Position, Navigation, and Timing Technologies in the 21st Century IEEE Press 445 Hoes Lane Piscataway, NJ 08854

IEEE Press Editorial Board Ekram Hossain, Editor in Chief

Jón Atli Benediktsson David Alan Grier Elya B. Joffe Xiaoou Li Peter Lian Andreas Molisch Saeid Nahavandi Jeffrey Reed Diomidis Spinellis Sarah Spurgeon Ahmet Murat Tekalp Position, Navigation, and Timing Technologies in the 21st Century

Integrated Satellite Navigation, Sensor Systems, and Civil Applications

Volume 1

Edited by Y. T. Jade Morton, University of Colorado Boulder Frank van Diggelen, Google James J. Spilker, Jr., Stanford University Bradford W. Parkinson, Stanford University

Associate Editors: Sherman Lo, Stanford University Grace Gao, Stanford University Copyright © 2021 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permissions.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com.

Library of Congress Cataloging-in-Publication Data

Names: Morton, Y. Jade, editor. | Van Diggelen, Frank Stephen Tromp, editor. | Spilker, James J., Jr., 1933- editor. | Parkinson, Bradford W., editor. Title: Position, navigation, and timing technologies in the 21st century : integrated satellite navigation, sensor systems, and civil applications / editors, Y. Jade Morton, University of Colorado Boulder, Frank van Diggelen, Google, James J. Spilker, Jr., Stanford University, Bradford W. Parkinson, Stanford University. Description: First edition. | Hoboken : Wiley/IEEE Press, [2020] | Includes index. | Contents: Introduction, early history, and assuring PNT (PTA) / Bradford W. Parkinson, Stanford University, USY, T. Jade Morton, University of Colorado Boulder, US, Frank van Diggelen, Google, US, James J. Spilker Jr., Stanford University, US – Fundamentals of satellite-based navigation and timing / John W. Betz, The Mitre Corporation, US – The Navstar Global Positioning System / John W. Betz, The Mitre Corporation, US. Identifiers: LCCN 2020044255 (print) | LCCN 2020044256 (ebook) | ISBN 9781119458340 (set ; hardback) | ISBN 9781119458418 (volume 1 ; hardback) | ISBN 9781119458494 (volume 2 ; hardback) | ISBN 9781119458463 (volume 1 ; adobe pdf) | ISBN 9781119458456 (volume 1 ; epub) | ISBN 9781119458425 (volume 2 ; adobe pdf) | ISBN 9781119458401 (volume 2 ; epub) Subjects: LCSH: Mobile geographic information systems. Classification: LCC G109.4 .P67 2020 (print) | LCC G109.4 (ebook) | DDC 910.285–dc23 LC record available at https://lccn.loc.gov/2020044255 LC ebook record available at https://lccn.loc.gov/2020044256

Cover Design: Wiley Cover Images: Global telecommunication network © NicoElNino/Getty Images, GPS Satellite © BlackJack3D/iStockphoto

Set in 9.5/12.5pt STIXTwoText by SPi Global, Pondicherry, India

10987654321 In Memory of: Ronald L. Beard Per Enge Ronald Hatch David Last James J. Spilker, Jr. James B. Y. Tsui

vii

Contents

Preface xiii Contributors xv

Part A Satellite Navigation Systems 1

1 Introduction, Early History, and Assuring PNT (PTA) 3 Bradford W. Parkinson, Y.T. Jade Morton, Frank van Diggelen, and James J. Spilker Jr.

2 Fundamentals of Satellite-Based Navigation and Timing 43 John W. Betz

3 The Navstar Global Positioning System 65 John W. Betz

4 GLONASS 87 S. Karutin, N. Testoedov, A. Tyulin, and A. Bolkunov

5 GALILEO 105 José Ángel Ávila Rodríguez, Jörg Hahn, Miguel Manteiga Bautista, and Eric Chatre

6 BeiDou Navigation Satellite System 143 Mingquan Lu and Zheng Yao

7 IRNSS 171 Vyasaraj Rao

8 Quasi-Zenith Satellite System 187 Satoshi Kogure, Yasuhiko Kawazu, and Takeyasu Sakai

9 GNSS Interoperability 205 Thomas A. Stansell, Jr.

10 GNSS Signal Quality Monitoring 215 Frank van Graas and Sabrina Ugazio

11 GNSS Orbit Determination and Time Synchronization 233 Oliver Montenbruck and Peter Steigenberger

12 Ground-Based Augmentation System 259 Boris Pervan viii Contents

13 Satellite-Based Augmentation Systems (SBASs) 277 Todd Walter

Part B Satellite Navigation Technologies 307

14 Fundamentals and Overview of GNSS Receivers 309 Sanjeev Gunawardena and Y.T. Jade Morton

15 GNSS Receiver Signal Tracking 339 Y.T. Jade Morton, R. Yang, and B. Breitsch

16 Vector Processing 377 Matthew V. Lashley, Scott Martin, and James Sennott

17 Assisted GNSS 419 Frank van Diggelen

18 High-Sensitivity GNSS 445 Frank van Diggelen

19 Relative Positioning and Real-Time Kinematic (RTK) 481 Sunil Bisnath

20 GNSS Precise Point Positioning 503 Peter J.G. Teunissen

21 Direct Position Estimation 529 Pau Closas and Grace Gao

22 Robust Positioning in the Presence of Multipath and NLOS GNSS Signals 551 Gary A. McGraw, Paul D. Groves, and Benjamin W. Ashman

23 GNSS Integrity and Receiver Autonomous Integrity Monitoring (RAIM) 591 Sam Pullen and Mathieu Joerger

24 Interference: Origins, Effects, and Mitigation 619 Logan Scott

25 Civilian GNSS Spoofing, Detection, and Recovery 655 Mark Psiaki and Todd Humphreys

26 GNSS Receiver Antennas and Antenna Array Signal Processing 681 Andrew O’Brien, Chi-Chih Chen and Inder J. Gupta

Part C Satellite Navigation for Engineering and Scientific Applications 717

27 Global Geodesy and Reference Frames 719 Chris Rizos, Zuheir Altamimi, and Gary Johnston

28 GNSS Geodesy in Geophysics, Natural Hazards, Climate‚ and the Environment 741 Yehuda Bock and Shimon Wdowinski Contents ix

29 Distributing Time and Frequency Information 821 Judah Levine

30 GNSS for Neutral Atmosphere and Severe Weather Monitoring 849 Hugues Brenot

31 Ionospheric Effects, Monitoring, and Mitigation Techniques 879 Y. Jade Morton, Zhe Yang, Brian Breitsch, Harrison Bourne, and Charles Rino

