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An Ionospheric Remote Sensing Method Using an Array of Narrowband Vlf Transmitters and Receivers

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An Ionospheric Remote Sensing Method Using an Array of Narrowband Vlf Transmitters and Receivers AN IONOSPHERIC REMOTE SENSING METHOD USING AN ARRAY OF NARROWBAND VLF TRANSMITTERS AND RECEIVERS A Dissertation Presented to The Academic Faculty By Nicholas C. Gross In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Electrical and Computer Engineering Georgia Institute of Technology December 2018 Copyright © Nicholas C. Gross 2018 AN IONOSPHERIC REMOTE SENSING METHOD USING AN ARRAY OF NARROWBAND VLF TRANSMITTERS AND RECEIVERS Approved by: Dr. Morris Cohen, Advisor School of Electrical and Computer Engineering Dr. Paul Steffes Georgia Institute of Technology School of Electrical and Computer Engineering Dr. Sven Simon Georgia Institute of Technology School of Earth and Atmospheric Sciences Dr. Mark Go lkowski Georgia Institute of Technology School of Electrical Engineering University of Colorado Denver Dr. Mark Davenport School of Electrical and Computer Date Approved: September 7, 2018 Engineering Georgia Institute of Technology To my parents Theresa and Brian and to my fianc´e Shraddha ACKNOWLEDGEMENTS I would first like to share my immense gratitude to my advisor, Professor Morris Cohen. He challenged me to pursue my own original ideas and was always available to give guidance and insight. His thoughtful mentorship and sound advice have been invaluable. I am also grateful to have had such an excellent undergraduate research advisor, Professor Mark Go lkowski. Thank you for introducing me to the VLF community and helping me build a strong foundation of electromagnetics and plasma physics knowledge. Thank you to my other three thesis committee members: Professor Sven Simon for further developing my understanding of plasma physics, Professor Paul Steffes for the won- derful conversations about RF and planetary atmospheres, and Professor Mark Davenport for teaching me statistical signal processing techniques. I also owe a special thank you to Dr. Ryan Said for his time and guidance on MSK demodulation. I am indebted to all the members of the Georgia Tech LF Radio Lab. This thesis would not exist if it were not for everyone’s contributions toward the construction, deployment, operation, and maintenance of the Lab’s radio receivers. Thank you to Jackson McCormick, Evan Worthington, Marc Higginson-Rollins, Nate Opalinski, Parker Singletary, Lee Thomp- son, and Nikhil Pailoor. You have all been tremendous friends. I deeply appreciate the love and support my family has given me. I thank my sister, Katie, for her friendship and my parents, Theresa and Brian, for the countless sacrifices they have made. I finally thank my lovely fianc´e, Shraddha, for her unwavering support and endless patience during my research endeavors. This work was supported by the National Science Foundation under grants AGS 1451142 and AGS 1653114 (CAREER) to the Georgia Institute of Technology. iv TABLE OF CONTENTS ListofTables ..................................... viii ListofFigures..................................... xi Chapter1: Introduction .............................. 1 1.1 ResearchPurpose................................. 1 1.2 VLF Radiation Sources . 2 1.2.1 Narrowband VLF Transmitters . 4 1.2.2 Radio Atmospherics . 6 1.3 DataAcquisition ................................. 8 1.3.1 ReceiverPerformance .......................... 9 1.3.2 Transmitter and Receiver Array . 10 1.4 VLF Waves in the Ionosphere . 11 1.4.1 D Region Plasma Physics . 12 1.4.2 Wait and Spies Two-Parameter Model . 20 1.5 Contributions................................... 24 1.5.1 Journal Publications . 25 1.5.2 Conference Presentations . 25 Chapter 2: VLF Propagation Modeling . 27 2.1 Ideal Waveguide Theory . 28 2.2 WaveguideReflections .............................. 31 2.2.1 Ionospheric Reflection Coefficient Matrix . 32 2.2.2 Ground Reflection Matrix . 33 v 2.2.3 The Booker Quartic . 34 2.2.4 The WKB Approximation . 37 2.2.5 Accounting for Earth’s Curvature . 39 2.2.6 Ionospheric Reflection Matrix . 41 2.3 ModeFinding................................... 44 2.4 ModeConversion................................. 48 2.5 LWPCExamples ................................. 52 2.6 VLF Remote Sensing History . 55 Chapter 3: Narrowband VLF Fundamentals . 58 3.1 Minimum-ShiftKeying.............................. 60 3.1.1 MSK Definition . 60 3.1.2 MSK Demodulation . 63 3.1.3 Coherent MSK Removal . 