Broadcast vs Precise GPS Ephemerides: A Historical Perspective

184 Pages·2002·3.57 MB·English

BROADCAST VS PRECISE GPS
EPHEMERIDES:
A HISTORICAL PERSPECTIVE

THESIS

David L.M. Warren, BEng (Elec)

Squadron Leader, Royal Australian Air Force

AFIT/GSO/ENG/02M-01

DEPARTMENT OF THE AIR FORCE
AIR UNIVERSITY
AIR FORCE INSTITUTE OF TECHNOLOGY

Wright-Patterson Air Force Base, Ohio

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED.
Report Documentation Page
Report Date Report Type Dates Covered (from... to)
26 Mar 02 Final Aug 2000 - Mar 2002
Title and Subtitle Contract Number
Broadcast vs Precise GPS Ephemerides a
Historical Perspective Grant Number
Program Element Number
Author(s) Project Number
Squadron Leader, David L. M. Warren, RAAF
Task Number
Work Unit Number
Performing Organization Name(s) and Performing Organization Report Number
Address(es) AFIT/GSO/ENG/02M-01
Air Force Institute of Technology Graduate School
of Engineering and Management (AFIT/EN) 2950
P Street, Bldg 640 WPAFB, OH 45433-7765
Sponsoring/Monitoring Agency Name(s) and Sponsor/Monitor’s Acronym(s)
Address(es)
Major David Goldstein, SMC/CZE 2435 Vela Way
Sponsor/Monitor’s Report Number(s)
Suite 1613 El Segundo, CA 90245-5500
Distribution/Availability Statement
Approved for public release, distribution unlimited
Supplementary Notes
The original document contains color images.
Abstract
The Global Positioning System (GPS) Operational Control Segment (OCS) generates predicted satellite
ephemerides and clock corrections that are broadcast in the navigation message and used by receivers to
estimate real-time satellite position and clock corrections for use in navigation solutions. Any errors in
these ephemerides will directly impact the accuracy of GPS based positioning. This study compares the
satellite position computed using broadcast ephemerides with the precise position provided by the
International GPS Service for Geodynamics (IGS) Final Orbit solution. Similar comparisons have been
undertaken in the past, but for only short periods of time. This study presents an analysis of the GPS
broadcast ephemeris position error on a daily basis over the entire period 14 Nov 1993 through to 1 Nov
2001. The statistics of these errors were also analysed. In addition, the satellite position computed using
the almanac ephemeris was compared to the IGS precise final orbit to determine the long-term effect of
using older almanac data. The results of this research provide an independent method for the GPS Joint
Program Office (JPO) and the OCS to gauge the direct impact of Kalman filter modifications on the
accuracy of the navigational information available to the GPS users. GPS engineers can compare future
Kalman filter changes to the historical baseline developed by this thesis and readily assess the significance
of each proposed engineering change.
Subject Terms
GPS, Global Positioning, Ephemeris, Ephemerides, Navigation, Error, Almanac, Broadcast
Report Classification Classification of this page
unclassified unclassified
Classification of Abstract Limitation of Abstract
unclassified UU
Number of Pages
182
The views expressed in this document are those of the author and do not reflect the official
policy or position of the U.S. Department of Defense, the U.S. Government, or the
Government of the Commonwealth of Australia.
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AFIT/GSO/ENG/02M-01

BROADCAST VS PRECISE GPS EPHEMERIDES:
A HISTORICAL PERSPECTIVE

THESIS

Presented to the Faculty
Department of Electrical and Computer Engineering
Graduate School of Engineering and Management
Air Force Institute of Technology
Air University
Air Education and Training Command
In Partial Fulfilment of the Requirements for the
Degree of Master of Science in Space Operations

