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FIFTH ANNUAL
ASAP '97 WORKSHOP


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Reduced Wavenumber
Synthetic Aperture

Ira Ekhaus
Synap Corporation
100 Wildwood Avenue
Arlington, VA 02174
email: ekhaus@synapcorp.com

Abstract Underwater acoustic imaging applications such as mine countermeasures and side scan sea floor surveys would benefit from an operational synthetic aperture sonar system. While the potential of high resolution Synthetic Aperture (SA) Sonar designs to provide unprecedented detection and discrimination performance is clear, the real world issues of residual platform motion and temporal/spatial variations in sound speed (SSP) have prevented operational deployment of existing designs. High azimuthal resolution requires a synthesized aperture of lengths approaching ten thousand wavelengths. This imposes tight constraints on residual motion error and environmental scenarios where present designs could potentially operate. These problems have provided much incentive for research in autofocusing and motion compensation techniques as they apply to the sonar environment. Synap Corporation has developed the Reduced Wavenumber Synthetic Aperture (REWSA) algorithm/design based on an interferometric preprocessing stage operating on multiple element baseband receptions, and a unique inversion stage. The system specification is shown to develop a reduced wavenumber characteristic that is inherently robust to the effects of the sonar environment. The REWSA algorithm is shown to be a quadratic generalization of wavenumber decomposition SA techniques. Where traditional SA techniques can be cast as a 2-D Fourier inversion, REWSA can be interpreted as a 3-D Fourier inversion where two dimensions of the data set form a two spatial (and frequency) correlation function that are inverted to obtain a final image estimate of the reflectivity of the target field. Simulation of time records for a variety of target scenarios reinforce a discussion of spatial sampling and aperture requirements.

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