32 GNSS Observation for Detection, Monitoring, and Forecasting Natural and Man-Made Hazardous Events 939 Panagiotis Vergados, Attila Komjathy, and Xing Meng

33 GNSS Radio Occultation 971 Anthony J. Mannucci, Chi O. Ao, and Walton Williamson

34 GNSS Reflectometry for Earth Remote Sensing 1015 James Garrison, Valery U. Zavorotny, Alejandro Egido, Kristine M. Larson, Felipe Nievinski, Antonio Mollfulleda, Giulio Ruffini, Francisco Martin, and Christine Gommenginger

Part D Position, Navigation, and Timing Using Radio Signals-of-Opportunity 1115

35 Overview of Volume 2: Integrated PNT Technologies and Applications 1117 John F. Raquet

36 Nonlinear Recursive Estimation for Integrated Navigation Systems 1121 Michael J. Veth

37 Overview of Indoor Navigation Techniques 1141 Sudeep Pasricha

38 Navigation with Cellular Signals of Opportunity 1171 Zaher (Zak) M. Kassas

39 Position, Navigation and Timing with Dedicated Metropolitan Beacon Systems 1225 Subbu Meiyappan, Arun Raghupathy, and Ganesh Pattabiraman

40 Navigation with Terrestrial Digital Broadcasting Signals 1243 Chun Yang

41 Navigation with Low-Frequency Radio Signals 1281 Wouter Pelgrum and Charles Schue

42 Adaptive Radar Navigation 1335 Kyle Kauffman

43 Navigation from Low Earth Orbit 1359 Part 1: Concept, Current Capability, and Future Promise Tyler G.R. Reid, Todd Walter, Per K. Enge, David Lawrence, H. Stewart Cobb, Greg Gutt, Michael O’Connor, and David Whelan

43 Navigation from Low-Earth Orbit 1381 Part 2: Models, Implementation, and Performance Zaher (Zak) M. Kassas x Contents

Part E Position, Navigation, and Timing Using Non-Radio signals of Opportunity 1413

44 Inertial Navigation Sensors 1415 Stephen P. Smith

45 MEMS Inertial Sensors 1435 Alissa M. Fitzgerald

46 GNSS-INS Integration 1447 Part 1: Fundamentals of GNSS-INS Integration Andrey Soloviev

46 GNSS-INS Integration 1481 Part 2: GNSS/IMU Integration Using a Segmented Approach James Farrell and Maarten Uijt Haag

47 Atomic Clocks for GNSS 1497 Leo Hollberg

48 Positioning Using Magnetic Fields 1521 Aaron Canciani and John F. Raquet

49 Laser-Based Navigation 1541 Maarten Uijt de Haag, Zhen Zhu, and Jacob Campbell

50 Image-Aided Navigation – Concepts and Applications 1571 Michael J. Veth and John F. Raquet

51 Digital Photogrammetry 1597 Charles Toth and Zoltan Koppanyi

52 Navigation Using Pulsars and Other Variable Celestial Sources 1635 Suneel Sheikh

53 Neuroscience of Navigation 1669 Meredith E. Minear and Tesalee K. Sensibaugh

54 Orientation and Navigation in the Animal World 1689 Gillian Durieux and Miriam Liedvogel

Part F Position, Navigation, and Timing for Consumer and Commercial Applications 1711

55 GNSS Applications in Surveying and Mobile Mapping 1713 Naser El-Sheimy and Zahra Lari

56 Precision Agriculture 1735 Arthur F. Lange and John Peake

57 Wearables 1749 Mark Gretton and Peter Frans Pauwels

58 Navigation in Advanced Driver Assistance Systems and Automated Driving 1769 David Bevly and Scott Martin Contents xi

59 Train Control and Rail Traffic Management Systems 1811 Alessandro Neri

60 Commercial Unmanned Aircraft Systems (UAS) 1839 Maarten Uijt de Haag, Evan Dill, Steven Young, and Mathieu Joerger

61 Navigation for Aviation 1871 Sherman Lo

62 Orbit Determination with GNSS 1893 Yoaz Bar-Sever

63 Satellite Formation Flying and Rendezvous 1921 Simone D’Amico and J. Russell Carpenter

64 Navigation in the Arctic 1947 Tyler G. R. Reid, Todd Walter, Robert Guinness, Sarang Thombre, Heidi Kuusniemi, and Norvald Kjerstad

Glossary, Definitions, and Notation Conventions 1971

Index I1

xiii

Preface

The ability to navigate has been an essential skill for applications. It starts with a historical perspective of GPS survival throughout human history. As navigation has and other related PNT developments. Part A consists of advanced, it has become almost inseparable from the ability 12 chapters that describe the fundamental principles and to tell time. Today, position, navigation, and timing (PNT) latest developments of all global and regional navigation technologies play an essential role in our modern society. satellite systems (GNSSs and RNSSs), design strategies that Much of our reliance on PNT is the result of the availability enable their coexistence and mutual benefits, their signal of the Global Positioning System (GPS) and the growing quality monitoring, satellite orbit and time synchroniza- family of Global Navigation Satellite Systems (GNSSs). Sat- tion, and satellite- and ground-based systems that provide ellite-based navigation and other PNT technologies are augmentation information to improve the accuracy of being used in the many fast-growing, widespread, civilian navigation solutions. Part B contains 13 chapters. These applications worldwide. A report sponsored by the US provide a comprehensive review of recent progress in satel- National Institute of Standards and Technology (NIST) lite navigation receiver technologies such as receiver archi- on the economic benefits of GPS indicated that GPS alone tecture, signal tracking, vector processing, assisted and has generated a $1.4 trillion economic benefit in the private high-sensitivity GNSS, precise point positioning and real- sector by 2019, and that the loss of GPS service would have a time kinematic (RTK) systems, direct position estimation $1 billion per-day negative impact.1 PNT has become a techniques, and GNSS antennas and array signal processing. pillar of our modern society. Knowledge and education Also covered are topics on the challenges of multipath-rich are essential for the continued advancement of PNT urban environments, in handling spoofing and interference, technologies to meet the increasing demand from society. and in ensuring PNT integrity. Part C finishes the volume That is the rationale that led to the creation of this book. with 8 chapters on satellite navigation for engineering and While there are many publications and several outstand- scientific applications. A review of global geodesy and refer- ing books on satellite navigation technologies and related ence frames sets the stage for discussions on the broad field subjects, this two-volume set offers a uniquely comprehen- of geodetic sciences, followed by a chapter on the important sive coverage of the latest developments in the broad field of subject of GNSS-based time and frequency distribution. PNT and has been written by world-renowned experts in GNSS signals have provided a popular passive sensing tool each chapter’s subject area. It is written for researchers, for troposphere, ionosphere, and Earth surface monitoring. engineers, scientists, and students who are interested in Three chapters are dedicated to severe weather, ionospheric learning about the latest developments in satellite-based effects, and hazardous event monitoring. Finally, a compre- PNT technologies and civilian applications. It also exam- hensive treatment of GNSS radio occultation and reflectom- ines alternative navigation technologies based on other sig- etry is provided. nals and sensors and offers a comprehensive treatment of The three parts in Volume 2 address PNT using alterna- integrated PNT systems for consumer and commercial tive signals and sensors and integrated PNT technologies applications. for consumer and commercial applications. An overview The two-volume set contains 64 chapters organized into chapter provides the motivation and organization of the six parts. Each volume contains three parts. Volume 1 volume, followed by a chapter on nonlinear estimation focuses on satellite navigation systems, technologies, and methods which are often employed in navigation system modeling and sensor integration. Part D devotes 7 chapters to using various radio signals transmitted from sources on 1 RTI International Final Report, Sponsored by the US National Institute of Standards and Technology, “Economic Benefits of the the ground, from aircraft, or from low Earth orbit (LEO) Global Positioning System (GPS),” June 2019. satellites for PNT purposes. Many of these signals were xiv Preface