70 3.2 2-Channel MSK Demodulation . 72 3.3 Polarization Ellipses . 76 3.3.1 Polarization Definition and Utility . 76 3.3.2 Angle of Arrival . 81 3.3.3 Ellipse Analysis Example . 86 3.3.4 Phase Stability of VLF Transmitters . 88 3.4 Ionospheric Remote Sensing . 90 3.4.1 Modal Changes Due to Terminator Effects . 90 3.4.2 Early/Fast Event . 92 3.4.3 LEP Event . 95 3.4.4 Solar Flare Event . 97 Chapter 4: Comparison of VLF Observations to LWPC . 99 4.1 LWPC Inverse Function Estimation . 99 4.2 Relating VLF Observations to Waveguide Properties . 100 4.2.1 Narrowband Phase Ambiguity Removal . 102 4.2.2 A Simple Approach to Inferring Waveguide Parameters . 106 vi 4.3 Radio Wave Scattering . 108 4.3.1 Briarwood Differential Phase Experiment . 108 4.3.2 SchererDifferentialPhaseExperiment . 111 Chapter 5: Concurrent Electron Density Inference . 116 5.1 Artificial Neural Network . 116 5.1.1 A LWPC Based Inverse Function for Multiple Paths . 116 5.1.2 ANN Basics . 118 5.1.3 High-noon Assumption . 121 5.2 Synthetic Training Data . 125 5.2.1 Synthetic Data Generation . 125 5.2.2 ANN Performance . 130 5.3 Quiet Day Analysis . 131 5.3.1 Optimal Seed Values . 132 5.3.2 Inferred Waveguide Parameters . 135 5.3.3 Comparison to Past Works . 138 5.3.4 Indirect Error Measurements . 140 5.4 Solar Flare Analysis . 144 5.4.1 Inferred Waveguide Parameters . 146 5.4.2 Indirect Error Measurements . 148 Chapter 6: Summary and Suggestions for Future Work . 151 6.1 Summary . 151 6.2 Suggestions for Future Work . 153 6.2.1 Polarization Ellipses . 153 6.2.2 Polarization Ellipsoids . 153 6.2.3 Carrier Phase Ambiguity . 153 6.2.4 Radio Wave Scattering . 154 6.2.5 Improvements to the ANN Method . 154 Bibliography...................................... 169 vii LIST OF TABLES 1.1 CCIR recommended daytime h′ and β profiles . 22 1.2 CCIR recommended nighttime h′ and β profiles . 22 2.1 Example daytime QTM mode values from LWPC . 53 2.2 Example nighttime QTM mode values from LWPC . 54 4.1 Briarwood to Baxley phase wrapping . 105 4.2 Briarwood to Lost Pines phase wrapping . 106 4.3 Briarwooddifferentialphasetestresults . 110 4.4 Schererdifferentialphasetestresults . 112 viii LIST OF FIGURES 1.1 Earth-ionosphere waveguide propagation . 3 1.2 AWESOMEreceiverblockdiagram. 9 1.3 Map of narrowband VLF transmitters . 11 1.4 Simple example for h′ and β frequencydependence . 23 2.1 Ideal parallel-plate waveguide . 29 2.2 World conductivity and relative permittivity map . 34 2.3 Circular to flat waveguide transformation . 40 2.4 LWPC waveguide segmentation . 49 2.5 ExampleLWPCoutput ............................. 53 2.6 LWPC examples with different h′ and β .................... 55 3.1 In-phase and quadrature symbol mapping . 61 3.2 Two MSK signals interfering with each other . 64 3.3 Narrowband lowpass filter response . 65 3.4 Comparison of original and reconstructed MSK signal . 68 3.5 Coherent MSK removal example . 71 3.6 Synchronized two channel MSK demodulation . 74 3.7 Comparison of unsynchronized and synchronized two channel MSK demod- ulation....................................... 75 3.8 Polarization ellipse geometric setup . 78 ix 3.9 Angle of arrival error correction . 84 3.10 Ellipse parameters for a full day . 87 3.11 Evidence of unstable carrier phase in a VLF transmitter . 89 3.12 Ellipse parameters for night-to-day terminator crossing . 91 3.13 Polarization ellipse technique applied to an Early/fast event . 94 3.14 Polarization ellipse technique applied to an LEP event . 96 3.15 Polarization ellipse technique applied to a solar flare event . 98 4.1 Trellis comparison between Briarwood and Baxley . 104 4.2 Observations compared with LWPC . 107 4.3 Briarwood differential phase map . 109 4.4 Schererdifferentialphase ............................ 113 4.5 Schererdifferentialphaseerror . 114 5.1 Simple ANN graph . 119 5.2 ANN graph for function estimation . 125 5.3 Waveguide parameter distribution on a global scale . 126 5.4 Synthetictrainingdata.............................. 129 5.5 ANN synthetic data performance . 131 5.6 Map of transmitters and receivers . 132 5.7 Minimumseederror ............................... 134 5.8 Quietdayinferredwaveguideparameters . 136 5.9 Waveguide parameters at minimum solar zenith angle . 137 5.10 Comparison of past works with inferred waveguide parameters . 139 5.11 Briarwood quiet day error measurement . 142 x 5.12 OX quiet day error measurement . 143 5.13 Ellipse analysis on 15 paths during a solar flare event . 145 5.14 Solar flare inferred waveguide parameters . 147 5.15 Briarwood solar flare.
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