### Browsing by Author "Denbigh, P N"

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- ItemOpen AccessA three dimensional imaging sonar(1986) Runciman, Peter Brian; Denbigh, P N
- ItemOpen AccessA bathymetric sidescan sonar system(1986) Roy, Stuart; Denbigh, P NThe design and construction of a bathymetric sidescan sonar system capable of operation over a 39 degree vertical sector is described. Field tests conducted at a dam site indicate that the completed system does not function as required. This is due to errors in the sonar depth measurement which are caused by reverberation resulting from the multiple scattering of sound from the dam floor. Recommendations are made so that future systems will be less susceptible to this interference source.
- ItemOpen AccessThe development of correlation log(1985) Horwitz, Adrian Miles; Denbigh, P NA measure of ship speed is needed for dead reckoning navigation, docking, and as an input to satellite navigation systems. Ship speed is also used as an input to fire control systems on Naval vessels. The need for an accurate speed measuring device, that measures ship speed relative to the sea bed is thus apparent. All non acoustic logs measure ship speed relative to the water, and absolute ship speed can only be estimated if a knowledge of water currents is available. An acoustic log that provides an absolute measure of ship speed at limited operating depths is the Doppler log. For deep water the Doppler log measures speed relative to the water and it is thus affected by currents. A new development in acoustic logs is the correlation log. The correlation log can measure absolute speed at much greater depths than can the Doppler log. This is because it utilises a wide beam pointing vertically at the sea bed. The.wide beam permits a low operating frequency to be used which implies low attenuation. The high backscattering strength at normal angles of incidence combined with the low attenuation, means that relative to the Doppler log, the correlation log can measure absolute speed at much greater depths. The correlation log consists of a transmitter, which utilises tone burst transmission, and two or more receivers in line with the direction of motion. The signals received by two transducers will be similar except for a time shift 'T', which is given by the equation T = d/2V, where V is the speed and d the transducer separation. A device based on these principles has been built and tested. Results have shown that the system concepts are viable and will lead to an absolute speed measuring device that can operate at great depth.
- ItemOpen AccessAn electronically focused multiple beam side scan sonar(1985) Fox, Peter; Denbigh, P NPhased arrays have been in use since World War II but their commercial application has been constrained by the bulk and cost of the beam forming system. High resolution side scan sonar systems have many operational parameters that may only be extended with the aid of phased arrays, the resolution and the imaging rate being the most important. This thesis describes a microprocessor controlled dynamically focused side scan sonar where high resolution and high image acquisition rates are achieved. Dynamic focusing prevents the depth of field limitations of fixed focus arrays by updating the array phases at regular intervals so as to create a focal point which recedes from the array in synchronism with the returning echoes from the transmitted pulse. A high image acquisition rate is achieved through the simultaneous formation of multiple beams. Using a microprocessor as a low-cost controller demands rapidly executable software and a little specialized hardware. Programmable quadrature phase shifters give phase and amplitude control. A beam forming board combines the phase shifted signals into a beam and samples it. A 'time domain multiplexed' transmitter solves the problem of efficient insonification of swaths. The system timing is complex; while image samples are captured data is formatted and presented for recording on a chart recorder. This occurs in real-time, while the focus of each of the multiple beams is changed almost every two meters. Tank tests of the completed system provide confirmation of the resolution predicted with theory and computer simulation. Sea trials confirm that resolution close to that predicted may be obtained under operational circumstances. The results obtained fully justify the assertion that low cost microprocessor controlled dynamically focused multiple beam phased arrays are both an attainable and an attractive solution to the problems faced by the designer of high resolution side scan sonar systems.
- ItemOpen AccessFish stock assessment by a statistical analysis of echo sounder signals(1986) Weintroub, Jonathan; Denbigh, P NA means of assessing the quantity of exploitable fish in the sea is a requirement for effective management of the resource. Sonar is widely used in this regard, as it provides a rapid means of assessment. Two acoustic assessment techniques currently used are the echo counting and echo integration· methods. The echo counting method requires that only single fish echoes are present in the backscatter from the shoal, while the echo integration technique requires an a-priori knowledge of the average target strength of the fish in the shoal. A novel method of assessment has been proposed. It relies on the relationship between the statistics of the backscatter from a volume distribution of scatterers and the number of scatterers contributing to the backscatter at any one time. The attraction of the method when applied to the estimation of number density of fish, is that estimates can be produced in the presence of overlapping echoes, and that knowledge of the target strength of the fish is unnecessary. The application of this method to acoustic fish stock assessment is investigated in this work. Current methods of assessment are reviewed and the theory of the statistical method is given. A computer simulation of the scattering problem gives a useful insight into the effects of sample size and density on the accuracy of the method. The method has been applied to the assessment of fish at sea, where it was run in tandem with an echo integrator. The results obtained with the two techniques are compared. Reasons for discrepancies are proposed and problems in the application of the method are identified.
