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Ocean, Platform, and Signal Processing Effects on Synthetic Aperture Sonar Performance

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Ocean, Platform, and Signal Processing Effects on Synthetic Aperture Sonar Performance AD-A267 288 N Ocean, Platform, and Signal Processing Effects on -• Synthetic Aperture Sonar Performance by Kenneth D. Rolt DTIC fS ELECTE Submitted in partial fulfillment AUG 0 3 1993 of the requirements for the degree of U A U Scientiae Magister at the Massachusetts Institute of Technology Cambridge, MA 02139 February 1991 © Kenneth D. Rolt, 1990; All rights reserved. The author hereby grants MIT and the C.S. Draper Laboratory permission to reproduce and/or distribute copies of this thesis in-whole or in-part. Signature of Author_________________________________ Department of Ocean Engineering 28 November 1990 Certified by Professor Jerome Milgram Thesis Co-Supervisor Certified by Professor Henrik Schmidt Thesis Co-Supervisor Accepted by Professor A. Douglas Carmichael Chairman, Ocean Engineering Graduate Committee This document ha- b"en approved fo: public relea-e and sale; its distribution is un1imite . 4k # Ocean, Platform, and Signal Processing Effects on Synthetic Aperture Sonar Performance by Kenneth D. Rolt keywords: synthetic aperturesonar (SAS), synthetic aperture radar(SAR), acousticalimaging, radarimaging ABSTRACT Synthetic Aperture Sonar (SAS), the underwater sound application of the aperture synthesis method borrowed from Synthetic Aperture Radar (SAR) is investigated by means of oceaa nitdium effects, platform motion effects, and signal processing effects. Synthetic aperture methods, both in radar and in sonar, allow large synthesized apertures, or equivalently synthesized arrays, to be formed from the combined use of signal processing and a small, real aperture transducer (e.g. a radar or a sonar transducer). Large synthetic apertures give rise to relatively narrow synthesized beams, which in turn allow very high resolution focused images to be formed from radar or sonar data. The target in the synthetic aperture sonar case could be the ocean bottom, and the objective could be, for example, ocean-bottom mapping or searching for a sunken vessel. While synthetic aperture radars have been extensively developed and used since the late 1950's, synthetic aperture sonars have been rare except for a number of academic papers and a few experimental systems. A computer-based model has been created to evaluate the imaging potential of a synthetic aperture sonar system within a horizontally stratified ocean. The model includes ocean refraction, spatial and temporal coherence, surface and bottom influences via multipath, deterministic and random platform motion, and a variety of processing options. This model, the most extensive ever used for SAS simulations, has been verified by comparison to a SAS ocean experiment performed by others. By the use of the model, the three dominant influences of ocean, platform, and signal processing may be studied on the performance of synthetic aperture sonar imaging. The ocean influence on SAS imagery is shown to depend on (1) the extent of spatial/temporal coherence which limits the useful aperture length that may be used for sharp imaging, and (2) the refraction profile which must be accounted for in image reconstruction to ensure a match between the real ocean and the estimated ocean in the computer model; this turns out to be a geometric matched fi!tcr. 2 DISCLAIMER NOTICE THIS DOCUMENT IS BEST QUALITY AVAILABLE. THE COPY FURNISHED TO DTIC CONTAINED A SIGNIFICANT NUMBER OF PAGES WHICH DO NOT REPRODUCE LEGIBLY. Platform motion is also shown to degrade the imagery, and the model confirms that lateral platform motion should be measured to within the canonical X/8 distance found in the SAR literature for the sharpest images, but also that the image persists even for lateral platform motion in excess of X/8. Finally, a number of processing effects are shown, with the most important being that: (1) synthetic aperture radars and sonars working exactly at the Nyquist sampling rate do not entirely null alias lobes (an assumption often made in the SAS literature); (2) the use of many sub-aperture lengths for a Nyquist sampled synthetic aperture will force the aliases to the level of the far sidelobes (hence making them invisible); (3) the bandwidth must be carefully chosen for a Nyquist-sampled synthetic aperture to minimize alias images; and (4) broadband operation is always useful for undersampled synthetic apertures, and a "ballpark" formula is developed to estimate the level of alias image smear. [Work supported by the Charles Stark Draper Laboratory]. Acce.;ion For NTiS CRk&l SlIC IAI L1 U:j:@iJ ,d B .......... .. f-C pT I-S:.i 3 TY'IC QUALITY INSPEfCTED 3 -- • M • iii i i | i i •3 ACKNOWLEDGEMENTS First, I acknowledge the guidance and support of my co-supervisors