Abstract

Sonars are essential for underwater sensing as they can operate over extended ranges and in poor visibility conditions. The use of a synthetic aperture is a popular approach to increase the resolution of sonars, i.e., the sonar with its N transducers is positioned at k places to generate a virtual sensor with transducers. The state of the art for synthetic aperture sonar (SAS) is strongly coupled to constraints, especially with respect to the trajectory of the placements and the need for good navigation data. In this article, we introduce an approach to SAS using registration of scans from single arrays, i.e., at individual poses of arbitrary trajectories, hence avoiding the need for navigation data of conventional SAS systems. The approach is introduced here for the near field using the coherent phase information of sonar scans. A Delay and Sum (D&S) beamformer (BF) is used, which directly operates on pixel/voxel form on a Cartesian grid supporting the registration. It is shown that this pixel/voxel-based registration and the coherent processing of several scans forming a synthetic aperture yields substantial improvements of the image resolution. The experimental evaluation is done with an advanced simulation tool generating realistic 2D sonar array data, i.e., with simulations of a linear 1D antenna reconstructing 2D images. For the image registration of the raw sonar scans, a robust implementation of a spectral method is presented. Furthermore, analyses with respect to the trajectories of the sensor locations are provided to remedy possible grating lobes due to the gaping positions of the transmitter devices.

Highlights

  • Sonar is an essential technology for underwater sensing

  • The following contributions are presented in this article: (a) the use of registration of single raw scans is proposed as a novel basis for Synthetic Aperture Sonar (SAS), (b) a suited algorithm for the registration is presented in the form of a spectral method based on our previous work, and (c) the concrete implementation of our new approach to synthetic aperture sonar (SAS) is completed with a Delay and Sum (D&S)

  • We introduced a new approach to coherent imaging for sonar, which uses the properties of a synthetic aperture of multiple sensor platform positions, i.e., the synthetic aperture is built from different sub-apertures of an antenna system

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Summary

Introduction

Sonar is an essential technology for underwater sensing. its spatial resolution depends on a combination of transducers to (roughly) approximate a sampling beam via interferences. In [19,20], for example, it is shown that a registration and alignment of sonar scan-data is possible even with severe interference and partial overlap between individual scans While this pairwise registration—which is the basis for the work presented in this article—is already very accurate, positional information over the entire aperture can even be further improved by Simultaneous Localization and Mapping (SLAM) [21]. The following contributions are presented in this article: (a) the use of registration of single raw scans is proposed as a novel basis for Synthetic Aperture Sonar (SAS), (b) a suited algorithm for the registration is presented in the form of a spectral method based on our previous work, and (c) the concrete implementation of our new approach to SAS is completed with a Delay and Sum (D&S).

Image Reconstruction
Wavefront Reconstruction
Coherent Image Superposition
Sampling the Aperture
Grating Lobes
Compressed Sensing for Aperture Gap Recovery
Pixel Based Scan Registration
Experiments and Results
Conclusions
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