David L.M. Warren, BEng (Elec)
Squadron Leader, Royal Australian Air Force

March 2002

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED
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Acknowledgments
I would like to express my sincere appreciation to my thesis advisor, Major John
Racquet for his support and encouragement over the course of my studies. I would also like
to thank my committee members, Dr. Steven Tragessor and Dr. Peter Maybeck for their
guidance and advice. I have relied heavily upon the support provided by each of the
committee members. I have been guided in this research by the efforts of Major David
Goldstein, Ph.D. (Chief, Engineering Branch, Navstar GPS JPO) and the staff of the GPS
JPO. I would also like to acknowledge the contribution made to my work by Angelyn
Moore, Ph.D. (Deputy Director, IGS Central Bureau) and Ms. Carey Noll (Manager, Crustal
Dynamics Data Information System) who provided the data files referenced throughout this
thesis.
I am very grateful to the Royal Australian Air Force for giving me the opportunity to
study at AFIT and experience, for twenty months, American life and culture. While studying
has been a privilege for me, my fondest memories will be of the people, social events and
places. I must extend a special thank you to Annette, Jo, Lynne and Janet in the
International Students Office for their untiring efforts in support of the international
families.
To my wife, I can only say ‘I love you. Thanks for sticking by me whilst I spent all
those nights studying’.
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Table Of Contents
Page
Acknowledgments.....................................................................................................................v
Table Of Contents...................................................................................................................vi
List Of Figures..........................................................................................................................x
List Of Tables.........................................................................................................................xii
Abstract.................................................................................................................................xiii
I. Introduction......................................................................................................................1
Research Objectives........................................................................................................2
Motivation.......................................................................................................................2
Summary.........................................................................................................................2
II. Background And Literature Review................................................................................4
Introduction.....................................................................................................................4
GPS System Overview...................................................................................................4
Background 4
Space Segment................................................................................................................6
Operational Control Segment (OCS) 8
User Segment 10
Space Segment Error Sources.......................................................................................10
Earth Oblateness Perturbations 11
Third Body Gravitational Perturbations 12
Solar Radiation Pressure (SRP) Torque 13
Yaw-Bias 13
Fixed Body Tides 14
Earth’s Albedo 14
Aerodynamic Torque / Drag 14
Gravity Gradient Torque 15
Satellite Clock Phase Error 15
Non Space Segment Error Sources...............................................................................16
Miscellaneous 16
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Foliage attenuation 16
Selective Availability (SA) 16
Relativistic Effects 17
Multipath 18
Ionosphere 18
Troposphere 19
Scintillation 19
Receiver Noise 20
Geometric Dilution Of Precision (GDOP) 20
GPS Navigational Errors..............................................................................................20
Operational Control Segment Performance Measures.................................................22
Observed Range Deviations (ORD) 22
Estimated Range Deviations (ERDs) 23
NAVigational SOLutions (NAVSOLs) 24
Smoothed Measurement RESidual Generator (SMRES) 24
A Posteriora Analysis...................................................................................................25
Orbit and Clock State Comparisons 26
Laser Ranging Residuals 27
Previous Analysis.........................................................................................................28
GPS OCS Performance Analysis and Reporting (GOSPAR) 28
University of New Brunswick Study 29
Other Studies 29
Orbit Generation...........................................................................................................30
Broadcast / Almanac Orbit 30
IGS Final Orbit 31
Data Formats.................................................................................................................32
Broadcast Ephemeris 32
Almanac Ephemeris 32
IGS Final Orbit 33
Summary.......................................................................................................................33
III. Analysis and Modelling Methodologies........................................................................34
Introduction...................................................................................................................34
Required Outputs..........................................................................................................34
Required Inputs.............................................................................................................35
Method of Analysis.......................................................................................................35
Overview of the Process...............................................................................................36
Broadcast Orbit 36
Almanac Orbit 37
Analysis Parameters and Assumptions.........................................................................39
Study Analysis Period 39
Ephemeris Data 39
The Sampling Interval 40
Orbit Analysis...............................................................................................................42
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Precise Orbit Analysis 42
Broadcast Ephemeris Analysis 42
Almanac Ephemeris Analysis 43
Error Analysis 43
Data Format..................................................................................................................49
Summary.......................................................................................................................49
IV. Presentation and Analysis of Results.............................................................................50
Introduction...................................................................................................................50
Broadcast Orbit Position Error Results.........................................................................50
Outlier Filtering 50
Along-Track, Cross-track and Radial 53
Statistical Analysis 60
3D ACR Broadcast Position Error 65
SISRE 69
Almanac Orbit Position Error Results..........................................................................72
Outlier Filtering 72
Along-Track, Cross-track and Radial 72
Statistical Analysis 75
SISRE 76
Summary.......................................................................................................................78
V. Summary and Conclusions............................................................................................79
Summary – Broadcast Ephemeris.................................................................................79
Summary – Almanac Ephemeris..................................................................................80
Conclusions...................................................................................................................81
Recommendations.........................................................................................................82
Appendix A: Data Format.......................................................................................................83
Poserr 83
Rms 84
Stats 85
Sisre 86
Appendix B: Along-Track, Cross-track and Radial Broadcast Position Error.......................87
1 Nov 1994...................................................................................................................87
1 Nov 1997.................................................................................................................100
1 Nov 2000.................................................................................................................113
Appendix C: RMS ACR Broadcast Position Error...............................................................127
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