Broadband Passive Sonar Signal Simulation in Shallow Ocean

The broadband plane wave model is valid only in the far-field of a point source under free-field propagating conditions. However the acoustics in ocean is characterized by multi-modal acoustic propagation due to its top-bottom limited boundary conditions. The effect of multi-modal field is to alter the source spectrum while the effect of dispersion is to modify the pulse shape. Moreover the use of a plane wave beamformer in a multi-modal field leads to a bias in the bearing estimates. These effects are highly dependant on the environment parameters and have important ramifications for target localization and classification in an ocean waveguide. We propose a more realistic simulator which essentially models these effects and therefore serves to provide test signals for first hand verification of signal processing algorithms to be developed for such scenarios. This model is to be understood as a better model than the naïve plane wave model which is entirely oblivious of even the gross features such as wave propagation in an oceanic waveguide. The channel parameter so estimated from the present simulation can be convolved with the radiated noise spectra of the source to generate the passive sonar signal.Defence Science Journal, 2011, 61(4), pp.370-376, DOI:http://dx.doi.org/10.14429/dsj.61.8

Localization of Multiple Moving Targets Based on Matched Field Processing

고소음의 간섭표적과 저소음의 표적이 존재하는 천해환경에서 다중표적의 분리탐지 성능을 향상시키기 위하여 두 가지의 정합장처리 기법을 제안하고 설명하였다. 첫 번째로, 여러 개의 구속조건을 동시에 줄 수 있는 MCM 기법의 성질을 이용하여 인접한 간섭표적의 영향을 줄이는 시도를 하였다. MCM 기법을 이용하면 조향위치 이외의 구속조건을 추가로 지정하여 간섭표적의 부엽을 효과적으로 필터링할 수 있다. 이를 위하여 다중 구속조건에 조향위치의 위치 벡터와 간섭표적의 위치벡터를 포함시킨 NDC를 이용한다. NDC 구속행렬은 각각의 신호단편에 포함된 간섭 부공간을 추정하여 제거하는 필터 역할을 할 수 있다. NDC는 고소음의 빠른 표적에 의해 잠식된 DOF를 복원시킴으로서 결과적으로 DOF가 증가되어 인접한 저소음 표적의 분리탐지가 가능하게 된다. 간섭 부공간에 대한 NDC를 추정하기 위하여 자유로운 위치 선택의 장점이 있는 복제음장을 이용하였다. MCM 기법의 수치실험으로부터 표적의 신호들이 서로 상관되어 있는 경우에도 간섭표적이 잘 제거되고 표적의 분리가 가능함을 확인할 수 있다. SWellEx-96 해상실험 자료를 처리한 결과 간섭표적의 출력과 주변의 부엽들이 함께 낮아짐으로써 저소음 표적의 경로가 분리됨을 확인하였다. 두 번째로 도파관 불변성 현상을 이용하여 빠르게 움직이는 간섭표적의 속도를 보정하는 알고리즘을 설명하였다. 표적의 이동을 정확하게 추정하기 위하여 조향빔처리 기법을 제안하였다. 또한 도파관 불변성이 미약해서 일관성이 낮은 간섭형태를 갖는 도파관에 적합한 이동보상 알고리즘으로써 긴 관측시간 동안의 신호단편들로 구성한 하나의 CSDM 대신에 여러 개의 CSDM으로 나누어 짧은 시간에 대한 이동보상 후에 비상관처리 하는 시간분할 이동보상 알고리즘을 적용하였다. 이동보상 알고리즘의 성능을 검증하기 위하여 수치실험을 수행하고, SWellEx-96 해상실험 자료를 분석하였다. 표적의 이동을 보상한 결과 SBNR이 개선되고, 부엽이 전반적으로 낮아짐으로써 탐지성능이 향상됨을 확인할 수 있었다.The propeller for propelling a ship is installed at the stern side end of propeller shaft protruding from its stern. Therefore the propeller shaft is deflected by the propeller weight or any external force and the stern side of stern tube bearing will be loaded larger load than its bow side end. This one-side load tends to cause local contact between the shaft and bearing and causes bearing such as wear, burning, etc. of bearing metal. In this study, to clarify the performance of the stern tube bearing, Reynolds equation under deflected condition is solved by using computer. Oil-film pressure distribution and variation of the bearing are also calculated. And the shaft declination within the stern bearing is expressed by declination curve obtained from the deflection calculations for the entire shafting system, calculated simultaneously with the stern tube bearing by ANSYS software. Oil film characteristics are obtained, based on the finite width hydrodynamic theory and solved through the convergence calculations.Abstract = Ⅱ 제 1 장 서 론 = 1 1.1 연구 목적 = 1 1.2 연구 배경 = 2 1.3 논문 구성 = 6 제 2 장 정합장처리 = 8 2.1 신호 모델 = 8 2.2 정합장 음원위치 추정 알고리즘 = 11 제 3 장 도파관 공간의 간섭표적 필터링 = 26 3.1 영 방향 구속조건을 갖는 MCM 알고리즘 = 26 3.2 모의 수치실험 = 30 3.3 해상실험 적용 결과 = 39 제 4 장 도파관 불변성 기반의 이동보상 = 51 4.1 도파관 불변성 이론 = 53 4.2 이동보상 알고리즘 = 58 4.3 모의 수치실험 = 62 4.4 해상실험 적용 결과 = 78 제 5 장 결 론 = 96 참고문헌 = 9

