Circular Synthetic Aperture Sonar Research Articles

Approximate extraction of late-time returns via morphological component analysis

A fundamental challenge in acoustic data processing is to separate a measured time series into relevant phenomenological components. In the setting of sensing elastic objects using active sonar, we wish to separate the early-time returns (e.g., returns from the object's exterior geometry) from late-time returns caused by elastic or compressional wave coupling. Under the framework of morphological component analysis (MCA), we compare two separation models using the short-duration and long-duration responses as a proxy for early-time and late-time returns. Results are computed for broadside geometries using Stanton's elastic cylinder model as well as experimental data taken from an in-air circular synthetic aperture sonar (AirSAS) system, whose separated time series are formed into imagery. We find that MCA can be used to separate early and late-time responses in both the analytic and experimental cases without the use of time-gating. The separation process is demonstrated to be robust to noise and compatible with AirSAS image reconstruction. The best separation results are obtained with a flexible, but computationally intensive, frame based signal model, while a faster Fourier Transform based method is shown to have competitive performance.

A simulation method on target strength and circular SAS imaging of X-rudder UUV including multiple acoustic scattering

Target strength (TS) and circular synthetic aperture sonar (CSAS) images provide essential information for active acoustic detection and recognition of non-cooperative unmanned undersea vehicles (UUVs), which pose a significant threat to underwater preset facilities. To access them, we propose an iterative physical acoustics (IPA)-based method to simulate the multiple acoustic scattered fields on rigid surfaces in high-frequency cases. It uses the Helmholtz integral equation with an appropriate Green's function in terms of the Neumann series, and then incorporates the ideas of triangulation and iteration into a numerical implementation. Then two approximate analytic formulae with precise physical meanings are derived to predict the TS and CSAS images of concave targets, respectively. There are no restrictions on the surface's curvature and the order of multiple scattering. The method is validated against the finite element method (FEM) for acoustic scattering from a sphere segment and against an experiment involving an X-rudder UUV's stern. On this basis, we simulate and analyze the TS and CSAS images of an X-rudder UUV. In addition, the influence of the angle of adjacent rudders on the multiple scattering characteristics is discussed. Results show that this method can potentially predict accurate UUV features, especially the multiple scattered features.

Three-Dimensional Imaging of Circular Array Synthetic Aperture Sonar for Unmanned Surface Vehicle

Synthetic aperture sonar (SAS) and interferometric synthetic aperture sonar (InSAS) have a range layover phenomenon during underwater observation, the AUV-mounted circular synthetic aperture sonar (CSAS) system, that insonifies targets using multiple circular scans that vary in height and can eliminate the layover phenomenon. However, this observation method is time-consuming and difficult to compensate. To solve this problem, the circular array synthetic aperture sonar (CASAS) based on the equivalent phase center was established for unmanned surface vehicles. Corresponding to the echo signal model of circular array synthetic aperture sonar, a novel three-dimensional imaging algorithm was derived. Firstly, the echo datacube was processed by signal calibration with near-field to far-field transformation and grid interpolation. Then, the sparse recover method was adopted to achieve the scattering coefficient in the height direction by sparse Bayesian learning. Thirdly, the Fourier slice theorem was adopted to obtain the 2D image of the ground plane. After the reconstruction of all height slice cells was accomplished, the final 3D image was obtained. Numerical simulations and experiments using the USV-mounted CASAS system were performed. The imaging results verify the effectiveness of the 3D imaging algorithm for the proposed model and validate the feasibility of CASAS applied in underwater target imaging and detection.

Spatially variant autofocus for circular synthetic aperture sonar.

Circular synthetic aperture sonar (CSAS) is a method for improving the resolution and target detection capabilities of a synthetic aperture sonar system. CSAS data are difficult to focus because of their large aperture sizes and elevation sensitivity. This difficulty has sometimes been addressed by using transponders or distributing isotropic scatterers in the field of view of the system; however, this comes at the cost of reduced practicality. As an alternative, map-drift based multipoint autofocus ("multilateration") was proposed by Cantalloube and Nahum [IEEE Trans. Geosci. Remote Sens. 49, 3730-37 (2011)] for autofocusing analogous circular synthetic aperture radar data. Multilateration also solves the problem of aberration spatial variance by providing a three-dimensional navigation correction. In circular synthetic aperture focusing problems, though, correcting aberrations is a joint navigation and elevation estimation problem, and the present work extends the multilateration approach to simultaneously solve both a navigation solution and coordinate corrections for the multilateration control patches. Additionally, methods for addressing the stability and behavior of the inverse problem are addressed, and an adaptive weighting scheme for reducing the influence of outliers is presented. The field results demonstrate near optimal point-spread functions on distributions of natural isotropic scatterers and robustness in regions with bathymetric variability.

