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How are spectral wave data derived from buoy motion measurements?
NDBC-reported wave measurements are not directly measured by sensors on board
the buoys. Instead, the accelerometers or inclinometers on board the buoys
measure the heave acceleration or the vertical displacement of the buoy hull
during the wave acquisition time. A Fast Fourier Transform (FFT) is applied to the
data by the processor on board the buoy to transform the data from the
temporal domain into the frequency domain. Note that the raw acceleration or
displacement measurements are not transmitted shore-side. Response amplitude
operator (RAO) processing is then performed on the transformed data to
account for both hull and electronic noise. It is from this transformation
that non-directional spectral wave measurements (i.e., wave energies with
their associated frequencies) are derived. Along with the spectral energies,
measurements such as significant wave height (WVHGT), average wave period
(AVGPD), and dominant period (DOMPD) are also derived from the
transformation.
Note that the wave measurements contained in the SeaBreeze CD-ROM only
include calculations of WVHGT, AVGPD, and DOMPD. To receive spectral wave
energy density data, see the Web page about obtaining
archived data.
For a more detailed explanation on FFT's, see:
Brigham, E.O., 1988: The Fast Fourier Transform and its
Applications. Prentiss Hall
International, 448 pp.
For a detailed description on the applications
of Fast Fourier Transforms to wave data, see:
Tucker, M.J., 1991: Waves in Ocean Engineering: Measurement,
Analysis, and Interpretation.
Ellis Horwood, LTD., 431 pp.
For more information about NDBC's wave measuring systems, refer to:
Steele, K.E. and T.R. Mettlach, 1993: NDBC wave data - current and planned.
Ocean Wave Measurement and Analysis - Proceedings of the Second
International Symposium. ASCE, 198-207.
NDBC also reports directional wave data for selected
stations. Besides buoy heave acceleration, measurements
of hull azimuth, pitch, and roll are also necessary for directional waves.
Three methods exist for the measurement of hull azimuth, pitch, and roll.
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The first method includes the use of a Datawell Hippy sensor to measure vertical
heave acceleration along with pitch and roll. A second sensor, a triaxial magnetometer,
is used to measure hull azimuth. For a more detailed description, refer to:
Steele, K.E., Teng, C-C., and D. W-C. Wang, 1992: Wave direction measurements
using pitch and roll buoys. Ocean Engineering, 19, 4, pp. 349-375.
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The second method (no longer used) uses only the triaxial magnetometer to measure
hull azimuth, pitch, and roll. Vertical heave acceleration is measured by an accelerometer.
For more information, refer to:
Steele, K.E. and M. Earle, 1991: Directional ocean wave spectra using buoy
azimuth, pitch, and roll derived from magnetic field components. IEEE Journal of
Ocean Engineering, 16, 4, pp. 427-433.
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The third method uses three orthogonal angular rate measurements to measure hull
pitch and roll. Vertical heave acceleration is measured by an accelerometer,
and a triaxial magnetometer is used to measure hull azimuth.
For more information, refer to:
Steele, K.E., D.W. Wang, M.D. Earle, E.D. Michelena, R.J. Dagnall, 1998: Buoy
pitch and roll computed using three angular rate sensor, Coastal Engineering,
35, Issues 1-2, October 1998, pp. 123-139.
The processing stream as applied to raw, directional measurements is similar
to that presented above for non-directional data: RAO's are applied to the
acceleration data after all Fourier processing is performed. The main
difference between directional and the non-directional wave data is that, for
directional data, four frequency-dependent parameters are calculated along
with the spectral measurements, WVHGT, AVGPD, and DOMPD. These other
parameters are ALPHA1 (mean wave direction), ALPHA2 (principle wave
direction), and R1 and R2 (parameters which describe the directional
spreading about the main direction).
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