intended for other functions, such as broadcasting, net- provided valuable input and comments to other chapters working, and imaging and surveillance. In Part E, there in the book. We also sought input from graduate students are 8 chapters covering a broad range of non-radio fre- and postdocs in the field as they will be the primary users quency sensors operating in both passive and active modes and represent the future of the field. We want to acknowl- to produce navigation solutions, including MEMS inertial edge the following individuals who have supported or sensors, advances in clock technologies, magnetometers, encouraged the effort and/or helped to improve the con- imaging, LiDAR, digital photogrammetry, and signals tents of the set: Michael Armatys, Penina Axelrad, John received from celestial bodies. A tutorial-style chapter on Betz, Rebecca Bishop, Michael Brassch, Brian Breitsch, multiple approaches to GNSS/INS integration methods Phil Brunner, Russell Carpenter, Charles Carrano, Ian is included in Part E. Also included in Part E are chapters Collett, Anthea Coster, Mark Crews, Patricia Doherty, on the neuroscience of navigation and animal navigation. Chip Eschenfelder, Hugo Fruehauf, Gaylord Green, Finally, Part F presents a collection of work on contem- Richard Greenspan, Yu Jiao, Kyle Kauffman, Tom porary PNT applications such as surveying and mobile Langenstein, Gerard Lachapelle, Richard Langley, Robert mapping, precision agriculture, wearable systems, auto- Lutwak, Jake Mashburn, James J. Miller, Mikel Miller, mated driving, train control, commercial unmanned air- Pratap Misra, Oliver Montenbruck, Sam Pullen, Stuart craft systems, aviation, satellite orbit determination and Riley, Chuck Schue, Logan Scott, Steve Taylor, Peter Teu- formation flying, and navigation in the unique Arctic nissen, Jim Torley, A. J. van Dierendonck, Eric Vinande, environment. Jun Wang, Pai Wang, Yang Wang, Phil Ward, Dongyang The chapters in this book were written by 131 authors Xu, Rong Yang, and Zhe Yang. The Wiley-IEEE Press from 18 countries over a period of 5 years. Because of the team has demonstrated great patience and flexibility diverse nature of the authorship and the topics covered throughout the five-year gestation period of this project. in the two volumes, the chapters were written in a variety And our families have shown great understanding, gener- of styles. Some are presented as high-level reviews of prog- ously allowing us to spend a seemingly endless amount of ress in specific subject areas, while others are tutorials with time to complete the set. detailed quantitative analysis. A few chapters include links This project was the brainchild of Dr. James Spilker, Jr. He to MATLAB or Python example code as well as test data for remained a fervent supporter until his passing in October those readers who desire to have hands-on practice. The 2019. A pioneer of GPS civil signal structure and receiver collective goal is to appeal to industry professionals, technologies, Dr. Spilker was truly the inspiration behind researchers, and academics involved with the science, engi- this effort. During the writing of this book set, several pio- neering, and application of PNT technologies. A website, neers in the field of GNSS and PNT, including Ronald Beard, pnt21book.com, provides chapter summaries; downloada- Per Enge, Ronald Hatch, David Last, and James Tsui also ble code examples, data, worked homework examples, passed away. This set is dedicated to these heroes and all select high-resolution figures, errata, and a way for readers those who laid the foundation for the field of PNT. to provide feedback. A comprehensive project of this scale would not be pos- Jade Morton sible without the collective efforts of the GNSS and PNT Frank van Diggelen community. We appreciate the leading experts in the field Bradford Parkinson taking time from their busy schedules to answer the call in Sherman Lo contributing to this book. Some of the authors also Grace Gao xv

Contributors

Zuheir Altamimi Jacob Campbell Institut National de l’Information Géographique et Air Force Research Laboratory, United States Forestière, France Aaron Canciani Chi O. Ao Air Force Institute of Technology, United States Jet Propulsion Laboratory, United States J. Russell Carpenter Benjamin W. Ashman National Aeronautics and Space Administration, National Aeronautics and Space Administration, United States United States Eric Châtre Yoaz Bar-Sever European Commission, Belgium Jet Propulsion Lab, United States Chi-Chih Chen Miguel Manteiga Bautista The Ohio State University, United States European Space Agency, the Netherlands Pau Closas John W. Betz Northeastern University, United States The MITRE Corporation, United States H. Stewart Cobb David Bevly Satelles, United States Auburn University, United States Simone D’Amico Sunil Bisnath Stanford University, United States York University, Canada Evan Dill Yehuda Bock National Aeronautics and Space Administration, Scripps Institution of Oceanography, United States United States

Alexei Bolkunov Gillian Durieux PNT Center, Russia Max Plank Institute for Evolutionary Biology, Germany

Harrison Bourne Alejandro Egido University of Colorado Boulder, United States Starlab, Spain

Brian Breitsch Naser El-Sheimy University of Colorado Boulder, United States University of Calgary, Canada

Hugues Brenot Per K. Enge Royal Belgian Institute for Space Aeronomy, Belgium Stanford University, United States xvi Contributors

James Farrell Zaher (Zak) M. Kassas Vigil Inc., United States University of California Irvine, United States

Alissa M. Fitzgerald Kyle Kauffman A.M. Fitzgerald & Associates, LLC, United States Integrated Solutions for Systems, United States

