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Towed array geometry estimation during ship's maneuvering

S.M. Jesus, P. Felisberto sjesus@ualg.pt , pfelis@ualg.pt
SiPLAB-FCT, Universidade do Algarve
PT-8000 Faro, Portugal
E. Ferreira Coelho oceanografia@hidrografico.pt
Instituto Hidrografico
PT-1000 Lisboa, Portugal

Comments: download pdf file .
Ref.: in Journal of the Acoustical Society of America,Vol. 96, No. 5, Pt. 2,  p.3312,  1994.

Abstract : Towed arrays of hydrophones are commonly used as receiving apparatus for determining the directionality of the underwater acoustic field. It's well known that a line array beam response has an inherent left-right ambiguity and that any deformation of the array will produce a distortion on the estimated acoustic field directionality. In particular, the array can not be operated during ship's maneuvering which is a potential drawback to its operational usage.In theory, if the array is not straight but the hydrophone's position are known at each time instant, the beamformer could be compensated in order to obtain a corrected beam response. More, depending on the array shape, the left-right ambiguity could also be resolved. In practice, it is extremely difficult to obtain suficientely accurate measurements of the hydrophone positions. This paper presents the measurements obtained at sea, with a 156 m aperture array, instrumented with several high precision tiltmeters, compasses, depth sensors and accelerometers. After filtering and preliminary analysis of the sensor position data it is concluded that the array is never a straight horizontal line. The array has approximately a catenary shape with vertical deviations at the tail up to 15 m at constant tow speed. Under maneuvering, the array is largely deformated and a consistent shape could be estimated on real time from the non position sensors. The use of the estimated geometry for acoustic data processing, shows that consistent beam responses (close to theoretical) could be obtained even under strong array distortion. It is also shown, with real data, that the knowledge of array geometry significantely improves full-field matching for source localization and/or bottom characterization.

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