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  • 1
    Online Resource
    Online Resource
    Institute of Electrical and Electronics Engineers (IEEE) ; 2011
    In:  IEEE Journal of Oceanic Engineering Vol. 36, No. 3 ( 2011-7), p. 454-461
    In: IEEE Journal of Oceanic Engineering, Institute of Electrical and Electronics Engineers (IEEE), Vol. 36, No. 3 ( 2011-7), p. 454-461
    Type of Medium: Online Resource
    ISSN: 0364-9059 , 1558-1691 , 2373-7786
    Language: Unknown
    Publisher: Institute of Electrical and Electronics Engineers (IEEE)
    Publication Date: 2011
    detail.hit.zdb_id: 2025369-2
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  • 2
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2016
    In:  Journal of the Acoustical Society of America Vol. 140, No. 4_Supplement ( 2016-10-01), p. 3169-3169
    In: Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 140, No. 4_Supplement ( 2016-10-01), p. 3169-3169
    Abstract: A series of underwater acoustic experiments utilizing a Robinson R44 helicopter and an underwater receiver station has been conducted in shallow (16.5 m) water. The receiver station consisted of an 11-element nested hydrophone array with a 12 m aperture configured as a horizontal line (HLA) 0.5 m above the seabed. An in-air microphone was located immediately above the surface. The main rotor blades of the helicopter produce low-frequency harmonics, the fundamental frequency being ~13 Hz. The tail rotor produces a sequence of harmonics approximately six times higher in frequency. The first experiment characterized the underwater sound signature of the helicopter with altitude and range. Using analytical and numerical 3-layer (atmosphere-ocean-sediment) acoustic propagation models a sediment geoacoustic inversion technique has been developed. This technique, requiring only knowledge of the relative location of the sensors and sound source (helicopter), uses the cross-correlation between HLA sensor pairs to produce the estimated time delay of the head wave. The results from the simulations and the latest experiment are presented. [Research supported by ONR, SMART(DOD), NAVAIR, and SIO.]
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2016
    detail.hit.zdb_id: 1461063-2
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  • 3
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 4 ( 2001-04-01), p. 1355-1366
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 4 ( 2001-04-01), p. 1355-1366
    Abstract: The authors demonstrate MFP using data-derived modes and the sound speed profile, using no a priori bottom information. Mode shapes can be estimated directly from vertical line array data, without a priori knowledge of the environment and without using numerical wave field models. However, it is difficult to make much headway with data-derived modes alone, without wave numbers, since only a few modes at a few frequencies may be captured, and only at depths sampled by the array. Using a measured sound speed profile, the authors derive self-consistent, complete sets of modes, wave numbers, and bottom parameters from data-derived modes. Bottom parameters enable modes to be calculated at all frequencies, not just those at which modes were derived from data. This process is demonstrated on SWellEx-96 experiment data. Modes, wave numbers, and bottom parameters are derived from one track and MFP based on this information is demonstrated on another track.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
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  • 4
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2004
    In:  The Journal of the Acoustical Society of America Vol. 115, No. 2 ( 2004-02-01), p. 607-619
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 115, No. 2 ( 2004-02-01), p. 607-619
    Abstract: The use of adjoint modeling for acoustic inversion is investigated. An adjoint model is derived from a linearized forward propagation model to propagate data-model misfit at the observation points back through the medium to the medium perturbations not being accounted for in the model. This adjoint model can be used to aid in inverting for these unaccounted medium perturbations. Adjoint methods are being applied to a variety of inversion problems, but have not drawn much attention from the underwater acoustic community. This paper presents an application of adjoint methods to acoustic inversion. Inversions are demonstrated in simulation for both range-independent and range-dependent sound speed profiles using the adjoint of a parabolic equation model. Sensitivity and error analyses are discussed showing how the adjoint model enables calculations to be performed in the space of observations, rather than the often much larger space of model parameters. Using an adjoint model enables directions of steepest descent in the model parameters (what we invert for) to be calculated using far fewer modeling runs than if a forward model only were used.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2004
    detail.hit.zdb_id: 1461063-2
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  • 5
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 110, No. 5_Supplement ( 2001-11-01), p. 2661-2661
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 110, No. 5_Supplement ( 2001-11-01), p. 2661-2661
    Abstract: Adjoint modeling is an inversion technique that has the potential to sharply reduce the number of modeling runs typically needed to achieve an inversion. Although adjoint models are being used in oceanographic and meteorological modeling, they seem to have been overlooked by the acoustics community. A single adjoint modeling iteration calculates an adjustment to forward model control inputs (initial conditions and boundary conditions) needed to correct for a mismatch between predicted and measured system parameters. We will present how the adjoint of an acoustic propagation model can be used to trace a mismatch in predicted and measured acoustic parameters back to the perturbations in the environmental parameters needed to correct for the mismatch.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
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  • 6
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 115, No. 5_Supplement ( 2004-05-01), p. 2468-2468
    Abstract: The Kauai Experiment (22 June–9 July 2003) was designed to study high-frequency (8–50 kHz) acoustics in a shallow-water waveguide. In contrast to much of the previous literature, emphasis was placed on multipath arising from multiple boundary interactions. Various participants were interested in different applications; however, a core theme was the role of the environment on acoustic communications. A great deal of effort was made to characterize the environment including the surface wave spectrum, 2D temperature structure along the propagation path, salinity, currents, and bottom properties. Most of these parameters were measured continuously over the 2 weeks of the experiment, providing information on the diurnal cycles. At the same time, extensive acoustic measurements were made using a variety of vertical line arrays, some of which spanned the entire water column. The acoustic measurements included channel probes to characterize the variation of the impulse response. These probes were interleaved with a variety of modulation schemes for communications including noncoherent methods such as MFSK (multifrequency shift keying), and DPSK (differential phase-shift keying), as well as coherent schemes such as QAM (quadrature amplitude modulation), OFDM (orthogonal frequency division modulation), and PPC (passive-phase conjugation) methods. Thus, the experiment provides a vast amount of information relating environment to acoustic propagation to modem performance. This talk will present an overview of key lessons learned to date.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2004
