"Very Long Baseline Interferometer"

The book will be published in June, 2000 by Ohmsha and IOS Press.

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In October 1896, one year after the success of Marconi's wireless telegraphy experiments, Japan started research into wireless telegraphy at the Electrical Technical Laboratory of the Ministry of Communications. The following year, Mr. Matsunosuke Matsushiro found success in his wireless telegraphy experiments. The Radio Research Laboratories, Ministry of Posts and Telecommunications, was established in 1952 after some reorganization of the laboratories researching radio communication technology. The Radio Research Laboratories was renamed the Communications Research Laboratory in 1988 to reflect the change in the environment for research following the privatization of the Nippon Telegraph and Telephone Corporation in Japan.

To celebrate 100 years of radio communications, we have spent several years at the Communications Research Laboratory planning and preparing to publish in book from the results of the research so far conducted. Entitled the "Wave Summit Course", it will contain a series of papers on various aspects of radio communications technology and is aimed at students of both the senior and graduate levels, that is, the generation of scientists who will lead the way in the 21st century.

Now the time for publication has arrived. This series consists of seven volumes: 'Satellite Communications', 'Very Long Baseline Interferometer', 'Science of Space Environment', 'Modern Millimeter-Wave Technologies', 'Digital Broadcasting', 'Mobile Communications', and 'Global Environment Remote Sensing'. We hope that these volumes will be widely read as useful guides to the different fields in which radio waves are used.

Finally, I would like to extend my deep gratitude to all those involved in enabling this project to come to fruition, including the authors, editors, and successive Director-Generals of the CRL and chairpersons of the publications committee, and of course to Ohmsha, Ltd. for their untiring assistance and encouragement.

This text, entitled "Very Long Baseline Interferometer", celebrates the 100th anniversary of the discovery of the wireless telegraph, and is one of the texts in the "Wave Summit Course" published by the Communications Research Laboratory (CRL), Ministry of Posts and Telecommunications. The material covered in this text is presented in as systematic a manner as is possible, and although aimed specifically for the student of VLBI, it should also be suitable for engineering school and graduate-level students as well.

VLBI is an acronym for Very Long Baseline Interferometer, and can be roughly divided into two areas of application. The first area is in geodesy, which is covered in considerable detail in the text. The second area is in astrometry, or radio astronomy, which is only briefly treated in this text.

Many people probably relate geodesy and land surveying more with geology and geography than with space-based measuring techniques; and at least up through the first half of the 20th century, geodesy was largely based on knowledge in these fields. Surveyors were generally considered as technicians or craftsmen. Modern GPS (Global Positioning Systems) is probably best known for its use in automobile navigation, but such space-based measuring systems have made tremendous advances in the field of geodesy as well.

In a 1996 speech announcing the United States' policy regarding GPS, the US Vice President, Albert Gore, boasted to the world that, in addition to the Internet, the US had achieved a level of technology in space-based measuring systems far superior to any other country on earth. Given Japan's present status in the high-tech field of modern geodesy, it appears that we have been relegated to following the US's lead. This is paricularly true with regards to GPS, which can be considered as a large engineering conglomerate consisting of both space and terrestrial systems. Such a project requires large financial support, but if not correctly evaluated and prioritized, simply having the correct scientific knowledge will not necessarily produce successful results. The same can be said for VLBI: at first glance it is easy to mistake the motives and outcomes of research of being for a scientific project because it deals primarily with the sciences of astronomy and geodesy. As with GPS, VLBI should also be considered as a project yielding superb scientific results because of a well-financed, flawlessly operated engineering system which allows the melding of hardware and software technologies.

The CRL has provided an environment where hardware and software can be properly evaluated from an engineering standpoint, and scientists and engineers have cooperated in the development of the world's most advanced VLBI technologies.

The word "geodesy" in Japanese is a term composed of the words, measurement, earth, and studies. The word itself strongly embodies the engineering elements of metering distances and determination of precision values. The study you are about to embark on in space-based surveying encompasses VLBI techniques in this case, but as its name implies, is also a new world of science that combines hardware and software to create a megatechnology. Radio wave and microwave technology form the core of the VLBI system. One can thus immediately see that the late marriage of VLBI and wireless communications technology which is over 100 years old really means that VLBI has a connection to the past, present, and to the future.