Grace Gao Yasuhiko Kawazu Stanford University, United States National Space Policy Secretariat, Japan

James Garrison Norvald Kjerstad Purdue University, United States Norwegian University of Science and Technology, Norway Christine Gommenginger National Oceanography Centre, United Kingdom Satoshi Kogure National Space Policy Secretariat, Japan Mark Gretton TomTom, United Kingdom Attila Komjathy Jet Propulsion Laboratory, United States Paul D. Groves University College London, United Kingdom Zoltan Koppanyi The Ohio State University, United States Robert Guinness Finnish Geospatial Research Institute, Finland Heidi Kuusniemi Finnish Geospatial Research Institute, Finland Sanjeev Gunawardena Air Force Institute of Technology, United States Arthur F. Lange Trimble Navigation, United States Inder J. Gupta The Ohio State University, United States Zahra Lari Leica Geosystems Inc., Canada Greg Gutt Satelles, United States Kristine M. Larson University of Colorado Boulder, United States Maarten Uijt de Haag Technische Universität Berlin, Germany Matthew V. Lashley Georgia Tech Research Institute, United States Jörg Hahn European Space Agency, the Netherlands David Lawrence Satelles, United States Leo Hollberg Stanford University, United States Judah Levine National Institute of Standard and Technology, Todd Humphreys United States University of Texas–Austin, United States Miriam Liedvogel Mathieu Joerger Max Plank Institute for Evolutionary Biology, Virginia Tech, United States Germany

Gary Johnson Sherman Lo Geoscience Australia, Australia Stanford University, United States

Sergey Karutin Mingquan Lu PNT Center, Russia Tsinghua University, China Contributors xvii

Anthony J. Mannucci Peter Frans Pauwels Jet Propulsion Laboratory, United States TomTom, the Netherlands

Francisco Martin John Peake Starlab, Spain Trimble Navigation, United States

Scott Martin Wouter Pelgrum Auburn University, United States Blue Origin LLC, United States

Boris Pervan Gary A. McGraw Illinois Institute of Technology, United States Collins Aerospace, United States Mark Psiaki Subbu Meiyappan Virginia Tech, United States NextNav LLC, United States Sam Pullen Xing Meng Stanford University, United States Jet Propulsion Laboratory, United States Arun Raghupathy Meredith E. Minear NextNav LLC, United States University of Wyoming, United States Vyasaraj Rao Antonio Mollfulleda Accord Software and Systems, India Starlab, Spain John F. Raquet Oliver Montenbruck Integrated Solutions for Systems, United States German Aerospace Center, Germany Tyler G. R. Reid Y.T. Jade Morton Stanford University, United States University of Colorado Boulder, United States Charles Rino Alessandro Neri University of Colorado Boulder, United States University of Roma TRE, Italy Chris Rizos Felipe Nievinski University of New South Wales, Australia UFRGS, Brazil José Ángel Ávila Rodríguez ’ Andrew O Brien European Space Agency, the Netherlands The Ohio State University, United States Giulio Ruffini ’ Michael O Connor Starlab, Spain Satelles, United States Takeyasu Sakai Bradford W. Parkinson National Institute of Maritime, Port, and Aviation Stanford University, United States Technology, Japan

Sudeep Pasricha Charles Schue, III Colorado State University, United States UrsaNav, Inc., United States

Ganesh Pattabiraman Logan Scott NextNav LLC, United States LS Consulting, United States xviii Contributors

James Sennott Frank van Graas Tracking and Imaging Systems, United States Ohio University, United States

Tesalee K. Sensibaugh Panagiotis Vergados University of Wyoming, United States Jet Propulsion Laboratory, United States

Suneel Sheikh Michael J. Veth ASTER Labs, Inc., United States Veth Research Associates, United States Todd Walter Stephen P. Smith Stanford University, United States The Charles Stark Draper Laboratory Inc., United States Shimon Wdowinski Andrey Soloviev Florida International University, United States QuNav, United States David Whelan James J. Spilker Jr. University of California San Diego, United States Stanford University, United States Walton Williamson Thomas A. Stansell, Jr. Jet Propulsion Laboratory, United States Stansell Consulting, United States Chun Yang Peter Steigenberger Sigtem Technology Inc., United States German Aerospace Center, Germany Rong Yang Nikolai Testoedov University of Colorado Boulder, United States PNT Center, Russia Zhe Yang University of Colorado Boulder, Peter J. G. Teunissen United States Curtin University, Australia and Delft University of Technology, The Netherlands Zheng Yao Tsinghua University, China Sarang Thombre Finnish Geospatial Research Institute, Finland Steven Young National Aeronautics and Space Administration, Charles Toth United States The Ohio State University, United States Valery U. Zavorotny Andrei Tyulin National Oceanic and Atmospheric Administration, PNT Center, Russia United States; Sabrina Ugazio University of Colorado Boulder, United States Ohio University, United States Zhen Zhu Frank van Diggelen East Carolina University, Google, United States United States 1

Part A

Satellite Navigation Systems

3

1

Introduction, Early History, and Assuring PNT (PTA) Bradford W. Parkinson1, Y.T. Jade Morton2, Frank van Diggelen3, and James J. Spilker Jr.1