Development of an in-air circular synthetic aperture sonar system as an educational tool

Synthetic aperture sonar (SAS) is an underwater acoustic imaging technique that is used to generate high quality imagery through the coherent combination of signals measured on a moving platform. The practical challenges of underwater experimentation and the lack of publicly available data sets make it difficult to teach SAS processing to students using experimental data. These challenges are compounded by the complicated signal processing that is required to compensate for the unwanted platform motion that is common in field data. This paper details the development of an in-air circular SAS system using commonly available laboratory equipment. By operating in air and using a circular geometry many of the challenges that students would otherwise encounter in SAS processing are either well controlled or completely eliminated. Imagery collected on the system is shown to demonstrate how meaningful data can be generated using a simple set of equipment. A number of applications of the system as a tool to teach array signal processing, pulse compression, and imaging concepts are also discussed.

Circular synthetic aperture sonar image resolution theory

The introduction of SAS (Synthetic Aperture Sonar) systems has been a game changer for underwater surveys. The gain in resolution, compared to traditional sidescan systems, created a paradigm shift as the information contained in a SAS image switches from shadows to highlights. SAS systems traditionally perform lawnmower type surveys, but the need for multiple views in MCM (Mine-Counter Measure) tasks, for example, opened the interesting problem of target re-acquisition patterns. In particular, circular patterns maximize the aperture, thus the overall image resolution of such system. The capability of CSAS (Circular SAS) has been demonstrated on the field, but the derivation of CSAS processing has not been fully developed. The non-uniform sampling of the circular pattern in particular introduces aberrations within the field of view and a non uniform PSF (Point Spread Function). In this talk, we propose a new spatial sampling scheme which makes the CSAS PSF perfectly uniform. The theoretical closed form solution of the PSF is then derived both in time and Fourier domain. The PSF derivation naturally leads to redefine the image resolution as an energy leakage problem. Thanks to the new sampling scheme and the uniform PSF, we also propose a deconvolution method based on atom waves which increases the CSAS resolution.

Increasing circular synthetic aperture sonar resolution via adapted wave atoms deconvolution.

Circular Synthetic Aperture Sonar (CSAS) processing computes coherently Synthetic Aperture Sonar (SAS) data acquired along a circular trajectory. This approach has a number of advantages, in particular it maximises the aperture length of a SAS system, producing very high resolution sonar images. CSAS image reconstruction using back-projection algorithms, however, introduces a dissymmetry in the impulse response, as the imaged point moves away from the centre of the acquisition circle. This paper proposes a sampling scheme for the CSAS image reconstruction which allows every point, within the full field of view of the system, to be considered as the centre of a virtual CSAS acquisition scheme. As a direct consequence of using the proposed resampling scheme, the point spread function (PSF) is uniform for the full CSAS image. Closed form solutions for the CSAS PSF are derived analytically, both in the image and the Fourier domain. The thorough knowledge of the PSF leads naturally to the proposed adapted atom waves basis for CSAS image decomposition. The atom wave deconvolution is successfully applied to simulated data, increasing the image resolution by reducing the PSF energy leakage.

Sonar image reconstruction of objects near a reflective boundary

The properties of reconstructed acoustic images, created from data obtained using backscattering synthetic aperture sonar (SAS) systems, have been studied for objects in the free field. The presence of a nearby reflecting boundary, such as the seafloor or surface, greatly increases the number of mechanisms or raypaths by which sound may be backscattered to the sonar system. Some of these raypaths involve one or more interactions with the reflecting boundary (multipaths), and others involve sound scattering multiple times from the same target (multiple scattering). Experiments were conducted using an air-water interface and simple cylindrical targets in order to examine the properties of the reconstructed circular SAS (CSAS) images, including effects due to vertical obliquity of the target [D. Plotnick et al., J. Acoust. Soc. Am. 137, 470-480 (2015)]. The CSAS image is by definition a two-dimensional representation of three-dimensional scattering; the image is thus highly dependent on the object’s 3-D shap...

Circular synthetic aperture sonar images and aspect dependent spectral properties of solid elastic cubes

While cubes may not be commonly found underwater, studying their aspect and material-dependent backscattering mechanisms may be useful in leads to understanding the scattering physics of other objects. Two solid cubes, made of either steel and or brass, were insonified in water using a circular synthetic aperture sonar (CSAS) system; Fourier based techniques were used to reconstruct target images. Identifiable backscattering mechanisms include edge-diffraction and elastic responses. Rayleigh waves, a class of surface elastic waves, are excited near a material specific incident angle (Rayleigh angle) that depends on the material of the cube. Those waves may retroreflect from the edges of the cube’s facee edges of the cube’s face in such a way as to enhance backscattering [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 105, 700-710 (1998)]. The steel cube displayed a strong Rayleigh wave response, easily visible in CSAS images; for the smaller brass cube that said mechanism was relatively subtle. Aspect dependent spectral properties can bewere extracted from the images or directly from the time-domain records. When the cube’s top ridge is slanted vertically, an aspect angle dependent splitting in the low frequency spectrum can bewas observed and associated with edge diffraction. The experimental results were compared to Kirchhoff-Integration based simulations, which did notthat don’t include elastic responses. [Work supported by ONR.]

