When the buoy is operating in the environment of a bimodal wave system, it 

 responds to the dynamics of the whole system. The analysis of the measured data 

 in this environment requires the knowledge of the interaction mechanisms of the two 

 wave systems. The corresponding hydrodynamic response of the buoy can then be 

 determined and the results can be estimated. Currently, bimodal wave spectra are 

 not immeditely analyzed as such. The data are analyzed without any assumption of 

 the specific wave model. The procedure can then be modified once the interaction 

 mechanisms of bimodal wave systems have been developed. 



ERROR CONSIDERATIONS 



Errors from mechanical and electrical configurations and components in the 

 measurement of wave height are considered to be manageable or negligible compared 

 to errors attributed to statistical errors and environmental effects. However, the 

 same cannot be said of wave direction measurement. The combined effects of the 

 inclinometers, compass, their alignments and sensitivities produce a wave direction 

 accuracy of +/- 10.0 degrees for the buoy associated with this analysis, as spe- 

 cified by the manufacturer, ENDECO. 



Statistical errors develop during the analysis of the data. The receiver and 

 the buoy's solid state memory sample the data at a rate of one hertz. This 

 sampling rate provides an un-aliased upper frequency limit of 0.5 hertz, though our 

 interest lies in frequencies below 0.33 hertz. When digitized from analog tape, 

 the sample rate is not limited to one hertz, but a practical limit of four hertz 

 has been imposed. The resolution of the receiver and digitizer in the solid state 

 memory yields less than an ,k percent uncertainty. 



Ultimately, the statistical uncertainty, or confidence limits, can be deter- 

 mined from the number of degrees of freedom. The degrees of freedom is a function 

 of the number of segments overlapped, the sample rate and the total time length of 

 the run. The number of degrees of freedom increase with sample rate and with the 

 length of the run, but decrease with the size of the individual segments. Given 

 the number of degrees of freedom, the confidence limits is determined from a chi- 

 square graph. 



Perhaps the greatest errors that can develop occur from environmental effects, 

 such as shallow water depth, current, and mooring. The depth effect is elimi- 

 nated through normalization using the dispersion relation, as previously mentioned. 

 The current effect was also discussed earlier. However, at the time of this 



21 



