

+ 2J1E[(x -

J1)]

+ J12 (2.4)

Section 2.2 • Sampling Distribution

11

case, the RMS (= {L~l x 2 j N ) and the biased estimate of the variance (s~ = L~l (Xi - m)2 IN) 0 a random variable x with mean m = L~l x.]N are related by In the

DISCRETE

N

(RMS)2 = L i=1

xl IN =

N

L(Xi - m + m)2 IN i=1

N

N

N

= L(Xi - m)2 IN + 2m L(Xi - m)IN + L m21N i=1 i=1 i=1 N

= L(Xi - m)2 I N i=1

+ m2

=s~+m2

(2.5)

In terms of the sample standard deviation s, we have N -1 2 (RMS) 2 =_-s2+ m N

(2.6)

Ifwe use the dataset from Example 2.1 with the M-file rmsx.m, we will obtain RMS = 5.2890 via the command RMS == rmsx(x).

2.2 SAMPLING DISTRIBUTION

The sample statistics defined in terms of the observations of a random variable are themselves random variables. Hence, one can talk about the sampling distribution, the PDF of the sample statistic. At the beginning of Section 2.1, we defined the sample mean and sample standard deviation in Equations (2.1) and (2.2), respectively, as and

s=

N

2

L(X; - m) I(N - 1) ;=1

where {x., X2, ... , XN} are N random samples of x. Note that nothing is said about the PDF of the random variable x. For the discussion of the sampling distribution of these two statistics, we will assume that the random variable x is normally distributed. We state without proof (see [1, 4, 6]) the following important results. Theorem 2.2.1. Suppose that x is a normal random variable with mean J1 and variance (J2. Let x., X2, ... , XN be N random observations ofx. Then the random variable m has the normal distribution with mean J1 and variance (J2 IN, and the random variable

has the X2 distribution with (N - 1) degrees offreedom. Furthermore, m and S2 are independent variables.

12


Theorem 2.2.2. Suppose that z is a standard normal random variable and v is a X2 random variable with n degrees offreedom, then if z and v are independent, the random variable z

~ has the student-t distribution with n degrees offreedom. Theorem 2.2.3. If x is a normal random variable with mean J1 and variance cr2, then the random variable m-J1

s/~ has the student-t distribution with (N - 1)degrees offreedom. The X2 and student-t distributions are discussed further in Appendix A.

2.3 INTERVAL ESTIMATE OF POPULATION MEAN

In the preceding section, point estimates of parameters have been considered. Often, however, one prefers an interval estimate that will express the accuracy of the estimate as well. This interval estimate provides a range into which the population parameter may fall. The end points of the interval are called the confidence limits for the parameter. Starting from a random sample of size N, we compute its mean m and standard deviation s. We then define an interval in terms of m and s, and determine whether the true population parameter can be located within this interval in a statistical sense. The confidence associated with the estimate, or how close the estimate is to the true but unknown value of the population parameter, depends on the frequency or probability that the interval estimate actually captures the population parameter of concern. As expected, the confidence will grow with the number of random samples that are taken. The accuracy associated with the confidence interval or the degree of confidence about this interval estimate is called the confidence coefficient. The degree of confidence we have for the population mean J1 to be captured in the interval (m - ES, m + ES) is expressed as the probability that the population parameter of interest falls into this interval centered at the sample mean m, and having a halfwidth of ES. Here S is the sample standard deviation and E is a specified fraction, where p

== P{J1

E

(m - ES, m + ES)}

Note that p is dependent on both m and S from the sample. This equation can be rearranged to the following equivalent statements p

== P{m - ES < J1 < m + ES} == P{-E < (m - J1)/s < E} == P{-E~ < (m - J1)~/s

< E~}

(2.7)

13

Section 2.3 • Interval Estimate of Population Mean

Since the random variable (m - /l)JN js has a student-t distribution with n == N - 1 degrees of freedom, by Theorem 2.3, we have l'

g(r) where n

== tN-I == (m - /l)JN js with PDF

= n(n + 1)/2] (1 + r 2 /n)-(n+2)/2, y'1mr(nj2)

for real r

(2.8)

== N - 1, and the gamma function rex) is defined as rex)

= J~ t x - 1 exp "

dt, for x > 0

Thus, the confidence coefficient p becomes p

== P{-8J"N -,JI99 == -14.11, when n == 100. The two curves practically coincide, justifying the computation of p using N(O, 1) instead of the X~ distribution. 2.5 SUMMARY OF M-FILES

The M-files used in this chapter are summarized in Table 2.1


20

f(x) =N(O, 1) is defined for all real u 0.5 ...--~----r---~--,.----r---. . . . .

- ---.-----r--------

0.4 ................. 0.3

• • • • • • ••

:

,

:

:

•••••••••

:

:

:

:

.

.

.

~

.

.

• .' • • • • • • • • • ,', • • • • • • • • s'

••••••••

0.2

. . .................. .

.

. .

. .

. .

.

,

.

.

0.1

:

:

:

:

:

:

-:

.

O'----.........~--=-_--&..-_....L---~-_....L--_-"""'----~ ......._ - - L_ _--J 4 5 2 3 -1 o -4 -3 -2 -5

x

g(u) is defined for

· 0.4 ........ :

.

.

.

. .

0.3

~ 0)

when n = 100

u~-14.11,

:

:

:

.

.

.

. .

.

~

.

.

•• • • • • • • • • • • •• • • • • • • • • • •• • • • • • • • • • 1 • • • • • • • •

· ., 0.2 ................... . ·

.

.,

. .

.

.

.

.

.

.

.

0.1

:

:

:

:

:

:

:

.

;

O"-_.........

~

-5

-4

Figure 2.6

:

_ _=-_--I-_ _....L-__

....L--__"""'---_....;;;;;::::I....

_ - - L_ _--J

...L-_ _

-3

-2

-1

o

2

u

3

4

Comparison of the Standard Normal PDP with the PDP g(u) for n == 100.

TABLE 2.1 FileName rmsx.m nfl.m tandn.m xandn.m tdis.m gofu.m x2df.m x2dg.m x2cdf.m cnf.m mucnf.m

M-files Used in Chapter 2 MATLAB Command y == rmsx(x) == nfl(xo) tandn xandn y == tdis(x) y == gofu(u) y == x2df(x) y == x2dg(x) p == x2cdf(x) r == cnf(f" n) p == mucnfie n)

y

5


21

The M-files tandn.m and xandn.m are used to generate Figures 2.3 and 2.6, respectively. The standard normal distribution is implemented in the M-file nfl.m. The student-r, distribution with n degrees of freedom (see Equation [2.8]) is implemented in the M-file tdis.m. It is invoked in the program tandn.m. The PDF g(u) of the random variable u (see Equation [2.14]) is implemented in the M-file gofu.m. It is invoked in the program xandn.m. The PDF and CDF of the X2 distribution with n degrees of freedom are implemented in the M-files x2df.m and x2cdf.m, respectively. The file x2dg.m is needed for x2cdf.m, according to Equation (A.3) in AppendixA.

One-Dimensional Error Analysis

We start error analysis with the simplest kind-one-dimensional error analysis. Taking measurements of the height of a person, for example, or any measurement with a single degree of freedom, gives rise to this type of problem. We will be concerned mainly with normally distributed random variables. The normal (Gaussian) distribution is useful because it describes the random observations of most experiments, and it describes well the distribution of the estimation of parameters from most probability models (via the central limit theorem; see Appendix A).

3.1 NORMAL DISTRIBUTION

The probability density function for one-dimensional errors, treated as a random variable t, is generally taken to be a normal distribution

get, 11, (1")

1

= .J2ii(1". exp

[-( t 2(1"2

p)2] ,for real t

where u is the mean and (J is the standard deviation of the random variable t. A short-hand notation for this is t N(f.1, (J2). The curves in the upper graph of Figure 3.1 show how the peak location of the PDF shifts as the mean value varies for (J == 1. The curves in the lower graph show how the dispersion (or scatter) changes as the standard deviation value varies for f.1 == o. t'.I

23

Chapter 3 • One-Dimensional Error Analysis

24

cr = 1, for all three curves

-.-

OL...-----&-----'---.L---.....I-----L.--"--_-1--_.....;;;;..L_---==L.---=.J

-5

0.4

-3

-4

-2

o

-1

J.J

4

5

=0, for all three curves

r------,r-----y----..,.----.---,---~----.-----.,

, ,. _ I

0.3 C\I

ob

3

2

0.2

~ 0.1

...... - -"

:

-..:- -

-. ~. -

: :

: :

: :

.~_

~ - -"

_

_

-.

. . ..

I

:

: :

\

\

I

~./......

f

=

\_···cr %

: :

_..'-:. _

\: • .

cr

.. :

:~/:

_

-2

-3

.

_

:

: :

_.. -

: :

=1\ ~ ..-_ _ j-_.,-

j

-

. . ..

;--. ::./t~::-::~···T~~:%:i\.: ··L~:·

L

0-===~__JL....___-....::J~

-4

~.,

:,

. ····;·········· ......1-......:::=-

_ _ L_ ____L_ ___L._ ___S::.._ _

o

-1

Figure 3.1

2

3

4

t

Normal Distribution.

After the mean J1 is removed, the new random variable 1" == t - u has the probability density function (PDF) hi», a)

= ~a· exp[~;:J for real r

(3.1)

This new random variable T is said to be normally distributed with zero-mean. Most random errors encountered in applications are of this type. A further scaling of the random variable T by the standard deviation (J will lead to a new variable x == T / (J with the probability density function f(x)

= ~. exp[

tl

for real x

(3.2)

The random variable x is said to have a standard normal distribution and is commonly abbreviated with the notation x~ N(O, 1). Note that I(x) is an even function in x and h(T, (J) is an even function in T. The standard normal distribution will be used throughout this book since any other normally distributed random variable t ~ N(j1, (J2) can be transformed to x-> N(O, 1) via the formula (see [5]): t - j1

x==-(J

(3.3)

25

Section 3.1 • Normal Distribution

This transformation first removes the mean f.l and then normalizes the difference == t - u with respect to the standard deviation (J. Figure 3.2 shows a bell-shaped normal PDF with f.l == 0 and (J == 1. This PDF has a peak value of Ij,J'fn ~ 0.3989 at x == 0 and slowly decreases toward zero for [x] greater than 4. The probability that the random variable t falls in the interval [-R + u, R + j1] is equivalent to the probability of finding the random variable l' in the interval [-R, R], and is also equivalent to the probability that the random variable x falls in the interval [-R/a, R/a]. This probability is found from

l'

== P{f.l -

p

R ~ t ~ f.l

+ R}

== P{-R b, decrease the dimensions of A, then go back to (2). If Po - P > b, increase the dimensions of A, then go back to (2).

Therefore, both the direct problem and the inverse problem require an efficient way to compute the double integral of Equation (4.2). Depending on the size of the ellipticity u, the shape of the integration region A, and the value of the correlation coefficient p, six cases are to be investigated: • Case I,A==A r , p==O, uE[O,I] • • • • •

Case Case Case Case Case

2, A == A c , 3, A == A c , 4, A == A c , 5, A == A e , 6, A == A e ,

p == 0, U == 1 p == 0, U E [0, 1] p E [-1,1], U E [0,1] p == 0, U E [0, 1] p E [-1,1], U E [0,1]

In order to easily refer to these cases without lengthy description, we rely on an abbreviation system using six characters (XXYYZP). The meaning of the sixcharacter string is as follows: XX: (ED, CD) YY: (RA, CA, EA) Z: (U, C) P: (D, I)

(Elliptical Distribution, Circular Distribution) (Rectangular, Circular, Elliptical Area) (Uncorrelated, Correlated) (Direct problem, Inverse problem)

Thus, the direct and inverse problems to be considered are: Case Case Case Case Case Case

1 2 3 4 5 6

(EDRAUD), (EDRAUI) (CDCAUD), (CDCAUI) (EDCAUD), (EDCAUI) (EDCACD), (EDCACI) (EDEAUD), (EDEAUI) (EDEACD), (EDEACI)

Section 4.3.

Case 1,

A = Ar ,

0,

p =

U E

[0,1]

39 Case 2

Case 1

~O'-'-'-'-'-i

I · I · I · I

.

. I . I . I .

_._.-._.--.~

Case 4

Case 3

ce-'-j)

Figure 4.4

" .... --. ,,'

r?Y __ ~

Case 5

Case 6

.

.,,'

Equal PDF Contour (solid line) and Boundary of Integration Region (dashed line).

These cases are studied in Sections 4.3 through 4.9. Figure 4.4 shows a representative geometry (equal PDF contour and integration region) for each of the six cases. When naming M-files developed for these six cases, we will also use the same abbreviation convention.

4.3 CASE 1, A == A r ,

p == 0,

[0,1]

U E

With the assumptions that the region of integration is rectangular and the two channels are uncorrelated, the double integral in Equation (4.2) can be written as

p ==

I J b

-b

-a

2 1

1UJ x (J Y

1-1

2

y2] exp -2 [X 2+2 (J x

(J

y

I

dxdy

(4.3)

which is then simplified to p

==

erf[_a].erf[_b] ,J2 (J x

,J2 a y

(4.4)

Even though four parameters (a, b, ax, a y) appear in the preceding equation, only two normalized variables (ao == a/ax, bo == b/a y) are needed to determine the probability p. The direct problem can be solved either from look-up in Table 4.1, containing the values of erf(Ro/ ,J2), or the M-file edraud.m can be used. The MATLAB command to use this file is p == edraud(a, b, a x, a y). Upon using Table 4.1, we should first split R o into an integer part i and a decimal part d, that is, R o == i + d. The value of erf(Ro/,J2) is then obtained at the intersection of column i and row d. Thus, if R o == 1.2, we have i == 1, d == 0.2, and erf(I.2/,J2) == 0.7699. We can also use the M-file nf2.m via the command nf2(1.2) to arrive at the same answer, since erf(r /,J2) == nf2(r).

40

Chapter 4 • Two-Dimensional Error Analysis TABLE 4.1

Values of erf(Ro/,J2)

Ro=i+d

d

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.0000 0.0797 0.1585 0.2358 0.3108 0.3829 0.4515 0.5161 0.5763 0.6319

0.6827 0.7287 0.7699 0.8064 0.8385 0.8664 0.8904 0.9109 0.928r 0.9426

2

3

0.9545 0.9643 0.9722 0.9786 0.9836 0.9876 0.9907 0.9931 0.9949 0.9963

0.9973 0.9981 0.9986 0.9990 0.9993 0.9995 0.9997 0.9998 0.9999 0.9999

An example follows. EXAMPLE 4.1 If a position error has an elliptical normal PDP with a, == 0.4 nmi and (Jy == 1 nmi, and errors in the x, y channels are uncorrelated, what is the probability that a measured position error will fall within a rectangle bounded by [x] ::; a == 0.6 nmi and Iyl ::; b == 1.2 nmi? The probability can be found from Table 4.1, with ao == (0.6/0.4) == 1.5 and bo == (1.2/1) == 1.2. Hence, P == erf(aol,J2)erf(bo/,J2) == nf2(ao)nf2(b o) == 0.8664 x 0.7699 == 0.6670 or enter the MA TLAB command:

p = edr aud (0 . 6, 1.2, O. 4, 1) which gives the same result.

Given (Jx, (Jy, and p, there are many (a, b) pairs that satisfy Equation (4.4). The inverse problem can result in multiple solutions of (a, b) if one does not specify the ratio v == a/b. Notice that, unlike the ratio u which is bounded by 0 and 1, the ratio v can have any value in [0, (0). Solutions for inverse problems are obtainable from edraui.m. The MATLAB command is [a, b] == edraui(p, v, a; (Jy). Let us look at another example. EXAMPLE 4.2 The joint PDP of a certain position error is elliptical normal with (Jx == 1.5 nmi and (Jy == 0.6 nmi, and errors in the x, y channels are uncorrelated. Determine length a along the x-axis and width b along the y-axis, if the specified ratio is v == (a/b) == 2, and also if the probability is 90% for a sample position error to fall inside this 2a x 2b rectangle. After entering the MA TLAB command

[a, b l = edr aui (0 . 9 , 2 . a, 1. 5 , a. 6) , we obtain a == 2.6650, b == 1.3325. Note that the requirement v == 2 == (a/b) == (2.66501 1.3325) is met. If v == (alb) == 1/2, we obtain, from

[a, b ]

= edr aui (0 . 9 , a. 5 , 1. 5 , a. 6)

Section 4.4 • Case 2,

A == A c ,

p ==

0,

U

41

== 1

the alternate results a == 2.4673, b == 4.9346. This demonstrates the necessity of clearly specifying v == alb for the inverse problem.

4.3.1 Limiting Situation: u

~

0

As the ellipticity u approaches zero, the two-dimensional joint distribution becomes that ofa one-dimensional PDF. We have in place of Equation (4.4):

1 /fC

p == u

J-u v 2n (J x

4.4 CASE 2, A == A c ,

p ==

0,

2]

X exp [22 dx == erf [

(J x

Ma

v 2 (J x

]

(4.5)

u == 1

When the random variables x, yare uncorrelated (p == 0) and have equal standard deviation (rr, == (Jy == (J), and the integration region is circular with radius R, Equation (4.2) becomes

P=

JJ 2:(j2 exp[ - 2~2 (x2 + l)] dx dy

(4.6)

Ac

with A c : x 2 + y2::; R 2. This double integral can be transformed to an equivalent polar form

2] 2] JR 1 [-r [-R P = 021r JR 0 2n(j2 exp 2(j2 r dr de = 0 h(r, rr) dr = 1 - exp 2(j2 J

(4.7)

Note that

r

[r

2

her, (J) == (J2 exp 2(J2 ] ' for r 2 0

(4.8)

which is the Rayleigh distribution function. The independent variable of this function corresponds, for example, to the random planar radial error, r == Jx2 + y2. Following the notation convention of Chapter 3 (page 25), where we used the notation her, (J) as the PDF of r, and H(R/(J) == 2f~h(r, (J)dr as the CDF, we now have her, (J) and H(R/(J) == f~h(t, (J)dt for PDF and CDF, respectively. Thus, with Equation (4.7) and the substitution of the normalized radius r == R/(J, we have p == H(R/(J) == 1 - exp(-R 2/ 2(J2) == H(r) == 1 - exp(-r2/ 2)

(4.9)

Geometrically, this means that the volume under f(x, y) over the region x 2 + y2::; R 2 is equivalent to the area under the Rayleigh PDF her, (J) over the interval [0, R]. Therefore in this special case, and this case alone, two-dimensional error analysis can be performed in terms of one random variable r, the planar (two-dimensional) radial error. Equation (4.9) provides the closed-form solution for the direct problem in this special case, and the MATLAB M-file for it is cdcaud.m. The command form in MATLAB is p == cdcaud(R, a). An example follows.

42

Chapter 4 • Two-Dimensional Error Analysis EXAMPLE 4.3 If a position error has a circular normal distribution with (J x == (J y == 0.8 nmi, and errors in the x, y channels are uncorrelated, what is the probability that a sample position error will fall within a circle of radius R == 1.2 nmi?

p = cdcaud(1.2,0.8) = 0.6753 There is a 67.53% chance.

