Pseudo-Random Codes for Single-Mode and Simultaneous Multi-Mode Operation in Ultrasonic Imaging Systems

Conventional pulse-echo imaging systems used in ultrasonics can become limited in average transmit power by transmitter, transducer, and medium peak-power limitations. In addition, imaging systems which use multi-element arrays are limited in speed by the necessity to transmit sequentially when scanning in more than one direction in order to avoid interfering echoes. A new system is studied which can overcome both the speed and power limitations by using correlation receivers and pseudo-random transmit codes. First, the performance of several single-mode correlation systems are compared to conventional pulse-echo systems in the presence of clutter and moving targets. The system which uses special pseudo-random codes called Golay codes is shown to provide the best overall performance. A multi-mode correlation system is then studied which images in many different modes (e.g. scan directions) simultaneously. This multi-mode system is studied under the effects of moving targets, clutter and background receiver noise. A comparison with the operation of Conventional sequentially-scanned phased array systems is made under a variety of signal-to-noise ratio (SNR) conditions and operating speeds to determine the optimal type of imaging system. Results indicate that under many conditions, a simultaneous multi-mode system can provide improved SNR and/or speed over conventional sequential multi-mode systems. The multi-mode system which uses Golay codes is shown to provide the best overall performanc

Bistatic sonar and a novel form of variable depth sonar

This thesis relates to the sonar system research undertaken for a Naval Requirement for proposals to improve the cost effectiveness of the defence of shipping against submarine attacks. Defence systems evolved as a function of the developing technology of the opposition which in this instance is the ability of a submarine to remain undetected below the sea surface while searching for, tracking and attacking its targets. An inherent problem for underwater detection with Escort Ships hull type sonars is its location on the air-sea interface with the need of a two-way propagation path to access the depth-range volume available to a submarine. As the power of an Escort's sonar is increased so is the size of ship, 5000 tons and more, to accommodate the optimum size of transducers required.Sonar system research at all times is a multi-discipline task and in this particular case was further broadened with a requirement to review the possibilities for sources of energy other than underwater acoustics. The research confirms the dominance of sonar for underwater detection and establishes the feasibility of a Bistatic Sonar concept which replaces the two-way propagation path of a hull type sonar with a one-way path, source-target-receiver with a variable depth directive towed line receiver on a small ship as a distant receiver. A second objective which became feasible with the development of an adequate towed source was a variable depth sonar which has now been produced by British Aerospace for a world market. A summary of the thesis is provided as an introduction to the subject matter of the different sections

Simulation Method for the Development of Mulit-Beam Echo Sounder

Seafloor mapping using acoustic remote sensing technique is an attractive approach due to its high coverage capabilities and limited costs. The Multi-Beam Echo-Sounder (MBES) system provides high-resolution bathymetry and backscatter information with relatively perfect coverage. This paper described how the MBES system works and with the objective to get simulation images of it. The main theory of this thesis is sonar equation. The backscatter target strength is what we really needed. We present a simulation of seafloor mapping through backscatter data per beam, and because its performance is sensitive to seafloor type variation along the MBES swath so corrections for the angular dependence are applied. This simulation can support a wide range of survey applications including: hydrography, search and locate, route survey and seafloor characterization. Also, a further discussion such as the doppler effect and sound speed correction should be discussed in the future.Contents i List of Figures iii Abstract v 1. Introduction 1 1.1 Background 1 1.2 Component of This Thesis 4 2. The System Composition and Theoretical Foundation 5 2.1 System Composition 7 2.1.1 Conversion 7 2.1.2 Signal Projection and Reception 8 2.1.3 Application of MBES System 8 2.2 Acoustic Principle of MBES 10 2.2.1 Sonar Equation 10 2.2.2 Beamforming 11 2.2.3 Beam Steering 15 2.3 The Operational Principle of MBES 19 2.3.1 Operational Principle 19 2.3.2 Mainly Measurement Error and Solution 22 3. The Design and Performance Analysis 24 3.1 Signal Selection 24 3.2 Backscattering Data Processing 26 3.1.1 Grazing Angle correction 26 3.1.2 Lambert's Law 28 3.3 Simulation and Results 30 4. Conclusions 32 References 3

Index to NASA Tech Briefs, 1975

This index contains abstracts and four indexes--subject, personal author, originating Center, and Tech Brief number--for 1975 Tech Briefs

Advances in Sonar Technology

The demand to explore the largest and also one of the richest parts 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, have lead to remarkable advances in sonar technology in the last years.The work on hand is a sum of knowledge of several authors who contributed in various aspects of sonar technology. This book intends 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 their applications. It is intended for scientist and engineers from a variety of backgrounds and even those that never had contact with sonar technology before will find an easy introduction with the topics and principles exposed here

The development and testing of a parametric SONAR system for use in sediment classification and the detection of buried objects

This thesis describes the work carried out in the development and testing of parametric sonar systems for application in the fields of seabed sediment characterisation and classification, and the detection of seabed embedded objects. Parametric sonar systems offer a number of advantages over conventional sonar systems. This is especially true of the conflicting requirements of both seabed delineation and penetration required for a practical sub-seabed profiling system. Echoes from sub-bottom layers vary in strength dependent on both the boundary acoustic reflectivity and the absorption characteristics of the layer above. Absorption effects are usually frequency dependent, allowing better penetration to lower frequency signals. [Continues.

Investigating the build-up of precedence effect using reflection masking

The auditory processing level involved in the build‐up of precedence [Freyman et al., J. Acoust. Soc. Am. 90, 874–884 (1991)] has been investigated here by employing reflection masked threshold (RMT) techniques. Given that RMT techniques are generally assumed to address lower levels of the auditory signal processing, such an approach represents a bottom‐up approach to the buildup of precedence. Three conditioner configurations measuring a possible buildup of reflection suppression were compared to the baseline RMT for four reflection delays ranging from 2.5–15 ms. No buildup of reflection suppression was observed for any of the conditioner configurations. Buildup of template (decrease in RMT for two of the conditioners), on the other hand, was found to be delay dependent. For five of six listeners, with reflection delay=2.5 and 15 ms, RMT decreased relative to the baseline. For 5‐ and 10‐ms delay, no change in threshold was observed. It is concluded that the low‐level auditory processing involved in RMT is not sufficient to realize a buildup of reflection suppression. This confirms suggestions that higher level processing is involved in PE buildup. The observed enhancement of reflection detection (RMT) may contribute to active suppression at higher processing levels