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A CTFM Synthetic Aperture Sonar 101 6.1.1 Projector Radiation Pattern 101 6.1.2 Hydrophone Response 103 X

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A CTFM Synthetic Aperture Sonar 101 6.1.1 Projector Radiation Pattern 101 6.1.2 Hydrophone Response 103 X CT M s THE C u E Michael P. Hayes, B.E. (Hons) A thesis presented for the degree of Doctor of Philosophy in Electrical and Electronic Engineering, at the University of Canterbury, Christchurch, New Zealand. September 1989. ENGINEERING L1~f\AP-Y iii st Thls thesis describes the theory and operation of a seafloor imaging Synthetic Aper­ ture Sonar (SAS), based on broadband Continuous Tone Frequency Modulation (CTFM). Narrowband synthetic aperture techniques are reviewed, and some of the limita­ tions of using such techniques in sonar applications are described. Aperture under­ sampling is a particular problem when towing single-beam sonars at realistic speeds. However, the mapping rate constraints may be relaxed by using broadband signals, but at the expense of increased self-clutter (or background). One such broadband signal that is suitable for synthetic aperture operation is CTFM. The signal processing requirements for the CTFM signal are investigated, and are shown to be considerably simplified by decomposing each echo sweep into an ensemble of narrowband components. Images reconstructed from each of these components may be combined in a variety of ways. The relative merits of these differing methods are examined using a computer simulation of a side-scan CTFM sonar. The temporal phase stability of the acoustic environment is vital to the formation of a synthetic aperture. An experiment was performed whlch indicates that the phase stability is much better than anticipated, and certainly adequate for the formation of undersea synthetic apertures. Thls prediction was confirmed by another experiment in which the prototype CTFM sonar was moved along a fixed cableway under realistic operating conditions. Images of a test target (an air-filled steel buoy) were successfully reconstructed using data measured from this experiment. Reconstructions obtained from a number of different algorithms are presented, for differing values of the various operating parameters. It is demonstrated that artefacts resulting from aperture undersampling are reduced by using broadband CTFM. v cknowle ments I sincerely thank my supervisor, Dr. Peter Gough ('the Chief'), for his advice, guidance, and enthusiasm during the course of my research. Maybe one day he too will go grey. I am grateful to my family and friends for all their support and understanding, especially during the duration of my Ph.D study. A big thanks is due to all those people who helped in the preparation of this thesis, in particular Bruce McCallum and Catherine Watson for their proof-reading, Peter Gardenier for his typesetting suggestions, my sister Lynley for help in the scissors and sellotape department, and Denise Fastier for her continual encouragement. I would also like to thank Bruce for his colourful contribution to the Department, and for the many useful discussions I had with him. Thanks are also due to my long suffering fiatmates, who have had to put up with my gui tar playing, the fire-crackers in the toaster, and the nocturnal nature of my attempts at thesis writing. The assistance of the technical staff in the development and construction of the sonar is gratefully acknowledged; in particular Art Vernon, Dermot Sallis, and Michael Cusdin. Thanks are also due to Professor J. W. Roy Griffiths and his group at Loughborough Technical University, England, for their assistance with the testing of the sonar. The project has been supported financially by the New Zealand Defence Scien­ tific Establishment, the New Zealand University Grants Committee, and Marconi Underwater Systems Ltd. I acknowledge the receipt of a New Zealand University Grants Committee Post­ graduate Scholarship and financial assistance from Marconi Underwater Systems Ltd. for the period of my research in the U.K. Lastly, but not least, I would like to humbly dedicate this thesis to all those whom lowe a beer. vii ont nts Abstract iii Acknowledgements v Preface Xl Glossary of Notation xv Symbols xv 1 Preliminary Concepts 1 1.1 Complex Signal Notation 1 1.1.1 Pre-Envelope 2 1.1.2 Band-Pass Signals 3 1.1.3 Complex Sampling of Real Signals 4 1.1.4 N arrow band Signals 4 1.2 Echo Ranging Systems 4 1.3 Matched Filtering 5 1.4 Effects of Sonar/Target Motion 6 1.4.1 Moving Target/Stationary Sonar 7 1.4.2 Moving Sonar/Stationary Target 8 1.4.3 The Distorted Echo Signal 9 1.4.4 The Doppler Effect 9 1.4.5 Signal Tolerance to Sonar/Target Motion 11 1.4.6 Resolution on the Basis of Sonar/Target Motion 12 1.5 Ambiguity Functions 12 1.5.1 Multiple Target Resolution 13 1.6 The Sonar Equation 14 1.6.1 Pulse Integration 15 1.6.2 Pulse Compression 15 1. 