1 Stanford University, United States 2 University of Colorado Boulder, United States 3 Google, United States

1.1 Introduction to illustrate such interconnecting relationships and the benefits that accrue to the user. Knowledge of your current location is now taken for granted Application Explosion For the user, GPS is simply a by people worldwide. This is largely due to the advent of sat- technique to assure PNT; there are significant other ways ellite-based navigation systems, particularly the Global Posi- to find location today, and more will become available in tioning System (GPS). These global navigation satellite the near future. This book will explore both current and systems (GNSSs) are still rapidly evolving with more capabil- future techniques, especially the other GNSSs and regional ityand evengreater robustness. Their fundamental purpose is navigation satellite systems (RNSSs). But GPS is now a determining location in four dimensions – three geographical name familiar to nearly every cell phone user in the world. positions plus time. A user is indifferent to the source of loca- By the year 2015, over 2 billion receiver sets had been pro- tion knowledge – any technique will do if it is reliable. While duced, and, driven by cell phone applications, these are much of this book is devoted to satellite-based navigation sys- increasing by over 1.4 billion per year. tems (satnavs), we intend to give full explanations of virtually Besides ubiquitous cell phones, GPS has stealthily crept all modern sources of position, navigation, and timing (PNT). into virtually every corner of our society. Even the early The classical definition of navigation is the act, activity, developers have been amazed by the countless applications. science, method, or process of finding a route to get to a Table 1.1 presents a partial list of application areas. Clearly, place when you are traveling in a ship, car, airplane, etc. any attempt to explore all applications of satnav and It involves the determination of position, course, and dis- PNT would require many volumes and be outdated as soon tance traveled [1]. A more contemporary, formal definition as it was published. However, we do intend to describe of navigation is determining positions, orientations, veloci- representative current and future applications for PNT in ties, and accelerations, all in three dimensions and in a this book. stated coordinate system, and time, as well as planning, GPS has been called “The Stealth Utility” because many finding, and following a route. applications are usually invisible to the user. Operationally, The goal for most satnav users is assured PNT; providers GPS availability has been over 99.9%, on a worldwide recognize that combining dissimilar sources of basic basis [2]. This pervasive availability drives the enormous PNT information leads to a much more robust positioning GPS benefits in terms of safety, productivity, and conven- capability – that is, greater PNT assurance. The United ience. For example, there are now over 3600 certified States Federal Aviation Administration (FAA) uses four cri- GPS runway approaches for aircraft in the United States. teria to measure PNT capability. These are (i) Availability, An economic study for the US National Space-Based PNT (ii) Accuracy, (iii) Integrity, and (iv) Continuity of opera- Advisory Board (PNTAB) calculated the mid-range value tions. They are useful measures for all users and all appli- of GPS at over $65 billion per year for the United States cations, not just aviation. In particular, compounding or alone [3]. These broad benefits have led to GPS being prop- augmenting systems (e.g. satnav + inertial) leads to greater erly described as “a system for humanity.” As such, GPS assurance of PNT in the face of deliberate or inadvertent raises some historical questions. How did it come into being radio interference. Thus, another purpose of this book is and what applications are likely to be developed in the

Position, Navigation, and Timing Technologies in the 21st Century: Integrated Satellite Navigation, Sensor Systems, and Civil Applications, Volume 1, First Edition. Edited by Y. T. Jade Morton, Frank van Diggelen, James J. Spilker Jr., and Bradford W. Parkinson. © 2021 The Institute of Electrical and Electronics Engineers, Inc. Published 2021 by John Wiley & Sons, Inc. 4 1 Introduction, Early History, and Assuring PNT (PTA)

Table 1.1 Twelve major application areas for satnav

Areas Example application

Aviation Area navigation, approach, landing up to Cat III, NextGen Agriculture Autofarming: crop spraying, precision cultivating, yield assessment Automotive Turn-by-turn guidance, concierge services, driverless cars Emergency and Rescue Services 911, ambulance, fire, police, rescue helicopters, emergency beacons, airplane and ship locaters Intelligent Transportation Train control and management, UAVs Military Rescue, precision weapon delivery, unit and individual location Recreation Geocaching, drones, hiking, boating, fitness Robotics and Machine Control Bulldozers, earth graders, mining trucks, oil drilling Scientific Earth movement and shape, atmosphere, weather forecasting, climate modeling, ionosphere, space weather, tsunami warning, soil moisture, ocean roughness and salinity, wind velocity, snow, ice, foliage coverage, etc. Survey and GIS Mapping, environmental monitoring, tagging disease outbreaks Timing Cell phone towers, banking, power grid Tracking Fleets, assets, equipment, shipments, children, Alzheimer’s patients, wildlife, livestock, pets, law enforcement, criminals, parolees, etc.