in this effort: Professor Jerome Milgram and Professor Henrik Schmidt. I also thank the Charles Stark Draper Laboratory for sponsoring the work. I must naturally recognize the previous work upon which this thesis is constructed, particularly by those efforts in radar and sonar wave propagation, and in synthetic aperture radar and sonar. Although the bibiiography in this thesis accounts for most of the published 'prior art' in synthetic aperture sonar, the list of contributors to the field is considerably longer. This work is, in its own way, a minor improvement on an already invented and constantly-improving wheel. Many individuals have helped me along during this work. At the start, Dr. George M. Walsh of the Raytheon Company, Submarine Signal Division, furnished me with three volumes of Submarine Signal Division reports on synthetic aperture sonar and medium stability. Dr. William Carey also furnished copies of papers on synthetic aperture sonar, particularly on passive synthetic arrays. Dr. Shahen A. Hovanessian kindly sent me a copy of his book on synthetic arrays. I had the pleasure of meeting Dr. Philippe de Hecring at the Acoustical Society of America Spring Meeting (May 1990), discussed synthetic arrays with him. de Heering also furnished me with a copy of his Ph.D. thesis on SAS. I was also fortunate in finding the Dec. 1989 J. Acoust. Soc. Am. paper on a prototype SAS by Dr. Peter T. Gough and Dr. Michael P. Hayes, of the University of Canterbury, Christchurch, New Zealand. This publication of synthetic aperture sonar images on a known target enabled me to verify my computer model. Gough and Hayes were both kind enough to correspond with me via mail concerning their work, and also sent me a copy of Dr. Hayes' Ph.D. thesis. Howard Zebker and W.T.K. Johnson of the Jet Propulsion Laboratory, California Institute of Technology, also provided me with SAR information and images containing azimuthal aliases, which also helped confirm my synthetic aperture model. Locally, I was also helped by many others. At MIT I was grateful for the discussions with DJ Tang, Brian Tracey and John Halsema in particular, and by many others in the office I share. Dr. Jeff Lozow and John Irza of the Charles Stark Draper Laboratory were most helpful. Dr. John L. Butler of Image Acoustics was most helpful in discussing all sorts of acoustics, including my work, and in keeping me active in the design of underwater sound sources. 4 My father, George H. Rolt, deserves some sort of award (perhaps part of my diploma) for the many discussions of my work and for requesting a rough draft of this for parental scrutiny. Parental scrutiny soon gave way to parental editing, in extremis. However, both the writer and the reader of this thesis greatly benefit from the editing. Thanks Dad. Christine Coughlin also gets an award for just-plain putting up with me during 1988-1990, and sticking with me because of my "future potential." The last note is extended to the reader of this thesis: since almost no one ever really reads these things, you deserve an award for getting at least this far. "Work like hell, tell everyone everything you know, close a deal with a handshake, and have fun." - Harold E. "Doc" Edgerton, a pioneer, inventor and educator in many areas, including imaging sonar. CONTENTS ABSTRACT ................................................................................................. 2 ACKNOW LEDGEM ENTS ..................................................................................................... 4 L IST O F F IG U R E S ................................................................................................................. 8 L IST O F T A B L E S ................................................................................................................ 1 1 NOTATION AND LIST OF SYM BOLS ................................................................... ......... 12 CHAPTER 1 INTRODUCTION ...................................................................................... 15 1.1 History and Overview ............................................................................................ 15 1.1.1 Synthetic Aperture Radar (SAR) ..................................................................... 19 1.1.2 Synthetic Aperture Sonar (SAS) ..................................................................... 21 1.2 Previous Work in SAS ........................................................................................... 22 1.2.1 'Low' Frequency Active SAS (below 1 MHz) .......................................... 24 1.2.2 'High' Frequency Active SAS (above 1 MHz) .......................................... 28 1.2.3 Passive SAS ....................................................................................... 30 1.3 Contributions of this Thesis to the Field of SAS .......................................................
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