7 Reverberation 16 1.7.1 Volume Reverberation 17 1. 7.2 Surface Reverberation 17 1.7.3 Signal to Reverberation 18 1.8 Undersea Acoustic Propagation 18 1.8.1 Attenuation 19 1.8.2 Multipath 19 1.8.3 Coherence 21 viii 1.9 Target Characteristics 21 1.9.1 Speckle 22 1.10 Undersea Imaging Systems 23 1.10.1 Acoustical Undersea Imaging 23 1.10.2 Imaging Sonars 24 1.10.3 Narrow Beam Side-Scan Imaging Sonars 25 1.10.4 Range Resolution 25 1.10.5 Maximum Range 26 1.10.6 Range Ambiguities 26 1.10.7 Azimuth Resolution 27 1.10.8 Azimuth Ambiguities 28 1.10.9 Towfish Instabilities 28 2 Synthetic Aperture Sonar 31 2.1 Synthetic Aperture Side-Scan Sonar Geometry 33 2.1.1 Apparent Target Motion 34 2.1.2 Target Phase History 35 2.1.3 Target Doppler History 36 2.2 Sampled Apertures 37 2.2.1 Real Arrays 38 2.2.2 Synthetic Arrays 39 2.3 Synthetic Aperture Resolution 39 2.3.1 Beam Compression 40 2.3.2 Doppler Processing 41 2.3.3 Close Target Separability 42 2.4 Ambiguities 44 2.4.1 Grating Lobe Suppression 44 2.4.2 Doppler Sampling 44 2.5 Area Mapping Rate 45 2.5.1 Multiple Horizontal Beams 46 2.5.2 Multiple Vertical Beams 46 2.5.3 Multiple Hydrophones 46 2.5.4 Living with Range Ambiguities 47 2.6 Broadband Methods 47 2.6.1 Smearing of Grating Lobes 48 2.7 Incoherent Synthetic Aperture Processing 49 2.8 Phase Errors and Motion Compensation 50 2.8.1 Motion Compensation 50 2.8.2 Passive Autofocusing 51 2.8.3 Effects of Motional Errors 51 2.8.4 Range Curvature 52 2.9 Summary 54 3 Continuous Transmission Frequency Modulation 57 3.1 The CTFM Signal 58 3.1.1 The CTFM Echo Signal 60 3.1.2 Ambiguity Function of the CTFM Waveform 60 ix 3.2 Demodulation of CTFM Echo Signals 62 3.2.1 Simple CTFM Demodulation 62 3.2.2 Quadrature Demodulation 64 3.2.3 Dual-Demodulation 65 3.2.4 The Effects of Target Motion on CTFM Signa] Processing 68 3.3 Spectral Analysis of Demodulated CTFM Signals 71 3.3.1 Range Resolution 71 3.3.2 Auditory Spectral Analysis 72 3.3.3 Analogue Spectral Analysis 72 3.3.4 Digital Spectral Analysis 72 3.4 CTFM Signal Generation 74 3.4.1 Analogue Methods 74 3.4.2 Digital Methods 75 3.5 Properties of CTFM Sonars 75 3.5.1 Maximum Range 75 3.5.2 Range and Frequency Equalisation 76 3.5.3 Phase Correction 76 3.5.4 Reverberation 77 3.5.5 Crosstalk 77 3.5.6 Beamwidth Considerations 78 3.6 Comparison of Pulsed and CTFM Sonars 78 4 The Prototype CTFM Synthetic Aperture Sonar 81 4.1 Electronic Design 81 4.1.1 The CTFM Sonar Front-End 81 4.1.2 Data Storage 84 4.1.3 Computer 85 4.2 Pre-Processing of the Raw Sonar Data 86 4.2.1 Chopping up the Sweep 86 4.2.2 Range Compression 88 4.2.3 Phase Response 88 4.2.4 Practical Considerations 89 4.3 CTFM Phase Monopulse 90 4.4 Summary 91 5 Measurement of the Acoustic Temporal Phase Stability 93 5.1 Background 93 5.2 The Effect of Noise on Phase Accuracy 94 5.3 Experimental Details 95 5.4 Experimental Results 97 5.4.1 Long Term Fluctuations 99 5.5 Summary 100 6 Simulation and Reconstruction 101 6.1 Simulation of a CTFM Synthetic Aperture Sonar 101 6.1.1 Projector Radiation Pattern 101 6.1.2 Hydrophone Response 103 x 6.1.3 The Effect of Sonar Displacement During Signal Propagation 103 6.1.4 The Simulation Algorithm 105 6.1.5 Approximations and Assumptions 106 6.2 CTFM Synthetic Aperture Reconstruction 106 6.2.1 Phase Correction 107 6.2.2 Azimuth Compression 107 6.2.3 Addition of Narrowband Images and Speckle Reduction 107 6.2.4 Narrowband Approximations 108 6.2.5 Algorithm Enhancements and Optimisations 109 6.3 Image Reconstructions from Simulated Data 110 6.3.1 Range Compression of Simulated Data 112 6.3.2 Azimuth Compression of Simulated Data 112 6.3.3 The Effects of Speed of Sound and Tow Speed Errors 119 6.4 Summary 119 7 Image Reconstructions from Sea Trials 125 7.1 Imaging the Test Target 125 7.2 Results of Image Reconstructions 126 7.3 The Effects of Speed of Sound and Tow Speed Errors 131 Summary 132 8 Conclusions and Recommendations for Future Research 137 8.1 Conclusions 137 8.2 Recommendations 139 A Specifications and Theoretical Performance 141 A.1 The Sonar Equation 141 A.I.1 Projector Source Level 141 A.1.2 Cavitation Limit 142 A.I.3 Pulse Compression Improvement 142 A.1.4 Noise 143 A.1.5 Target Strength 144 A.1.6 Signal to Noise Ratio 144 A.2 Signal to Reverberation Ratio 145 A.2.1 Volume Reverberation 145 A.2.2 Surface Reverberation 146 A.3 Noise Limited Range 147 A.4 CTFM Sonar Dynamic Range 148 B Sonar Parameters 149 C Image Reconstruction Parameters 151 References 153 Xl Preface The applications of side-scan sonar are diverse.
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