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Contribution to the development of wide-band signalprocessing techniques for new sonar technologies

Abstract : With the increase in the coastal activities, there is an increasing need for high-resolution bathymetry and sidescan imagery. Many types of acoustic measurement equipment are available to investigate the sea bottoms. Our main interest is in the study of interferometry techniques of measurement. The wide swath given by an interferometer makes it a desirable tool in shallow water environments, where time and cost of survey plays an important role. In this research work we aim to investigate the major quality degradation issues from the signal processing point of view, and we propose some ways to improve the current techniques.Using narrowband (single-frequency) acoustic signals requires a trade-off between range and resolution. A longer transmitted pulse contains higher energy; hence the intensity of far range backscattered echoes is also higher and distinguishable in ambient noise. However, a longer pulse also makes a bigger footprint (insonified area) on the sea bottom, and degrades resolution: the ability to measure the two closely placed targets (scatters on sea floor).The other problem lies with the ability to measure the depths accurately in a noisy environment. Depth measurement with interferometric technique is done by measuring the incident angle, which is calculated from phase difference measurements at receive array. So, any error in phase measurements results in inaccurate bathymetry. That makes it highly sensitive to all phase related noise sources. We give an uncertainty model from the signal processing point of view. Here we focus on different noise sources in the overall bathymetry degradation.We propose the use of wideband signals to overcome the problems introduced in this work. We start with solving the range-resolution trade off by doing an assessment of different signals, e.g. narrowband and wideband signals. Then in the second section, we propose wideband signals to overcome the previously introduced bathymetry degradation sources. We explore each noise source separately and compare the improvements in individual noise components given by wideband pulses. We conclude by assessing the overall improvement from using wideband signals, while introducing the limitations and issues with this approach.
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Jitendra Singh Sewada. Contribution to the development of wide-band signalprocessing techniques for new sonar technologies. Signal and Image Processing. Université Grenoble Alpes [2020-..], 2020. English. ⟨NNT : 2020GRALT003⟩. ⟨tel-02926986⟩

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