future? The history will be addressed in this introductory ensure integrity for aviation, WAAS became operational on chapter‚ and selected applications will be summarized 10 July 2003. This pioneering system performs real-time here, but expanded in later chapters. measurements of all GPS satellites and sends the user an While GPS has been the pioneer in satnav, other nations integrity message in near real time that also corrects any are in the process of fielding their own systems. Three exam- real-time ranging errors. An example of the GPS measure- ples of newer Global satnavs are the upgraded GLONASS ment accuracy performance: for the 22 WAAS ground sta- built by the Russians, the EU-sponsored system called tions in the third quarter of 2015, it was better than 2.2 m Galileo,1 and the Chinese system, called BeiDou (formerly: of horizontal error at the 95th percentile [4]. The European Compass). In addition, a number of countries are fielding Geostationary Navigation Overlay Service (EGNOS) and Regional satnav systems (RNSSs). In Volume I, Part A of this Japanese Multi-functional Transport Satellite (MTSAT) book, we devote a chapter to each of these global and regional Space-based Augmentation System (MSAS) perform simi- systems. With these systems, an individual user is able to use lar functions. All of these are examples of satellite-based well over 40 satellites simultaneously for determining posi- augmentation systems (SBAS) for satnavs. A similar, more tion. The key enabler for the user is that the satnav systems local augmentation technique, called the Local Area Aug- are at least interoperable, if not interchangeable. The simila- mentation System (LAAS), and other ground-based aug- rities and differences among the GNSSs as well as the chal- mentation systems (GBAS) and techniques are designed lenges for interoperability are also addressed in this book. for high-integrity, blind landing of aircraft. Furthermore, Reliance on satnav alone is imprudent for many users. numerous ground-based accuracy augmentation networks, What techniques and processes can be used to increase such as the Continuously Operating Reference Station robustness and accuracy? This first chapter will introduce (CORS) networks, are the results of combined efforts by the topic‚ and later chapters will expand on this in depth. government organizations, self-funding agencies, universi- The program to ensure PNT availability has been a major ties, and research institutions from over 100 countries subject for the US PNTAB and is called “PTA.” PTA stands [5–7]. These networks and their combined super-networks for Protect, Toughen, and Augment and will be further ela- play a fundamental role in enabling broad areas of appli- borated in this introduction. cations. These essential GNSS augmentation systems will An example of GNSS augmentation is the US FAA’s Wide be discussed in this book. Area Augmentation System (WAAS). Driven by the need to Numerous innovative PNT algorithms and metho- dologies have been developed since the inception of the 1 The recent Brexit will probably change Galileo management. There GPS concept. Volume I of this book focuses on the is the possibility of an added British navigation satellite constellation. progress made and future trends in satnav technologies 1.2 A Brief History Prior to SatNav 5 and applications, while Volume II focuses on non-GNSS source with a known location. Practical RDF devices were sensors, integrated PNT systems, and applications. The fun- in use by the early 20th century. RDF is an example of damental purposes of these volumes are to offer technical bearing-measurement systems that includes the modern explanations of the many satnav and other techniques that VHF omni-directional range (VOR) used by FAA. The provide civil PNT and to explore selected applications that other four classes of radio navigation system are beam sys- are useful to the global community. The chapters have been tems, transponder systems (including distance measuring written by world-class experts on the current state of the art equipment (DME)), hyperbolic systems, and satnavs. of these PNT technologies. The most well known of the hyperbolic systems is prob- ably LORAN (Long-Range Navigation). A modernized variant of this is eLORAN (enhanced LORAN). The under- 1.2 A Brief History Prior to SatNav lying technique is to measure the difference in time of arrival (TOA) between pairs of transmitted pulses. Each sta- tion pair produces a hyperbolic line of position. The user’s 1.2.1 Early Navigation Techniques location is determined by the intersection of two or more As humans migrated across the globe, the ability to navi- such lines. This system is only two-dimensional (2D) gate was an absolute prerequisite to survival. The Polyne- with accuracies of about 20 m in a calibrated differential sians developed techniques of using the observation of mode. It is appealing as an augmentation for GPS because stars and planets to navigate across vast areas of the Pacific its powerful RF signal is in an entirely different radio Ocean with legendary accuracy. In 1976, these voyages spectrum band. were replicated using a newly built war canoe of ancient Polynesian design. This vessel, named the Hokulea, was 1.2.3 Inertial Navigation navigated solely by the stars from Hawaii to Tahiti. It is a fascinating story that elaborates on the ancient techniques Inertial navigation is another method of providing posi- of using the heavenly bodies [8]. tioning information. During World War II, the Germans About 1000 years ago, the Chinese introduced the mag- deployed an elementary guidance system for the V-2 rocket, netic compass‚ which was particularly useful for voyages but this was not a generally useful configuration. The first in overcast conditions. On cloudy days, the Vikings may purely inertial, generalized system was invented and devel- have used cordierite or some other birefringent crystal oped by Dr. Charles Stark Draper at MIT in the early 1950s to determine the Sun’s direction and elevation from the [10]. The basic idea is to mount very precise accelerometers polarization of daylight through cloudy skies. on a gyroscopically stabilized platform (in strapped-down Then a series of techniques were developed to measure mode‚ such stabilization is maintained in software). By the altitude (angle above the horizon) of stars and other doubly integrating the accelerometer outputs and correct- heavenly bodies to calculate position, culminating in the ing for the effect of gravity, which cannot be sensed, posi- invention of the sextant by British Vice-Admiral John Camp- tion can be determined. This requires very accurate initial bell in 1757. Latitude could be determined by the elevation of conditions (both position and velocity) as well as careful the Sun above the horizon at high noon or the elevation of alignment with an inertial coordinate frame. The nature the star Polaris, but longitude required accurate time, syn- of double integration magnifies small sensor biases into chronized to an observatory that had published a nautical error growth that is proportional to time or time squared, almanac. The challenging requirement for synchronized so periodic reset is essential for most applications. time soon led to the development of highly accurate ship- Draper’s inertial navigation systems were very successful borne clocks (called chronometers), with the initial success- and quickly became the basic navigation device for the ful version by a Yorkshire carpenter, John Harrison‚ in 1761. Navy’s ballistic missile submarines. Professor Walter Wrig- His clock was accurate to better than 1 second per day. This ley has written a history of inertial navigation and said: history was documented in the excellent book Longitude by “Notwithstanding the work of those previously discussed, Dava Sobel [9]. the MIT Instrumentation Laboratory under Professor C. S. Draper was the main spearhead in the development of inertial navigation systems and components for aircraft, 1.2.2 Radio Navigation ships, missiles, and spacecraft” [11]. Professor Wrigley With the discovery and exploitation of radio waves, new bases this statement on an earlier article in the American navigation techniques could be developed. Perhaps the Journal of the Institute of Navigation by H. Hellman [12]. most elementary is radio direction finding (RDF), which These state-of-the-art inertial navigators still required peri- allows a user to determine the line of bearing to a radio odic updates of position and velocity to maintain the 6 1 Introduction, Early History, and Assuring PNT (PTA)

desired accuracy. This led to the first space-based radio nav- 1.3 Initial GPS Development: Key igation system‚ called Transit, which is discussed below. Milestones in the Early Development Modern micro-electro-mechanical systems (MEMS) of Worldwide 3D satnav for PNT which measure accelerations and rotations are now common in many applications including automobiles. Figure 1.1 depicts the major events in GPS and related devel- Major efforts are being made to improve the accuracy opments in two major segments: 1957–1983 and 1989–2020. and stability of these devices. A parallel development of The focus of this chapter is on the early history and develop- chip-scale atomic clocks, which are both inexpensive and ment of GPS. While major events related to other GNSSs and accurate, is also a major advance. MEMS devices and RNSSs and future development are touched upon as well, these clock technologies are discussed in Volume II. these topics are fully addressed in later chapters.

GPS DEVELOPMENT: 1957–1983

Getting, Aerospace Corp CDMA signal structure US Air Force First GPS satellite President Regan Proposed 3D satnav system & Gold Code designated to develop launched formally guaranteed Joint Navigation System GPS civil signal availability Woodford & Nakamura, Pentagon “Lonely Halls” US Air Force 621B study meeting. GPS defined Initial operational control system White Sands tests, DSARC 6 satellites in space Inverted ranges approved GPS Median accuracy 7m

57 58 1960 62 66 67 1970 73 74 78 79 1980 82 83 Transit, US Navy NTS-1, NRL Guier & Weiffenbach Timation (quartz oscillator) First attempt to place World’s first First GLONASS track satellite using Doppler. Navy Research Labs (NRL) atomic clock in space satnav system satellite launched Transit conceived. Experienced early Cicada, Sputnik, USSR radiation-induced failure USSR

RELATED DEVELOPMENT

GPS DEVELOPMENT: 1989–2020

GPS declared President Clinton First Block IIR First Block IIF fully operational discontinued satellite satellite Selective Availability broadcasting L1M, broadcasting L5 L2C, L2M signals signals launched launched First GPS III satellite broadcasting L1C launched

24 satellites in space Began broadcasting SPS PS RMS 4m CNAV messages 19 GPS satellites launched SPS PS RMS 6m

89 1990 93 95 96 2000 01 05 08 2010 12 13 14 17 18 2020 NavIC: Galileo, QZSS: first GLONASS: Beidou I: Galileo: first test 7 satellites NavIC, QZSS satellite launched 24 satellites in space limited test system satellite GIOVE-A launched operational operational launched Galileo: first Beidou II: operational satellite 14 satellites launched in space Beidou III operational global OTHER GNSS & RNSS DEVELOPMENT operation

Figure 1.1 Timeline of major development in GPS, GNSS, RNSS, and related technologies. 1.4 The Seminal System Study of Alternatives for Satellite-Based Navigation Sponsored by the Air Force and Ivan Getting 7