The authors of this text are members of the research staff at the Communications Research Laboratory. We have also written this book from the perspective of a group of scientists who, until reaching the university level had never had the opportunity to utilize such technologies as we have available to us now. Consequently, we consider ourselves to be members of a group whose primary purpose is to determine how to best realize scientific results from the technology-heavy equipment found in VLBI systems.

The most notable results obtained by VLBI so far has been the global-scale measuring of the movements of the tectonic plates which cover the surface of the earth. Details of this achievement are discussed in the text, but the primary focus of the material covered here remains an investigation of how VLBI can conduct these measurements with such a high level of precision. The text also explains how the various aspects of system hardware, software, and data analysis techniques can be effectively combined to yield a measurement accuracy that is four orders in magnitude better than conventional surveying techniques. VLBI requires knowledge in many areas of science and engineering. To the students of this text: as you carefully study the processes by which these highly precise measurements are achieved, you should gain an appreciation of how each process relates to the various areas of engineering and science. We also believe that the text will be very useful to researchers having prior knowledge of GPS techniques.

If this text proves useful in overcoming to some degree the huge advantage that the US presently enjoys in GPS and other measuring technologies, we will be gratified beyond our expectations.

Finally, the authors wish to express our gratitude to Mr. Wayne Hughes, the translator of this text.

