Advances in Sonar Technology Advances in Sonar Technology Edited by Sergio Rui Silva I-Tech IV Published by In-Teh In-Teh is Croatian branch of I-Tech Education and Publishing KG, Vienna, Austria. Abstracting and non-profit use of the material is permitted with credit to the source. Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published articles. Publisher assumes no responsibility liability for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained inside. After this work has been published by the In-Teh, authors have the right to republish it, in whole or part, in any publication of which they are an author or editor, and the make other personal use of the work. © 2009 In-teh www.in-teh.org Additional copies can be obtained from: publication@ars-journal.com First published February 2009 Printed in Croatia p. cm. ISBN 978-3-902613-48-6 1. Advances in Sonar Technology, Sergio Rui Silva Preface The demand to explore the largest and also one of the richest part of our planet, the advances in signal processing promoted by an exponential growth in computation power and a thorough study of sound propagation in the underwater realm, lead to remarkable advances in sonar technology in the last years. Since the use of imaging system that rely on electromagnetic waves (optical, laser or radar) is restricted to only very shallow water environments, and given that the good propagation of sound waves in water is known from at least the writings of Leonardo da Vinci, the sonar (sound navigation and raging) systems are the most widespread solution for underwater remote sensing. Sonar systems can be divided into two major types: passive sonar systems that enable detection of a sound emitting target and active sonar systems that use the properties of a signal reflected on the targets for its detection and image formation. As system complexity increases, the study of the way sound is used to obtain reflectivity and bathymetry data from targets and submersed areas becomes fundamental in the performance prediction and development of innovative sonar systems. Because of the many similarities between sonar and radar, algorithms created for the latter found application in sonar systems which made use of the advances in signal processing to overcome the barriers of the problematic underwater propagation medium and to challenge the resolution limits. In particular, synthetic aperture methods, applied with so much success in radar imagery, were adapted to sonar systems. This in turn enabled a considerable increase in sonar image quality and system robustness. Target detection developments lead to the use of multiple transducer sensors and sophisticated beam forming techniques with also excellent results. High quality sonar imagery with reduced noise and enhanced resolution enables more complex applications. Leaving the traditional real of military applications, sonar civil applications arise for the study of biology ecology and related fields. Moreover integration and data fusion of different sensors is becoming more and more common, being it navigation data integration and enhancement for synthetic aperture, sonar systems with different propagation characteristics or optical image integration for the improvement of object detection. But, not unlike natural evolution, a technology that matured in the underwater environments is now being used to solve problems for robots that use the echoes from air- acoustic signals to derive their sonar signals. The work on hand is a sum of knowledge of several authors that contributed in various different aspects of sonar technology. This book intends therefore to give a broad overview of the advances in sonar technology of the last years that resulted from the research effort of the authors in both sonar systems and its applications. It is destined to scientist and VI engineers from a variety of backgrounds and, hopefully, even those that never had contact with sonar technology before will find an easy entrance in the topics and principles exposed here. The editor would like to thank all authors for their contribution and all those people who directly or indirectly helped make this work possible, especially Vedran Kordic who was responsible for the coordination of this project. Editor Sergio Rui Silva University of Porto Contents Preface V Side-looking Sonar 1. Simulation and 3D Reconstruction of Side-looking Sonar Images 001 E. Coiras and J. Groen Synthetic Aperture Sonar 2. Synthetic Aperture Techniques for Sonar Systems 015 Sérgio Rui Silva, Sérgio Cunha, Aníbal Matos and Nuno Cruz 3. Motion Compensation in High Resolution Synthetic Aperture Sonar (SAS) Images 043 R. Heremans, Y. Dupont and M. Acheroy Sonar Image Enhancement 4. Ensemble Averaging and Resolution Enhancement of Digital Radar and Sonar Signals 075 Leiv Øyehaug and Roar Skartlien Sonar Detection and Analysis 5. Independent Component Analysis for Passive Sonar Signal Processing 091 Natanael Nunes de Moura, Eduardo Simas Filho and José Manoel de Seixas 6. From Statistical Detection to Decision Fusion: Detection of Underwater Mines in High Resolution SAS Images 111 Frédéric Maussang, Jocelyn Chanussot, Michèle Rombaut and Maud Amate Sonar Sensor Integration 7. Multi-Sonar Integration and the Advent of Senor Intelligence 151 Edward Thurman, James Riordan and Daniel Toal VIII 8. On the Benefits of Using Both Dual Frequency Side Scan Sonar and Optical Signatures for the Discrimination of Coral Reef Benthic Communities 165 Tim J Malthus and Evanthia Karpouzli Air-acoustics Sonar Systems 9. Outdoor Sonar Sensing 191 Fernando J. Álvarez Franco and Jesús Ureña Ureña 10. Mobile Robot Localization using Particle Filters and Sonar Sensors 213 Antoni Burguera, Yolanda González and Gabriel Oliver Side-looking Sonar [...]