Thesis (M.S.)--Naval Postgraduate School, 2002. / Thesis advisor(s): Steven R. Baker, Thomas G. Muir. Includes bibliographical references (p. 63-65). Also available online.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, March 2003. / Thesis advisor(s): Lawrence J. Ziomek, Ziaoping Yun. Includes bibliographical references (p. 63). Also available online.

The scattering and attenuation caused by fish schools has been extensively studied for applications in fisheries management and naval sonar. The literature contains extensive in situ measurements of scattering by fish schools, however significant uncertainties exist with respect to characterizing the size, quantity, and distribution of fish within the schools, that confounds accurate measurement-model comparison. Hence there is a need for application of measurement techniques that can more precisely characterize the acoustic properties of fish schools and the variations intrinsic to live subjects in continual motion. To begin to address this deficiency, measurements of the sound speed through collections of live fish were conducted in a laboratory setting. The species chosen for measurement were zebrafish (Danio rerio). The sound speed was investigated using a resonator technique which yielded inferences of the phase speed within the fish school though measurements of the resonances of a one-dimensional waveguide. The waveguide was calibrated to compensate for finite wall impedance and for finite reflections from the ends of the waveguide. Fish densities were investigated ranging from 8.6 to 1.7 fish lengths per mean free path. Measurements agree well with a predictive model that is based on shell-free spherical bubbles, which indicates that the phase speed is not significantly affected by the fish flesh or swimbladder morphology for the species studied. The variation in phase speed due to individual fish motion within the model school was measured to be up to ± 5.6 %. This indicates that precise knowledge of the fish position is required to achieve greater model accuracy. To compliment the phase speed measurements, the attenuation through a cloud of encapsulated bubbles was evaluated through insertion loss measurements. Multiple arrangements of balloons of radius 4.68 cm were used to surround a projector. The insertion loss measurements indicated an amplification of around 10 dB at frequencies below the individual balloon resonance frequency and an insertion loss of around 40 dB above the individual balloon resonance frequency. Analytical modeling of the bubble collection predicted both the amplification and loss effect, but failed to accurately predict the level of amplification and insertion loss. Effective medium models and full scattering models (requiring knowledge of bubble size and position) were evaluated for a model fish school. The two models agree for forward scattering for all frequencies except those immediately around the individual bubble resonance frequency. Back scattered results agree at low frequencies, however as soon as the wavelength becomes smaller than four mean free paths between fish the models diverge. Ramifications of these findings and potential future research directions are discussed. / text

Firefighters routinely have to go through buildings with reduced visibility due to smoke. Moving through even the simplest apartment building can become a perilous task when you remove visual sensing, and introduce fires, toxic gasses and extremely high temperatures. While a number of tools, both low and high tech, exist to aid firefighters, none of them are perfectly able to solve this problem. This thesis proposes using sonar to supplement those tools, and documents the development of a flame-penetrating sonar. To the authors knowledge, no previous effort has been made to develop sonar for firefighting applications. Traditional ultrasonic range finders were found unable to penetrate flames, and thus a pulse-compression based sonar is presented here. A prototype sonar is developed to allow experimental testing of this technique. A number of experiments were conducted to understand the limitations of this device. This sonar was able to overcome the scattering of the flame, and even detect the extents of the flame. A number of applications of this technology can be imagined other than assisting firefighters. Military personnel, or anyone else needing to navigate obscured environments could use this technology. Other applications could be two-dimensional and three-dimensional temperature field reconstruction for industrial applications. The author believes combination sensors using sonar, thermal-imaging, global positioning system, dead reckoning, etc, are the only way to solve the problem of firefighter navigation. / text

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No description available.