Digital Signal Processing Research Program

Contains table of contents for Section 2, an introduction, reports on sixteen research projects and a list of publications.Bose CorporationMIT-Woods Hole Oceanographic Institution Joint Graduate Program in Oceanographic EngineeringAdvanced Research Projects Agency/U.S. Navy - Office of Naval Research Grant N00014-93-1-0686Lockheed Sanders, Inc./U.S. Navy - Office of Naval Research Contract N00014-91-C-0125U.S. Air Force - Office of Scientific Research Grant AFOSR-91-0034AT&T Laboratories Doctoral Support ProgramAdvanced Research Projects Agency/U.S. Navy - Office of Naval Research Grant N00014-89-J-1489U.S. Navy - Office of Naval Research Grant N00014-93-1-0686National Science Foundation FellowshipMaryland Procurement Office Contract MDA904-93-C-4180U.S. Navy - Office of Naval Research Grant N00014-91-J-162

Blind deconvolution in multipath environments and extensions to remote source localization

In the ocean, the acoustic signal from a remote source recorded by an underwater hydrophone array is commonly distorted by multipath propagation. Blind deconvolution is the task of determining the source signal and the impulse response from array-recorded sounds when the source signal and the environment’s impulse response are both unknown. Synthetic time reversal (STR) is a passive blind deconvolution technique that accomplishes two remote sensing tasks. 1) It can be used to estimate the original source signal and the source-to-array impulse responses, and 2) it can be used to localize the remote source when some information is available about the acoustic environment. The performance of STR for both tasks is considered in this thesis. For the first task, simulations and underwater experiments (CAPEx09) have shown STR to be successful for 1.5-4 kHz broadcast signal. Here STR is successful when the signal-to-noise ratio is high enough, and the receiving array has sufficient aperture and element density so that conventional delay-and-sum beamforming can be used to distinguish ray-path-arrival directions. Also, an unconventional beamforming technique (frequency-difference beamforming) that manufactures frequency differences from the recorded signals has been developed. It allows STR to be successful with sparse array measurements where conventional beamforming fails. Broadband simulations and experimental data from the focused acoustic field experiment (FAF06) have been used to determine the performance of STR when combined with frequency-difference beamforming. For the source localization task, the STR-estimated impulse responses may be combined with ray-based back-propagation simulations and the environmental characteristics at the array into a computationally efficient scheme that localizes the remote sound source. These localization results from STR are less ambiguous than those obtained from conventional matched field processing in the same bandwidth. However, when the frequency of the recorded signals is sufficiently low and close to modal cutoff frequencies, STR-based source localization may fail because of dispersion in the environment. For such cases, an extension of mode-based STR has been developed for sound source ranging with a vertical array in a dispersive underwater sound channel using bowhead whale calls recorded with a 12-element vertical array (Arctic 2010).PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102443/1/shimah_1.pd

