1,597 research outputs found
WHOI acoustic telemetry project interim report 12/1/88 - 6/1/89
This interim report covers the progress of the acoustic telemetry project
during the period 12/1/88 to 5/15/89. In general, the work followed the
format specified in WHOI proposal No. 5674.1. The major exception was
the deletion of the transmitter array development task and a corresponding
funding decrease from 170,000. In addition, the period for the
funding was extended to June 30, partly due to a two month delay in project
startup.
The telemetry project was centered around the construction, programming
and testing of a digital receiver prototype capable of supporting future
signal processing algorithms in real-time over ocean acoustic channels. The
baseline receiver consists of a two-channel analog quadrature demodulator,
and interface to a multiprocessor receiver for digital signal processing.
The software developed includes routines for command and control of the
analog demodulator, data handling and formatting, and minimal software
to digitally implement an incoherent MFSK demodulator, synchronizer and
data decoder. Data storage and display programs were also completed to
facilitate the performance analysis of the unit during testing.
The system was tested in Woods Hole harbor at data rates up to 4800
bits/sec. The acoustic channel was time-dispersive Rayleigh fading, and
performance close to theoretical expectations was achieved. We are confident
that the system error behavior is arising from channel-caused effects and
known deficiencies in system performance, such as excessive synchronizer
steady-state jitter.Funding was provided by the Office of Naval Research
under contract Number N00014-86-K-0751, and
by the Charles Stark Draper Laboratory Inc
Design study for a moored surface-scanning sonar
This report contains the results of a design study for a surface scanning sonar instrument capable of long-term deployment on
ocean moorings. The instrument is intended to sample the bubble field just below the ocean's surface and compute the
backscattered intensity and Doppler velocity in small unit volumes. The principal motivation for the development of such an
instrument is to enhance the study of upper ocean processes by utilizing the ability of the sonar to detect surface waves and
Langmuir circulation. Important design parameters for the instrument are investigated and a detailed design proposed. Key technical
issues such as the trade-offs among spatial resolution, temporal resolution, velocity precision, total range, and power are discussed.
The azimuthal motion of the instrument on a mooring is considered as a potential problem, and possible solutions are discussed.
Matlab functions used for the investigations are included in an appendix.Funding was provided by a grant from the Webster Foundation to the
Woods Hole Oceanographic Institution
Measured and modeled acoustic propagation underneath the rough Arctic sea-ice
Author Posting. © Acoustical Society of America, 2017. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 142 (2017): 1619-1633, doi:10.1121/1.5003786.A characteristic surface duct beneath the sea-ice in the Marginal Ice Zone causes acoustic waves to
be trapped and continuously interact with the sea-ice. The reflectivity of the sea-ice depends on the
thickness, the elastic properties, and its roughness. This work focuses on the influence of sea-ice
roughness on long-range acoustic propagation, and on how well the arrival structure can be predicted
by the full wave integration model OASES. In 2013, acoustic signals centered at 900 Hz
were transmitted every hour for three days between ice-tethered buoys in a drifting network in the
Fram Strait. The experiment was set up to study the signal stability in the surface channel below the
sea-ice. Oceanographic profiles were collected during the experiment, while a statistical description
of the rough sea-ice was established based on historical ice-draft measurements. This environmental
description is used as input to the range independent version of OASES. The model simulations
correspond fairly well with the observations, despite that a flat bathymetry is used and the sea-ice
roughness cannot be fully approximated by the statistical representation used in OASES. Longrange
transmissions around 900 Hz are found to be more sensitive to the sea-ice roughness than the
elastic parameters.The fieldwork was performed
under funding from the Research Council of Norway
through the UNDER-ICE (Grant No. 226373) project and
ENGIE E&P Norway providing additional support. The data
analysis, modeling and preparation of the publication has
been carried out under funding from the Office of Naval
Research (Global) (Grant No. N62909-14-1-NO33) and
UNDER ICE (Grant No. 226373) projects. The U.S. Office
of Naval Research provided partial support for this work
under Grant No. N000141210176 to the Woods Hole
Oceanographic Institution
A comprehensive set of simulations of high-velocity collisions between main sequence stars
We report on a very large set of simulations of collisions between two main
sequence (MS) stars. These computations were done with the ``Smoothed Particle
Hydrodynamics'' method. Realistic stellar structure models for evolved MS stars
were used. In order to sample an extended domain of initial parameters space
(masses of the stars, relative velocity and impact parameter), more than 15000
simulations were carried out. We considered stellar masses ranging between 0.1
and 75 Msun and relative velocities up to a few thousands km/s. To limit the
computational burden, a resolution of 2000-30000 particles per star was used.