Volumetric Acoustic Imaging via Circular Multipass Aperture Synthesis

In this paper, volumetric imaging via multipass circular synthetic aperture sonar (CSAS) is demonstrated using an autonomous underwater vehicle (AUV). A multidimensional aperture is synthesized by performing a series of circular scans at varying grazing angles around targets and coherently combining the backscattering information from the set of scans to form high-resolution volumetric images. A data-driven technique for precision alignment of the individual scans comprising the multipass set enables synthesis of a multidimensional array. To beamform in the vertical dimension using the irregular and undersampled multipass aperture, a compressive-sensing-based approach is adopted which is similar to methods used in analogous synthetic aperture radar tomography applications but modified to accommodate for the wider fractional bandwidth of the synthetic aperture sonar (SAS) system. The modification exploits a joint sparsity assumption in the vertical scattering profile at different subbands and adapts a standard joint sparse solving algorithm to the relevant case in which the sparsity profile is common between solution vectors but the sensing matrices are different. Results are shown for a variety of targets, including proud and obliquely buried unexploded ordnance, a 2-1 solid aluminum cylinder, and a steel oil drum.

Autofocusing circular synthetic aperture sonar imagery using phase corrections modeled as generalized cones.

Circular synthetic aperture sonar (CSAS) is a coherent aperture synthesis technique that utilizes backscattered acoustic information from an encircled scene to generate information rich, high-resolution imagery. The aperture length required for image synthesis is much longer than in its linear synthetic aperture sonar counterpart and can result in challenging phase delay and navigation estimation constraints. Residual uncorrected phase errors manifest as focus aberrations in reconstructed CSAS imagery. This paper demonstrates that phase error in image patches can be approximated as an aspect variant linear phase shift representable as a generalized cone in wave-number space. If the geometry of the generalized cone is known, it can be applied as the spectral phase of an inverse filter for aberration correction. A method is derived for reconstructing the error cone geometry from independent estimates of its local curvatures, which are found via a series of one-dimensional line searches that maximize the focus of CSAS sub-aperture images. This approach is applied to real and simulated CSAS data containing aperture distortions, and the results successfully demonstrate estimation and correction of the underlying focus aberrations.

Fast nearfield to farfield conversion algorithm for circular synthetic aperture sonar.

Monostatic circular synthetic aperture sonar (CSAS) images are formed by processing azimuthal angle dependent backscattering from a target at a fixed distance from a collocated source/receiver. Typical CSAS imaging algorithms [Ferguson and Wyber, J. Acoust. Soc. Am. 117, 2915-2928 (2005)] assume scattering data are taken in the farfield. Experimental constraints may make farfield measurements impractical and thus require objects to be scanned in the nearfield. Left uncorrected this results in distortions of the target image and in the angular dependence of features. A fast approximate Hankel function based algorithm is presented to convert nearfield data to the farfield. Images and spectrograms of an extended target are compared for both cases.

Fast nearfield to farfield conversion for circular synthetic aperture sonar

Monostatic circular synthetic aperture sonar (CSAS) images are formed by processing azimuthal angle dependent backscattering from a target at a fixed distance from a collocated source/receiver. In the laboratory data is taken by fixing the source location and spinning the target via a rotating mount. Typical CSAS imaging algorithms [Marston et al., Proc. IEEE Oceans (2011); Ferguson et al., J. Acoust. Soc. Am. 117, 2915 (2005)] assume the scattering data are taken in the farfield. Experimental constraints may make farfield measurements impractical and thus require a target to be scanned in the nearfield. If left uncorrected, this results in distortions of the target image and possible distortions of the angular dependence of features. A fast approximate Hankel function based algorithm is presented that converts nearfield data to farfield data. Images and spectrograms of an extended target are compared for both cases. Spatial sampling requirements for data collection are also considered. [Work supported by ONR.]