The inverse problem can also be solved from Equation (4.9) analytically in this particular case. When p is given, R

== (JJ-2In(1 - p)

(4.10)

The MATLAB M-file cdcaui.m is implemented for solving this inverse problem. Its command form is R == cdcaui(p, (J). An example is given next. EXAMPLE 4.4 If position error has a circular normal distribution with (J x == (J y == 1.8 nmi, and the errors in the x, y channels are uncorrelated, find the radius of a circle such that 50% of the time a measured position error will fall inside this circle?

R=cdcaui(0.5, 1.8) =2.1193 Thus, the radius of this SO%-probability circle is 2.1193 nmi. The radius for a p == 50% circle, R SOOfo , is also called the CEP (see next section).

The M-files cfl.m, cf2.m, and cf3.m carry out the mathematics in Equations (4.8) through (4.10), respectively; cfl.m is for the Rayleigh PDF, cf2.m is for direct problems, while cf3.m is for inverse problems. They all assume (J == 1. Hence, cf2.m and cf3.m are the normalized counterparts of cdcaud.m and cdcaui.m, respectively. This means that the two commands p == cdcaud(R, (J) and R == cdcaui(p, (J) can be obtained from p == cf2(r) and r == cf3(p), with r == R/(J being the normalized radius. 4.5 TWO-DIMENSIONAL ERROR MEASURES

Because of its simplicity, the Rayleigh distribution in Equation (4.8) will be used to illustrate several two-dimensional error indicators. These include mean radial error (MRE), root mean square radial error (RMSR), distance root mean square error (DRMS), standard radial error (SRE), and circular error probable (CEP, see [8]). 4.5.1 Mean Radial Error (MRE)

Mean radial error (MRE, /lr) is defined as

MRE = u;

= E[r] =

J:

rh(r, ai dr =

~a ~ 1.2533a

(4.11)

Section 4.5 • Two-Dimensional Error Measures

43

4.5.2 Root Mean Square Radial Error (RMSR) The mean square radial (MSR) error is defined as MSR

= E[r 2] = J~ r 2h(r, a) dr = 2a2

The root mean square radial error (RMSR), also called radial error (RE) in [8], or radial position error (RPE), is obtained by taking the square root of the MSR, RMSR == RE == RPE == j£[r2] == J2(J ~ 1.414(J

(4.12)

To avoid confusion with the random variable introduced in Section 4.4, the author will not use the term radial error for the type of error shown above.

4.5.3 Distance Root Mean Square Error (DRMS) DRMS is defined as the square root of the sum of the x and y variances as DRMS

= Ja~+a~ = ~ax

(4.13)

When u == 1 (or a, == (Jy == (J), DRMS ==

J2 (J == RMSR

but, in general, DRMS i= RMSR. The GPS horizontal dilution of precision (HDOP) is defined in terms of this two-dimensional DRMS (see Appendix D).

4.5.4 Standard Radial Error (SRE) The variance of the radial error r is (4.14)

The standard radial error is the square root of this variance; thus, SRE == a, ~ 0.6551(J. These three error indicators (SRE, RMSR, MRE) are related as (J; == (SRE)2 == (RMSR)2 - (MRE)2

(4.15)

Notice that RMSR (or DRMS) is not equal to (J or a..

4.5.5 Circular Error Probable (CEP) The circular error probable (CEP) is defined to be the particular radius of a circular region A c over which the double integral in Equation (4.6) produces a probability of 50%. Under these conditions, we see from Equation (4.10) that CEP == Rso% == (JJ -2In(1- 0.5) == (J j21n(2) == 1.1774(J

44

Chapter 4 • Two-Dimensional Error Analysis

0.9 0.8

~

~

:

.: :. .

· ·:

~

: · . . ..: . : :

8

0.7 ...........

:s:

0.6

LL-

0.5

II

0.4

~

0.3

-c

0.2 ... /

LL

II

:r: o a..

--

~

0.1

:

.••••••••••: •.•••

~

~

.

. . . .. .. .

"';"-~-'40",:'

f

..... ....~. H(r) = ~ .h(t, 1)dt ..

···

.

••..••

...

..

~

.

~

RMSR ~ . ..•••..••••.:•••...•••••• : ..•••...••. ~

.........y// _. ~~~~~········_···········I···········_,,···········_· r:

/

"

o0

"

~

L

~

~

~ .

~

.

", j

~

1~5

;

~

2~5

=1 in Rayleigh

r (assume o Figure 4.5

~

:.......•...•~.,.••......

~

~

.

~

~~.j.~(~!}

~ ~ j .:.......•.... ; ....•.•.... 0~5

~

L l>., ; .

.\..

J...•....

,~

.... 3- - -

3.5

4

PDF)

PDF and CDF for the Rayleigh Distribution.

We can also use one of the MATLAB commands R == cdcaui(0.5, 1) or r == cf3(0.5), to obtain 1.1774. Figure 4.5 shows the PDF her, a) with (J == 1, and the CDF H(r) ofa Rayleigh random variable. The RMSR, MRE, and CEP are marked with "*", "+", and "0", respectively. Other points of interest are the 2(J and 3(J probabilities, which are 86.470/0 and 98.89%, respectively. Upon substituting (J == CEPI )2In(2) into Equation (4.9), we have p == 1 _

2-(RjCEP)2

which is easier to use when CEP is given in place of (J. Since the variable r in H(r) is to be interpreted as a normalized quantity (r == RI(J), the solid CDF curve in Figure 4.5 is valid for all positive values of (J. The variable r in her, (J) should not be treated as a relative quantity. Thus, the dashed PDF curve in Figure 4.5 is valid only for (J == 1. In other words, the variable used for the horizontal axis has two interpretations, one for the CDF and another for the PDF. Figure 4.6 shows the PDF her, (J) of Equation (4.8) with (J == 1. The shaded area under the h(r, 1) curve in the [0,1] interval (or R == 1) is the probability p == H(1/1) == 1 - exp(-1/2) == 0.3935, according to Equation (4.9). However, if (J == 2, then her, 2) will be different than that in Figure 4.6 and we have p == H(1/2) == 1 - exp(-1/8) == 0.1175. Some military agencies use the term dispersion (£5) to mean the diameter (not radius) of a 75% probability circle where l5 == dispersion == 2 * R75 /o == 2 * 1.6651(J == 3.3302(J 0

The terms I-SIGMA (1(J) error, 2-SIGMA error, and so on, in two-dimensional error analysis should be used only for the special circular normal distribution case. In all other cases, it is not even defined. For example, a statement such as "the position accuracy of a system is 60 m (2(J)" could leave the reader wondering

45

Section 4.5 • Two-Dimensional Error Measures Area under her, 1) for r in [0,1] is 0.3935 0.7 r--,-----.----,---..----'------.--....:...,--'---,--,....---,-----,

0.6 ;::- 0.5

..... ....... J::

u.- 0.4

o

0...

s:

.~ 0.3

~ a: 0.2 0.1

0.5

Figure 4.6

1.5

2

2.5

r

3

3.5

p = J~h(r, (J) dr for the Rayleigh PDF, (J

4

4.5

5

= I.

whether the distribution is circular, in which case (from the Rayleigh distribution) the numbers describe the 86.47% probability circle. It could also be interpreted as one-dimensional sigmas along each axis, in which case (from the standard normal distribution) the 95.45% probability circle is implied (assuming the distribution to be circular normal, which actually may not be so). Extra care is needed to avoid confusion. Table 4.2 contains the R-to-p pairs for the direct problem in this special case. R is expressed in three forms : R = k . (J, R = k · MRE, and R = k.- CEP. Table 4.3 shows the p-to-R pairs for the inverse problem, where, for a given p, R is expressed as multiples of (J, MRE, and CEP; that is, k = R/(J, k = R/MRE, and k = R/CEP, respectively . The probabilities for R = 1 DRMS and 2 DRMS are, respectively , 63.21% and 98.17%, see [14]. Conversion between these error indicators is possible via the two M-files, r2p2d.m and p2r2d.m. The former (r2p2d .m) takes as input a multiple (k) of SIGMA, MRE, DRMS, and CEP, and then produces the corresponding probability p for R = k- SIGMA, R = k. MRE, R = k - DRMS, and R = k - CEP, respectively . For example , entering r2p2d(2) in MATLAB results in For r = 2*SIGMA

P = 0.8647 For R = 2*MRE

P = 0.9568 For R = 2*DRMS (RMSR)

P

= 0.9817

For R = 2*CEP

P = 0.9375

46


TABLE 4.2 k

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0

Two-Dimensional Direct Problem, from R to p p = H(R/a) R=k·a

p = H(R/a) R=k·MRE

0.0000 0.0050 0.0198 0.0440 0.0769 0.1175 0.1647 0.2173 0.2739 0.3330 0.3935 0.4539 0.5132 0.5704 0.6247 0.6753 0.7220 0.7643 0.8021 0.8355 0.8647 0.8897 0.9111 0.9290 0.9439 0.9561 0.9660 0.9739 0.9802 0.9851 0.9889 0.9918 0.9940 0.9957 0.9969 0.9978 0.9985 0.9989 0.9993 0.9995 0.9997

0.0000 0.0078 0.0309 0.0682 0.1181 0.1783 0.2463 0.3194 0.3951 0.4707 0.5441 0.6134 0.6773 0.7348 0.7855 0.8292 0.8661 0.8967 0.9215 0.9413 0.9568 0.9687 0.9777 0.9843 0.9892 0.9926 0.9951 0.9967 0.9979 0.9986 0.9991 0.9995 0.9997 0.9998 0.9999 0.9999 1.0000 1.0000 1.0000 1.0000 1.0000

p = H(R/a) = k·CEP

R

0.0000 0.0069 0.0273 0.0605 0.1050 0.1591 0.2208 0.2880 0.3583 0.4296 0.5000 0.5677 0.6314 0.6901 0.7430 0.7898 0.8304 0.8651 0.8942 0.9181 0.9375 0.9530 0.9651 0.9744 0.9815 0.9869 0.9908 0.9936 0.9956 0.9971 0.9980 0.9987 0.9992 0.9995 0.9997 0.9998 0.9999 0.9999 1.0000 1.0000 1.0000

47

Section 4.5 • Two-Dimensional Error Measures

TABLE 4.3 P

0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 0.990

Two-Dimensional Inverse Problem, from p to R

k == R]o

k == R/MRE

k == R/CEP

0.0000 0.2250 0.3203 0.3949 0.4590 0.5168 0.5701 0.6203 0.6680 0.7140 0.7585 0.8020 0.8446 0.8866 0.9282 0.9695 1.0108 1.0520 1.0935 1.1352 1.1774 1.2202 1.2637 1.3082 1.3537 1.4006 1.4490 1.4993 1.5518 1.6069 1.6651 1.7272 1.7941 1.8671 1.9479 2.0393 2.1460 2.2761 2.4477 2.7162 3.0349

0.0000 0.1795 0.2556 0.3151 0.3663 0.4123 0.4549 0.4949 0.5330 0.5697 0.6052 0.6399 0.6739 0.7074 0.7406 0.7736 0.8065 0.8394 0.8725 0.9058 0.9394 0.9736 1.0083 1.0438 1.0801 1.1175 1.1561 1.1963 1.2381 1.2821 1.3286 1.3781 1.4315 1.4897 1.5542 1.6272 1.7122 1.8160 1.9530 2.1672 2.4215

0.0000 0.1911 0.2720 0.3354 0.3899 0.4389 0.4842 0.5268 0.5674 0.6064 0.6442 0.6811 0.7173 0.7530 0.7883 0.8235 0.8585 0.8935 0.9287 0.9642 1.0000 1.0363 1.0733 1.1111 1.1498 1.1896 1.2307 1.2734 1.3179 1.3647 1.4142 1.4670 1.5238 1.5857 1.6544 1.7321 1.8226 1.9331 2.0789 2.3069 2.5776

48


The latter (p2r2d.m) takes p as input and generates R expressed in terms of SIGMA, MRE, DRMS, and CEP. For example, entering p2r2d(O.5) in MATLAB results in

For p = 0.5 r = 1.177*SIGMA r = O. 9394*MRE

r = O. 8326*DRMS (RMSR) r = l*CEP

This is a form of the inverse problem.

4.5.6 CEP Rate and RPE Rate The time rates of change of CEP and RPE are called CEPR and RPER, respectively. Under the assumption that (Jx == (Jy == (J, we have CEP == 1.1774(J and RPE == ,J2(J from Subsections 4.5.2 and 4.5.5. This means

CEP = 1.1774

~E =

0.8325 RPE

and thus the relationship between CEPR and RPER is CEPR == 0.8325 RPER when we divide the equation, CEP == 0.8325 RPE, by unit time.

EXAMPLE 4.5

If the circular error on 95% of system flights is increasing at a rate of 2 nmi/hr, what are the rates of CEP and RPE? Since R95% == 2.44770" == 2 nmi, we can find 0" == 2/2.4477 == 0.817 nmi. Thus, CEP == Rso% == 1.17740" == 0.9620 nmi, and RPE == ,J20" == 1.1555 nmi. The corresponding rates are CEPR == 0.9629 nmi/hr RPER == 1.1555 nmi/hr

Section 4.6 • Case 3, A == A c ,

4.6 CASE 3, A == A c ,

p

p == 0,

== 0,

U E

U E

49

[0,1]

[0,1]

In this case, the probability in Equation (4.2) becomes p

==

1 1-1 [x

I

2 y2] dxdy 2 exp -2 2+2 ax a y JJ naxay

(4.16)

Ac

where the area of interest A c is a circle of radius R, x2

+ y2 S R 2

J

After we substitute u == a y/ a x, A == r/ ax == x 2 + y2 / a x, and the rectangular-topolar transformation x == r cos 0, y == r sin 0, into the above equation, it becomes p=

2~uJ:1t 1:/

ux

exp[

;2

S(B, U)}dAdB

(4.17)

where S(O, u) == cos? 0 + sin 2 0/u 2 • Integrating with respect to A, we obtain p

== _1 J21t {l- exp[ -(R/(JxiS(B, u)/2]) dB 2nu

0

S(O, u)

(4.18)

One can also perform integration with respect to 0 in Equation (4.17), and apply the relationship lo(t)

1 J2n exp[ ± t cos(20)] dO == -1 In exp[ ± t cos 0] di) == -2

non

0

to obtain (4.19) in terms of lo(t), which is the zero order modified Bessel function of the first kind; see [12]. Equation (4.18) is easier to use than Equation (4.19) for two reasons. First, it is a definite integral; second, the integrand function does not involve the modified Bessel function 10 • The MATLAB M-file gf2.m carries out the integration in Equation (4.18) for u E (0, 1) via Simpson's rule (which is implemented in simprule.m). Thus, note that the probability p depends not only on the normalized radius R/ ax, but also on the ellipticity u == ay/ax, which is the ratio of the smaller standard deviation to the larger standard deviation and is also a derivable or known parameter. Numerical integration and root searching techniques are needed to solve both the direct and inverse problems. They are implemented in the M-files edcaud.m and edcaui.m. Notice that when u == 1, case 3 specializes to case 2. As will be seen in the next few sections, cases 4-6 are transformable to case 3 by rotation or scaling. Therefore, case 3 is the heart of this chapter.


50

4.6.1 Limiting Situation: u ~ 0

As u approaches 0, Equation (4.16) becomes p=

R

J-R

V

[-x 22 dx (J 2

1 r;:c

2n (J x

exp

]

x

= erf [

hR

v 2 (J x

]

The M-file nf2.m can be used to evaluate p. The MATLAB command is p = nf2(r), where r = R/(J x is the normalized radius. Figure 4.7 shows how the probability p varies when u is increased from 0 to 1, for the uncorrelated case corresponding to p = o. The curve on the left (R = 1DRMS) indicates that p decreases as u increases; the one on the right (R = 2 DRMS) displays a different behavior: p increases as u increases. Recall that as u varies, 1 DRMS = ,JT+U2 (Jx does not remain constant. Later we will see a similar pattern for curves in the correlated, p =1= 0, case (Figure 4.11). The normalized radius R/ (J x, as a function of ellipticity u, is plotted in Figure 4.8 for various probability values. The curves in Figure 4.9 show the relationship between probability and radius for u = 0 to u = 1 in steps of 0.2. Note that the curve corresponding to u = 0 is identical to the one-dimensional normal CDF and the curve corresponding to u = 1 is identical to the Rayleigh CDF. Figure 4.10 repeats the bottom curve (p = 0.5) of Figure 4.8, along with the two approximations suggested by Pitman (see [10]): CEP/(Jx = 0.589(1 + u)

CEP/(Jx = 0.562 + 0.615u

0.69,...------,.....-----....

0.68

:

.

::1

u)

u)

~

~

cr:

~

o s 0.66

~

"'C :::J

.

0.97

~ "'C :::J

as

~ 0.965

o

"'C

"'C

Q)

Q)

~

0.98

.

__ 0.975

'5 0.67

II

0.985 ,...---------.,...------....

0.65

.

II ~

0.96 0.64

0.63

:

L.--

L--

o

0.5 u= cry/crx

Figure 4.7

.

----'

0.955

0.95

0

0.5 u= cry/crx

Probability Versus Ellipticity for p == 0 and R == 1 DRMS and R == 2DRMS.


p

== 0,

U E

51

[0, 1]

2.6 2.4 2.2

................ ;

.. . .

:

.. .. . . ......... ,

:

~

:: ........ ; :

3~

~

2

.

"

..

: :

.

.. ..

"

"

:

..

..

.

1.8

"0

::J en 1.6 o

"0 Q)

II \"""

1.4 1.2

.........~

0.8 0.6 0

0.1

0.2

0.3

0.4

·1·

'1'

..

..

..

0.7

0.8

0.9

~

..

.

0.5

0.6

....

·1·

.

" " " " " "

u =cry/crx Figure 4.8

Normalized Radius Versus Ellipticity for p == 0 and Various p Values.

We conclude this section with an example of finding the radius of a 50% probable error circle. EXAMPLE 4.6 Suppose a certain position error has an elliptical normal distribution with (J x == 2 m, (J y == 3 m, and that errors in the x and y channels are uncorrelated. Can you find the radius of a circle such that 50% of the time a measured position error will fall inside this circle? R = edcaui(O.5, 2,3) = 2.9264

Thus, we have R so% == 2.9264 m.

0.9 ................:

; . ..

··

0.8

3-

i~ ..::=:.9

:

)

0.7

)

. .

:

: :

: j

~

:.

;

.

~

.

t

.

:: 0.6 ~

"0

~ 0.5 o

"0

..

~ 0.4 Q.

0.3 .............·~ 0.2 ... ... .. 0.1

"

..: .. .

~ ~ ..::=:.1

: . ···

..

~

L

~

~ ..

.. ..

.

.

...

.

.

r·············(·············"(············:············ ···r ············ 0.5

Figure 4.9

.

1.5

r

2

2.5

3

Probability Versus Normalized Radius for u == 0 : 0.2 : 1 and p ==

o.


52

1.2 r-----r----,.---r--.....,...--r---~---,.-------

1.1

.

1

:

.

