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UMI Number. 9961957 UMI* UMI Microform9961957 Copyright 2000 by Bell & Howell Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. Bell & Howell Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. © Copyright 2000 by Arlen D. Schmidt All Rights Reserved ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a disser­ tation for the degree of Doctor of Philosophy. <9 uq i— _____________________________ Prof. Howard A. Zebker (Principal Adviser) I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a disser­ tation for the degree of Doctor of Philosophy. ( j t - L T \£m , Prof. G. Leonard Tyler I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a disser­ tation for the degree of Doctor of Philosophy. D\wtmt G- Nishimura Approved for the University Committee on Graduate Studies: in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Abstract Earth-based radar im a g in g of orbiting satellites at frequencies below about 1 GHz requires compensation of the dispersive effects of the ionosphere. Without the appropriate com­ pensation, image resolution is limited to about 10—100 m at radar frequencies of 300 MHz, depending on ionospheric conditions. W ith compensation, the resolution expected in the absence of ionospheric dispersion can be achieved. For stable ionospheric conditions and frequencies above about 200 MHz, a 1 // phase model characterizes the propagation path to sufficient accuracy to allow mitigation of the ionospheric effects, provided that a sufficiently accurate estimate of total electron content (TEC) is available. The information required to estimate TEC is inherent in the radar observations. An initial estimate, accurate to about ± 2 x 1016 e~/m2, is obtained from group-delay difference measurements across frequency subbands. That initial estimate is refined to the requisite accuracy by using radar image contrast as a measure of performance. A phase correction, calculated from the TEC esti­ mate, provides compensated data from which full-resolution images are produced, provided the radar echo is not scintillating and the SNR is above about 10 dB. The accuracy of the TEC estimates obtained through im a g in g depends on center frequency, bandwidth, and angular aperture. Radar im a g in g through the ionosphere is demonstrated with full-resolution ultra-wide bandwidth images of the Mir Space Station produced from data collected between January, 1998, and June, 1998, using SRl-Intemational dish antennas at Stanford University and on Ascension Island. The two sites provided observations through both the benign mid-latitude ionosphere (Stanford), and the thicker, more-turbulent equatorial ionosphere (Ascension). Range and cross-range resolutions of 0.7 m and 0.5 m, respectively, are achieved in 200 to 400 MHz radar images of Mir as well as other satellites, confirming that, for these examples, the ionospheric effects are mitigated. The corresponding TEC estimates are self-consistent across 30-degree subaperture images to better than about 10 15 e"/m2, in agreement with v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the accuracy predicted by analysis. Thus, the experimental results are consistent with the predicted performance, both in terms of resolution and TEC estimation accuracy. vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgments It has truly been a privilege to work with a professional and highly-skilled team of engineers at SRI on the project that led to this thesis. I was fortunate in being granted the respon­ sibility for analyzing the radar data and developing techniques for producing VHP satellite images. That role was particularly rewarding in that I was able to see, and benefit from, the team’s efforts to collect high-quality data—the fundamental ingredient for high-resolution radar images and unambiguous validation of the ionospheric compensation techniques. The research described in this thesis would have been impossible without the efforts of each member of the team; I gratefully acknowledge their many contributions, ha particular. Mr. Philip Bentley designed most of the RF hardware and was responsible for system installa­ tion and subsystem testing. Dr. David Bubenik provided software for producing ephemeris and tracking control files from the two-line element sets, as well as guidance and tutelage in the area of orbital mechanics. Mr. George Carpenter, my supervisor at SRI, completed most of the initial system design. Dr. Michael Cousins designed and supervised the construction of the Ascension Island radar site, and managed the Stanford and Ascension Island radar sites- Dr. Kenneth Dreyer sold and managed the project, and nurtured the idea of forming VHF satellite images. Mr. Teije Oseberg wrote most of the software for the data acquisition system. I have enjoyed immensely working with the members of this team, whom I now count as good friends. Foremost, I gratefully acknowledge the many contributions of my friend and mentor, Mr. Gary Price, an integral part of the SRI team. Several ideas presented herein originated with Gary. He made countless direct contributions to this research, at both macroscopic and microscopic levels, especially in the areas of ionospheric physics and signal processing. I thank my reading committee, Dr. Howard Zebker, Dr. G. Leonard Tyler, and Dr. Dwight Nishimura. Without their efforts, this document would be much more difficult to read and understand. vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Finally, I could not have completed this thesis without the unending love and support of my wife, Joyce, and our children, Jacob, Adam, and Nicola. They made as many sacrifices and put as much effort into seeing this research through to completion as I. Thank you, Joyce. Thank you, Jacob. Thank you, Adam. Thank you, Nicola. viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Contents Abstract v Acknowledgments vii 1 Introduction 1 1.1 Motivation ..................................................................................................................... 2 1.2 H i s t o r y ............................................................................................................................ 4 1.3 Inverse Synthetic Aperture Radar ............................................................................. 5 1.4 Satellite Imaging at V H F .......................................................................................... 6 1.5 Estimating and Removing Ionospheric Effects ...................................................... 6 1.6 Experimental Verification .......................................................................................... 8 1.7 Contributions .................................................................................................................