The primary goal of this study was to build a complete database from which the
result of any collision can be interpolated. This allows us to incorporate the
effects of stellar collisions with an unprecedented level of realism into
dynamical simulations of galactic nuclei and other dense stellar clusters. We
make the data describing the initial condition and outcome (mass and energy
loss, angle of deflection) of all our simulations freely available on the
Internet. We find that the outcome of collisions depends sensitively on the
stellar structure and that, in most cases, using polytropic models is
inappropriate. Published fitting formulas for the collision outcomes,
established from a limited set of collisions, prove of limited use because they
do not allow robust extrapolation to other stellar structures or relative
velocities.Comment: 45 pages, 44 figures. Modified to reflect the changes in the
published version (MNRAS). PDF version with high-res figures at
http://obswww.unige.ch/~freitag/papers/article_collisions.pdf, simulation
data at http://obswww.unige.ch/~freitag/MODEST_WG4/FB_Collision_Data/, movies
at http://obswww.unige.ch/~freitag/collisions/animations/index.htm
Multicarrier communication over underwater acoustic channels with nonuniform Doppler shifts
Author Posting. © IEEE, 2008. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 33 (2008): 198-209, doi:10.1109/JOE.2008.920471.Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose a two-step approach to mitigating the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem and 2) high-resolution uniform compensation of the residual Doppler. We focus on zero-padded orthogonal frequency-division multiplexing (OFDM) to minimize the transmission power. Null subcarriers are used to facilitate Doppler compensation, and pilot subcarriers are used for channel estimation. The receiver is based on block-by-block processing, and does not rely on channel dependence across OFDM blocks; thus, it is suitable for fast-varying UWA channels. The data from two shallow-water experiments near Woods Hole, MA, are used to demonstrate the receiver performance. Excellent performance results are obtained even when the transmitter and the receiver are moving at a relative speed of up to 10 kn, at which the Doppler shifts are greater than the OFDM subcarrier spacing. These results suggest that OFDM is a viable option for high-rate communications over wideband UWA channels with nonuniform Doppler shifts.B.
Li and S. Zhou are supported by the ONR YIP grant N00014-07-1-0805
and the NSF grant ECCS-0725562. M. Stojanovic is supported by the ONR
grant N00014-07-1-0202. L. Freitag is supported by the ONR grants N00014-
02-6-0201 and N00014-07-1-0229. P. Willett is supported by the ONR
grant N00014-07-1-0055
Ocean acoustical ray-tracing : Software Ray
A new computer program for accurate calculation of acoustic ray paths through a range-varing ocean sound channel has been
written. It is based on creating a model of the speed of sound in the ocean, consistent with input data, that produces the smoothest
possible wavefronts. This scheme eliminates "false caustics" from the wavefront. It may be useful in calculating an approximate
solution to the full wave equation at megameter ranges.Funding was provided by the Office of Naval Research under contract N00014-86-C-0358 and the
Office of Naval Technology under contract N00014-90-C-0098
Inversion of surficial sediment thickness from under-ice acoustic transmission measurement
Author Posting. © Acoustical Society of America, 2021. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 149(1), (2021): 371, https://doi.org/10.1121/10.0003328.The under-ice acoustic transmission experiment of 2013, conducted under ice cover in the Fram Strait, was analyzed for bottom interactions for the purpose of developing a model of the seabed. Using the acoustic signals, as well as data from other sources, including cores, gravimetric, refraction, and seismic surveys, it was deduced that the seabed may be modeled as a thin surficial layer overlaid on a deeper sediment. The modeling was based on the Biot–Stoll model for acoustic propagation in porous sediments, aided by more recent developments that improve parameter estimation and depth dependence due to consolidation. At every stage, elastic and fluid approximations were explored to simplify the model and improve computational efficiency. It was found the surficial layer could be approximated as a fluid, but the deeper sediment required an elastic model. The full Biot–Stoll model, while instrumental in guiding the model construction, was not needed for the final computation. The model could be made to agree with the measurements by adjusting the surficial layer thickness.The fieldwork was performed under funding from the Research Council of Norway through the UNDER-ICE (Grant No. 226373) project and ENGIE E&P Norway providing additional support. This analysis was supported by the United States Office of Naval Research, Ocean Acoustics Program.2021-07-1
Superconductivity enhanced conductance fluctuations in few layer graphene nanoribbons
We investigate the mesoscopic disorder induced rms conductance variance
in a few layer graphene nanoribbon (FGNR) contacted by two
superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we
observe pronounced conductance fluctuations superimposed on a linear background
of the two terminal conductance G. The linear gate-voltage induced response can
be modeled by a set of inter-layer and intra-layer capacitances.
depends on temperature T and source-drain voltage .
increases with decreasing T and . When lowering , a
pronounced cross-over at a voltage corresponding to the superconducting energy
gap is observed. For |V_{sd}|\ltequiv \Delta the fluctuations are
markedly enhanced. Expressed in the conductance variance of one
graphene-superconducutor (G-S) interface, values of 0.58 e^2/h are obtained at
the base temperature of 230 mK. The conductance variance in the sub-gap region
are larger by up to a factor of 1.4-1.8 compared to the normal state. The
observed strong enhancement is due to phase coherent charge transfer caused by
Andreev reflection at the nanoribbon-superconductor interface.Comment: 15 pages, 5 figure
The Gene-Lifestyle Interaction on Leptin Sensitivity and Lipid Metabolism in Adults: A Population Based Study
Background: Obesity has been associated with leptin resistance and this might be caused
by genetic factors. The aim of this study was to investigate the gene-lifestyle interaction betwee
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