0.9

.

0.8

.

:.:

b

a:

LU

o

/

/

-:

0.7

..

/ /

0.6

l'

~ /'.

/

~.:

~'.~.' ~

:

:

r: : .

/

:

.

/

o

Approxlmation, ---"_ _

Figure 4.10

4.7 CASE 4, A == A c ,

p

E

~

.

=0.562+0.615u

_ _L . _ _ . . . . . . L . _ _ - - - - L_ _....L___

__1__

~_---L-

0.4

0.3

0.2

0.1

:

Approximation, = 0.589(1 +u)

/

0.5 """--

:

Exact

/

)'

/.~.:

:.

0.5

0.7

0.6

0.8

___J

0.9

CEP / a x versus u: Exact and Two Approximations.

[-1,1],

[0,1]

U E

Under these assumptions, Equation (4.2) becomes

p ==

I

2

I

(4.20)

y2]

(4.21)

1 -1 [x- - 2pxy exp - + y2] - dxdy A 21C(Yx (Jy ~ 2(1 - p2) (J~ (J x (J y (J~

11 c

with the area of integration being the interior of a circle

Let Ax

2

2

+ Bxy + Cy = (1 -

1

[x

2

2pxy

2) 2: - - - + 2 P (Jx (Jx(Jy (Jy


A

== A c ,

p

E

[-1,1],

U E

53

[0,1]

We can eliminate the second term (Bxy) in the above quadratic expression by a suitable angular rotation f),

Substituting the rotation transformation pair

== ~ cos f) - f/ sin f) y == ~ sin f) + f/ cos f)

(4.22)

x

(4.23)

into Equation (4.21), we obtain

Ax2 + Bxy + Cy2 == Al ~2

+ C i f/2

(4.24)

where the new coefficients AI, C I are related to the old ones (A, B, C) via

Al == A cos 2 f) + B cos f) sin f) + C sin 2 f) CI

== A sin

2

f) -

B cos f) sin () + C cos

2

()

(4.25) (4.26)

This is equivalent to using a similarity transformation on the quadratic form

to obtain

where AI, C I are the eigenvalues of the matrix

A B [ B/2 A/2] and the two corresponding eigenvectors

[

:~:]

and

are used in the transformation from (x, y) to

(~,

[ •• (~,

f/)

The circular boundary of the integration area remains a circle in the new f/)-coordinate system,


54

Note that the problem is now expressed in

(~,

1]) and shows no dependency on

I

p. Thus, the problem can be solved by the method used in case 3 as

P ==

1

II 2ita A'c

~

a 1]

1]2] d~ dn exp 1-1 -2 [~2 2" + 2" a~

a 1]

where a~ == 1/ AI, a~ == I/C I and the area of integration A~ is ~2

+ 1]2 ~ R 2

Direct problems are solved by edcacd.m, with the MATLAB command p == edcacd(R, ax, a y, p), whereas inverse problems are solved by edcaci.m via the command R == edcaci(p, ax, "s- p). In edcacd.m, the MATLAB program abc2ac.m is

utilized for the needed rotation transformation before calling edcaud.m. Figure 4.11 shows the variation of the probability p as u is increased from 0 to 1, for the correlated case corresponding to p == 0.3. The curve on the left (R == 1 DRMS) indicates that p decreases as u increases; the one on the right (R == 2 0.685

0.98

0.68

_

,....-----~-----

. 0.975

.

0.675

(t)

o

0.67

(t)

0.665

en ~

o:j

s

T-

T-

en ~ a::

a::

o

o 0.965 C\J U o

T-

U o (lj

0.66

(lj

o

o

"0

"0

Q)

Q)

II

II

~

0.97

0.655

~

0.96

0.65 0.955

.

.

.

0.645

0.64

""'------....I--------"tU

o

Figure 4.11

0.95 ""'--

o

Probability Versus Ellipticity for p = 0.3 and R R=2DRMS.

---..r

--..I-

= 1 DRMS

and


p

E

[-1,1],

U E

55

[0,1]

DRMS) displays a different behavior: p increases as u increases. Recall that as u varies, 1 DRMS = v'f+U2 ax does not remain constant. We see similarity in the behavior of these curves and those of Figure 4.7. Notice that when p == 0, case 4 reduces to case 3; and when both p == 0 and u == 1, case 4 reduces to case 2. 4.7.1 Limiting Situation: Ipl

~

1

When p == 1, the two random variables x, yare perfectly correlated by the linear equation

Each of the random points will fall on this line; thus, we have a one-dimensional with == (1 + U2)(J~, where t represents the random radial distance error t ~ N(O, from (0,0) to (x, y), t2 == x2 + y2. Our double integral problem becomes a single integral

a;),

a;

[-t2(Jf

1 p == R --exp -

J

- R

,J2n(Jt

2 ]

dt == erf [ -R-]

,J2(J t

Figure 4.12 illustrates the situation. y

--+--------~'-----_...L..---_A__I_--x

Figure 4.12

Geometry for Limiting Situation: p == 1.


56

If p == -1, the two random variables x, yare also perfectly correlated; they have the following linear relationship

and all other relationships apply, including Figure 4.12, as for the p == 1 case. For fixed R == 1, the probability p versus correlation coefficient p curves, using the ellipticity u as a parameter, are shown in Figure 4.13. Since the probability is symmetric about p == 0, the horizontal axis is labeled as Ipl. The probability value increases as Ipl approaches 1 and as u approaches 0.

4.8 CASE 5,

A

== A e ,

p

== 0,

U E

[0, 1]

In this case, the probability in Equation (4.2) becomes (4.27)

0.7 r---_r__-~-~--or--_r__-__.._-_.,..--r__-_r_-_ : u=o: ·

0.65

.

.................................................................. _

..

"e

.

..

..

.

~

..

0.6 ..

......: ···

.............. ,...

15' o as o

~

:

. ..

: . ..

.

: .

0.5

II

Q..

· . . ................................... ·· .. ..

0.45

......

0.4L..-_--

0.35

..

0.1

.. ..

.. .. .. .. .. ..

..

0.3

0.4

.. ..

_.0

.. .. .. ..

..

.. .. ...

..

..

.. .. ..

.. .. .. .. ..

.. .. .. .. .. ..

..

..

.. ... .. .

" .. ..

..

..

..

..

.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. -----._ _a - - _ . . . a - _ - - ' - _ - - - L_ _' - - - _ . . . a . - _ - - '

~_...L..-

o

.. ~

..

·· ··· ··

0.2

0.5

0.6

0.7

0.8

0.9

Ip I Figure 4.13

Probability Versus Correlation Coefficient for R == 1, U == 0 : 0.1 : 1.


A

= Ae ,

p

= 0,

U E

57

[0, 1]

where the area of interest is an ellipse A e , x2

a2

y2

+ b2 < 1

We solve this problem by using a pair of scaled variables xl==x/a

and

x2==y/b

so that Equation (4.27) becomes (4.28) where a1 == ax/a, a2 == ay/b, and the area of integration A~ is a unit circle in the new coordinates and

Once again we have converted the direct problem of Equation (4.27) to one that is solvable by the method used in case 3. The M-files edeaud.m and edeaui.m have been developed to solve the direct and inverse problems. The routine edeaud.m includes the needed scaling before calling edcaud.m. The MATLAB commands are p == edeaud(a, b, ax, a y) and [a, b] == edeaui(p, v, ax, ay). Notice that when a == b, case 5 is simplified to case 3; when a == band u == 1, case 5 is reduced to case 2. The following example shows how to find a 500/0-errorellipse. EXAMPLE 4.7 If a certain position error has an elliptical normal distribution with (J x = 2 m, (Jy = 3 m, and errors in the x and y channels are uncorrelated, find the semi-major and semi-minor axes (a, b) of an ellipse such that 50% of the time a sample position error will fall inside this ellipse. FIrst, assume v = (a/b) = 2/3; next, try v = (a/b) = 0.8.

[a, b l

= edeaui(0.5,

2/3, 2,3)

=

[2.3548, 3.5322]

Thus, the 500/0-error ellipse has a semi-major axis of 3.5322 m in the y direction and a semiminor axis of 2.3548 m in the x direction. In this case, since v = 2/3 coincides with the ellipticity (J x/(Jy = 2/3, the two ratios a/(J x and b/ (Jy should also be identical; indeed, they are both equal to 1.1774. Why this particular phenomenon occurs (when alb = (Jx/(Jy) is explained in Subsection 4.8.1 of this chapter. However, when we set v = 0.8 and determine a and b from

[a, b]

= edeaui(0.5,

0.8, 2,3)

=

[2.5871, 3.2338]

we can no longer expect the two ratios aj (J x and bj (Jy to be equa1.

Figure 4.14 shows the 500/0-error circle and 500/0-error ellipse for p == 0.5. The values of the parameters used are taken from Examples 4.6 and 4.7; that is, ax == 2, a y == 3, and p == 0.5. The semi-major axis and semi-minor axis for the 500/0 error


58

4r---'---""T""""""-~-~---,------r----...-----,

3 2

,0

-1

-2 -3

~4

-3

-2

o

-1

2

x

3

4

Figure 4.14 Ellipse, p ==

p-Error-Circle and p-Error50o~.

ellipse are a == 2.3548 and b == 3.5322, respectively, whereas the radius for the 500/0 error-circle is R == 2.9264. It is interesting to note that under the same joint PDF, and for the same probability p, the area associated with the p-error-circle is larger than that of the p-errorellipse. In Figure 4.14 A c == nR 2 == 26.9040, while A e == nab == 26.1312. As the ellipticity u approaches 1, the two areas become closer and eventually coincide. Figure 4.15 shows this relationship between the areas of the 500/0-error-circle and 500/0-errorellipse. 4.5 r---~--,------r------r-----..,.----.....---_-_---r'-----' 4 ~

~ II

35 .

.

3

ct

Q)

co

~

~

+

j

j

j

:

:

:

:

:

:

: f

: :.:

·f:

~

~

~

~

~

r·········r········ .~

2·

oq:

-g

~

:

:

:

:

:

'~.~"""" '~."""" ·1.·········~··· .~.~:

::

: .!

.",'.

~ 2.5 ~

~

:

1.5

:: ~

~

:

........~

: : ~circle :

~

~

~

~

':"

j

:,-'

' .,,:..: ~,-'- ~

,-,~

~

;..,.::':'.~.i..

~

~

: ~

~

0-'

o

Figure 4.15

0.1

~

~

···f········~;:~······f········T········l·········T···

~

~ ""~lIipse! ~ ~ ~ ··.>:r-~:·····l·········,·········:··········r········r·

0.5

0.2

0.3

~.~ •....... .

.

~

~

~

~

~

_ ~

.

1' ~

1' ~

.

~ ;:~"4:"""" ·~··········t······· ·1········ :)'-::::

:

:

,.~:

.

0.4

0.5

0.6

~

~

~

~

-:-

~ 1'

~ 1'

0.7

0.8

0.9

. .

Area of 50 %-Error-Circle and Area of 50o~-Error-Ellipseversus Ellipticity.

Section 4.9 • Case 6, A == A e ,

p

E

[-1,1],

U E

[0,1]

59

4.8.1 Limiting Situation: afb == (Jx/(Jy When atb == (Jx/(Jy, Equation (4.28) can be greatly simplified to p

== 1 - exp( _k2 /2), with k == afo; == bjo,

because (JI == (Jx/ a == (Jy/b == (J2. This last equation is similar to Equation (4.9). Thus, an elliptical normal PDF over an elliptical region (such that a/b == (Jx/(Jy) can be treated just as the situation of a circular normal PDF over a circular region.

4.9 CASE 6, A == A e ,

p

E

[-1,1],

U E

[0,1]

The only difference between the present case and case 4 is that the integration area is now the inside of an ellipse

instead of a circle x 2 + y2 ~ R 2. Therefore, if we rescale the variables x and y by Xl

==x/a

X2 ==y/b then Equation (4.2) becomes P-

JJ A'c

2n(J1 (J2

1JT=P2 {-I 2(1 exp

[XI- - -2p- -X2+x~] - }dXI dX2 XI

p2) (JI

(JI

(J2

where (JI == (Jxl a, (J2 == (Jylb, and the area of integration (x}, X2) coordinates and

(J~

A~

(4.29)

is a circle in the new

xi+x~~1 We can see that this equation is almost identical to Equation (4.20), except radius R in that equation is replaced by 1 here. The M-files edeacd.m and edeaci.m have been developed to solve the direct and inverse problems, respectively. The two MATLAB commands are p == edeacd(a, b, (Jx, (Jy, p) and [a, b] == edeaci(p, v, (Jx, (Jy, p). Notice that when p == 0, case 6 is specialized to case 5; when a == b, case 6 simplifies to case 4; when p == and a == b, case 6 collapses to case 3; when p == 0, a == b, and u == 1, case 6 reduces to case 2. This is the most general case among cases 2 through 6. For the case of a == 2 and b == 1.5, probability p versus correlation coefficient p curves, using ellipticity U as a parameter, are shown in Figure 4.16. Since the probability is symmetric about p == 0, the horizontal axis is labeled as Ipl. The probability value increases as u approaches 0.

°


60

0.96 0.94 0.92

"2 ::i

-

,.....

0.90

................ :

~

..

.. .. .. ..

..

..

..

0.88

:-

:

..

.. .. .. .. ..

.. .. .. .. ..

.. ..

..

..

..

:

.

..

..

..

..

..

..

.......................................................... .. .. .. ·.. . .

·· .. .. ··.. .. .. .. .. .. .. .. C\i 0.86 ............................................................................................. .. .. .. .. .. .. .. .. .. 1J ... .. .. .. .. .. .. .. .. o .. .. .. .. .. .. .. 0' ,.....

o

Cd Q)

"C Q)

II Q..

..

..

..

..

0.84 ............ ;..

;

..

..

..

..

~

~

~

..

..

..

:

: .... ..

..

.. ..

.. .. .. ..

~

~

..

..

....

.."

..

........ ~

.. ..

.. .. " . .. . .. 0.80 ........ :

0.78

:

;

·

··· · a

·

Figure 4.16

..

... ..

0.3

0.4

0.5

a

a ••• ~

0.2

..

.. ... .

~

......... ~ ••••••••• ~

0.1

:

.. ..

... .

.. ..

..

..

~

~

~

..

..

..

:

..

: ...

:

~

~

0.7

0.8

0.9

..

,.

..

..

..

..

..

.. .. .. .. .. ..

..

..

:

~

..

..

..

.."

~

..

..

:

..

..

..

0.82

..

•

•

~ ••••••••• :

I pi

: ••• ~

0.6

..

...

.. .. ..

.. ..

. . .. .. . . ".. . .. "' .. . "

:

.. ... •

.. •

...

.

.

•

..

.

.

Probability Versus p for a == 2, b == 1.5, and u == 0 : 0.1 : 1.

4.9.1 Limiting Situation: Ipl

~ 1

When p == 1, the two random variables x, yare perfectly correlated by the linear equation y == xu == xtanO == x(ay/a x ) Each of the random points will fall on this line. Thus, we have a one-dimensional error t ~ N(O, tJ';), with a; == (1 + u2)a~, where t represents the radial distance from (0, 0) to (x, y). Our double integral problem becomes a single integral 2

o

1 [-t --exp - ] dt == erf [ -to-] J-to J"2irtJ't 2tJ'~ ,j2tJ't where the integration limit (to) is found as follows: First, solve y == xu == x tan 0

p ==

t

(4.30) (4.31)

and obtain

a2 b2 a2u 2 + b2 y2==

a2 b2u2 _

a2u 2 + b2

Section 4.10 • PDF of the Angular Position of a Random Point

61 y

---+----------,jl"----~-_A__+_-x

Figure 4.17 Geometry for Limiting Situation: p == 1.

Thus,

Figure 4.17 illustrates the situation. If p == -1, the only difference is in the line y

== - xu == - x tan 0 == - x((J y/ (J x)

4.10 PDF OF THE ANGULAR POSITION OF A RANDOM POINT

In the preceding sections we have studied the probability that a random radial error falls in some particular region surrounding the origin. Here we look at the probability distribution for angular position of a random point on the plane. Assume that the rectangular coordinates (x, y) of a random point are independent random variables with probability density functions N(llx' (J~)

(4.32)

Y N(lly, (J;)

(4.33)

X

1'-1

1'-1

What is the probability density function (PDF) of the polar angle of a random point in the plane relative to the origin (0, O)? The problem can be solved by first converting the two-dimensional joint PDF in rectangular coordinates (x, y) into polar coordinates (r, 0), and then integrating with respect to r from 0 to 00 to find h(O), the PDF of the random variable 6. This PDF obviously is a function of 0, but it also depends on four other quantities: llx' (J x, lly' (J y.

62


I-!

I

The joint probability density function for the two random variables (x, y) is f( X, y) --

1 exp 21UJx (Jy

2

[(X - 2flx)2 + (y -

fly)2] 2 (Jy

(Jx

Using the Jacobian operator o(x, y) oCr, e)

== I oxjor oxjoe I == I cos e -r sin e I == r oyjor

sin ()

oyjoe

I-!

r cos ()

we obtain the joint PDF g(r, ()) for the two new random variables (r, 6) g(r,

e)

== o(x, Y)f( ~(e) r,

u

X,

y

)

== _ r _

21[(Jx(Jy exp

2

[(r cos () - flx)2 (Jx2

+ (r sin e(Jy2-

fl y)2]

I

Defining cos? e sin 2 e

A== -(J2 - - +(J2

B==-2

(4.34)

y

x

[

fl Xcos e fly sin ()] +(J2 (J2 -

(4.35)

y

x

(4.36)

we simplify g(r, e) to

~

!!-)2+(c -

g(r, ()) == _r_expl_ [A(r + 21[(J.oy 2 2A

2

B ) 4A

]

I

(4.37)

The one-dimensional PDF, h(e), for the random variable 0 is obtained by integrating Equation (4.37) with respect to r from 0 to 00, h(e) ==

!!-)2+(c -

Jooo _r_expl_! [A(r + 2n(Jx(Jy 2 2A

2

B ) 4A

]

I

dr

(4.38)

After a sequence of algebraic operations and simplifications, we have h(8) = 2

1 exp(-Cj2) [1 - wJ1texp(w 2)erfc(w)] An(Jx(Jy

(4.39)

where erfc, the complementary error function, and ware defined as 2 2 Joo erfc(t) = .fii t exp( _u ) du

B W==--

J8A

(4.40) (4.41)

The M-file pdft.m performs the operations required to compute h(e) when flx' fly' (Jx, and (Jy are given. The command form in MATLAB is y == pdft«(), flx' fly' (J x, (Jy). Figure 4.18 shows h(e) versus e for flx == 5, fly == 3, (J x == 0.4, and (Jy == 0.25.

63

Section 4.11 • Summary of M-files 8r-------,r-----,---...,.----....--------,-----, C)

at ~

·..

= arctan.(3/5) = 30:96° = 0.5~04 radia~s

..

7

..... ~

6

"

~

.."

.. .. ..

....

... ..

"Jlx

.

= 5,

.. " ..

..

:

:

:

.. .. ..

...

.. " ..

..

