(Dr. Peter Rogers, advisor)

"The Determination of Far-Field Backscatter Using a Near-Field Calibration Array"

Abstract

Active sonar relies on underwater acoustic backscattering.  An object in the path of a submarine can be identified and located based on its backscattered directivity pattern.  Most often, the submarine is in the far-field of the scattering object, since the distance to the object is much larger than the size of the object.  In the attempt to study far-field scattering in the laboratory, one is often limited by a distance constraint provided by tank walls.  It is therefore desirable to extrapolate far-field scattered patterns from measurements made in the near-field of a scattering target.

The goal of this thesis is the design, development, and analysis of a near-field measurement system capable of obtaining near-field scattering data for synthesis of a far-field backscattered directivity pattern.  The main components of this measurement system are a near-field transmit array (NFTA), precision XY positioner, and receive line array.  The receive line array is used to form a cylindrical synthetic aperture near-field calibration array (NFCA).

Three scattering targets are used in this investigation:  a single sphere, two spheres at broadside incidence, and two spheres at head-on incidence.  Styrofoam spheres are used so that a pressure release boundary condition is approximated underwater, as this boundary condition can be modeled theoretically.  Both near-field and synthesized far-field backscatter results are compared to models.

Single sphere scattering results provide a measurement of the error associated with the near-field measurement system.  The single sphere target is also used to determine how closely Styrofoam approximates the pressure release boundary condition.  Two-sphere scattering targets give rise to complex backscatter directivity patterns in both the near-field and far-field.  Analysis of backscatter from the two-sphere targets provides an indication as to the ability of the near-field measurement system to synthesize a far-field with interference patterns.