Beng Chun Lee

Wave Measurements Using GPS Velocity Signals

This study presents the idea of using GPS-output velocity signals to obtain wave measurement data. The application of the transformation from a velocity spectrum to a displacement spectrum in conjunction with the directional wave spectral theory are the core concepts in this study. Laboratory experiments were conducted to verify the accuracy of the inversed displacement of the surface of the sea. A GPS device was installed on a moored accelerometer buoy to verify the GPS-derived wave parameters. It was determined that loss or drifting of the GPS signal, as well as energy spikes occurring in the low frequency band led to erroneous measurements. Through the application of moving average skill and a process of frequency cut-off to the GPS output velocity, correlations between GPS-derived, and accelerometer buoy-measured significant wave heights and periods were both improved to 0.95. The GPS-derived one-dimensional and directional wave spectra were in agreement with the measurements. Despite the direction verification showing a 10° bias, this exercise still provided useful information with sufficient accuracy for a number of specific purposes. The results presented in this study indicate that using GPS output velocity is a reasonable alternative for the measurement of ocean waves.

EXPERIMENTAL STUDY OF THE REVERSE CONVERSION OF WAVE SPECTRA

The observed deep-sea wave data are most appropriate to use in the data assimilation technique. For wave stations (e.g., the Longdong data buoy station in the northeastern Taiwan Sea near the coast), wave data must be reverse-calculate deep-sea wave data for data assimilation applications because seafloor slopes in the northeastern Taiwan Sea are acute and the distances between wave interactions during propagation are shorter. Consequently, nonlinear effects arising from the topography become insignificant as waves approach the coast from the open sea. The proposed technique to reverse-calculate the deep-sea wave spectrum from the near-shore wave spectrum, based on first-order wave spectrum theory, is verified based on physical experiments. The results indicate that this method is effective and applicable to calculations of the energy at wave spectrum peaks.