Theory and Application of Autoproducts

Acoustics is a branch of physics largely governed by linear field equations. Linearity carries with it the implication that only the frequencies broadcast by acoustic sources can be measured in the surrounding acoustic medium. However, nonlinearities introduced not in the physical world, but in the mathematical and signal processing realm, have the potential to change frequency content. In this dissertation, nonlinear mathematical constructions termed ‘autoproducts’ are created which have the potential to shift frequencies from the measured, in-band frequencies to other higher or lower frequencies which may no longer be in-band. These out-of-band autoproduct fields did not physically propagate in the environment, and yet, this research has found that autoproducts can nonetheless mimic genuine out-of-band fields in a number of different acoustic environments. Approximately half of this dissertation addresses the theory of autoproducts. More specifically, mathematical analyses and simple acoustic models are used to uncover the reasons for how this frequency-shifting behavior works, and what its limitations are. It is found that there are no inherent limitations on the frequencies considered, and that in single-path environments, like plane or spherical waves, autoproducts mimic out-of-band fields in all or nearly all circumstances, respectively. However, in multipath environments, the mimicry of out-of-band fields by autoproducts is no longer so complete. Though, with bandwidth averaging techniques, it is found that the difference in time-of-arrivals of multiple paths is an important parameter: if it is larger than the inverse of the bandwidth available for averaging, then autoproducts can succeed in mimicking out-of-band fields. Other theoretical considerations include the effects of diffraction behind barriers and the effects of strong refraction. Strengths and limitations of autoproducts are assessed with a variety of simple acoustic models, and conclusions are drawn as to the predicted capabilities of autoproduct-based techniques. The other half of this dissertation covers applications of autoproducts. More specifically, it focuses on the use of autoproducts to perform physics-based source localization, especially for applications in the shallow ocean. Existing techniques are well-known to be very sensitive to uncertainties in the acoustic environment (e.g. the sound speed), especially at high frequencies (nominally greater than 1 kHz in the shallow ocean). Through the use of autoproducts, measured fields at high frequency can be shifted to much lower frequencies, where they can be processed with much more robustness to environmental uncertainties. In one of the main results of this dissertation, it is shown that a remote acoustic source broadcasting sound between 11 and 33 kHz in a 106-meter-deep, downward refracting sound channel could be localized using measurements from a sparse array located 3 km away. The data from the method suggest that autoproduct-based source localization can make physics-based array signal processing robust at arbitrarily high frequencies – a novel and important contribution to existing literature. Overall, by developing the theory for, and exploring applications of, these nonlinear mathematical constructions, the extent to which autoproducts are fundamentally limited is assessed, and new signal processing techniques are developed which have the potential to significantly improve the robustness of source localization algorithms for uncertain multipath environments. Through this study, significant portions of the necessary theoretical foundation have been laid, which will aid in the further development of robust, autoproduct-based signal processing techniques.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145865/1/bworthma_1.pd

Array design considerations for exploitation of stable weakly dispersive modal pulses in the deep ocean

© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Journal of Sound and Vibration 400 (2017): 402-416, doi:10.1016/j.jsv.2017.03.035.Modal pulses are broadband contributions to an acoustic wave field with fixed mode number. Stable weakly dispersive modal pulses (SWDMPs) are special modal pulses that are characterized by weak dispersion and weak scattering-induced broadening and are thus suitable for communications applications. This paper investigates, using numerical simulations, receiver array requirements for recovering information carried by SWDMPs under various signal-to-noise ratio conditions without performing channel equalization. Two groups of weakly dispersive modal pulses are common in typical mid-latitude deep ocean environments: the lowest order modes (typically modes 1–3 at 75 Hz), and intermediate order modes whose waveguide invariant is near-zero (often around mode 20 at 75 Hz). Information loss is quantified by the bit error rate (BER) of a recovered binary phase-coded signal. With fixed receiver depths, low BERs (less than 1%) are achieved at ranges up to 400 km with three hydrophones for mode 1 with 90% probability and with 34 hydrophones for mode 20 with 80% probability. With optimal receiver depths, depending on propagation range, only a few, sometimes only two, hydrophones are often sufficient for low BERs, even with intermediate mode numbers. Full modal resolution is unnecessary to achieve low BERs. Thus, a flexible receiver array of autonomous vehicles can outperform a cabled array