Synthetic Aperture Sonar Processing Research Articles

Synthetic aperture sonar applied to a multibeam volume imaging sonar

The possibility of using synthetic aperture sonar (SAS) processing techniques on a multibeam volume imaging sonar (the Buried Object Search Sonar, or BOSS) is explored. A system algorithm is developed which involves first forming several parallel synthetic apertures from a translated array oriented in the cross-track direction, then beamforming the synthetic apertures to form a fully 3D volumetric representation. The validity of the technique is verified by taking data from a subaperture of an experimental cable-guided multibeam system and applying the SAS processing string. Test targets are examined, including spheres and cylinders. The process proved quite robust, particularly considering that the system was not originally designed for SAS processing, and no sophisticated motion compensation schemes were employed. Implications for multibeam SAS processing of future AUV-based systems are discussed. [Work supported by ONR’s National Research Enterprise Internship Program, and performed at Naval Surface Warfare Center, Panama City, FL.]

Coherent backscatter simulation for synthetic aperture sonar

COSIM is a coherent backscatter simulator for generic high frequency sonars developed by QinetiQ. This model simulates acoustic backscattering in the time domain from various types of seafloor and targets, in various environmental conditions. The parameters of the model include pulse length and type, sonar characteristics, environmental properties concerning water column and sea surface and physical seafloor properties. In this paper we present descriptions of the various simulation stages, from the seabed generation to the signal simulation. Our main focus in thi paper is the introduction of spatial and temporal fluctuations within the water column and on the seabed and their detrimental effect on synthetic aperture sonar processing. Prior work on seabed clutter (Dugelay and Brothers, Proceedings Oceans 2002) and statistical analysis (Dugelay, Brothers and Heald, 142nd ASA Proceedings) is introduced into the seabed simulation module and initial results on the effect of seabed clutter density are presented. More particularly, the introduction of water column fluctuations is studied: the simulation of realistic fluctuations will be described and the effects on the backscattered signals will be analyzed. Finally, synthetic aperture sonar imaging will be shown in a variety of environmental cases and the image degradation will be compared to the environmental fluctuations.

Test results for a synthetic aperture sonar (SAS)

A special purpose synthetic aperture sonar (SAS) system has been developed and built by Hughes Aircraft Co. and Dynamics Technology, Inc. The main features of the system are a 3.2-m-long, 32-element linear array, and a data acquisition system capable of recording the elemental data in real time while preserving the phase and amplitude fidelity required by SAS processing. High-resolution, large (1002 m2) and small (32 m2) SAS images of various objects will be presented that demonstrate resolutions of 20 cm at 1000 m. The innovation of the work is that resolution achievable only with short-range, high-frequency sonar has been demonstrated at ranges obtainable only with a low-frequency sonar. [Work supported by DARPA.]

A synthetic aperture parametric sonar for buried object detection

This paper describes an experimental comparison between a parametric sonar and a conventional sonar for the detection of small objects on the seabed. First, a quick description of the parametric sonar used is given. Second, rail and sea experiments are described. The parametric array was mounted on a rail which was able to move at a constant speed (0.25 m/s). As a first experiment the sonar was kept fixed on the rail. A comparison of the detection of the object put on the seabed is performed between conventional sonar and the parametric one. The obtained results show a great improvement brought on by the parametric sonar which is less affected by bottom reverberation and which allows very precise identification of the detected object. As a second experiment, the sonar was moving on the rail in order to be able to perform synthetic aperture sonar processing with the parametric array. Significant improvements in resolution and signal-to-noise ratio were obtained when integrating together successive pings transmitted while the sonar was moving. In addition, a similar experiment performed at sea with a towed fish is described and results are given.

Real-time synthetic aperture sonar processing system

A real-time, shipborne synthetic aperture sonar processing system is presented. The system is based on an array of 49 transputers and is capable of synthetic-aperture processing and displaying a seabed swath of up to 700 m in real time. The limitations and advantages of synthetic aperture sonar imaging and bathymetry processing using correlation techniques are discussed. The results of trials carried out in the Mediterranean sea are presented. They show that the application of synthetic aperture processing to sonar systems is viable, providing good quality images of the seabed with a constant, high level of resolution in azimuth across the whole swath.

Alternate schemes in synthetic aperture sonar processing

Suboptimal processing schemes, the application of which is not widespread in synthetic aperture sonar processing, are described with reference to seafloor imaging. It is shown that their application can result in a significant increase of the azimuth resolution of the sonar system with respect to the resolution due to its physical beamwidth, without imposing unreasonable constraints on the sonar platform trajectory.