" " ..

: .. .... .. .... .. .... ....

..

..

..

.

..

..

..

.. ..

.. .. ..

..

" .. .. ..

.. .. ................... .,

5

...

..

..

Jly = 3, crx = 0.4, cry = 0.25···

" .. ..

.. ..

"'

.

LL

o

0Il

· .... . ............................................................................................

4

~

.c:

3

:

:

" "

~

...

...

..

;

;

.

...

..

.. ..

... ..

....

... .

... .

..

..

2

1

\

. .

··.. ·.. ~ ·· ..

: ....

:

:

· ..

... ;

.. ..

. ..

:

... ." ... ;

..

. :

..

..

..

;

..

.

.. .. ..

..

..

;

. ..

~

.. .

.. .

.. ..

.

:

· ·· ··

.

.

.

..

.. ..

.. ..

:

.

:

. :

4

5

O.......--ooA..----'"'----------"""'-----'------A.----'---......

o

2

Figure 4.18

When flx == 0,

fly

3

o(radians)

7

Probability Density Function of Polar Angle 8.

== 0, and a, ==

(Jy

==

(J,

A, B, C, g(r, e) become

A==~ (J2 B ==C == w == g(r, 8) =

2:0-

2 ex

°

p[-

;:2]

and the PDF in Equation (4.39) reduces to a constant over the interval [0,2n] as

1 h(e) == 2n ' for

eE

[0, 2n]

(4.42)

In this case, the random angular position (measured in radians) has a simple, uniform distribution over [0, 2n].

4.11 SUMMARY OF M-FILES

The M-files used or generated in this chapter are summarized in Table 4.4. Recall that the true length R and the normalized length r are related by

r == R/max((J x, (J y)

64

C h a p t e r 4 II T w o - D i m e n s i o n a l E r r o r Analysis

TABLE 4.4


File Name

MATLAB Command p -- nf2(r) y -- cfl (r) p = cf2(r) r = cf3(p) p -- 9f2(r, u) s = simprule(y, h)

nf2.m cfl .m cf2.m cf3.m gf2.m simprule.m r2p2d.m p2r2d.m edraud.m edraui.m edca.m cdcaud.m cdcaui.m edcaud.m edcaui.m edcacd.m edcaci.m edeaud.m edeaui.m edeacd.m edeaci.m abc2ac.m pdft.m

r2p2d(k) p2r2d(p) p = edraud(a, b, ax, ay) [a, b] = edraui(p, v, ax, try) p = edca(r, u) p = cdcaud(R, ax) R = cdcaui(p, try) p = edcaud(R, a~, ay) R = edcaui(p, ax, ay) p = edcacd(R, ax, ay, p) R = edcaci(p, ax, ay, p) p = edeaud(a, b, ax, ay) [a, b] = edeaui(p, v, a~, ay) p = edeaud(a, b, ax, ay, p) [a, b] = edeaui(p, v, ax, ay, p) [a l, cl] = abc2ac(a, b, c) y = pdft(O, inx, fly, ~Tx, tTy)

T h e M - f i l e e d c a . m is i n v o k e d in t h e p r o g r a m e d c a u d . m . F i g u r e 4.19 d e s c r i b e s t h e r e l a t i o n s h i p s b e t w e e n t h e m a j o r M - f i l e s d e v e l o p e d in this c h a p t e r .

edeaci.m

7a

edeaui.m

edcaci.m

6a

5c

cdcaci

8a

cdcacd

8

. . . . . .

edcacd.m "~I edeacd'm II I

"-I "1 edeaud.m

7

6

edraui.m

5b

9a

edcaui.m 5a

edraud

i

edca.m

..I nf2.m

Figure 4.19

edcaud.m

cf2.m

Relationships Between Major M-files.

gf2.m

Three-Dimensional Error Analysis Errors in the measurement of distance in three-dimensional space are treated in this chapter. This type of error is the result of a root sum square process of three onedimensional random errors in three different channels.

5.1 THREE-DIMENSIONAL NORMAL DISTRIBUTION

In three-dimensional error analysis, the most general trivariate normal joint probability density function of random variables (x, y, z) is

(Jx(Jy _ 2(pxz - PxyPyz)(x - Jlx)(z - Jlz)] } (Jx(J z

where Jlx and a; are the mean and standard deviation for random variable x; Jl y and (Jy are the mean and standard deviation for random variable y; Jlz and a, are the mean and standard deviation for random variable z; and Pxy' Pyz' Pxz are the correlation coefficients between the xy, yz, xz channels, respectively. 65

66

Chapter 5 • Three-Dimensional Error Analysis

After the means flx' fly, and flz are removed, the joint normal PDF is simplified to 1 f( x, y, z) == --;::::::.=================================--J(2rr)3(1 + 2pxypyZPxz - P~y - P~z - p~z) (Jx(Jy(Jz X

X

-1 2 2 2 2(1 + 2pxypyzPxz - Pxy - Pyz - Pxz) (1 - p;z)x 2 (1 - p~z)y2 (1 - p~y)z2 2(pxy - Pxzpyz)xy 2 + 2 + 2 [ ax ay az axa y exp {

_ 2(p yz - PxyPxz)yz _ 2(pxz - PXyPyz)XZ]} aya z axa z

(5.1)

We assume, without loss of generality, that ax 2: a y 2: a z and use u and v to represent the ratios ay/a x and az/a x, respectively. Since (Jx 2: (Jy 2: «; then 1 2: u 2: v 2: O. The two ratios U==(Jy/(Jx V

== az/(Jx

will be called the first and second ellipticities, respectively. When a, == (Jy == a z == a (or U == v == 1) and Pxy == Pyz == Pxz == 0, the joint PDF becomes

1 [-1

f(x, y, z) = ~ exp V(2n)3a 3

2(J2

(x 2 + Y2 + z2 )]

(5.2)

This PDF is called the spherical normal distribution since each volume of equal probability density has the shape of a sphere x 2 + y2 + z2 == ka". All other values for the a's and p's lead to an ellipsoidal normal distribution since each volume of equal probability density has the shape of an ellipsoid.

5.2 DIRECT AND INVERSE PROBLEMS The goal of the direct problem in three-dimensional error analysis is to evaluate the probability p via the triple integral off(x, y, z), shown in Equation (5.1), over a given volume V p=

HIf(X, y, z)dxdydz v

where the integration volume V can be one of three types Box Sphere Ellipsoid

V ==

Jtb: [x] :::; a, Iyl:::; b, Izl:::; c

V == ~: x 2 + y2 + z2 :::; R 2 V == ~: (x/a)2

+ (y/b)2 + (z/c)2 :::; 1

(5.3)

67

Section 5.2 • Direct and Inverse Problems

For inverse problems we start with a specified probability Po and then proceed to find the dimensions of the integration volume-that is, the width, length, and height of a box; the radius of a sphere; or the three semi-axes of an ellipsoid. The solution is found via a repetitive trial-and-comparison procedure: (1) Assume an initial dimension(s) for V; choose a reasonably small tolerance limit b. (2) Carry out the triple integration in Equation (5.3) to obtain p. (3) Compare p with Po. (4) If Ip - Pol < b, accurate dimensions of V have been found, stop. (5) If P - Po > b, decrease the dimensions of V, then go back to (2). (6) If Po - P > b, increase the dimensions of V, then go back to (2). Therefore, both the direct problem and the inverse problem require an efficientway to compute the triple integral in Equation (5.3). Depending on the size of the ellipticities u, v, the shape of the integration volume V, and the value of the correlation coefficients Pij' there are six cases to be investigated: • • • • • •

Case Case Case Case Case Case

1, 2, 3, 4, 5, 6,

== Vb, V == ~, V == ~, V == ~, V == ~, V == ~, V

Pxy Pxy

== Pyz == Pxz == 0, == Pyz == P xz == 0, == Pyz == Pxz == 0,

U, V E

[0, 1]

u==v==1

u, v E [0, 1] Pxy' Pyz' Pxz E [-1, 1], u, V E [0, 1] u, V E [0, 1] Pxy == Pyz == Pxz == 0, Pxy' Pyz' Pxz E [-1, 1], u, V E [0, 1]

Pxy

In order to easily refer to these cases without lengthy description, we rely on an abbreviation system using six characters (XXYYZP). The meaning of the sixcharacter string is as follows: XX: (ED, SD) YY: (BY, SY, EY)

z:

(U, C) P: (D, I)

(Ellipsoidal Distribution, Spherical Distribution) (Box, Sphere, Ellipsoidal Volume) (Uncorrelated, Correlated) (Direct problem, Inverse problem)

Thus, the direct and inverse problems to be considered are: Case Case Case Case Case Case

1 2 3 4 5 6

(EDBYUD), (EDBYUI) (SDSYUD), (SDSYUI) (EDSYUD), (EDSYUI) (EDSYCD), (EDSYCI) (EDEYUD), (EDEYUI) (EDEYCD), (EDEYCI)

68


These cases are studied in Sections 5.3 through 5.9. When naming M-files developed in this chapter, we will also follow the same abbreviation convention. Cases 2 and 3 have been treated in [9].

5.3 CASE 1,

V == Jth,

Pxy == Pyz == Pxz == 0,

U, V E

[0, 1]

Assuming that the volume of integration is a box and that the three channels are uncorrelated, we can write the triple integral in Equation (5.3) as

p == .

JI I c -c -b

-a

1

2n(J x (Jy (J z

2 1-1 [X-+-+y2 Z2] I

exp -

2

(J~

(J~

dxdydz

(J~

(5.4)

which is separable and can be simplified to p

== erf[_a]. erf[_b]. erf[-C]

-J2 (J x

-J2 (J y

-J2 (J z

(5.5)

Even though six parameters a, b, C, (Jx, (Jy, (Jz appear in Equation (5.5), only three normalized variables ao == a/(Jx, bo == b/(Jy, and Co == cia, are needed to determine the probability p. The direct problem can be solved either by look-up in Table 5.1 (which is identical to Table 4.1; see instructions on page 39) containing the values of erf(Ro/-J2), or by using the M-file edbvud.m. The MATLAB command to use this file is p == edbvud([a, b, c], [(Jx, (Jy, (Jz]). An example follows.

TABLE 5.1

Values of erf(Ro/ J2)

Ro = i-i- d

d

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.0000 0.0797 0.1585 0.2358 0.3108 0.3829 0.4515 0.5161 0.5763 0.6319

0.6827 0.7287 0.7699 0.8064 0.8385 0.8664 0.8904 0.9109 0.9281 0.9426

2

3

0.9545 0.9643 0.9722 0.9786 0.9836 0.9876 0.9907 0.9931 0.9949 0.9963

0.9973 0.9981 0.9986 0.9990 0.9993 0.9995 0.9997 0.9998 0.9999 0.9999

EXAMPLE 5.1

If a position error has an ellipsoidal normal PDP with a; == 2 nmi, (Jy == 1 nmi, (Jz == 0.25 nmi, and errors in the x, y, z channels are uncorrelated, what is the probability that a measured position error will fall within a box bounded by [x] :s a == 1.4 nmi, /yl :s b == 1 nmi, and [z] :s c == 0.5 nmi?


and

V ==~,

Pxy == Pyz == Pxz ==

0,

U

== v == 1

69

The probability can be found from Table 5.1 with ao == (1.4/2) == 0.7, bo == III == 1, (0.510.25) == 2. Hence,

Co ==

p == erf( aol,J2)erf(bol ,J2)erf(col,J2) == 0.5161 x 0.6827 x 0.9545 == 0.3363,

or we can enter either of the two MATLAB commands:

p = nf2(0. 7)*nf2(1)*nf2(2)

p

= edbvud ( [1.4,

1, 0. 5], [2, 1, 0. 25] )

and obtain the same result p == 0.3363.

Given (Jx, (Jy, a., p, there are many (a, b, c) triplets that satisfy Equation (5.5). The inverse problem could have multiple solutions of (a, b, e) if one does not specify the two ratios k1 == alb, k2 == b]«: Notice that unlike the ratios u, v, which are bounded by and 1, the ratios k 1 , k2 can have any value in [0, (0). Solutions for inverse problems are obtainable from edbvui.m. The MATLAB command is [a, b, e] == edbvui(p, k 1 , k2 , [(Jx, «; (Jz]). Let us look at another example.

°

EXAMPLE 5.2

Suppose that the joint PDP of a certain position error is ellipsoidal normal with (J x == 1.5 nmi, "» == 0.6 nmi, (Jz == 0.5 nmi, and that errors in the x, y, z channels are uncorrelated. Determine the length a along the x-axis, width b along the y-axis, and height c along the z-axis when the specified ratios are k1 == (alb) == 2, k2 == (blc) == 1, and the probability for a sample position error to fall inside this 2a x 2b x 2c box is 90%. After entering the MA TLAB command

[a, b , c]

= edbvui (

°.9, 2, 1, [1. 5 , °.6 , °.5] )

we obtain a == 2.7049, b == 1.3525, c == 1.3525. If, however, k, == (alb) == 1/2, k2 == (blc) == 1, then using

[a,b,c] = edbvui(0.9, 0.5,1, [1.5,0.6,0.5]) we obtain a == 2.4673, b == 4.9346, c == 4.9346, instead. This demonstrates the necessity of clearly specifying k1 == (alb), k 2 == (blc) for the inverse problem.

5.4 CASE 2,

V ==~,

Pxy == Pyz

== Pxz == 0,

u == v == 1

When the random variables x, Y, and z have the same standard deviation ((Jx == (Jy == (Jz == (J), the general joint PDF is reduced to a spherical normal distribution

1 [-1

f(x, y, z) = ~ exp 2a 2 (2 x V(2n)3(J3

+ y2+ Z2)]

(5.6)

70


The triple integral of f(x, y, z) over a spherical volume x 2 + y2 + z2 ~ R 2 represents the probability of a sample point falling in this region. Equation (5.3) can be transformed to an equivalent form in spherical coordinates

p=

InrnJR ~ exp[~r:]r2drSin(c/J)dc/Jd() = JR h(r, a)dr o

0

0

(2n)3 a 3

a

(5.7)

0

with her, a) being the Maxwell distribution function

[2;:l

h(r,a)=A::exP

forr20

(5.8)

The independent variable of this Maxwell probability density function is the random radial error r = Jx2 + y2 + Z2. The M-file sfl.m implements the Maxwell PDF for a = 1. Following the notation convention of Chapter 3 (page 25), we will designate the associated CDF as H(R/a), where

H(Rja)

=

r

h(t, a)dt

since we have used her, a) as the PDF of r. Thus, carrying out the integration in Equation (5.7), we have

p = H(Rja) = = _

J~A~: eXP[2;~] dt

Vf!.:~ !!.exp a

[-R+ erf(~) 2

2a2

]

V2a

(5.9)

Geometrically, this means that the integral of f(x, y, z) over the volume

x2

+ y2 + z2 ~ R 2 is equivalent to the area under the Maxwell PDF, h(r, a), over the

interval [0, R]. Therefore, in this special case, and this case alone, three-dimensional error analysis can be performed in terms of a single random variable r, the spatial (three-dimensional) radial error. Equation (5.9) provides the closed-form solution for the direct problem. We can rewrite Equation (5.9) in the following form

p = H(r) = H(Rja) - Arex p[ -:]

+ erf(~)

(5.10)

Note that r = R]o is the radius normalized with respect to (J. The M-file sf2.m, with the MATLAB command form p = sf2(r), can be used to find the probability p. The inverse problem offinding R from

p == or finding r from

AR

[-R2] + erf (-V2a R) -

--exp - na 2(J2

Section 5.5 • Three-Dimensional Error Measures

71

with given p, cannot be solved analytically. Numerical iteration is used to obtain the solution. This numerical inversion procedure is implemented in sf3.m. The MATLAB command is r == sf3(p). Another command, r == sf4(p), achieves the same goal using the Newton-Raphson method; see Equation (1.6). The corresponding general forms of sf2.m and sf3.m are sdsvud.m and sdsvui.m, respectively. The MATLAB commands to use these files are p == sdsvud(R, (Jx) and R == sdsvui(p, (Jx).

5.5 THREE-DIMENSIONAL ERROR MEASURES Because of its simplicity, the Maxwell distribution will be used to illustrate several three-dimensional error indicators.

5.5.1 Mean Radial Error (MRE) The mean radial error (J1r) is defined as MRE = u, = E[r] = [rh(r, 0") dr =

2A

0"

~ 1.59580"

(5.11)

5.5.2 Root Mean Square Radial Error (RMSR) The mean square radial (MSR) error is similarly defined as

J~ r2h(r, O")dr =

MSR = E[r 2] =

30"2

The RMSR is obtained by taking the square root of the MSR RMSR == JE[r 2 ] == v'3(J ~ 1.7321(J

(5.12)

5.5.3 Distance Root Mean Square Error (DRMS) DRMS is defined as the square root of the sum of all three x, y, z variances as DRMS

= JO"~ + O"~ + 0"; = J1 + u2 + v2 O"x

(5.13)

with u == (J y/(J x and v == (Jz/(J x- When (J x == (J y == (Jz == (J, DRMS ==

v'3 (J == RMSR

The GPS position dilution of precision (PDOP) is defined in terms of this three-dimensional DRMS (see Appendix D).

5.5.4 Standard Radial Error (SRE) The variance of the radial error r is

2

0"; = E[(r - Ilri] = E[r ]- (Ilri =

(3 -~)0"2

(5.14)

The standard radial error is the square root of this variance; thus, SRE == (J r ~ O.6734(J.

72


These three error indicators (SRE, RMSR, MRE) are related by

a; == (SRE)2 == (RMSR)2 - (MRE)2

(5.15)

Notice that RMSR is not equal to (J or are

5.5.5 Spherical Error Probable (SEP) The SEP is defined as the particular radius of the spherical volume V over which the triple integral in Equation (5.7) produces a probability of 50%, SEP

p = 0.5 =

J

her, a)dr

0

Figure 5.1 shows the PDF and CDF of the Maxwell distribution. The RMSR, MRE, and SEP points are marked with "*", "+", and "0", respectively. Other points of interest are the lrr, 2a, and 3a probabilities, which are 19.870/0,73.850/0, and 97.070/0, respectively. Since the variable r in H(r) is to be interpreted as a normalized quantity (r == R/a), the solid CDF curve in Figure 5.1 is valid for all positive values of a. The variable r in h(r, a) should not be treated as a relative quantity. Thus, the dashed PDF curve in Figure 5.1 is valid only for a == I. In other words, the variable used for the horizontal axis has two interpretations, one for the CDF and another for the PDF.

u,

0.9

.

.

0.8

.

... .H(r~ =

S 0.7 II ~

.

0.6

o

~

'" '-'

0.4

.

,

~

::-

..........

::-- 0.3

~

.

~

::t u; 0.5

~,'

0.2

~'/~'

/

/

.

~.I••••••••••

,r ::

···~·~'··T·····

0.1

o ". o

0.5

................

....

.. ~

: :

.. .. .. .. .. .. ..

:

f; h(t, 1!dt .....

!

:

.

: :

:

..

: :

···~·~·+<M~RE~SR

j

j

.

.........

~

~

.