- ItemOpen AccessOn the synthesis of optimum monopulse antenna array distributions(1986) McNamara, Derek Albert; Denbigh, P NThe stringent specifications of modern tracking systems demand antennas of high performance. For this reason arrays are finding increasing application as monopulse antennas. A new exact procedure is introduced for the synthesis of optimum difference distributions for linear arrays of discrete elements, with a maximum sidelobe level specification. The method is based on the Zolotarev polynomial, and is precisely the difference mode equivalent of the Dolph-Chebyshev synthesis for sum patterns. When the interelement spacings are a half-wavelength or larger the element excitations are obtained in a very direct manner from the Chebyshev series expansion of the Zolotarev polynomial. For smaller spacings, a set of recursive equations is derived for finding the array excitation set. Efficient means of performing all the computations associated with the above procedure are given in full. In addition, a set of design tables is presented for a range of Zolotarev arrays of practical utility. A novel technique, directly applicable to arrays of discrete elements, for the synthesis of high directivity difference patterns with arbitrary si delobe envelope tapers is presented. This is done by using the.Zolotarev space factor zeros and correctly relocating these in a well-defined manner to effect the taper. A solution to the direct synthesis of discrete array sum patterns with arbitrary sidelobe envelope tapers is introduced. In this case the synthesis is also done by correct placement of the space factor zeros. The above techniques enable high excitation efficiency, low sidelobe, sum and difference pat terns to be synthesized independently. Contributions to the simultaneous synthesis of sum and difference patterns, subject to specified array feed network complexity constraints, are also given. These utilise information on the excitations and space factor zeros of the independently optimal solutions, along with constrained numerical optimisation. The thesis is based on original research done by the author, except where explicit reference is made to the work of others.
- ItemOpen AccessProjection of acoustic fields using the Fourier transform(1985) Yudelman, Errol; Denbigh, P NA method is presented for projecting acoustic fields using the Fourier transform. It is shown that the source velocity distribution can be represented by a number of sinusoids. Each sinusoid gives rise to a plane wave whose direction is given by the wavelength of the sinusoid. The plane waves are summed at the plane of interest to obtain the resulting pressure field. Errors are introduced when projecting pressure fields by large distances. These are circumvented by observing that the farfield pressure pattern can be found by simply taking the Fourier transform of the nearfield pressure distribution. A rectangular source is simulated on the computer; the Fourier transform technique of projecting fields is compared to a direct integration technique. The Fourier transform technique is used to back project a measured pressure pattern to detect defects on the transducer face. Measurements of pressure are made in the nearfield of a circular transducer. These measurements are forward and back projected to give the pressure and velocity at other planes.
- ItemOpen AccessSonar beamforming based upon monaural localisation techniques(1987) Tollman, Peter Alan; Denbigh, P NSonar beamforming is usually accomplished using a multi-element transducer array. To obtain high resolution, such a system is costly and complex. In contrast, many mammals are capable of good angular resolution using only a single active element surrounded by an irregular reflector – the ear. A study of monaural localisation was therefore undertaken, with a view to the development of a novel beamforming system which uses only a single active element. Computer simulations have shown that the direction of a source can be determined by cross –correlating the output signal spectrum with the known spectral responses of the receiving system for all angles.
- ItemOpen AccessSynthetic aperture sonar for sub bottom imaging(1986) Dutkiewicz, Marek Karol; Denbigh, P NThe major problems associated with implementing synthetic apertures in sonar may be overcome by applying the technique to sub-bottom imaging. The use of a low frequency aids both penetration of the sediment and the attainment of long coherent apertures. The implementation of synthetic apertures realises a narrow fan beam. By directing this beam out sideways, a more rapid survey rate is possible than that achieved by the normal echo-sounding mode used in sub-bottom profiling. The synthetic aperture technique appears to offer significant advantages over that of parametric arrays, for high resolution sub-bottom imaging. Synthetic apertures have been implemented in a laboratory tank, under controlled conditions at a fresh water reservoir, and under realistic operational conditions at sea. Encouraging results have been obtained.