Imaging objects with variable offset in an evanescent wave field using circular synthetic aperture sonar and spectroscopy

Imaging properties of objects suspended in an acoustic evanescent wave field are examined. Evanescent waves are generated using a tank containing immiscible liquids and an appropriately directed acoustic beam [C. F. Osterhoudt et al., IEEE J. Oceanic Eng. 33, 397-404 (2008)]. The source and receiver transducers are in the liquid having the higher sound velocity. Object(s) are spun about a vertical axis while scattering is measured. The object(s) offset into the wave field is then varied and the experiment repeated. In this work small spheres and other objects are used to gain insight into imaging properties as a function of the object or object(s) position in the evanescent field. Data is examined using circular synthetic aperture techniques. Additionally, a comparison is made between spectral data and a heuristic model in the case of two spheres. The spectral evolution in that case is affected by the interference from the two scatterers. Cases where the source and receiver are collocated (monostatic) and those where the source and receiver are separated (bistatic) are compared. [Work supported by ONR.]

Bistatic scattering by scaled solid metallic objects: Circular line-scan measurements and modeling

Multistatic sonar systems can be used to obtain target scattering information that cannot be measured monostatically. This information has potential benefits for detection and classification schemes. Bistatic and monostatic scattering measurements have been performed simultaneously on several scaled targets using the circular line-scan system of the small scale test bed facility at NSWC PCD. Water tank measurements were made for targets in the free-field and resting proud on or buried in simulated scaled sediment composed of spherical glass beads. Targets were placed in the center of the circular scan line. Circular synthetic aperture sonar and frequency-aspect target strength (acoustic color) results will be presented for a solid steel sphere, solid aluminum cone, and solid aluminum cylinders. Results for cylinders and the sphere are compared with T-matrix simulations to facilitate interpretation of features observed. Comparisons between back and forward scattering results demonstrate particularly strong features for each target in the forward scattering direction. Forward scattering by the sphere and cylinders also exhibits responses that arrive earlier in time than sound that travels directly from the source to the receiver. In the case of the sphere, this is attributed to elastic target responses involving leaky Rayleigh waves. [Research supported by ONR.]

Scattering by square and rectangular metallic cylinders: Circular synthetic aperture sonar images and spectral coupling loci.

Reversible circular synthetic aperture sonar (CSAS) is one approach to displaying and studying the evolution of monostatic sonar data with changes in the orientation of the viewed object. The present investigation concerns the evolution of backscattering by a solid slender square brass cylinder and a rectangular brass cylinder, both objects being examples of a horizontal rectangular parallelepiped elastic waveguide. In both cases, substantial elastic glints are visible in CSAS images for orientations suitable for generating surface guided elastic waves. The square cylinder is also useful for examining the effect of changing the azimuthal orientation relative to the cylinder’s axis and the direction of the horizontal illumination. While rotating the azimuthal angle by 45 deg greatly modifies the magnitude of the backscattering, some of the frequency domain coupling loci (functions of rotation of the cylinder’s axis about a vertical axis) are in common for both orientations. After azimuthal rotation, an apex line of the cylinder is viewed by the transducer instead of the cylinder’s long flat side. This kind of spectral investigation is relevant to characterizing the coupling loci for elastic waveguides lacking the rotational symmetry of a circular cylinder. Additional insight is obtained from the CSAS images. [Work supported by ONR.]

Comparisons of evanescent wave and propagating wave scattering based on circular synthetic aperture sonar and spectroscopy.

A convenient way to generate acoustic evanescent waves is to use a tank containing immiscible liquids and an appropriately directed acoustic beam [Osterhoudt et al., IEEE J. Oceanic Eng. 33, 397–404 (2008)]. This is useful for exploring the nature of scattering induced by evanescent acoustic waves by suspending an object (or set of objects) close to the horizontal interface between the two liquids. In the present work the scattering is studied for a pair of metal spheres and other metallic objects using reversible circular synthetic aperture sonar in which the object (or set of objects) is spun around a vertical axis while the scattering is measured. In some cases it is also possible to investigate spectral features and make comparisons with measurements using ordinary propagating waves or the interference pattern produced by sets of such incident waves. These experiments are useful for gaining insight into scattering phenomena and signal processing options. [Work supported by ONR.]

Circular synthetic aperture sonar results from autonomous underwater vehicle trials

The principle of synthetic aperture sonar (SAS) is to combine successive pings coherently from different observation angles in order to increase the azimuth resolution. By collecting data along a circular track, a circular SAS (CSAS) image, or tomographic image, can be made. The image reconstruction can be done either incoherently, fully coherently, or partially coherently where each subaperture of coherent processing consists of a section of the circle. In target classification, CSAS has several benefits: the object is observed from all aspect angles giving a better perception; the resolution in the image increases. In this paper, we calculate required accuracy in navigation, bathymetry and sound velocity for successful circular SAS. Finally, we show circular SAS images of small targets from real data collected by the HUGIN autonomous underwater vehicle carrying the Kongsberg HISAS 1030. We test different beamforming strategies, and show the effect of coherent and incoherent tomographic imaging.