;

.:

;

SEp·~, .~

"~

........... ~

t

~

...

···..

.. .. .:

........ i ~ .

:

':"~""""':""""'"

~

-, h(r, 1)

;

.

~

······.···········-:······~····r············:-·········

:

:

::

:: : ::

1.5

2

: ~

" :

~,

:

""

:

: ,,: : ;

:

~.,.:

:',:

:

2.5

r (assume o =1 in Maxwell

3

. .

3.5

4

PDF)

Figure 5.1 PDF and CDF for the Maxwell Distribution.

Figure 5.2 shows the PDF her, (J) of Equation (5.9) with a == I. The shaded area under the her, I) curve in the [0,1] interval (or R == I) is the probability p == H(I/I) == -J2/nexp(-1/2) + erf(I/J2) == 0.1987, according to Equation (5.10). However, if a == 2, then her, 2) will be different from that in Figure 5.2 and we have p == H(I/2) == -J2/n exp(-1/8) + erf[I/(2,J2)] == 0.0309. The terms I-SIGMA (la) error, 2-SIGMA error, and so on, in three-dimensional error analysis should only be used in the special spherical normal distribution case. In all other cases, it is not even defined. A statement such as "the position accuracy of a system is 60 m (2a)" could cause the reader to wonder whether the


73

Area unde r her, 1), for

r

in [0,1] is 0.1987

0.5

l?

~ 0.4 J::

u.:

~ 0.3

Q)

3:

~ 0.2 :2 0.1

0.5

1.5

2.5

2

Figure 5.2 P = f~h(r, u)dr

3

3.5

4

4.5

5

for the Maxwell PDF, a = 1.

measure is a spherical error, in which case the numbers describe the 73.85% probability sphere. It can also be interpreted as a one-dimensional sigma along each axis, in which case the 95.45% probability sphere is implied (assuming the distribution to be spherical normal, which actually may not be so). Extra care is needed to avoid confusion. Table 5.2 contains the R-to -p pairs for the direct problem in this special case . R is expressed in three forms: R = k . a, R = k . MRE, and R = k . SEP. Table 5.3 shows the p-to-R pairs for the inverse problem. In this table, for a given p, R is expressed as multiples of a, MRE, and SEP; that is, k = Ria, k = RIMRE, and k = RISEP, respectively. The probabilities for R = I . DRMS and 2 . DRMS are 60.21% and 99.26%, respectively. For direct problems, we use the M-file r2p3d.m, which takes a multiple k of SIGMA, RMSR, MRE, and SEP, and then produces the corresponding probability p for R = k . SIGMA, R = k . MRE, R = k . RMSR , and R = k . SEP, respectively. For example, entering r2p3d(2) in MATLAB results in For R

= 2*SIGMA

p = 0.7385 For R

= 2*MRE

p=0.9829 For R

= 2*RMSR

p = 0.9926 For R

= 2*SEP

p = 0.9763

74


TABLE 5.2

Three-Dimensional Direct Problem, from

Rtop

p == H(R/(J) k

R==k·(J

P == H(R/(J) R == k- MRE

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0

0.0000 0.0003 0.0021 0.0070 0.0162 0.0309 0.0516 0.0789 0.1128 0.1529 0.1987 0.2494 0.3038 0.3608 0.4192 0.4778 0.5355 0.5911 0.6439 0.6932 0.7385 0.7795 0.8161 0.8482 0.8761 0.8999 0.9200 0.9368 0.9506 0.9617 0.9707 0.9778 0.9834 0.9877 0.9909 0.9934 0.9953 0.9966 0.9976 0.9984 0.9989

0.0000 0.0011 0.0084 0.0273 0.0613 0.1120 0.1786 0.2584 0.3473 0.4405 0.5331 0.6207 0.7003 0.7695 0.8275 0.8745 0.9111 0.9387 0.9589 0.9732 0.9829 0.9895 0.9937 0.9963 0.9979 0.9988 0.9994 0.9997 0.9998 0.9999 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000

p == H(R/(J) == k· SEP

R

0.0000 0.0010 0.0075 0.0245 0.0554 0.1016 0.1629 0.2372 0.3210 0.4101 0.5000 0.5867 0.6670 0.7384 0.7996 0.8504 0.8911 0.9227 0.9466 0.9639 0.9763 0.9848 0.9905 0.9942 0.9965 0.9980 0.9989 0.9994 0.9997 0.9998 0.9999 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000


75

TABLE 5.3 Three-Dimensional Inverse Problem, from ptoR p 0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 0.990

k==R/a

k == R/MRE

k == R/SEP

0.0000 0.4645 0.5931 0.6870 0.7644 0.8321 0.8932 0.9496 1.0026 1.0529 1.1012 1.1478 1.1932 1.2377 1.2812 1.3244 1.3672 1.4098 1.4524 1.4951 1.5382 1.5817 1.6257 1.6706 1.7164 1.7635 1.8119 1.8621 1.9144 1.9691 2.0269 2.0884 2.1544 2.2264 2.3059 2.3957 2.5003 2.6277 2.7955 3.0575 3.3682

0.0000 0.2911 0.3717 0.4305 0.4790 0.5214 0.5597 0.5951 0.6283 0.6598 0.6900 0.7193 0.7477 0.7756 0.8029 0.8299 0.8568 0.8835 0.9102 0.9369 0.9639 0.9912 1.0188 1.0469 1.0756 1.1051 1.1355 1.1669 1.1997 1.2340 1.2702 1.3087 1.3501 1.3952 1.4450 1.5013 1.5668 1.6466 1.7518 1.9160 2.1107

0.0000 0.3020 0.3856 0.4466 0.4970 0.5409 0.5807 0.6174 0.6518 0.6845 0.7159 0.7462 0.7757 0.8046 0.8330 0.8610 0.8888 0.9165 0.9442 0.9720 1.0000 1.0283 1.0569 1.0861 1.1159 1.1465 1.1780 1.2106 1.2446 1.2802 1.3177 1.3577 1.4007 1.4474 1.4991 1.5575 1.6255 1.7083 1.8174 1.9878 2.1897

76


For inverse problems, the M-file p2r3d.m takes p as input and generates R expressed in terms of SIGMA, RMSR, MRE, and SEP. This may be considered as a conversion tool between these four types of three-dimensional errors. For example, entering p2r3d(0.5) in MATLAB results in

For p = O. 5 R

= 1.5382*SIGMA

R =

R

0 . 9693 *MRE

= o. 8881 *RMSR

R=l*SEP

5.6 CASE 3,

V ==~,

Pxy == Pyz == Pxz == 0,

U, V E

[0, 1]

When the random variables x, y, and z are uncorrelated and have unequal standard deviations, the general joint PDF becomes

f(x, y, z) =

~ exp[~l (:~ + ~~y + ;~)] (2n)30"xO"yO"z x

(5.16)

z

Because the three standard errors 0"x, 0"y, and o"z are not equal, we assume that O"x2:O"y2:O"z and use U and v to represent the ratios O"y/O"x and O"z/O"x, respectively. Equation (5.16) is changed to

1

f(x, y, z) = ~ exp V(2n)3uvo"~

[-1 (2 + y2 + z2) ] 20"2

x

u2

v2

(5.17)

x

Replacing rectangular coordinates with spherical coordinates in Equation (5.3), we obtain

p = Ifff(X, y, z)dxdydz ~

= I: I: I:"f(r sin ¢ cos f), r sin ¢ sin f}, r cos ¢ )r2 sin ¢ df} d¢ dr

f"rfllT

(5.18)

which, after the substitution of A == r/O"x, U == O"y/O"x, and v == O"z/O"x, simplifies to p=

~

(2n)3uv

.

0

0 0

X

exp

{2~~;2 [u2v2sin2¢cos2f} (5.19)


V

==~,

== Pyz == Pxz == 0,

Pxy

77

[0, 1]

U, V E

Letting w == cos cP, then p can be changed to _ p-

4

v'fn3 2n 3uv

JR/(Jx 0

2

A exp

(-A?) Jl exp [(V 2- 2 2 0

2]

1))?W 2 v

(5.20)

Thus, p depends not only on the ratio R/(J x, but also on the ellipticities u == (Jy/(Jx and v == (Jz/(Jx, which are derivable or known parameters. No analytic solutions are possible for both the direct and the inverse problems in this case. Numerical integration and root searching techniques must be used. The M-files tf2.m and tf3.m accomplish these goals in normalized form. The MATLAB commands are p == tf2(r, u, v) and r == tf3(p, u, v), respectively. The corresponding general (non-normalized) forms are p == edsvud(R, [(Jx, (Jy, (Jz]) and R == edsvui(p, [(Jx, (Jy, (Jz]), respectively. For example, p

= edsvud(2,

[1.5,1.6,1]) produces p

= 0.4573,

R= edsvui(0.4573, [1.5,1.6,1]) produces R= 2.0000.

It takes longer to run edsvui.m than edsvud.m, as is usually true for inverse problems. Figure 5.3 shows how the probability p changes when u and v are varied from oto 1 for a R == 1 . DRMS. Recall that as u, v vary, I DRMS == VI + u2 + v2 a, does not remain constant. 0.69

1 DRMS, r = R/crx

r------,-----~----r-----_,__---__. · .

~ 1 DRMS = ~"--1-+-u2- +- V- 2 (J'x

v= 0 0.68

v = 0.2:-

0.67

.............................. :..

.

.. .. .. .. ..

·

...... iu ~

·· ·

.. .. .. .. .. .. .. ..

..

..

. . · "

··

-. 0.66

.

.

.. . · ··

.. .. ..

v = crz/crx ~

~

..

~

.

-:-

..

..

. ...

.

· . ..........................................

r;:'

::i'

~ 0.65 ...............·: ·· :c ··· :::J > ~ 0.64 ·· Q) ·· II · ·· ~~

:

... . ..· ... .. ..

. .

• • • • • • • • • • • • • • • : • • • • • • • • • • • • • • • •:

Q.

0.63

0.62 .......................... ·: ·· ··· 0.61 .............. :.. ..

.. .. ..

0.6

..

'I. • • • • • • •

v = 0.6

~

· .. .

. . . .. . . .. . ..

:

:

..

..

~

... .. .. .. ..

..

~

..

..

.. .. ..

"----~----..L..-_-----JL--

o

0.2

0.4

0.6

:. . . . . ..

..

..

v= 0.8:

.

: .. ..

.. ..

..

..

v= 1

.. ....L--

0.8

Figure 5.3 Probability Versus u with v as Parameter for R = 1 . DRMS.

--J


78

Figure 5.4 is similar to Figure 5.3, except that it applies to the case of

R

== 2· DRMS.

We notice the resemblance of Figures 5.3 and 5.4 to the two graphs in Figure 4.7 for the two-dimensional case. The normalized radius r == RI(J x, as a function of both ellipticities u, v, is plotted in Figures 5.5 and 5.6 for various fixed probability values p == 0.5 and p == 0.95, respectively. We conclude this section with an example to find the radius of a 500/0 probable error-sphere. EXAMPLE 5.3 If a certain position error has an ellipsoidal normal distribution with (J x = 2.5 m, (J y = 2 m, = I m, and errors in the x, y, z channels are uncorrelated, find the radius of a sphere such that 500/0 of the time a measured position error will fall inside this sphere. (Jz

R= edsvui(O.5, [2.5,2,1]) = 2.8441 Thus, the SEP or Rso%

= 2.8441 m.

Figure 5.7 shows the volume (~ == 4nR 3/3) versus u and v, for p == 50% and a, == 1, O:::;u, v:::; 1. The maximum (~== 15.2444) occurs when R == 1.5382, corresponding to u == v == 1 or when all o's equal one. R = 2 DRMS, r = R/crx

0.99

:

:

;

~

··· · ··

0.985

:

.. ...

. ... v = 0.6·: ..

.

..

.

·· .. ··· ... ·· .. ........................................ 0.98

.

~

::i'

..............:

--lo......... 0.975 U :J >

~

: ..

.......

0.97

.

.

\

.

Q)

II Q

. . ................................................

0.965

· ........................... ,

0.96

....... :

0.955 0.95

v= 0

·

:·

·

0

0.2

.

.

:. :. . .

.

·· .. ~ 1 DRMS = ~1 + u2 + v2 ax ~

:·

: u ~ v = crz/crx: 0.4

u = crz/crx

0.6

:

.. 0.8

Figure 5.4 Probability Versus u with v as Parameter for R == 2 . DRMS.

1.6

r

= edsvui(0.5,[1 , u,v]) vs \I, with

u as parameter

r------:.--~---.,...----.,...----_r_---_r______.

1.5

~ .,.LO

8·5

1.4

.

1.3 1.21"1:L-_~-'QolI

1.1

> en

~-~-~

"'C Q)

II Q..

0.9""""-_ _

0.8

u = cry/crx

0.7 0.6

u 0

=0

~

0.2

0.4

0.6

0.8

v = cry/crx

Figure 5.5 Normalized Radius for Various Ellipticities when p == 0.5. r

= edsvui(0.95,[1 , u, v

2.7

-

]) vs v, with u as parameter

.

2.6

.

a

~

.

.............

::i 2.5 -

u = 0.8

.,.-

LO 0>

8·5

> en

"'C Q)

II ~

=1 .

:

2.4 2.3

........ : "'u

2.2 2.1

= 0.6·~·············

.......................... ~

..

2 1.9

u=O 0

0.2

0.6

0.4

0.8

Figure 5.6 Normalized Radius for Various Ellipticities when p == 0.95. Spherical Volume,

P =0.5

20 15

10 5

o 1

Figure 5.7 Spherical Volume for Various u and v Values when p == 50% •

79

80


5.7 CASE 4,

V ==~,

Pxy' Pyz' Pxz E [-1,1],

U,

v E [0,1]

Under the assumptions for the present case, Equation (5.3) becomes p=

III

J(2n)3(1

~ P~y P~z p~z)

+ 2pxypyzPxz

{

-

-

UxUyU z

-1

with the volume of integration being the interior of a sphere x

2

+ y2 + z2 :::; R 2

Let

Ax

+By +Cz +DIXY +D2YZ +D3 XZ =(1 + 2PxyPyzPxz -1 P2xy - P2yz - P2) xz

222

(1 - p;z)x 2 x

2

[

ax

+

(1 - p;z)y2 2

ay

+

(1 - p;y)z2 2

az

2(pxy - Pxzpyz)xy axay

_ 2(p yz - PxyPxz)yz _ 2(pxz - PXyPyz)XZ] aya z axa z

(5.22)

The correlation between x, y, Z errors can be removed by eliminating the three mixed-product terms (D1xy, D2yz, D3xz) in the preceding expression. This is carried out with eigenrotation of the axes or an appropriate similarity transformation on the quadratic form

B

Ax2 + y2 + Cz 2 + DIXY + D2YZ +

Dvxz == [x Y z] [D:/2

D~2 ~:~~] [;]

D3/2 D2/2 to obtain Ax2 + B y 2 + Cz 2 + DIXY + D2YZ + D3 XZ == [x Y z]

[~:: ~~: ~::] [~l q13

q23

q33

0

;1 0

~] [~:: ~:~ ~::]

C1

q31

q32

=[~ ({~l ~l ~J[~]=Ale+BI1J2+Cle 1J

q33

C

z

[;] Z

(5.23)


V

==~,

Pxy' Pyz' Pxz

E

[-1,1],

U,

81

v E [0,1]

Note that AI, B 1 , C I are the eigenvalues of the matrix

and the three corresponding eigenvectors

are used in the transformation from (x, y, z) to

(~,

(~,

1'/, ,)

The spherical surface of the integration volume remains a sphere in the new 11, ,) coordinate system,

Note that the problem is now expressed in (~, 11, ,) coordinates and shows no dependency on the p's; it can be solved by the method used in case 3 as

P=

1 JJ Vr;:::;::; (2n) a A~

~ (J 11 a,

where a~ == 1/AI, a~ == 1/ B 1 , az == ~2

exp

1-12 [~2 + 1'/2 + ,2] I (J2

(J2

~

11

(J2

d~ dn

,

l/e I , and the volume of integration A~ is

+ 1'/2 + ,2 ~ R2

The direct problems are solved by edsvcd.m, with the MATLAB command p == edsvcd(R, [ax, (Jy, (Jz], [PXy, Pyz' Pxz]), whereas the inverse problems are solved by edsvci.m via the command

R == edsvcd(p, [(J x, (Jy, (J .L [Pxy, Pyz' Pxz]). For example, p

= edsvcd (2 ,

[1. 2 , 1 . 4 , a. 5] , [0. 5 , a. 7 , a. 4] ) results in

p = 0.6866, R = edsvci (0.6866, [1.2, 1.4, 0.5] , [0.5, 0.7 ,0.4] )

results in R

= 2. 0000.

Notice that when Pxy == Pyz == Pxz == 0, case 4 is specialized to case 3; and when both Pxy == Pyz == Pxz == 0, and u == v == 1, case 4 is reduced to case 2.

82


5.8 CASE 5,

V ==~,

Pxy

==

P yz

==

Pxz

== 0,

U, V E

[0, 1]

In this case, the probability in Equation (5.3) becomes

p ==

JJJ

1

2

1UJ x

(Jy (Jz

1-1

2

I

y2 z2] dxdy exp -2 [X 2"+2"+2" (Jx

(Jy

(Jz

(5.24)

~

where the volume of interest

~

is an ellipsoid

y2

x2

z2

-+-+- probability results, the corresponding value for r == R/ (J is seen from the horizontal axis to be 1.1774. Thus, for this particular example, the CEP would be r x (J == 1.1774 x 5 == 5.887 m. One can also use the M-files cf2.m and cf3.m to obtain the same results via the commands p = c f 2 ( 1) and p = c f 3 ( 0 . 5 ) .

101

Section 7.1 • Single Error-Ellipse Analysis

0.9 : H(r) =f 0 h(t,1) dt

.

:

:

:

.

RMSR ~ : ~

: :

: ~ .

.

.........:.

0.8 ~

o

II

~

~ ~

0.7

~

0.6

~. ~ ;.-: ""

0.5

··········~·/······f·· ~~~~~

.

0.4

.........

I

/

.,{.

:

:

:

/ .. L

~ 0.3

0.2 .../

!.

f '

0.1

I

"

00

J

.

-:

L

l

,-.

.

.

~

j

J\ L :

L

:

j

.

.

:

~~.:.h.rr,.1J. i

:

.

i

:

.

~

.

L. 1.'~.,

~.. ~ ..........• ~

~

\:

~

:

~

~

~

~

0.5

1

1.5

2

,

.

'-,

~.,

~

.

~ ',,~2.5

3 - - 3.5

r (assume a = 1 in Rayleigh PDF)

4

Figure 7.4 PDF and CDF for the Rayleigh Distribution.

7.1.1 Approach 1. Fictitious Sigma and Cut Angle

In this approach, it is assumed that we can find fictitious values of (J such that the two different (Jl and (J2, originally given, may be replaced by two new a's of equal value, indicated as (J*. Thus, a new and fictitious angle of intersection ('1.* replaces the old one ('I.. Figure 7.5 illustrates the original and new values for the standard errors and the cut angle.

Figure 7.5 Old and New Standard Deviations and Cut Angles.