This thesis contains the complete end-to-end simulation, development, implementation, and calibration of the wide bandwidth, low-Q, Kiwi-SAS synthetic aperture sonar (SAS). Through the use of a very stable towfish, a new novel wide bandwidth transducer design, and autofocus procedures, high-resolution diffraction limited imagery is produced. As a complete system calibration was performed, this diffraction limited imagery is not only geometrically calibrated, it is also calibrated for target cross-section or target strength estimation. Is is important to note that the diffraction limited images are formed without access to any form of inertial measurement information. Previous investigations applying the synthetic aperture technique to sonar have developed processors based on exact, but inefficient, spatial-temporal domain time-delay and sum beamforming algorithms, or they have performed equivalent operations in the frequency domain using fast-correlation techniques (via the fast Fourier transform (FFT)). In this thesis, the algorithms used in the generation of synthetic aperture radar (SAR) images are derived in their wide bandwidth forms and it is shown that these more efficient algorithms can be used to form diffraction limited SAS images. Several new algorithms are developed; accelerated chirp scaling algorithm represents an efficient method for processing synthetic aperture data, while modified phase gradient autofocus and a low-Q autofocus routine based on prominent point processing are used to focus both simulated and real target data that has been corrupted by known and unknown motion or medium propagation errors.

Synthetic aperture sonar (SAS) has become a well developed imaging technique for imaging shallow water environments. Aperture synthesis provides high along-track resolution imagery, with range independent resolution. However, mapping of the seafloor using traditional SAS is limited to a two-dimensional surface. To provide the third dimension (height), an interferometric synthetic aperture sonar (InSAS) is formed, comprising of two or more vertically displaced hydrophone arrays. Each of the interferometric receiver datasets are processed using standard SAS algorithms, with motion compensation and corrective processing applied equally to each channel, preserving the underlying interferometric time delays. By then estimating the time delay of the incoming wavefronts across the interferometric receiver array, the height of the seafloor can be inferred from the side-scan geometry of the system. The InSAS approach is similar to the radar equivalent (InSAR), however, significant differences in geometry and medium properties limit the applicability of InSAR algorithms to the sonar equivalent. A height estimate from interferometric data is formed by estimating the time difference between the receiver elements of the interferometric array. Therefore, for an accurate estimate of the time-delay, the signals of the receivers must contain significant 'common' information. Presented in this thesis is an analysis of coherence as applicable to an InSAS system. The coherence of an InSAS system can be decomposed into five 'coherence components': additive acoustic noise, footprint misalignment, baseline decorrelation, temporal decorrelation, and processing noise. Of these, it is shown footprint misalignment has the greatest effect for an InSAS system if it is not corrected for. The importance of maintaining high coherence between the receiver channels is presented; small losses in coherence from the ideal of unity will have a significant impact of the accuracy of the resulting height estimate. To reduce the sensitivity of the height accuracy losses, multiple estimates of the height can be formed from independent 'looks' of the scene. Combining all these estimates into one height estimate is shown to significantly improve the height estimate. The design and signal processing of an InSAS system is of high importance to the generation of high accuracy height estimates of the seafloor. Several parameters of design are explored, in particular the effect of aperture sampling. Low along-track aperture sampling rates are shown to cause a significant decrease in signal coherence, caused by the generating of 'grating lobes' from the synthetic aperture processing. Substantial improvements can be made by careful selection of transmitter and receiver element sizes, relaxing the requirements of a highly sampled aperture. An analysis of interpolation schemes on interferometric quality is also presented. The effect of footprint misalignment can be reduced by first resampling the data from each receiver onto a common ground-plane. However, this requires prior knowledge of the seafloor height, an unknown parameter before an interferometric height estimate is made. One possible method to form an initial height estimate is through the use of belief propagation, a technique applied from the field of stereo imaging. Belief propagation is used to estimate an initial height surface, albeit at discrete height intervals. This initial low resolution height surface can then be used to remap the data, partially eliminating the detrimental effects of footprint misalignment. The combination of all the independent estimates of the scene can be combined using maximum likelihood estimation. This framework allows the individual estimates to be combined into one overall cost function. Searching of the cost function for minimum cost yields a single interferometric time-delay estimate, from which a single height estimate can be inferred. This framework allows looks formed from many different sources to be combined, including multiple imaging frequency bands, and the use of more than one interferometric pair of receivers.

Proefschrift Universiteit van Amsterdam. / Met lit. opg. - Met samenvatting in het Nederlands.

Thesis (Ph.D.)--University of Melbourne, Dept. of Electrical and Electronic Engineering, 2005. / Typescript. Includes bibliographical references (leaves 151-158).