Analyses of the statistical characteristics of bottom backscatter, measured in shallow water off San Diego, California, are presented. (The initial results of the experiment were presented by T. G. Goldsberry, S. P. Pitt, and R. A. Lamb at the 104th Meeting of the Acoustical Society of America, Orlando, FL, 8–12 November 1983.) The experimental sonar, mounted on the sea bottom, was operated at 30 kHz to gather data over a wide sector of the bottom. The bottom was patches of coarse and fine sand. The distribution function and probability of false alarm function of the detected envelope of widebeam and narrowbeam signals were measured. Some spatial and temporal correlation functions of the signal amplitudes were measured. A limited attempt was made to compare the results with existing theoretical models.
Methods of developing real and complex interpolatory representations of a bandlimited signal in which the quadrature components of the signal are explicitly exhibited are presented. Applied to the case of a bandpass signal, the real formulation is a new interpolatory result that has useful computational properties. Exact and approximate envelope and phase calculations are used to demonstrate some of these properties. In a similar application, the complex formulation gives the well-known result of Goldman and Woodward.
Acoustic backscattering measurements on a sand bottom were made at grazing angles in the range of about 2°–10° in water depth of approximately 15.5 m near San Diego, CA [T. G. Goldsberry, S. P. Pitt, and R. A. Lamb, J. Acoust. Soc. Am. Suppl. 1 72, S74 (1982)]. Data from these measurements have been analyzed to determine the mean value and standard deviation of the bottom backscattering strength per m2 as a function of grazing angle, insonified area, transmit signal type, and frequency. A curved ray path propagation model and measured sound speed profiles were used to determine grazing angle versus time. The mean value followed Lambert's law for the range of grazing angles measured and for all frequencies used. No significant differences in mean value were observed when the insonified area and transmit signal type were varied. The observed frequency dependence of the bottom backscattering strength per m2 falls in the range from f1.5 to f1.8 for this relatively flat, sandy bottom. [Work supported by NAVSEA 63R and NORDA Code 113.]
Acoustic backscattering measurements on a sand bottom were made at grazing angles in the range of about 2°–10° in water depth of approximately 15.5 m near San Diego, California (reported by T. G. Goldsberry, S. P. Pitt, and R. A. Lamb, 104th Meeting of the Acoustical Society of America, Orlando, FL, 8–12 November 1982). Data from these measurements have been analyzed to determine the mean value and standard deviation of the bottom backscattering strength per square meter as a function of grazing angle, insonified area, transmit signal type, and frequency. A curved ray path proportional model and measured sound speed profiles were used to determine grazing angle versus time. The mean value followed Lambert’s law for the range of grazing angles measured and for all frequencies used. No significant differences in mean value were observed when the insonified area and transmit signal type were varied. The observed frequency dependence of the bottom backscattering strength per square meter falls in the range from f 1.0 to f 1.5 for this relatively flat, sandy bottom.
This paper describes the design, implementation, and performance of a digital beamformer conceived in the late 1960's to replace the mechanical video scanning switch found in many sonar systems of that time. The design goals included preserving the wide dynamic range typical of high powered sonars, increasing the scanning rate to be commensurate with system bandwidth, and providing sonar data in digital form for studies of postbeamforming processing. In Sec. I, a digital beamformer which uses geometric quantization of quadrature sampled stave data is described, and its properties are obtained via computer simulation for a circular array geometry. The combination of digital phase shift beamforming with circular array geometry leads naturally to a serial implementation, which amounts to a "finite impulse response (FIR)" filter, to obtain a scanned beam output. One version of this implementation was built and tested. Brief descriptions of the hardware and its measured properties are given in Sec. II.
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