The first development occurred on 4 October 1957, when Table 1.2 Transit characteristics the entire world was fascinated by the launch of the Rus- First operational 1962 sian Sputnik satellite. The American public greeted this prototype event with both apprehension and curiosity. In 1958, the Operational 1964–1996 Applied Physics Laboratory (APL) of Johns Hopkins Uni- Orbit versity employed an extremely competent team of engi- Circular polar orbit at ~1000 km altitude in 5+ nominal orbital neers and scientists. Two of those scientists, Drs. William planes Guier and George Weiffenbach, began to study the orbits Transmit frequencies 150 and 400 MHz to correct for of the new Sputnik satellites. The satellites were broadcast- ionospheric delays ing a continuous tone signal, so their velocity, relative to the Time for a position fix ~15 min ground, created a Doppler shift of the signal that was Time between fix Periodic, ~90 min unique. After some innovative work, Guier and Weiffen- 2D accuracy ’ For moving ship, 200 to 500 m, bach discovered they could determine the Sputnik s orbit needs velocity correction with a single pass of the vehicle. For stationary user, 80 m At that point, Frank McClure of APL made a very crea- tive suggestion: why not turn the problem upside down? Using a known satellite position, a navigator could deter- mine their location anywhere in the world after receiving navigators of the United States Submarine Ballistic Missile and processing the satellite signal for 15 minutes. His Force then being deployed. These submarines were a major insight became the basis for the Navy’s Transit satellite deterrent during the Cold War. Transit was first tested in program, also known as the Navy Navigation Satellite 1960 and by 1964 the system was operational. System. This pioneering satellite system was developed Although limited in scope as a 2D shipboard navigation under the leadership of Dr. Richard Kershner, head of satellite system, Transit was a major contributor to APL (see his photo with Dr. Bradford Parkinson, who led satellite navigation as the first worldwide operational nav- the development of GPS in Figure 1.2). Transit’s main igation satellite system. Table 1.2 lists key information on purpose was to provide position updates to the inertial Transit.

1.4 The Seminal System Study of Alternatives for Satellite-Based Navigation Sponsored by the Air Force and Ivan Getting [13]

New Satellite-Based Navigation Systems Proposed 1962–1970 By 1962, Dr. Ivan Getting, president of the Aer- ospace Corporation, saw the need for a new satellite-based navigation system. While he did not have a specific imple- mentation, he envisioned a more accurate positioning system that would be available in 3 dimensions, 24 hours a day, 7 days a week. He had direct access to the highest levels of the Pentagon and was a tireless advocate for his vision. Getting’s energy and foresight in the early 1960s were essential to gaining Air Force support to study system alter- natives. As a result, the Air Force formed a new satellite navigation program that was later named 621B. His efforts were recognized in 2003 when he shared the Charles Stark Draper Prize of the National Academy of Engineering Figure 1.2 Dr. Richard Kershner (left) who led the development of “ ’ ” Transit. On his right is Col. Bradford Parkinson, who led the (known as the Engineer s Nobel ) with Dr. Bradford development of GPS. Parkinson. 8 1 Introduction, Early History, and Assuring PNT (PTA)

By 1962, engineers at Aerospace, under Air Force spon- USAF/621B 1964 Study of sorship, were heavily immersed in studying the system 12 Alternative GNSS Architectures alternatives for a new navigational satellite system. From 1964 to 1966, the Air Force directed Aerospace to perform an extensive, formal system study whose principal authors

were James Woodford and Hiroshi Nakamura, both highly Competing NRL proposal–User regarded space-systems engineers. required an atomic The Woodford/Nakamura study was summarized as a clock DoD (Department of Defense) secret briefing in August 1966. As a result of the classification, it was unavailable GPS selected the most Challenging to anyone outside the project until 13 years later in 1979, of 12 Alternatives – 4D Position 2 with no user need for an atomic when it was finally declassified. Figure 1.3 shows the front clock 12 (Demonstrated by USAF 621B– page of the GPS system study. 1971)

Figure 1.4 Key summary of alternatives for satellite-based navigation systems. The USAF Program Office selected the 12th alternative. The competing Naval Research Laboratory (NRL) proposal (Option #2) was two-dimensional and relied on an atomic clock at the user’s station.

with “X.”“A” shows that the user needs an atomic clock. “X” shows the user needs only a crystal clock. The option that was later selected for GPS is #12, designated with the green box. This technique is the 3 Δρ (four satellites) that eliminated the need for the user’s atomic clock and Figure 1.3 Front page of the seminal GPS system study performed provided three-dimensional positioning (really four- from 1964 to 1966 by USAF 621B program. Originally classified dimensional since it also captured time). This technique SECRET, it was declassified after the initial GPS constellation had been launched. This was the essential foundation for the GPS is illustrated in Figure 1.5 (taken from [14]). design concept. 1.4.1 The 621B Era – Additional USAF Studies This report was a very complete system study that exam- ined the following topics: From 1966 to 1972, program 621B continued with trade-off studies including signal modulation, user data process- • Capabilities and limitations of then current DoD naviga- ing techniques, orbital configuration, orbital prediction, tion systems. receiver accuracy, error analysis, system cost, and comprehen- • Tactical applications and utility of improved positioning sive estimates of the tactical mission benefits. In late 1968, accuracy. the Air Force’s NavSat program in the Plans Office (XR) at • Comprehensive analysis of alternative system configura- the Space and Missile Systems Organization (SAMSO) in tions and techniques for positioning using satellites. Los Angeles was re-designated as 621B. All of the various proposals that went forward from SAMSO to Headquarters Since the full survey of alternative system configurations came henceforth from the 621B office in XR. Over 90 NavSat was extremely important in selecting an optimum system reports completed by USAF/Aerospace during this period configuration for GPS, we reproduce the summarizing fig- were filed in the Aerospace Corporation library. ure in Figure 1.4. Twelve major alternatives were studied. Note that the “COMPUTATION PERFORMED BY USER” 1.4.2 The Code Division Multiple Access (CDMA) is split into two columns. The reader should focus on the or PRN Signal Structure columns of the 1-WAY passive ranging techniques with the red outline. These alternatives can have an unlimited Of these studies, the most important were those aimed at number of users – i.e. there is no system constraint. In this selecting the best passive ranging technique for the naviga- column, there are two “user boxes,” one with “A” and one tion signal. By 1967, it appeared that the best method was 1.4 The Seminal System Study of Alternatives for Satellite-Based Navigation Sponsored by the Air Force and Ivan Getting 9

(x|, y|, z|)