102

Chapter 7 • Position Location Problems

The values of (J* and rx* are computed from the two formulas

a"

= sin(2p)·

J + a~ aT

(7.3)

~ rx*

== arcsin[sin(2fJ) . sin(rx)]

(7.4)

with fJ == arctan((J2/(Jl)' We also define the sigma factor as Fa == (J* /maX(O"I, (J2). These two equations plus sigma factor may be computed using the M-file fsca.m. To obtain (J* and rx* from (Jl, (J2, and a, simply enter [scaia», 0"2, rx) in MATLAB. For example, entering fsca(2, 3, 30) or fsca(3, 2, 30) will produce (J* == 2.3534, rx* == 27.49 Fa == 0.7845. If 0"1 ::; 0"2, Figure 7.6 shows the sigma factor Fa == (J* / (Jl as a function of the ratio u == (J2/0"1. Figure 7.7 displays the fictitious cut angle rx* versus the original cut angle rx, with u == (J2/(Jl as a parameter. Figure 7.8 shows the probability that a measurement lies within a circle of radius r == R/ max((Jx, (Jy) under the new joint probability density function 0

,

(7.5)

0.9

;

;

··· ... · . . . : ;

0.8

.. ..

.. ..

..

..

. . . . ·:. ~............

0.7

· .... . ·.. ............................... -. ..

o

(3

0.6

.. ..

..

·· ..

........................... :... .... .. .. ...... ......

~ 0.5

.. .. .. ..

0)

II

...

.. ..

ctS u,

en

..

..

.. ..

....................... :.. ..... ...

0.4

~

t, 0.3

.............. :

0.2

:

··· ·· ·: ··

0.1

·

··

..

. ;

. .

. :

.:

.. . ..

.

..

.... :

..

.~

..

.. ..

... ,.

.. ..

. ..

..

.. ..

.. .. ..

~

. . . . ..

. .

.. .. .. .. .. ..

..

:.. ..

~

~

:

.

:

.

,

:

··· · ··: .. ... .

~

.. . . . ·

..

:

. ...

.

. :

... .. .

..

.. .. ..

.. .. .. .. ..

~

... . .. -:.

.. ... .:

~

...

.. ... .

..

. .

Orwt------.JL-..-----'----.....L----.J-.-----J

o

0.2

0.8

Figure 7.6 Sigma Factor Versus Ellipticity u.

Section 7.1 • Single Error-Ellipse Analysis 90

103

----.....-,r---.....----...,---r-----r---r----r-~

u= 0.9

~

80

.

70

.

60

_

.

~

c

«

:5 50

.

o

(J)

:::3

~ 40 C3

tr

~ 30 t$

20

u = 0.1 10

10

20 a

30

40

50

60

70

80

= Cut Angle Between LOPs (0)

Figure 7.7 Fictitious Cut Angle Versus Original Cut Angle.

0.9 0.8

0.7

:

0.6

:

:

.

.

..-.....

*

~"' 0.5

-......:..

Q.

0.4 0.3 0.2

·

0.1

~

~

.

: ··

:

:

:

1.5

2

2.5

.........:

· 0.5

" "

~

..

"

r

3

3.5

Figure 7.8 Probability Versus r, with Cut Angle as Parameter.

104


where

o; ==

(J*

~ . sin(a* /2)

and

(Jy

==

(J*

~ . cos(a*/2)

with the fictitious variable (J* computed from Equation (7.3) and (J* from Equation (7.4), using the original cut angle and the unequal standard errors (JI and a2. The next example shows the procedure for calculation. EXAMPLE 7.2

Assume that a position location system has provided the following data: rJ. == 50°, 0"1 == 20 m, and 0"2 == 15 m. What is the probability that the location of the point is within a circle of 25 m radius?

• Start MATLAB. • Enter p == seml(25, 15,20,50) or p == seml(25, 20,15,50) Note that the order of appearance for the two o's in the input argument list of seml.m is reversible. • p == 0.5035 or 50.350/0 is the answer.

Figures 7.6-7.8 can be used to solve problems similar to that shown in Example 7.2 for those who prefer approximate solutions from graphs rather than precise solutions from MATLAB M-files. Locations marked with the symbol "*" in Figures 7.6-7.8 correspond to steps 2, 4, and 6, in the following procedure: 1. 2. 3. 4.

Compute the ratio u == (J2/(JI == 15/20 == 0.75. Find the sigma factor Fa == 0.8485 from Figure 7.6, for u == 0.75. Obtain (J* == Fa . (JI == 0.8485 X 20 == 16.97 m. Find a* == 47.34° from Figure 7.7, for u == 0.75 and a == 50°.

5. 6.

Find r == Rfo" == 25/16.97 == 1.473. Find p == 0.5035 from Figure 7.8, for the fictitious cut angle a* == 47.34° and normalized radius r == 1.473.

7.1.2 Approach 2. New Sigmas Along the Major and Minor Axes A different transformation is used to find new sigmas along the two principal axes of the ellipse from the given sigmas and cut angle (see Figure 7.9). The two new sigmas (Jx, a y are computed from

2 ==

(J x

2 ==

(J y

. 12

2

SIn

2

SIn

. 12

(a)

(a)

.

[2 (J 1 + (J 2 2+

J 2+

(J 2

.

[2 (J 1 + (J 2 2 -

J 2+

(J 2

(J

(J

1

1

2 - 4· SIn2() a (J 21 (J 22]

(7.6)

2 - 4· 2] SIn 2(a ) (J 2 1 (J 2

(7.7)

Section 7.2 • Consideration of Geometrical Factors

105

Figure 7.9 Old a's with Cut Angle and New a's Along Orthogonal Axes.

We demonstrate this second approach using the same information as given in Example 7.2. • Start MATLAB. • Enter p == sem2(25, 15,20,50) or p == sem2(25, 20,15,50) and obtain 29.89,O"y == 13.1, and r == 0.8364. • p == 0.5035 or 50.35% is the answer.

(Jx

==

Alternately, we can use graphic approximation as follows: 1. 2. 3. 4.

Compute o, == 29.89 and O"y == 13.1 from Equations (7.6-7.7). Find ellipticity u == 0"y/ 0"x == 0.4382. Obtain r == R/O"x == 0.8364. Find p = 0.5035 from Figure 7.10.

Figure 7.10 shows the probability versus radius curves for various ellipticity values. This graph is used for direct problems. Figure 7.11 shows the radius of the circle with specified probability as a function of ellipticity. The values used for pare 50%, 600/0, 70%, 80%, 900/0, and 950/0. This graph is used for inverse problems. The radius r is a normalized quantity (normalized with respect to the maximum of o"x, O"y). To use either one of these two figures, we need to compute the ellipticity u == min(O"x, O"y)/ maxur,; O"y).

7.2 CONSIDERATION OF GEOMETRICAL FACTORS

Notice that the cut angle plays an important role in each of the two methods described earlier. We will develop curves for constant values of initial error showing

106


0.9

.

0.8 0.7

3' 1--"'

~

0.6

'0

~ 0.5

c

'0 CD

II Q..

0.4 0.3 ...

0.2

..

...

.......................... !

··

.:-

0.1

· ··

~

...

...

:

-:

.. .. .. . . ..

...

1.5

0.5

...

...

O-------..L-----L.---~---...&-_--.Io..-.

o

.

. :-

~

..

.

...

...

:

... ... :-

·· ··

2.5

2

r

__

___J

3

Figure 7.10 Probability Versus r with Ellipticity u as Parameter.

2.6,r----...-----------,..------------.----. 2.4

........ .~

.~

~

·

.. .. .

. ... .

.

... ... -: ... .

0.1

0.2

0.3

0.4

... ...

2.2

:.

· ··;. · ··· ·

;.

...

.. . ...

:

...

:..

:.

~

. ..: ..

..

.

;.

:

0.5

0.6

...

...

.

:. ~ .~~.?:~~. . ... ... ...

.

...

.

...

...

3' 1.8 1--"'

~ '0

~

o

1.6

'0 CD

~

1.4 1.2

0.8 0.6 0

u = cry/crx

0.7

0.8

0.9

Figure 7.11 Radius of Circle with Specified Probability Versus Ellipticity u.

107

Section 7.2 • Consideration of Geometrical Factors

that the radius of a circle with a fixed value of probability varies as a function of the intersection angle of the lines of position. To simplify the investigation of geometrical factors, let us assume 0'1 = 0'2 = 0'. In this case, the formulas for a; and (Jy reduce to O'x

=

(J

,J2. sin(aj2)

(J

and

Hence, the ellipticity is u = tan(aj2) or u = cot(aj2), depending on the sizes of a; and (Jy. In any problem involving position, it is well known that the best results are obtained when the lines of position intersect at right angles. We are interested in the impact when the cut angle is not 90 0

TABLE 7.1

a e) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175

•

Probability Versus Cut Angle for Constant Radius ax

ay

u

p[r(500/0, 90 0 ) ] (0/0)

p[r(90°/0, 90 0 ) ] (°/0)

16.2108 8.1131 5.4174 4.0721 3.2670 2.7321 2.3515 2.0674 1.8478 1.6732 1.5314 1.4142 1.3160 1.2328 1.1615 1.1001 1.0467 1.0000 1.0467 1.1001 1.1615 1.2328 1.3160 1.4142 1.5314 1.6732 1.8478 2.0674 2.3515 2.7321 3.2670 4.0721 5.4174 8.1131 16.2108

0.7078 0.7098 0.7132 0.7180 0.7243 0.7321 0.7414 0.7525 0.7654 0.7802 0.7972 0.8165 0.8384 0.8632 0.8913 0.9231 0.9591 1.0000 0.9591 0.9231 0.8913 0.8632 0.8384 0.8165 0.7972 0.7802 0.7654 0.7525 0.7414 0.7321 0.7243 0.7180 0.7132 0.7098 0.7078

0.0437 0.0875 0.1317 0.1763 0.2217 0.2679 0.3153 0.3640 0.4142 0.4663 0.5206 0.5774 0.6371 0.7002 0.7673 0.8391 0.9163 1.0000 0.9163 0.8391 0.7673 0.7002 0.6371 0.5774 0.5206 0.4663 0.4142 0.3640 0.3153 0.2679 0.2217 0.1763 0.1317 0.0875 0.0437

4.50 8.97 13.35 17.61 21.72 25.63 29.33 32.77 35.95 38.83 41.41 43.66 45.59 47.17 48.40 49.29 49.82 50.00 49.82 49.29 48.40 47.17 45.59 43.66 41.41 38.83 35.95 32.77 29.33 25.63 21.72 17.61 13.35 8.97 4.50

9.89 19.60 28.96 37.81 46.03 53.54 60.28 66.22 71.38 75.78 79.46 82.50 84.94 86.84 88.26 89.24 89.81 90.00 89.81 89.24 88.26 86.84 84.94 82.50 79.46 75.78 71.38 66.22 60.28 53.54 46.03 37.81 28.96 19.60 9.89


108

First, let us consider the direct problem: what is the probability of the errorcircle when the radius of the circle is minimal and the intersection is a right angle, that is, rx =90°? Define the normalized radius of the error circle as

R(po,rx) r(po,rx) = = - - - max(ax , a y)

J-2In(1 - Po) max(ax , a y)

Thus, R(po, rx) == r(po, rx)max(a x , ay), in particular, CEP == R(50%, «). Table 7.1 shows the probabilities for error circles of constant radius r( 50 %, 90°) and r(900/0, 90°) for various cut angles. Figure 7.12 is the graphic representation of the content of the two rightmost columns in Table 7.1. Here we see that the probability reaches a peak at 90° and decreases to zero as the cut angle approaches 0° or 180°. The MA TLAB command p == ang2p(a, Po) is used to generate Table 7.1. For example, p == ang2p(10, 0.5) results in 8.97. Next, consider the inverse problem: for a given probability value, how does the radius of the error-circle change as the cut angle varies? Define the error factor as the ratio R(p, rx)/ R(p, 90°). Table 7.2 shows significant parameters of the error-ellipses (ax, a y, u, r(50°A> , rx), R(50 0/0, rx) or CEP) and the error factor as a function of the intersection angle rx (measured counterclockwise from line of position #1 to line of position #2), for p == 50%. Table 7.3 is similar to Table 7.2, except that it is generated for p == 90%. 100 , . - - - - . - - - - - r - - - - , - -__-----.,.....--__--r--__.._----.

:

90

: ... ..

:

:

:

.. .. ..

.. ..

..

..

..

..

.. ..

.

..

....

80

..

.. ..

70

: r (90 0/0,900/0) ;......... .

..

..

-: ..

..

..

. ..

:

..

..

..

... ..

..

.....

..

.. ..

..

...

...

:

:

..

..

..... ..

..

....

:

~

.

:-

~

.. .. ..

.. .. .. .. ..

.. ..

.

.. : : ..

.. .. ..

...

.. : : ..

.. ..

..

:-

:

.

...:

..

..

..

..

..

. ..

:

.. .. ..

..

-. 60

r (500/0,90% ................................... .. .. .. ..

~

:0 50 ca

..

.. .. ..

.. .. ..

20

40

60

..

.c

e

.. . .. .

a.. 40

)

.. .. ..

..

30 20 10

00

80

a

= Cut

100

120

140

.

.. .. ..

.. ..

..

~ ~

.

.. .. ....

160

Angle (0)

Figure 7.12 Probability Versus Cut Angle for Constant Radius.

.

109

Section 7.3 • Analysis of Multiple Error-Ellipses TABLE 7.2

Significant Parameters of Error-Ellipse, P = 50%

(X CO)

ax

ay

u

r(500/0, (X)

R(50 0/0, (X)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170

8.113 5.417 4.072 3.267 2.732 2.351 2.067 1.848 1.673 1.531 1.414 1.316 1.233 1.162 1.100 1.047 1.000 1.047 1.100 1.162 1.233 1.316 1.414 1.531 1.673 1.848 2.067 2.351 2.732 3.267 4.072 5.417 8.113

0.710 0.713 0.718 0.724 0.732 0.741 0.752 0.765 0.780 0.797 0.816 0.838 0.863 0.891 0.923 0.959 1.000 0.959 0.923 0.891 0.863 0.838 0.816 0.797 0.780 0.765 0.752 0.741 0.732 0.724 0.718 0.713 0.710

0.087 0.132 0.176 0.222 0.268 0.315 0.364 0.414 0.466 0.521 0.577 0.637 0.700 0.767 0.839 0.916 1.000 0.916 0.839 0.767 0.700 0.637 0.577 0.521 0.466 0.414 0.364 0.315 0.268 0.222 0.176 0.132 0.087

0.680 0.688 0.698 0.714 0.734 0.758 0.786 0.817 0.849 0.883 0.919 0.957 0.996 1.038 1.081 1.128 1.177 1.128 1.081 1.038 0.996 0.957 0.919 0.883 0.849 0.817 0.786 0.758 0.734 0.714 0.698 0.688 0.680

5.519 3.725 2.844 2.332 2.005 1.783 1.625 1.509 1.421 1.353 1.300 1.259 1.228 1.205 1.190 1.180 1.177 1.180 1.190 1.205 1.228 1.259 1.300 1.353 1.421 1.509 1.625 1.783 2.005 2.332 2.844 3.725 5.519

R(50 % , (X)/ R(50 o/0, 90°) 4.687 3.164 2.416 1.980 1.703 1.514 1.380 1.282 1.207 1.149 1.104 1.070 1.043 1.024 1.010 1.003 1.000 1.003 1.010 1.024 1.043 1.070 1.104 1.149 1.207 1.282 1.380 1.514 1.703 1.980 2.416 3.164 4.687

The last column in Table 7.2 and in Table 7.3 contains the error factor which is the ratio of R(p, ex) to R(p, 90°) for p == 50% and p == 90%, respectively. Note that R(p, 90°) == min{R(p, ex)lex E [0°, 180°]}; hence, the error factor will always be greater than or equal to 1. Figure 7.13 shows the error factor as a function of cut angle for p == 50% and p == 900/0. 7.3 ANALYSIS OF MULTIPLE ERROR-ELLIPSES

We have developed tools in the preceding sections for specifying individual errorellipses about a single point. The realistic position location problem shown in Figure 7.1, however, involves consideration of the combination of errors from a number of sources. Each of these various error sources can be expressed as an errorellipse. Since each ellipse can be expressed in terms of the standard deviations along

110

Chapter 7 • Position Location Problems TABLE 7.3 Significant Parameters of Error-Ellipse, p == ex e) 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170

(Jx

(J y

u

r(900/0, ex)

8.113 5.417 4.072 3.267 2.732 2.351 2.067 1.848 1.673 1.531 1.414 1.316 1.233 1.162 1.100 1.047 1.000 1.047 1.100 1.162 1.233 1.316 1.414 1.531 1.673 1.848 2.067 2.351 2.732 3.267 4.072 5.417 8.113

0.710 0.713 0.718 0.724 0.732 0.741 0.752 0.765 0.780 0.797 0.816 0.838 0.863 0.891 0.923 0.959 1.000 0.959 0.923 0.891 0.863 0.838 0.816 0.797 0.780 0.765 0.752 0.741 0.732 0.724 0.718 0.713 0.710

0.087 0.132 0.176 0.222 0.268 0.315 0.364 0.414 0.466 0.521 0.577 0.637 0.700 0.767 0.839 0.916 1.000 0.916 0.839 0.767 0.700 0.637 0.577 0.521 0.466 0.414 0.364 0.315 0.268 0.222 0.176 0.132 0.087

1.647 1.650 1.654 1.660 1.668 1.677 1.689 1.704 1.722 1.747 1.778 1.816 1.863 1.918 1.983 2.059 2.146 2.059 1.983 1.918 1.863 1.816 1.778 1.747 1.722 1.704 1.689 1.677 1.668 1.660 1.654 1.650 1.647

900~

R(900/0, ex) 13.364 8.940 6.737 5.424 4.556 3.944 3.492 3.148 2.882 2.675 2.514 2.390 2.296 2.228 2.182 2.155 2.146 2.155 2.182 2.228 2.296 2.390 2.514 2.675 2.882 3.148 3.492 3.944 4.556 5.424 6.737 8.940 13.364

R(900~,

ex)/R(900/0, 90°)

6.227 4.166 3.139 2.527 2.123 1.838 1.627 1.467 1.343 1.246 1.171 1.114 1.070 1.038 1.017 1.004 1.000 1.004 1.017 1.038 1.070 1.114 1.171 1.246 1.343 1.467 1.627 1.838 2.123 2.527 3.139 4.166 6.227

its major and minor axes, the problem is to properly combine more than one ellipse. Determining the proper method needed to combine a number of individual errors in order to obtain the total error at some desired point is discussed later in this chapter. In the general case, the ellipses will be randomly oriented relative to one another. The exact consideration of this random orientation of axes complicates the analysis but is necessary in order to obtain the correct result. 7.3.1 Mutually Parallel Axes

It will be helpful to investigate the special and unlikely case where the major axis of every ellipse involved is parallel (thus all minor axes are also mutually parallel). The analysis for the more general case will then be considered. From the problem formulation in Figure 7.1, there are four error-ellipses of interest (with standard deviations for the two axes of each ellipse listed):

111

Section 7.3 • Analysis of Multiple Error-Ellipses

6

5

,,

: :

\ : .. "f .:

:

, ::

... . . \ ':

:: ::

\:

:

\

\

2

, :.~

:

· ··

···

00

20

40

•••••••

~

:..

