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.
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.
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).