Using GPS at sea to determine the range between a moving ship and a drifting buoy to centimeter-level accuracy

Doutt, James A.; Frisk, George V.; Martell, H.

doi:10.1109/oceans.1998.726287

Cited by 12 publications

(4 citation statements)

References 2 publications

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“…Drifter GPS positions are internally recorded at 1 Hz, with an absolute position error of about 64 m (George and Largier 1996). Postprocessing using carrier phase information reduces the absolute error to 61 m (Doutt et al 1998). Technical descriptions of the drifters and their response are found in Schmidt et al (2003).…”

Section: Observationsmentioning

confidence: 99%

Lagrangian Drifter Dispersion in the Surf Zone: Directionally Spread, Normally Incident Waves

Spydell

Feddersen

2009

Journal of Physical Oceanography

107

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Lagrangian drifter statistics in a surf zone wave and circulation model are examined and compared to single- and two-particle dispersion statistics observed on an alongshore uniform natural beach with small, normally incident, directionally spread waves. Drifter trajectories are modeled with a time-dependent Boussinesq wave model that resolves individual waves and parameterizes wave breaking. The model reproduces the cross-shore variation in wave statistics observed at three cross-shore locations. In addition, observed and modeled Eulerian binned (means and standard deviations) drifter velocities agree. Modeled surf zone Lagrangian statistics are similar to those observed. The single-particle (absolute) dispersion statistics are well predicted, including nondimensionalized displacement probability density functions (PDFs) and the growth of displacement variance with time. The modeled relative dispersion and scale-dependent diffusivity is consistent with the observed and indicates the presence of a 2D turbulent flow field. The model dispersion is due to the rotational components of the modeled velocity field, indicating the importance of vorticity in driving surf zone dispersion. Modeled irrotational velocities have little dispersive capacity. Surf zone vorticity is generated by finite crest-length wave breaking that results, on the alongshore uniform bathymetry, from a directionally spread wave field. The generated vorticity then cascades to other length scales as in 2D turbulence. Increasing the wave directional spread results in increased surf zone vorticity variability and surf zone dispersion. Eulerian and Lagrangian analysis of the flow indicate that the surf zone is 2D turbulent-like with an enstrophy cascade for length scales between approximately 5 and 10 m and an inverse-energy cascade for scales of 20 to 100 m. The vorticity injection length scale (the transition between enstrophy and inverse-energy cascade) is a function of the wave directional spread.

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Section: Observationsmentioning

confidence: 99%

Lagrangian Drifter Dispersion in the Surf Zone: Directionally Spread, Normally Incident Waves

Spydell

Feddersen

2009

Journal of Physical Oceanography

107

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“…Drifter GPS positions are internally recorded at 1 Hz with absolute position error of about Ϯ4 m (George and Largier 1996). Postprocessing using carrier-phase information reduces the absolute error to Ϯ1 m (Doutt et al 1998). Gaps in the time series of drifter positions occur when waves pass over the drifter, interrupting the satellite communication.…”

Section: Observationsmentioning

confidence: 99%

Observing Surf-Zone Dispersion with Drifters

Spydell

Feddersen

Guza

et al. 2007

Journal of Physical Oceanography

126

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Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10–20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height Hs was equal to 0.5 m and mean alongshore currents |υ| were moderate (<0.1 m s−1). On the second day, the obliquely incident waves were larger, with Hs equal to 1.4 m, and at some surf-zone locations |υ| was greater than 0.5 m s−1. The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along- and cross-shore directions, the asymptotic diffusivity is reached sooner within, rather than seaward of, the surf zone. Two-particle statistics indicate that relative dispersion grows like D2(t) ∼ t3/2 and that the relative diffusivity is scale-dependent as μ ∼ l2/3, with l being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [D2(t) ∼ t3 and μ ∼ l4/3]. Separations have a non-Gaussian self-similar distribution that is independent of time. The two-particle statistics are consistent with a nonconstant-coefficient diffusion equation for the separation probability density functions. The dispersion is explained by neither irrotational surface gravity waves nor shear dispersion. The observations imply the existence of a 2D eddy field with 5–50-m length scales, the source of which is speculated to be alongshore gradients in breaking-wave height associated with finite crest lengths.

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“…At any given time, no more then four frequencies were transmitted by a projec- GPS data were recorded and subsequently processed to achieve absolute positioning accuracy to within 1 meter. Further, by establishing a local differential GPS system between the source ship and each buoy, relative positions between the source and each receiver were measured to sub-meter accuracy (19J (20).…”

Section: Experiments Overviewmentioning

confidence: 99%

Geoacoustic inversion in laterally varying shallow-water environments using high-resolution wavenumber estimation

Becker¹

2002

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Sound propagation in shallow water is highly dependent on the interaction of the sound field with the bottom. In order to fufly understand this problem, it is necessary to obtain reliable estimates of bottom geoacoustic properties that can be used in acoustic propagation codes. In this thesis, perturbative inversion methods and exact inverse methods are discussed as a means for inferring geoacoustic properties of the bottom. For each of these methods, the input data to the inversion is the horizontal wavenumber spectrum of a point-source acoustic field. The main thrust of the tliesis work concerns extracting horizontal wavenumber content for fully threedimensionally varying waveguide environments. In this context, a high-resolution autoregressive (AR) spectral estimator was applied to determine wavenumber content for short aperture data. As part of this work, the AR estimator was examined for its ability to detect discrete wavenumbers in the presence of noise and also to resolve closely spaced wavenumbers for short aperture data. As part of a geoacoustic inversion workshop, the estimator was applied to extract horizontal wavenumber content for synthetic pressure field data with range-varying geoacoustic properties in the sediment. The resulting wavenumber content was used as input data to a perturbative inverse algorithm to determine the sound speed profile in the sediment. It was shown using the high-resolution wavenumber estimator that both the shape and location of the range-variability in the sediment could be determined. The estimator was also applied to determine wavenumbers for synthetic data where the water column sound speed contained temporal variations due to the presence of internal waves. It was shown that reliable estimates of horizontal wavenumbers could be obtained that are consistent with the boundary conditions of the waveguide. The Modal 1 Mapping Experiment (MOMAX), an experimental method for measuring the full spatial variability of a propagating sound field and its corresponding modal content in two-dimensions, is also discussed. The AR estimator is applied to extract modal content from the real data and interpreted with respect to source/receiver motion and geometry. For a moving source, it is shown that the wavenumber content is Doppler shifted. A method is then described that allows the direct measure of modal group velocities from Doppler shifted wavenumber spectra. Finally, numerical studies are presented addressing the practical issues associated with using MOMAX type data in the exact inversion method of Gelfand-Levitan.

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Using GPS at sea to determine the range between a moving ship and a drifting buoy to centimeter-level accuracy

Cited by 12 publications

References 2 publications

Lagrangian Drifter Dispersion in the Surf Zone: Directionally Spread, Normally Incident Waves

Lagrangian Drifter Dispersion in the Surf Zone: Directionally Spread, Normally Incident Waves

Observing Surf-Zone Dispersion with Drifters

Geoacoustic inversion in laterally varying shallow-water environments using high-resolution wavenumber estimation

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