... dropped and replaced by a final image level scaling Computations can be performed directly in image space, removing also the need for the FFT when working in frequency domain, resulting in the following expression for the observed pixel intensity at surface point r: ˆ ˆ I ( r ) = K ( r ) ∑ ( n k ⋅ r ) Rk ( r ) χ k ( r ) (11 ) k Where the sonar is assumed at the coordinate origin, K is a normalization... testing and developing signal processing algorithms for sonar image analysis, such as object detectors and classifiers An example is illustrated in Fig 1, where a measured synthetic aperture sonar (SAS) image of a cylinder sitting on the seafloor and a simulated image of a similar object at the same range are shown 2 Advances in Sonar Technology Fig 1 (a) NURC’s test cylinder (b) Image of the cylinder... side-looking sonar (backscatter strength) are studied The characterization of this sonar imaging process can be used in two ways: by applying the forward image formation model, sonar images can be synthesized from a given 3D mesh; conversely, by inverting the image formation model, a 3D mesh can be estimated from a given side-looking sonar image The chapter is thus divided in two main parts, each discussing... Side-looking sonar imaging geometry (adapted from (Coiras 2007)) In order to model the scattering process we use the traditional Lambertian (Zhang 19 99) model already described in Eq 11 , which permits one to derive the returned intensity from the parameters defining the observed scene This simple model for diffuse scattering assumes that the returned intensity depends only on the local angle of incidence... MUSCLE’s synthetic aperture sonar (SAS) (c) 3D computer model of a cylinder and (d) its corresponding sonar image simulated with the SIGMAS model 2 .1 Sonar fundamentals The basic idea behind any sonar system is as follows: an acoustic signal (or ping) is emitted by the sonar into an area to be observed; the sonar then listens for echoes of the ping that have been produced when bouncing back from the objects... beam-pattern Φ Following (Coiras 2007), with the coordinate system centered at the sensor in o, the x axis being the across-track ground distance and y pointing along the sensor’s trajectory, we have: r = ( x, 0, Z ( x, y ) ) (13 ) ⎛ ∂Z ⎞ ∂Z n = ⎜ − ( x, y ) , − ( x, y ) ,1 ∂x ∂y ⎝ ⎠ Where the y coordinate in r is 0 because the side-scan sonar pulse Φ is shaped so that only the points contained in the x-z plane... 1 + ⎜ ⎟ ⎜ ⎝ ∂y ⎟ ⎟ ⎠ ⎠ ⎝ 2 (15 ) Where the explicit dependencies on (x, y) have been dropped for clarity 3.2 Sonar inversion Equation 14 provides a direct formula for estimating the returned intensity given the model parameters But the inverse problem—obtaining the model parameters from the observed 9 Simulation and 3D Reconstruction of Side-looking Sonar Images intensities—is clearly under-determined,... for sonar image modelling for frequencies much higher than ten kilohertz 4 Advances in Sonar Technology Ray tracing Ray tracing (Bell 19 97) is a method to calculate the path of acoustic waves through the system of water, sea bottom, sea surface and objects of interest When the sound speed cannot be assumed constant in the water column refraction of the rays results in bent rays focused to certain places... illuminating sound pulse, and not on the direction of observation or on the frequency of the pulse For the problem to be manageable the surface describing the observed scene has to be univalued, which forces to replace the expression in Eq 11 for the following simpler one: 8 Advances in Sonar Technology I (r ) = K Φ (r ) R (r ) n ⋅r = K Φ ( r ) R ( r ) cos (θ ( r ) ) n r (12 ) Where Φ represents the intensity.. .1 Simulation and 3D Reconstruction of Side-looking Sonar Images E Coiras and J Groen NATO Undersea Research Centre (NURC) Italy 1 Introduction Given the limited range and applicability of visual imaging systems in the underwater environment, sonar has been the preferred solution for the observation of the seabed since its inception in the 19 50s (Blondel 2002) The images . Advances in Sonar Technology Advances in Sonar Technology Edited by Sergio Rui Silva I-Tech IV Published by In- Teh In- Teh is Croatian. replaced by a final image level scaling. Computations can be performed directly in image space, removing also the need for the FFT when working in frequency domain, resulting in the following expression. of Underwater Mines in High Resolution SAS Images 11 1 Frédéric Maussang, Jocelyn Chanussot, Michèle Rombaut and Maud Amate Sonar Sensor Integration 7. Multi -Sonar Integration and