Contents

Chapter 1 Introduction
	1.1 The Birth of Radio Astronomy		1
	1.2 The Concepts of Radio Interferometry		3
	1.3 The Evolution of the Atomic Clock		4
	1.4 Developments in Large-Scale Antenna Technology		6
	1.5 Advances in Signal Processing Technology		7
	1.6 The Advent of Geodetic VLBI		9
	1.7 Chapter Outlines		11
	. Chapter Questions		12
	. Chapter References		12
Chapter 2 Basic Concepts: A Quick Tour of VLBI
	2.1 The Basic Principles of Radio Interferometry		13
	2.2 Galactic Radio Sources and Coordinate Systems		15
	2.3 Observational Coordinate Systems		17
		2.3.1 Surveying the Earth's Shape from Outside the Solar System	18
	2.4 Propagation Delays		21
		2.4.1 Atmospheric Propagation Delay	22
		2.4.2 Solar and Planetary Effects	23
	2.5 Basic Principles of VLBI		23
	2.6 VLBI System Components: A Historical Sketch		27
	2.7 VLBI Observations		33
		2.7.1 Observation Equipment	33
		2.7.2 Observation Schedules	37
	2.8 Data Processing Flow in Geodetic VLBI		40
	2.9 Collaboration in VLBI Experiments		41
	. Chapter Questions		44
	. Chapter References		44
Chapter 3 Data Processing Techniques
	3.1 The Fourier Transform and Applications		45
		3.1.1 Defining the Fourier Transform	46
		3.1.2 Time Shifts and Fourier Transforms	47
		3.1.3 Correlation Theorem	47
		3.1.4 The Convolution Theorem	49
	3.2 A VLBI Equivalent Signal Model		51
	3.3 One-Bit Sampling		54
	3.4 Maximum Likelihood Estimation of Delay		57
		3.4.1 Maximum Likelihood Estimation Principle and Least- Squares Method	57
		3.4.2 Cross-Correlation as the Maximum Likelihood Esti- mation of Delay: A Time-Domain Approach	59
		3.4.3 Cross-Correlation as the Maximum Likelihood Esti- mation of Delay: A Frequency-Domain Approach	62
	3.5 Cross-Correlation Function in VLBI		65
		3.5.1 Cross-Correlation Function (without frequency conversion)	65
		3.5.2 Cross-Correlation Function of the Video Signal	67
	3.6 Correlator Processing		70
		3.6.1 Correlation Processing Principles	70
		3.6.2 Delay Tracking	72
		3.6.3 Fringe Stopping	75
		3.6.4 Bit Shifts and 90-degree Phase Jumps	79
		3.6.5 Fractional Bit Correction	83
		3.6.6 Phase Calibration Signal Detection	84
	3.7 Precise Determination of Delay and Delay Rates		85
		3.7.1 Normalizing Correlator Output	86
		3.7.2 Coarse Delay Search	88
		3.7.3 Precise Delay Search using Bandwidth Synthesis	91
		3.7.4 Ambiguities in Bandwidth Synthesis	93
	3.8 Verification of VLBI Measurement Precision		96
		3.8.1 Scattering and Bias of Measurement Values	96
		3.8.2 Signal-to-Noise Analysis	96
		3.8.3 Theoretical Delay Errors	100
		3.8.4 Error Verification using Independent Correlation Systems	104
	3.9 Real-Time VLBI		105
	. Chapter Questions		107
	. Chaptert References		107
Chapter 4 Data Analysis
	4.1 Overview of Data Analysis		109
	4.2 VLBI Reference Frames		114
		4.2.1 Inertial Reference Frame of the Solar System	114
		4.2.2 Terrestrial Reference Frame	116
		4.2.3 Time Systems	118
	4.3 The Physical Models		119
		4.3.1 The Earth's Rotation	119
		4.3.2 Propagation Delay Corrections	128
		4.3.3 Tidal Corrections	132
		4.3.4 Relativistic Effects	134
		4.3.5 Other Physical Effects	135
	4.4 Estimating Parameters		138
	4.5 Analytical Methods		139
		4.5.1 Analysis Procedures	139
		4.5.2 Kalman Filter Estimations	142
		4.5.3 Analysis Procedures	144
	4.6 VLBI Errors		147
		4.6.1 Observation Errors	147
		4.6.2 Errors of Estimated Values	149
	4.7 Improving Analysis Procedures		150
	. Chapter Questions		151
	. Chapter References		151
Chapter 5 Geodetic VLBI Experiments
	5.1 The Japanese Geodetic Coordinate System		155
	5.2 Plate Tectonics and Earthquakes		158
	5.3 Measuring Plate Motions		161
	5.4 Crustal Deformations near Plate Boundaries		164
	5.5 The Himalayan Collision Zone and the Japan-China Converging Zone		165
	5.6 Plate Motion near Japan		166
	5.7 VLBI Experiments in Japan		168
	5.8 VLBI Experiments in Antarctica		171
	5.9 Earth Rotation		172
		5.9.1 Earth Rotation Observations	172
		5.9.2 Results of Earth Rotation Observations	173
	5.10 Other Experiments		178
	. Chapter Questions		179
	. Chapter References		179
Chapter 6 Applications of VLBI Techniques
	6.1 Radio Astronomy		181
		6.1.1 Determining Radio Source Positions	181
		6.1.2 Radio Source Structures	186
	6.2 Precise Measurements of Earth Rotation		191
		6.2.1 Measuring Polar Motions and Fluctuations in Earth Rotation	191
		6.2.2 Fluctuations in Earth Rotation Speed and Atmospheric Angular Momentum	193
	6.3 Regional Deformation Monitoring System for the Tokyo Metropolitan Area		195
		6.3.1 Geographical Locations of Observation Stations	196
		6.3.2 Observation System	197
		6.3.3 Automation of Data Analysis	203
	6.4 Deep-Space Measurements		207
	6.5 Applications in Ionospheric Measurements		208
		6.5.1 Correcting for Ionospheric Delay	208
		6.5.2 Using Least-Squares Estimation to Determine Total Electron Content of the Ionosphere	212
	. Chapter Questions		216
	. Chapter References		216
Chapter 7 In Conclusion
	7.1 The Transition from Centimeter Accuracies to Millimeter Accuracies		219
	7.2 Beyond the Earth		220
Addendum
	Table 1 Wahr Nutation Model Coefficients		221
	Table 2 Yoder UT1 Earth Tides Correction Coefficients		223
	Table 3 Dickman UT1 Earth Tides Correction Coefficients		224
	Table 4 VLBI Station Positions in the ITRF Coordinate Frame		226
	Table 5 Selected Radio source Positions for VLBI Observations		228
Answers to Chapter Questions			229
Postscript			234
Author's Profile			235
Index			237

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