: :

.

,

i L!

.

............... :-

.

, , I

•••...•.••..•.• :• ....••••' J.••••• : :/

:

:

~

;

~,

.

:

:

.. . ... . ..

···

• • • • • • • o'

: :

:' : : " ,.. ..

::

: :

:\

:

.. : ,

:

.

............... ~

.

............. .. .

• • • • • • • • • • • • "':"";" •• ~-_-=-.-;

.

"-1

l-

/

')~.~

.

.

~" p; 0.5

:

--"~

...

. ..

60 80 100 120 a = Cut Angle (0)

140

160

Figure 7.13 Error Factor Versus Cut Angle.

• • • •

Weapon dispersion with a; =3 m, O"y =40 m Gun location with a, = 10 m, O"y = 15 m Forward observer location with a; = 15 m, O"y =20 m Target location with respect to forward observer with a;

=30 m, O"y = 10 m

We wish to obtain the probability of damage to the target assuming the shell must land within a circle of20-m radius in order to meet the desired damage level. For this example, it is assumed that each error-ellipse has its axes mutually parallel and, without loss of generality, aligned north and east. The method of obtaining the total error at the target consists of finding the sum of the variances in the two directions and converting these two sums to the standard deviations of the total error-ellipse at the target. The computation steps are as follows:

• o, == J3 2 + 102 + 152 + 302 == J1234 == 35.1283 m 2 2 2 2 • O"y == J40 + 15 + 20 + 10 == J2325 == 48.2183 m • U == minur; O"y)jmax(O"x, O"y) = 35.1283/48.2183 = 0.7285 • r == Rjmax(O"x, O"y) =20/48.2183 =0.4148 • Use bf2.m by entering p==bf2(r,u) in MATLAB to obtain p= 11.11%. One can also use gf2.m, first introduced in Chapter 4, to accomplish the same goal. Each of these steps is implemented in noval.m for a system with N errorellipses; that is, the command p == noval(R), with R == 20, will carry out all five steps

112


to determine p. This program requires an ASCII input file nsd.dat to store the two standard deviations for each of the N error-ellipses. For the example above, N == 4, and the input file nsd.dat consists of

3 40 10

15

15

20

30

10

It is interesting to compare the contribution to total error from the dispersion of the weapon to that contributed by location measurement errors of the gun, the forward observer, and the target. Table 7.4 contains the individual contributions from each of these four sources. Thus, with perfect location of all elements, the gun (with the stated dispersion) has a 37.89% probability of landing a shell within a circle of 20-m radius. When the three location errors are taken into account, however, the probability falls to 11.11% when the gun dispersion is considered. This method of adding variances along the two orthogonal axes (down range and cross range) is the standard method of preparing an error budget for a weapon system. However, it is not sufficient when one wishes to combine error-ellipses having random orientations of their axes. Since statistical distributions are involved, simple trigonometric resolution from one set of axes to another is not appropriate.

7.3.2 Randomly Oriented Axes In the general case of N error-ellipses with randomly oriented axes, a more complex procedure for combination is needed. Briefly, a reference set of axes must be chosen, and the orientation of each error-ellipse relative to these axes must be determined. Subsequently, the standard deviations along these axes must be computed, which is a procedure that involves cross-product terms. The formulas for this calculation are given below without derivation [8]. Figure 7.14 illustrates the situation. The three smaller ellipses are the inputs to the problem, and the large ellipse represents the final combination of the three smaller ones. In the formulas below, the letter i represents a quantity associated with the general ith ellipse, i == 1,2,3, ... ,N, where N is the number of ellipses involved. Each of these small ellipses is described in terms of its (J xi, (J yi and the angle f}i between the x-axis and the arbitrarily selected reference axes (the w- and z-axes). The TABLE 7.4

Contribution from Each Error Source

Source

u

Probability (0/0)

Weapon dispersion Gun location Observer location Target location

3/40 2/3 3/4 1/3

37.89 71.76 48.19 42.26

113

Section 7.3 • Analysis of Multiple Error-Ellipses Ellipse #2

Ellipse #1

30 r - - - - - - . - - - - - - - ,

15

o -15

-3~30

30 r-------...--....----.

@---15

0

15

15

~--

o -15

-3~30

30

Ellipse #3

30 r-----.---------,

30

15

15

o

o

-15

-15

-3~30

-15

0

15

-15

0

15

30

Final ellipse __-.....,

r - - - - . . -__-

-30 '-------~...I -30 -15 0 15 30

30

Figure 7.14 Input Error-Ellipses and Final Ellipse.

numerical values of the sigmas and orientation of the three input ellipses in Figure 7.14 are tabulated in Table 7.5. TABLE 7.5

1

Parameters of Input Error-Ellipses

10 20 20

15 10 10

2

3

To obtain the elements of the final ellipse, variances for each ellipse along the w- and z-axes are calculated. Since the W-, z-axes are not aligned with those of the in-

dividual ellipses, in general, an additional cross-product coefficient is required. These three elements (O'~i' O';i' Pi for each ellipse) are then combined to obtain the corresponding elements in the final ellipse (O'}, O'}, Pf). The subscript f is used to denote parameters associated with the final ellipse. The terms O'wf' O'zf' Pf may then be converted to the 0' xf, 0' yf of the final ellipse along its major and minor axes. First let us define the auxiliary functions to be used in the formulas. These are: 2(}

_ COS

i

. 2(}

SIn

i

Ai--2-+--20' xi

. 2(} i _ cos 2(}i SIn Bi - - 2 - + - 2 0' yi

Ci

== cos (}i sin (}i

(7.8)

0' yi

(7.9)

0' xi

(~ - ~) 0' yi

0' xi

(7.10)

114


Ci

Pi = ,JAiBi 2

1 (1 - pr)A i

(7.12)

1 (1 - PT)B i

(7.13)

a .=---WI

2

a .=---zi

(7.11)

The last three equations give the cross-product coefficient and variances for each ellipse in the reference w- and z-axes. These are then combined according to the next three equations to obtain the corresponding elements for the final ellipse. N '"" a wi 2 i=l

c:

2 awf

=

a2zf

= '"" c: a2zi

N

(7.14)

(7.15)

i=l

(7.16) We now have all the parameters needed for the final ellipse in terms of the wand z-axes. To eliminate the cross-product coefficient Pi' and to obtain axf' ayf along the major and minor axes of the final ellipse, we use the following formulas: (7.17)

(7.18) The orientation of the final ellipse relative to the w- and z-axes is given by the formula (7.19) The tedious computations associated with these equations are included in the M-file novall.m for MATLAB users. An input ASCII data file nsdl.dat containing the elements for the error-ellipses involved is needed for this program. For the example in Figure 7.14, the content ofnsdl.dat consists of

15 10

45

10 20

60

10 20 150

115


The CEP (radius of p == 50% circle) corresponding to the final ellipse in this example is found, via the MATLAB command R == noval1(0.5), to be 30.26 m. The program computes many other intermediate parameters in the process; these are O"~f 0"xf

== 662.5000,

== 26.9258,

0"yf

O";f

== 662.5000,

== 24.4949,

== 0.0943,

Pf

== 45°,

Of

u == 0.9097

7.4 SUMMARY OF M-FILES The M-files used or generated in this chapter are summarized in Table 7.6. Recall that the radius R and the normalized radius r are related by TABLE 7.6

M-files Used in Chapter 7 MATLAB Command

FileName

y == bfl(A) P == bf2(r, u) y == bf3(r) r == bf4(p, u) P == cf2(r) r == cf3(p) p == ang2p(ex, Po) fscaia«, 0"2, ex) p == semleR, aI, 0"2, ex) p == edcaud(R, 0"1, (2) p == sem2(R, 0"1,0"2, ex) p == noval(R)

bfl.m bf2.m bf3.m bf4.m ef2.m eO.m ang2p.m fsea.m seml.m edeaud.m sem2.m noval.m novall.m

R

r ==

== noval1(p)

R/ maxtrrx,

0"y)

The M-file bfl.m is used to implement the integrand of the integral defined in Equation (4.19) and repeated here

_!J.. P-

R/(Jx

U 0

2(1 [-A + U2)] [A 2(1 - U2)] Aexp 4 1 4 dA 2

U

0

2

U

whereas bf2.m evaluates the integral to determine the probability p. Both sem2.m and noval.m call bfz.m. The file novall.m calls bf4.m which, in turn, calls bf3.m; seml.m calls edcaud.m.

Risk Analysis

When a customer (buyer) purchases a high-technology system, it is necessary to know whether the system is as accurate as it is specified. A clearly defined set of test procedures, including pass/fail criterion, is required to determine if the system is acceptable to the customer. Because of the cost and time associated with each test, only a finite number of tests can be performed. The test results lead to estimates of the true characteristics of the system. Hence, a risk (based on using test statistics in decision making) is involved on the buyer's side in purchasing a bad system (true quality below specification), even though it passes the test. The seller also has his share of risk. That is, a good system (true quality equal to or better than the specification) can fail the test. The test pass / fail criterion therefore plays an important role in the risks of both buyer and seller. As more tests are performed, the test estimates become closer to the true system quality. In addition, given the specification, a conservative pass/fail criterion results in high buyer's risk, while a tight pass/fail criterion results in high seller's risk. It is beneficial to both sides to understand how the pass/fail criterion is set [16]. Thus, the risks associated with the buyer as well as the seller can be made both reasonable and acceptable. This chapter outlines the procedures for determining the pass/fail criterion and buyer's risk for a given seller's risk utilizing an associated specification supplied by the buyer. We will assume that the system under consideration is an Inertial Navigation Unit (INU) and that the particular parameter of concern is the CEP of the INU. The true INU CEP can be denoted as rO.5 and the computed/ estimated CEP, from an ensemble of N tests, as rO.5' We formulate the problem in the fashion of hypothesis testing, with the hypotheses stated in terms of the true population CEP, rO.5. The null hypothesis is

H«

rO.5

== Co 117

118

Chapter 8 • Risk Analysis

that is, the true CEP of the INU meets the specification, Co, and the alternate hypothesis is HI: rO.5 == C 1 > Co

that is, the true CEP of the INU exceeds the specification, Co. Note that the symbol C1 is used for rO.5, when rO.5 > Co. Given a specification Co, the seller's risk a is the conditional probability of rejecting (based on test result) a good INU (true CEP rO.5 == Co). Assume that an ensemble of N tests produces a computed CEP exceeding the pass/fail criterion (rO.5 2 L). That is, a == p{ro.5 2 Llro.5 == Co}

The buyer's risk fJ is the conditional probability of purchasing (based on test result) a bad INU (true CEP rO.5 == C1 > Co), if an ensemble of N tests shows that the computed CEP is below the set pass/fail criterion (rO.5 < L). Thus,

fJ == p{ro.5

1 L == pass/fail criterion == KaCo K, == normalized pass/fail criterion == L/Co

x == INU east position error y == INU north position error

== true INU standard deviation of x or y position error N == number of tests S2N == sample standard deviation from N tests rO.5 == true INU Circular Error Probable rO.5 == estimated Circular Error Probable from an ensemble of N tests X~ == chi-square distribution with m degrees of freedom X~,a == the point to the right of which a random variable with X~ distribution (J

has an area a under the PDF X~ l-b

== the point to the left of which a random variable with X~ distribution has an area b under the PDF

P == probability of the occurrence of some event

== P{event A under the condition that event B has occurred} a == seller's risk == p{rO.5 2: Llr o.5 == Co} f3 == buyer's risk == p{rO.5 < Llro.5 == Cll

P{AIB}

Notice that we have expressed Land C 1 in terms of the specification Co as L == KaCo and C1 == ACO, respectively. The CEP concept is introduced in Chapter 4, and the basic properties of the chi-square PDF are described in Chapter 6 as well as in Appendix A. The geometric meaning of X~,a and X~,l-b is shown in Figure 8.1. The area to the right of the point X~,a' and under the X~ curve is a. Similarly, the area to the left of the point X~,l-b' and under the X~ curve is b.

8.2 SELLER'S RISK

According to Theorem 2.2.1 in Chapter 2, we know that the random variable 2NS~N/(J2 has a X2 distribution with (2N - 1) degrees of freedom; the new random


120

0.2 r - - - - - - - - - - - - - - . - - - - - - - - - - - ,

Shaded area under X; PDF = a

0.1

0.2 r - - - - - - - - - - - - - - - - r - -- - - - - - - 0.15

0.1

Shaded area under

x; PDF = b

Figure 8.1 Geometric Meaning of X~.a and X~,I -b '

variable 2N(1.1774S 2N)2 j(1.1774oi (obtained by multiplying 1.17742 to both the numerator and the denominator of the old random variable 2NS~Nj(2)

2N(1.1774S 2N)2 j(1.1774ai = 2N(ro.d jr~.s also possesses the same X2 distribution with (2N - 1)degrees of freedom. The relationship between the pass /fail criterion L (or its normalized version K a) and the seller's risk a can be derived from

a = p{ro.s 2: Ll ro.5

= Co}

= P{2N(ro.5jro,si 2: 2N(Ljro.5)2 Iro.5

= Co}

= P{X~N-l 2: 2N(Lj CO)2}

(8.1)

Or using the convention defined in Figure 8.1, we have

a = P{X~N-l 2: X~N-l .a}

(8.2)

with X~N-l .a = 2N(LjCof Thus, determination of the seller's risk can be classified as a direct problem:

Given Co, L, N, determine the probability «from Equation (8 .2) . The solution process for the direct problem is implemented in cln2a.m .

121

Section 8.3 • The Pass/Fail Criterion

8.3 THE PASS/FAIL CRITERION Finding the pass/fail criterion L, when we know the seller's risk a, the specification Co, and the number of tests N, corresponds to the inverse problem: Find X~N-l rx' given the seller's risk a and the number oftests N.

The pass/fail criterion can then be obtained from

L

= JX~N-l'rJ: C 2N 0

(8.3)

or the normalized pass/fail criterion from

KrJ: = t.ic; = JX~N-l'rJ: 2N

(8.4)

Notice that K; is a function of a and N. The M-file an2ka.m computes K; via the MA TLAB command K, == an2ka( a, N). Since X~N-l,rx == 2N(Lj Co)2, we see from Equation (8.2) that the seller's risk a increases when Co decreases. Also, a decreases when N or L increases. Figure 8.2 shows the dependency of K, with N and the parameter a (seller's risk). Two facts are obvious from Figure 8.2: 1. For fixed N, K, increases as a decreases. 2. For fixed a, K, approaches 1 as N increases toward 1.4 1.3

0.. ····· ~· ~

'T· ~

00.

r T' ·..] ·:

·[ ·.. T

~

·T

~

~

~

~

~

~

~

:

:

~

;~

~

:

j

ooj'"

~

c:

0

2

c:5 '(ij

~ C/)

1.0

C/)

as 0.9

o,

'"0 Q)

.~

(ij

E

0.8

............

0

(

J:

: ..t···

z

2

~

'j"

::

.

~

~

~

4

6

8

00

••

~j

i

~j

~

r~

i ~

: : : : '" : : joo .. 'j' ooj j .~

10 N

~

00 • • •

12

14

16

18

Figure 8.2 Normalized Pass/Fail Criterion K(/. as a Function of the Number of Tests N, with Seller's Risk (X as Parameter.

20

122


8.4 BUYER'S RISK

Once the pass/fail criterion L is set, the corresponding buyer's risk from

f3 == p{ro.s

f3 can be derived

< Llro.s == CI}

== P{2N(ro.s/ro.s)2 < 2N(L/ro.s)2 Iro.s == C I } == P{X~N-l < 2N(L/C 1)2} == P{X~N-l < 2N[L/(,1Co)]2} == P{X~N-l < 2N(K a/,1)2}

(8.5)

Hence, (8.6)

with 2

X2N-l,1-{3

== 2N (

Ka

2

T)

(8.7)

which depends on N, K ex , and ,1. Given 11, N, we can determine K, from Equation (8.4) of the previous section. Thus, when 11, N, A are given, we know N, K ex , A, and thus X~N-I,l-P. The M-file anl2b.m carries out all the necessary computations to obtain f3 via the MATLAB command f3 == an12b(11, N, A). Substituting K, of Equation (8.4) into Equation (8.7), we have

2

2

X2N-l,1-{3

X2N-l,a

== ~

Therefore, A, which equals the ratio C1/CO, also satisfies 2

1 _ /L -

X2N-l,a 2

(8.8)

X2N-l,1-{3

Figure 8.3 shows how f3 varies with K ex , for a fixed N == 8, using A as parameter. Two facts are obvious: 1. For fixed N, regardless of A, buyer's risk increases as K, increases. 2. For fixed K ex , buyer's risk decreases as Aincreases. Figure 8.4 shows the relationship between the normalized pass/fail criterion K, and N, the number of tests, using seller's risk as a parameter. From this figure, we see that 1. For fixed N, K, decreases as 11 increases. 2. For fixed 11, K ex decreases as N increases.

123

Section 8.5 • A Practical Example

·1

N=8

0.9

.

0.8

.

0.7 ..... r •••••••••••••

~

, 0.6 .................. .. ..

. ..

..

.

.

..

en

a:

en

1i5

~

0.5

•••••••••••••••••••

.

.,

.

..

· 1.3

111

.

::::J

m

cri:

0.4

..... ,.

,.

.

. .......................... III

..

, . · 1.4

, ..

..

0.3

0.2

: 1.6 0.1

2 0.95

1.05 1.1 1.15 Kat Normalized Pass/Fail Criterion

1.2

1.25

Figure 8.3 Buyer's Risk versus K rx, for N == 8, with A as Parameter.

8.5 A PRACTICAL EXAMPLE Suppose we are given (1) N == 8 == the number of tests (2) Co == 0.8 == the CEP specification (3) L == 1.0 == the CEP pass/fail criterion The seller's risk

rx == p{rO.5 2: Ll ro.5 == Col and the buyer's risk

p == p{rO.5

Co}

124

Chapter 8 • Risk Analysis . ......................................... , .. .. . ...

1.7

,

. ..

,

..

0.02 .. .. .. .. . . ............................................................... ,

..

..

..

.............................. _.. .. . ..

.. .. ..

.. .. .. ...... ..

.. . ... . . . .

..

.....

..

.. ..

.. ..

"

"

.

.. ..

..

.. ..

.. ..

.

.. ..

....

.. ..

..

.

.. .. .... .. ................................................................ ..

..

..

..

II

..

