Towed array plane and non-plane wave beamforming under ship's maneuvering

P. Felisberto, S.M. Jesus and
Universidade do Algarve
Campus de Gambelas
PT-8000 Faro, Portugal

Comments: download pdf file .
Ref.: in 3rd European Conf. Underwater Acoustics (ISBN 960-85709-2-1),  p.483-488, Heraklion, Greece, 1996.

Abstract: Towed hydrophone arrays are commonly used for determining the spatial characteristics of the underwater acoustic field. The assumption that the hydrophones lie in a straight and horizontal line is often made when beamforming the hydrophone outputs. However, due to tow vessel motion, ocean swells and currents the array adopts a nonlinear shape and the beamformer output is degraded. In order to estimate the positions of the hydrophones, an array was instrumented with a set of positioning sensors: compasses, tiltmeters, accelerometers and pressure gauges. This paper presents the array deformations recorded at sea when the tow vessel is turning and along straight line tracks. The influence of the observed deformations on the performance of the conventional beamformer output is discussed and illustrated with simulated and real acoustic data. In particular it is shown that the sensor positioning information allows for introducing the appropriate corrections on the plane-wave beamformer and obtain an unambigous localization of sound sources in the 360-degree plane under heavy ship's maneuvering. Non-plane wave beamforming using a normal mode model shows that towed array deformations are also extremely important for source localization. In this case, the influence of towed array's deformation in the vertical plane is studied in particular, since this deformation is significant even when the array is being towed along straight line tracks.

ACKNOWLEDGMENT: this work was partially supported by the EU project MAS2-CT920022.