    detail.hit.zdb_id: 1461063-2
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  • 7
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2008
    In:  The Journal of the Acoustical Society of America Vol. 123, No. 5_Supplement ( 2008-05-01), p. 3749-3749
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 123, No. 5_Supplement ( 2008-05-01), p. 3749-3749
    Abstract: Gaussian beam tracing is an approach that constructs full-wave beams around the skeleton of conventional ray theory. This Gaussian beam approach leads to a very simple algorithm and provides remarkable accuracy and speed. For high-frequency, broadband applications, Gaussian beams are often the only practical approach, as the standard full-wave modeling alternatives are often thousands of times slower. Gaussian beams are derived using high-frequency asymptotics, and therefore fit naturally to certain current areas of interest in HF acoustics, such as acoustic communications. However, what often surprises people who are not familiar with the technique is that it works quite well at lower frequencies, depending on the water depth. That fact, is really a consequence of 20 years of continued advances in the Gaussian beam method. We will review those developments in the context of HF variability, considering effects of boundary, volume, and source/receiver dynamics.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2008
    detail.hit.zdb_id: 1461063-2
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  • 8
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2017
    In:  The Journal of the Acoustical Society of America Vol. 141, No. 5_Supplement ( 2017-05-01), p. 3991-3991
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 141, No. 5_Supplement ( 2017-05-01), p. 3991-3991
    Abstract: The array invariant proposed for robust source-range estimation in shallow water is based on the dispersion characteristics in ideal waveguides, utilizing multiple arrivals separated in beam angle and travel time for broadband signals. Recently, the array invariant was extended to general waveguides by incorporating the waveguide invariant β, referred to as a generalized array invariant. In range-dependent environments with a sloping bottom, the waveguide invariant β is approximately proportional to the source range via the water depth. Assuming knowledge of the bottom slope, the array invariant can be applied iteratively starting with β = 1 in shallow water, which converges toward the correct source range in a zigzag fashion. The iterative array invariant approach is demonstrated in a sloping-bottom shallow-water waveguide using a short-aperture vertical array (2.8 m) from the Random Array of Drifting Acoustic Receivers 2007 experiment (RADAR07), where a high-frequency source (2-3.5 kHz) close to the surface (6-m) was towed between 0.5 to 5 km in range with the corresponding water depth being 80 and 50 m, respectively.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2017
    detail.hit.zdb_id: 1461063-2
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  • 9
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2008
    In:  The Journal of the Acoustical Society of America Vol. 123, No. 5_Supplement ( 2008-05-01), p. 3891-3891
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 123, No. 5_Supplement ( 2008-05-01), p. 3891-3891
    Abstract: We investigate performance of turbo coded multiple-input multiple-output (MIMO)-OFDM systems with layered space time (LST) architectures for underwater acoustic (UWA) channels by using simulations and results from the AUVfest experiment performed in June 2007. MIMO systems have been shown to be promising in the sense that they increase the reliable transmission rates significantly without consuming additional bandwidth and power. Robustness of OFDM systems with cyclic prefix or zero padding to ISI channels are also well known; so the combination of MIMO techniques and OFDM can be regarded as a promising technology for shallow water UWA communications which is characterized by severe bandwidth limitations and long intersymbol interference (ISI) spans. The paper reviews necessary components of a MIMO-OFDM communication system, including, time and frequency synchronization, channel estimation, and tracking of the varying channel parameters. Modifications necessary to make the system suitable for UWA channels are summarized. Results of the AUVFest 2007 experiment are very promising; for instance, 2×2 MIMO-OFDM can reach up to 60 Kbps transmission for a bandwidth of 16 KHz with simple receiver structures for a range of 2000 m. In addition to the coherent system, differential and unitary space-time coded MIMO-OFDM scenarios are also considered.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2008
    detail.hit.zdb_id: 1461063-2
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  • 10
    Online Resource
    Online Resource
    Acoustical Society of America (ASA) ; 2022
    In:  The Journal of the Acoustical Society of America Vol. 152, No. 4_Supplement ( 2022-10-01), p. A156-A156
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 152, No. 4_Supplement ( 2022-10-01), p. A156-A156
    Abstract: Ocean dynamics can result in significant sound speed variability that are not well captured by deterministic numerical models and can impact differently high to low frequency acoustic propagation and bottom interactions by constraining horizontal and vertical paths. Uncertainty in the sound speed can, therefore, cause inaccuracies in acoustic based tactical decision tools and severely impact the accuracy of algorithms using sound propagation to estimate ocean volume states (e.g., ocean tomography or data assimilation). In this work, we propose to use a non-intrusive Reduced Order Modelling solution that consists of building a dictionary of static modes from historical ocean simulations with different settings and resolutions and climatology to derive an expedite uncertainty model along a central deterministic forecast. The system will then, through Orthogonal Matching Pursuit along the dictionary, use the available ocean model-data mismatches and/or acoustic innovations (e.g., arrival numbers and times, acoustic energy distribution, etc.) to define an ensemble of possible sound speed volume distributions and associated acoustic propagation outcomes. We will show results using a simplified simulation experiment that extracts “observations” from a nature reference field to document the procedure and benchmark results to reconstruct 3D sound speed fields from ocean-acoustic measurements.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    RVK:
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2022
    detail.hit.zdb_id: 1461063-2
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