0.1

Figure 8.4 Normalized Pass/Fail Criterion K; as a Function of the Number of Tests N, with Seller's Risk a as Parameter.

can be found from Figures 8.3 and 8.4 as follows: 1. Compute K, == L/Co == 1/0.8 == 1.25. 2. From Figure 8.4, find the intersection point of the horizontal line at K, == 1.25 and the vertical line at N == 8. Now estimate the value of the parameter rlw associated with the curve passing through that point. This produces the seller's risk at rlw ~ 50/0. 3. Assume C 1 == 1.12 so that A == C1/CO == 1.12/0.8 == 1.4. 4. From Figure 8.3, find the intersection point of the curve associated with A = 1.4 and the vertical line at K, = 1.25. The ordinate of this intersection point is the buyer's risk, /3 ~ 38%. Note that f3 is dependent on the choice of Aand hence the value of C 1. To avoid visual interpolation with figures, we can use the MATLAB M-file cln2a.m and cln2b.m to find the seller's risk and buyer's risk, a; /3, respectively. Employing the values of Co, L, and N shown above, we enter the command

alpha = cln2a(O.8, 1,8)

125

Section 8.6 • Generalization For N = 8, L = 1 and Co

100

90

~

··:

80

~

:

:

.

..

..

..

,

70

= 0.8: Ka = 1.25, Seller's Risk is a = 4.994%

r----r--...-or----r--r----r---~-__r--r__-...,....-....,

..:

:

. .: :

... ' "

:

:

:

·· •••••••• :

1

60

:

.

..

• • • • • • • • • • • • • • • - :• • • • • • • • • • : • • • • • • • • • - : • • • • • • • • • •: • • • • • • • • • •: - • • • • • • • • •: • • • • • • • • • •: • • • • • • • •

..

..

=

:

..

"'~""""

.. .:

..

.. :

:

:

~

:

.

· . . . . . . . . ....................................................................................................... .. .. .. .. .. .. ........ ·,

..

,

.

..

;

;

.;

: '" .. -:

..

..

:

-:

..

30

..

20

:

·

10

"

~'

..

..

"'~""

..

.'"

'~

..

-:

..

..

..

:-

:-

:-

:

:-

:-

..

.

..

-:

.

,..

'~"

..

.~

:-

.

.

..

: "

.:

.: .

.

O---.-....---------.a...-..----......I----a.--.&..--~----'

1

1.1

1.2

1.3

1.4

1.5 A

1.6

1.7'

1.8

1.9

2

Figure 8.5 Buyer's Risk for K, == 1.25 and N == 8, with A as Parameter.

and find a == 4.99430/0. The general form of this command is lI. == cln2a(Co, L, N). With the particular value of A == 1.4 (assume C1 == 1.12), we can obtain the buyer's risk f3 == 37.8794% via the command beta= cln2b(O.8, 1,8,1.4)

Thus, the values extracted from Figures 8.3 and 8.4 for f3 and a, respectively, are good approximations. If we let A vary from 1 to 2, we will obtain the buyer's risks as displayed in Figure 8.5.

8.6 GENERALIZATION In the preceding sections we considered the seller's risk, assuming rO.5 == Co. A reasonable question is, "What is the chance that the system passes the test but has a

126


quality p{rO.5

better

than

2: L I rO.5 < Co}·

the

specification?"

That

is,

find

the

probability

Thus, the more general problem should be formulated to include inequalities for rO.5 such as (J.

== p{rO.5

f3 == p{rO.5 Let us first define

rO.5

rO.5

2: Ll ro.5 < Co}
Co}

== KoCo, with K o:::; 1 in the expression for

(8.9) (8.10) (J.,

and

== K 1Co, with K 1 > 1 in the expression for f3. Also, if we define A == K1/Ko, then

we can obtain the expressions

K = a

2

X2N-l,I-/J

JX~N-l,rt K 2N Ka

0

(8.11)

2

= 2N ( Kl )

(8.12)

Since K o:::; 1, the normalized pass/fail criterion KrJ. will be scaled down when compared with Equation (8.4). By treating K 1Co as C 1 in the preceding section, we have K 1 == A. Therefore, X~N-l,l-P will be smaller on comparing Equations (8.7) with (8.12), resulting in a lower buyer's risk. Note that with A == K 1/K o, and the last two equations, A satisfies

A --

2 X2N-l,a 2

(8.13)

X2N-l,1-{3

just as with Ain Equation (8.8) of Section 8.3.

8.7 SUMMARY OF M-FILES

The M-files used or generated in this chapter are summarized in Table 8.1. Notice, however, that the direct problems (given R, find p == f~ g(t) dt, with g(t) being the PDF oft""X~) and the inverse problems (given p, find R) involved in risk analysis are solved by an interpolation technique utilizing auxiliary files. The details are explained as follows.

TABLE 8.1


FileName

MA TLAB Command

cdfchi2.m ichi2.m an2ka.m anl2b.m cln2a.m cln2b.m

p == cdfchi2(x, N), N 0 & sy > 0 p=nf2(a/sx)*nf2(b/sy); end

edraui.m function [a,b]=edraui(p,v,sx,sy)

%edraui. m 2-07-96 % % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

%

164

Appendix E • Listing of Author-Generated M-files

%Given the probability sx, sy, v = alb, p %Find a, b

x

%2D j oint elliptical normal pdf with sx, sy, rho = 0 %over a rectangular region: -a < x < a, -b < Y < b.

x

b=max(sx,sy); a=v*b; pO=edraud(a,b,sx,sy);

%%%%%%%%% searching for a, b %%%%%%%%%%%%%%%% for j=1:2:5 while (pO> p) b=b- (0 .1) - j ; a=v*b pO=edraud(a,b,sx,sy); end while (pO < p) b=b+(0.1)-j ; a=v*b pO=edraud(a,b,sx,sy); end end

edca.m function p=edca(r .mu) 1-23-96

% edca.m %

% Spatial Error Analysis ToolBox Version 1.0, October 5, 1997

% Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

% %p = probability of ED (elliptical normal pdf) over CA (circular area) %mu = min(sx,sy)/max(sx,sy) %r = R/max(sx,sy) %x = theta angle % Normalized version of edcaud.m {p = edcaud(R,sx,sy) } if mu>1 I mu=mlim & mu p) R=R-(0.1)"j; pO=edcaud(R,a,b); end while (pO < p) R=R+(0.1)"j; pO=edcaud(R,a,b); end end

edcaui.m functionp=edcaud(R,sx,sy)

Section E.4 • M-files Used in Chapter 4

%edcaud. m 1-23-96 % % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997


% [email protected]

% %Elliptical PDF, Uncorrelated, Circular region %edcaud.m, Find probability p, given R,sx,sy. %bf1b.m is the normalized version %function p = edcaud(R,sx,sy) % 1>= mu >=0 if R1 I sxl I p1 p=1; end

tf3.m functionr=tf3(p,u,v) %tf3.m, 2-20-96 % % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved. % [email protected]

% %solve for r from F(r,u,v) - p = 0, %si.g(x) - = sig(y) - = s igfz) if p>l I pu errore 'input error for tf3(p) ,) end if p==l r=Inf; else r=1; pO=tf2(r,u,v);

178


%%%%%%%%% searching for a, b %%%%%%%%%%%%%%%% for j=1:2:5 while (pO> p) r=r-(O.1)~j;

pO=tf2(r,u,v); end while (pO < p) r=r+(O.1)~j;

pO=tf2(r,u,v); end end end

r2p3d.m function r2p3d(k)

%r2p3d. m 2-26-96 % % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

% % %for 3-dim normal distribution f(x,y,z) with ai.gmaCx) = sigma(y) = sigma(z)

%given k = multiple of SIGMA %given k = multiple of MRE %given k = multiple of RMSR %given k = multiple of SEP %r = k * SIGMA (MRE = Mean Spherical Radial Error) %r = k * MSRE (RMSR = Root Mean Square Spherical Radial Error) %r = k * RMSR (SEP = Spherical Error Probable) %r = k * SEP %find p = F(r) %where F (r) = integral of f (x) from 0 to r. % Maxwell pdf used for f (x) here. % disp(' ') disp(' ') ic=menu ( , Options: ' , ' k*SIGMA' , ' k*MRE' , ' k*RMSR' , ' k*SEP' , ' all the above'); if ic==1 r= k; p=sf2(r) disp ( [ , For r=' ,num2str (k), ' * SIGMA'] )

Section E.5 • M-files Used in Chapter 5 disp ( t- p=' , num2str (p)] ) end if ic==2 r= k* 2*sqrt (2/pi) ; p=sf2(r) disp( [, For r=' ,num2str (k), ' * MRE'] ) disp ( [, p=' , num2str (p)] ) end if ic==3 r= k*sqrt (3) ; p=sf2(r) disp(['Forr=' ,num2str(k),' *RMSR']) disp ( [, p=' , num2str (p)] ) end if ic==4 r=k*sf4(O.5); p=sf2(r) ; disp(['For r=' ,num2str(k), ' * SEP']) disp( [, p=' '. num2str (p)] ) end if ic==5 r= k; p=sf2(r) disp(['For r=' ,num2str(k), ' * SIGMA']) da sp C]' p=', num2str(p)]) r= k* 2*sqrt (2/pi) ; p=sf2(r) disp(['Forr=' ,num2str(k), ' *MRE']) disp([' p=', num2str(p)]) r= k*sqrt (3) ; p=sf2(r) disp(['For r=' ,num2str(k), ' * RMSR']) dd sp C]' p=', num2str(p)]) r=k*sf4(O.5); p=sf2(r) da sp ( ['For r=' ,num2str(k), ' * SEP']) dfsp C] ' p=', num2str(p)]) end

179

180


p2r3d.m function p2r3d(p)

%p2r3d. m 2-26-96 % % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

% %r = r3ng(p) %given p and f(x), find r such that F(r) - p = 0, %express r in terms of SIGMA, MSRE, SEP, RMSR %where F (r) = integral of f (x) from 0 to r. % Maxwell pdf used for f (x) here. % % d i spf ' ') df sp C' ') df.sp C] ' For p=', num2str(p) ])

ic=menu ( , options: " above'); r=sf4(p);

' k*SIGMA', ' k*MRE', ' k*RMSR' , ' k*SEP' , ' all 4

if ic==1 disp ( [' r=' , num2str (r), ' times SIGMA'] ) end if ic==2 r=r/sqrt(8/pi); disp( ['r=', num2str(r), ' times MRE']) end if ic==3 r=r/sqrt(3); disp ( [' r=' , num2str (r), ' times RMSR'] ) end if ic==4 r=r/sf4(0.5); disp( ['r=', num2str(r), ' times SEP']) end if ic==5 disp ( [' r=' , num2str (r), ' times SIGMA'] ) r1=r/sqrt(8/pi); d i sp ( ['r=', num2str(r1), ' times MSRE'])

Section £.5 • M-files Used in Chapter 5

181

r2=r/sqrt(3); disp(['r=', num2str(r2), ' times RMSR']) r3=r/sf4(0.5); disp ( [' r=' , num2str (r3), ' times SEP']) end

edbvud.m functionp=edbvud(abc,sxyz)

%edbvud. m 2-20-96 %

% Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

% %The probability of a %3D j oint ellipsoidal normal pdf with (sx , sy, sz) , rho = 0 %over a box volume: x < a, Iyl < b, < c. % abc = [ a, b, c] %sxyz = [sx , sy, sz] l

l

l z

l

% a=abc(1); b=abc(2); c=abc(3); sx=sxyz(1); sy=sxyz(2); sz=sxyz(3); if sx p) x=x-(0.1)-j; pO=x2cdf(x); end while (pO < p) x=x+(0.1)-j; pO=x2cdf(x); end end xo=x; %%%%%%%%%%%%%%%%%%%%%%%%%%%%% r(ii,jj)=sqrt(xo/n) pause end end DAT=[l r]; save \mfile\chi .mat 1 r fname='c:\mfile\chi.dat' fid=fopen(fname,'wt') fprintf(fid, '%8.2f %8.4f %8.4f %8.4f %8.4f\n' ,DAT') fclose(fid) end %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% if ic==2 %part II load c:\mfile\chi.mat NQ =4 adj= [ 0.7 0.5 0.15 -0.1]; semilogx(l,r) xlabel ( "n") ylabel (' R(p) /RMS ') %t i t Le C' R(p)/RMS vs n, p=0.5, 0.8, 0.9, 0.95') grid for kk=1:NQ if kk==1 p=O . 5 elseif kk==2 p=O. 8 elseif kk==3 p=O. 9 elseif kk==4 p=O. 95 end text(.5,r(5,kk)+adj(kk),['p=' num2str(100*p) '%'])

195

196

Appendix E • Listing of Author-Generated M-files end axis ( , square' ) figure(l) pause pf3('f62') end

chi2.m % chi2.m 10-1-97

% % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved. % [email protected]

% %R(p) /RMS=r vs GM/RMS=z clear; elf; %result from psi1.m load c: \mf ile\psi . dat z=psi(:,2); %result from chi 1 . m load c: \mfile\chi .dat r=chi(:,2:5); DAT=[z r] save c: \mfile\psichi . mat z r

fname='c:\mfile\psichi.dat' fid=fopen(fname,'wt') fprintf(fid,'%8.4f%8.4f%8.4f%8.4f%8.4f\n',DAT') fclose(fid) plot(z,r) grid adj = [0 0 0.06 0.02]; for kk=1:4 if kk==l p=O. 5 else if kk==2 p=O. 8 ; elseif kk==3 p=O. 9 ; elseif kk==4 p=O. 95 ; end

Section E.6 • M-files Used in Chapter 6 text(0.2,r(7,kk)+adj(kk), ['p=' num2str(100*p) '%']) end xlabel (' x=GM/RMS ') ylabel (' y=R(p) /RMS ') % t i.t Le C' R(p)/RMS vs z=GM/RMS, for p=0.5, 0.8, 0.9, 0.95') axis ( , square' ) figure(1) pf3( 'f63')

gmrms2n.m functionn=gmrms2n(gm,rms) %gmrms2n.m 3-14-96


% fid=fopen('c:\mfile\psi.dat','r'); dat=f scanf (f id, , %g %g' , [2, inf] ) ; fclose(fid); ratio=gm/rms n2=dat(1,:); r2=dat(2,:); n=interp1(r2,n2,ratio,'spline');

n2gmrms.m function ratio=n2gmrms (n) %n2gmrms.m 3-14-96


% y=psin(n/2); ratio=exp(y/2)/sqrt(n/2);

r2pc6.m functionp=r2pc6(gm,rms,r) %r2pc6. m 6-22-96

%

197

198


% Spatial Error Analysis ToolBox Version 1.0, October 5, 1996 % Copyright 1996-1997 by David Y. Hsu All rights reserved. % [email protected]

% %when n = 0.9358 and r = 14.7497 %gm=5.81, rms=11.53 ---> p=0.8000. clear global n global n fid=fopen('c:\mfile\psi.dat' ,'r'); dat=f scanf (f id, , %g %g' , [2, inf] ) ; fclose(fid); ratio=gm/rms; n2=dat(1, :); r2=dat(2, :); n=interp1(r2,n2,ratio,'spline'); ms=rms . . 2; x=n*r . . 2/ms; p=x2cdf(x)

p2rc6.m functionR=p2rc6(gm,rms,p) %p2rc6. m 1-2-96


% %when n = O. 9358 %gm=5.8054, rms=11.5296, p=0.5 --->r=7.542 clear global n global n fid=fopen('c:\mfile\psi.dat','r'); dat=f scanf (f id, , %g %g' , [2, inf] ) ; fclose(fid); ratio=gm/rms; n2=dat(1,:); r2=dat(2,:); n=interp1(r2,n2,ratio,'spline');

199


%%%%%% use

fsolve.m (from optimization toolbox)

%%%%%%%

%global p

%x = fsolve('x2disc2' ,1) ;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%

iteration %%%%%%%%%%%% if p==1 x=Inf; else x=1; %initial guess pO= x2cdf (x) ; for j=1:2:5 while (pO> p) x=x-(O.1) . . j; pO= x2cdf (x) ; end while (pO < p) x=x+(O.1) . . j; pO= x2cdf (x) ; end end end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%% disp ( [' n = , , num2str (n) ] ) R=rms*sqrt(x/n);

gmrms.m function [gm , rms, ratio] =gmrms (f'name) 6-25-96

%gmrms.m

% % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997


% [email protected] % xo=myfun1(fname) [m,n]=size(xo); if n==1 x=xo end

if n==2 x=xo(: ,1) ..... 2 + xo C: ,2) ..... 2 x=sqrt(x); end

200


if n==3 x=xo ( : , 1) . - 2 + xo ( : ,2) . - 2 + xo ( : ,3) . - 2 ; x=sqrt(x); end tmp=prod(x); gm=tmp-(1/m); x2=x. -2; tmp2=sum(x2); rms=sqrt(tmp2/m); ratio=gm/rms;

myfun1.m function x=myfun1 (f'name) %myfun1.m 2-14-96

% % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected] %

s1=['load' 'c:\mfile\' fnameJ eval(s1) eval ( [' x = , , fname (1 : 3) ';' J )

E.7 M-FILES USED IN CHAPTER 7

bf1.m function z=bf1 (r) r>=O, 1-15-96

%bf1.m %

% Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

% %define fer) = 2-dim normal pdf, rho = 0, sigx and sigy may be unequal. global mu k1= r.-2*(1+mu-2)/(4*mu-2) k2= r.-2*(1-mu-2)/(4*mu-2)


z= r /mu. * exp (-k1) . *besseli (0, k2)

bf2.m functiony=bf2(r,mu)

%bf2.m 1-15-96 % % Spatial Error Analysis ToolBox Version 1.0, October 5, 1997 % Copyright 1997-1998 by David Y. Hsu All rights reserved.

% [email protected]

% %Find probability p %function p = bf2(r,mu) %where Ff r ) = integral of f(r,mu) over [O,r] clear global mu global mu if r1 I mu= 0 ' ) disp( , 0

Spatial Error Analysis: A Unified Application-Oriented Treatment

Spatial Error Analysis: A Unified Application-Oriented Treatment

Spatial Error Analysis: A Unified Application-Oriented Treatment

Error analysis: a comprehensive bibliography

Error Analysis and Interlanguage

A Unified Theory of Party Competition: A Cross-National Analysis Integrating Spatial and Behavioral Factors

Visual and Spatial Analysis

Spatial Analysis and GeoComputation

Models in spatial analysis

Spatial Analysis in Epidemiology

Visual and Spatial Analysis

A Unified Theory of Voting: Directional and Proximity Spatial Models

A Unified Theory of Voting: Directional and Proximity Spatial Models

A posteriori error analysis via duality theory

Error Analysis of a Temperature Measurement System

A Posteriori Error Analysis via Duality Theory

Spatial analysis: A guide for ecologists

Spatial Analysis: A Guide for Ecologists

SPATIAL ANALYSIS. A Guide for Ecologists

SPATIAL ANALYSIS. A Guide for Ecologists

A Unified Approach to the Finite Element Method and Error Analysis Procedures

Adjustment Computations: Spatial Data Analysis

Adjustment Computations: Spatial Data Analysis

Adjustment Computations: Spatial Data Analysis

Local Models for Spatial Analysis

Adjustment computations. Spatial data analysis

Unified

A Queen's Error

Perspectives on Spatial Data Analysis (Advances in Spatial Science)

Design-Oriented Analysis of Structures A Unified Approach

Spatial Databases: A Tour

Spatial Error Analysis: A Unified Application-Oriented Treatment