[Sonar, Underwater Acoustics and Signal Processing]

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Acknowledgement

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Table of Contents

CHAPTER 1: INTRODUCTION1

Sonar Imaging3

Imaging by backprojection4

Angular resolution5

Imaging techniques for phased arrays6

SAS sampling and coverage rate8

Undersampling of arrays9

Multi-element receiver arrays in SAS10

Design of Synthetic Aperture Sonar12

Resolution12

Frequency dependence14

Design parameters15

Challenges in SAS16

Navigation17

Topographic errors18

Ocean environment19

Vehicle stability19

Multipath environment19

Signal processing of SAS data20

Properties of SAS images22

Applications of SAS24

The HISAS 1030 interferometric SAS25

Underwater archaeology28

Search for small objects30

Summary31

REFERENCES32

Chapter 1: Introduction

SONAR is an acronym for Sound Navigation and Ranging. The basic principle of sonar is to use sound to detect or locate objects, typically in the ocean. Sonar technology is similar to other technologies such as: RADAR = Radio Detection and Ranging; ultrasound, which typically is used with higher frequencies in medical applications; and seismic processing, which typically uses lower frequencies in the sediments. There are many good books that cover the topic of sonar (Burdic, 1984; Lurton, 2010; Urick, 1983). There are also a large number of books that cover the theory of underwater acoustics more thoroughly (Brekhovskikh & Lysanov, 1982; Medwin & Clay, 1998).

The principle of Synthetic aperture sonar (SAS) is to combine successive pings coherently along a known track in order to increase the azimuth (along-track) resolution. A typical data collection geometry is illustrated in Fig. 1. SAS has the potential to produce high resolution images down to centimeter resolution up to hundreds of meters range. This makes SAS a suitable technique for imaging of the seafloor for applications such as search for small objects, imaging of wrecks, underwater archaeology and pipeline inspection. SAS has a very close resemblance with synthetic aperture radar (SAR). While SAS technology is maturing fast, it is still relatively new compared to SAR. There is a large amount of SAR literature (Carrara et al., 1995; Cumming & Wong, 2005; Curlander & McDonough, 1991; Franceschetti & Lanari, 1999; Jakowatz et al., 1996; Massonnet & Souyris, 2008).

This chapter gives an updated introduction to SAS. The intended reader is familiar with sonar but not SAS. The only difference between traditional sonar and synthetic aperture is the construction of the aperture (or array). We start by giving a review of sonar imaging and describe the backprojection method. Then, we derive the angular resolution. We describe the multi-element receiver concept and calculate the area coverage rate for SAS. We explain the frequency dependence in SAS, and discuss some of the choices and trade-offs in SAS design. We list some of the specific challenges in SAS, and suggest possible solutions. We give an overview of the signal processing involved in a SAS processor, and we discuss properties of SAS ...