RN R|

R1 b + (x, y, z) Measurements: Pseudoranges {R|}

Given: Satellite positions {(x|, y|, z|)}

2 2 2 Ri = (x| – x) + (y| – y) + (z| – z) – b i = 1, 2,...., N Unknown: User Position (x, y, z) Receiver clock Bias b

Figure 1.5 Illustration of the principle of satellite navigation (from [14]). The user-satellite ranging measurements are based on the times of transmission and receipt of signals. They are biased by a common time offset and are called pseudoranges. Four pseudoranges are required. Source: Reproduced with permission of Ganga-Jamuna Press.

pseudorandom noise because the encoded (but repeated) sequence appears to be random transitions of +1 and −1. It is called Code Division because each satellite is assigned its own coded signal. Each was a binary (digital) sequence selected to be uncorrelated with other signals and also uncorrelated with time shifts of the signal itself. The expected, powerful advantage of this technique was that all satellites would broadcast on exactly the same fre- quency. It was clear that it would lend itself to digital signal processing. Furthermore, and very importantly, any time shifts induced by the receiver for the various sat- ellite signals would be identical and effectively eliminated. However, there were still a number of significant ques- tions concerning CDMA that needed to be resolved. These included the following:

1) Could such a signal be easily acquired in the face of time uncertainty and Doppler shifts? 2) Was there a technique to encrypt the military signal so Figure 1.6 Dr. James Spilker Jr., one of the creative engineers who that unauthorized users could not gain access? led development of the GPS digital signal structure. 3) How would the codes be easily selected to avoid a false lock and also allow additional satellites to be added without interfering with existing satellite signals? a variation of a new communications modulation known 4) Would the anticipated complexity of the receiver drive as CDMA. Pioneering this signal were several outstand- costs to unacceptable levels? ing scientists and engineers, including Dr. James Spilker 5) Was the signal resistant to accidental or deliberate (Figure 1.6) and Dr. Fran Natali (both of Stanford Telecom), interference? as well as Dr. Charlie Cahn and Bert Glaser (both of 6) Could this signal accommodate communication capa- Magnavox). bility for satellite location, satellite clock correction, This signal has many names. In addition to CDMA, it is and other parameters? sometimes called “spread spectrum” or “spreading code” since the energy of the signal was spread over a wide range Fortunately, in 1967, a technique for selecting ortho- of spectrum frequencies. It is also sometimes called PRN or gonal codes was invented by an accomplished applied 10 1 Introduction, Early History, and Assuring PNT (PTA)

mathematician, Dr. Robert Gold of the Magnavox Corp. Nat- capability. To reduce both of these risks, the Air Force urally these are now known as the Gold codes, and they par- had developed a plan. tially resolved the third CDMA issue stated above. But that This included a proposal in early 1972 to deploy a four- was not the whole story. satellite “demonstration system.” This proposal addressed both risks. It would reduce the technological readiness risk in the clocks by launching simple L-band transponders. 1.4.3 The White Sands Missile Range Tests and The navigation signal would be generated on the ground Confirmation 1970–1972 and transponded to users with a “bent-pipe” in the satellite. To address the remaining problems, the USAF 621B pro- At the same time, it would save substantial money, thereby gram developed two prototype versions of CDMA naviga- reducing the political and budgeting risk. tion receivers (Magnavox and Hazeltine) for testing at the In many circles, this proposal was erroneously thought of White Sands Missile Range (WSMR). For the tests, 621B as the 621B operational proposal because it came from that arranged four transmitters in a configuration known as office. In fact, the operational concept for 621B never con- the Inverted Range. These transmitters broadcast CDMA templated or advocated using transponders in the final signals from locations that were geometrically similar to operational system. The use of transponders had been a satellite configuration except that they were broadcast rejected for the operational system because they could be from the ground, i.e. “upside down.” By 1972, program easily jammed from the ground. Such a jamming signal 621B had successfully proven the effectiveness and would overpower the transponder and steer all of the trans- accuracy of the CDMA signal by demonstrating that such mitted energy away from the transponded navigational sig- a configuration would achieve 5 m, three-dimensional nal. This enemy jamming would shut down the entire navigational accuracy. These test results answered most system, clearly an unacceptable risk. of the remaining questions regarding the CDMA signal. 621B Proposed Initial Satellite Constellation To dem- The tests confirmed the enormous technical value of the onstrate four-satellite, passive ranging capability, 621B had modulated signal by showing that all satellite signals could, studied a number of orbital configurations, including indeed, be received simultaneously on the same frequency. geosynchronous and inclined, lower orbits. They proposed These tests also corroborated the expectation that ranging placing a constellation of four synchronous satellites in to four satellites eliminated the need for a highly precise orbits over the United States. This array would allow user atomic clock, while still supporting full, three- extended periods of four-satellite testing without commit- dimensional navigation. This became an extremely impor- ting to a full global employment. If this demonstration were tant feature of GPS. If each user had required an atomic- successful, the next step would have been to add three clock-class frequency standard, no inexpensive user equip- more longitudinal sectors, each with its own array. Again, ment could have been produced within the technology the principal redeeming feature of this approach was that horizon visible at that time. This is still true today. there was some hope of it being funded. The Air Force in All this evidence supported CDMA as the passive ranging the Pentagon was placing enormous pressure on the signal of choice and was available to the Air Force’s 621B 621B program to come up with the absolutely cheapest team when the system configuration was selected at the way to demonstrate the four-satellite approach. In addition, September 1973 meeting that will be discussed later. they wanted any initial configuration to provide the begin- ning of a full global system. This proposed constellation design was a reasonable 1.4.4 Distinguishing Between the 621B Demo compromise, given the boundary conditions of a four- Configuration and the 621B Preferred Operational satellite demonstration and absolutely minimal cost. It is Configuration interesting that the Japanese, with a requirement to supple- From the time of the 1966 Woodford/Nakamura study on, ment GPS with satellite signals to improve coverage in the Air Force and Aerospace advocated the use of atomic urban areas (where there are high shading angles), have clocks in the operational satellites with the modulation also been deploying a very similar constellation. The Japanese originating in the satellites. There were two significant risks configuration is intended to improve coverage restricted to placing atomic clocks on the satellites: first, the technol- to their longitudinal sector of the globe. The new system ogy readiness risk (no radiation-hardened atomic clocks is called Quasi-Zenith Satellite System (QZSS), and the had ever been designed and flown), and second, the polit- Japanese have announced the four-satellite constellation ical and budgeting risk associated with gaining approval for is now operational (2018) [15]. The system is further a development/demonstration program of the full described in Chapter 8.