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Radar, Sonar and Navigation, IEE Proceedings -

Issue 3 • Date June 1996

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Displaying Results 1 - 12 of 12
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  • Phased array acoustic imaging in ground co-ordinates, with extension to synthetic aperture processing

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    The authors describe a digital implementation of time delay sonar imaging for linear swept frequency (chirp) sources. The technique, based on phase shifting of match filtered individual transducer echoes is a direct equivalent of analogue `phased array' processing. While the technique can be used for forward, side, or downward looking sonar, the authors concentrate on side scan imaging in a ground stabilised frame of reference. This allows the sonar record to be generated in the survey coordinate system without rectification or scan conversion, Computing and sampling costs are minimised by sparse population of the receiver aperture, using a separate transmission array with controlled beamwidth to suppress grating lobes. Synthetic aperture processing uses essentially the same computation, and is carried out for a wideband signal without frequency decomposition. When different arrays are used for transmission and reception, azimuthal resolution depends on transmission array length, but maximum movement between pings without aliassing is set by the length of the receiver array, so high resolution is compatible with fast survey. As receiver aperture length increases, the resolution of the physical aperture images which combine to form the synthetic image also increases, and self-registration of these images to reduce positioning requirements becomes interesting. This possibility is discussed briefly View full abstract»

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  • Rapid calculations of acoustic propagation loss in range dependent oceans

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    Calculation of transmission loss in range-dependent ocean environments usually involves CPU-intensive models that offer little insight into the problem. The authors present a new approach based on hybrid ray and mode concepts. A set of incoherent intensity formulas is given for different acoustic components of the propagation. The key points are the conservation of energy flux and the exploitation of the ray invariant to model weak range dependence. Predictions from these formulas are compared with benchmark solutions to demonstrate both their accuracy and the insight provided by the individual components View full abstract»

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  • Seabed classification through multifractal analysis of sidescan sonar imagery

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    The authors present a technique for the classification and analysis of seabed sediments from sidescan sonar images, the origins of which lie in the body of fractal theory. Six seabed types were analysed, namely clay, mud, sand, gravel, stones and rock. These data sets have previously been analysed by several authors who have used techniques based on the power spectrum. The method proposed in the paper allows frequency information to be obtained but without the use of large windows which are generally required for frequency domain measurements. Results are presented for the classification of individual ground truthed sediments and the segmentation of composite images containing these sediments. Correct classification rates of greater then 99% have been obtained and good segmentation accuracy achieved View full abstract»

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  • Side scan sonar using phased arrays for high resolution imaging and wide swath bathymetry

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    The design of a new bathymetric sidescan sonar dedicated to the survey of the continental shelf is described. It uses linear phased arrays in order to perform dynamically the control of the resolution through the focusing of the beams, and beam steering to compensate for yaw and pitch. The 4 m long array, working at 112 kHz, yields high resolution imaging. In addition, an interferometric doublet, with a 2λ spacing, provides a wide swath bathymetric capability. Both functions take advantage of a fast digital pulse compression process. Results of sea trials with the hull mounted system (high resolution images and topographic maps) are discussed View full abstract»

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  • Signal and reverberation prediction for active sonar by adding acoustic components

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    A practical, versatile approach to sonar performance prediction is presented based on addition of acoustic components. Often reverberation is written as a decibel sum of a propagation term and a scattering term. This separation is not possible when back-scattering strength is angle-dependent because the reverberation is an integral over angle. For this reason a linear form of the reverberation sonar equation is used. Examples are presented for a typical deep water problem including a calculation against frequency showing an optimum detection frequency which varies with the target range View full abstract»

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  • Efficient digital signal processing algorithm for sonar imaging

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    An algorithm is presented which can be efficiently applied to a variety of sonar imaging modes including forward looking, side-looking and synthetic aperture sonar, The digital focused beamforming (DFB) algorithm for pixel based imaging utilises key attributes of both time and frequency domain techniques. DFB for forward looking imaging is described in detail and adaptation of the basic concept in application to the other imaging modes is outlined. Examples of both simulated and real images are presented View full abstract»

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  • Acoustic correlation sonar for vertical profiling of ocean currents to a range of 1 km

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    Ocean current profilers, based on spatial or temporal correlation analysis of sound backscattered from passive targets drifting with the current, have several potential advantages over the more common Doppler profilers. Correlation profilers can operate over a longer range with smaller, lighter and cheaper transducers. However, the signal processing is more complex and computationally intensive, increasing transmission intervals and leading to difficulties understanding when and why the performance is poor. Using data gathered from RRS Discovery in early 1995 the authors describe the performance of a 22 kHz acoustic correlation current profiler and compare its results with a lowered acoustic Doppler profiler. At low ship speeds, the signal to noise ratio was sufficient to allow profiling to 1 km depth, with the achievable range reducing as speed increased. Bias and decorrelation occurred in the upper 160 m owing to unknown causes and greatly reduced data quality View full abstract»

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  • Real-time synthetic aperture sonar processing system

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    A real-time, shipborne synthetic aperture sonar processing system is presented. The system is based on an array of 49 transputers and is capable of synthetic-aperture processing and displaying a seabed swath of up to 700 m in real time. The limitations and advantages of synthetic aperture sonar imaging and bathymetry processing using correlation techniques are discussed. The results of trials carried out in the Mediterranean Sea are presented. They show that the application of synthetic aperture processing to sonar systems is viable, providing good quality images of the seabed with a constant, high level of resolution in azimuth across the whole swath View full abstract»

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  • Towed-array beamforming during ship's manoeuvring

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    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. To estimate the positions of the hydrophones an array was instrumented with a set of positioning sensors: compasses, tiltmeters, accelerometers and pressure gauges. The authors present 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 View full abstract»

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  • Detection and tracking of returns in sector-scan sonar image sequences

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    The authors report the development of algorithms for the detection and tracking of object returns in noisy sector-scan sonar image sequences. Static objects are first removed using spatial and frequency domain filtering. The optical flow of the resulting images (containing only dynamic returns) is calculated. Significant dynamic returns are detected and segmented using adaptive thresholding. The average optical flow of each significant return is used by a tracking algorithm both to constrain search window radii and to derive a similarity measure. A tree of possible tracks is maintained in which the cumulative similarity measure is used to identify the most likely tracks and to prune out the least likely object sequences. The results of experiments using real scan sequences are presented to show the utility of the proposed tracking system View full abstract»

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Aims & Scope

IEE Proceedings Radar, Sonar and Navigation covers the theory and practice of systems involving the processing of signals for radar, radio location, radio navigation and surveillance purposes.

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