We appreciate the comments and discussion by Earle, Steele and Wang in which they call attention to our inadequate understanding of NDBC rationale and methods. The discussion addresses several aspects of wave direction analysis from buoy motions: (i) whether using a single heave-tilt phase response function is a valid approach or separate determinations of heave-pitch and heave-roll phase response functions are required; (ii) NDBC's choice of wave number determination (from the linear dispersion equation or from the linear wave relationship between displacement and slope co-spectra); and (iii) incomplete description of buoy configuration, equipment and methods employed lends uncertainty to interpretations of the data.
We take full responsibility for not providing a detailed description of our 3 m buoy in Huang and Chen (1998). Fig. 1 illustrates this 3 m buoy configuration. The basic parameters of this buoy are: weight 1295 kgf; pitching moment of inertia 1085 kgm-m 2 ; and height of center of gravity 0.46 m. Three equally spaced bridal chains (1 inch size and 1.5 m long) are connected to a swivel and then to a 1 inch mooring chain.
Due to cost considerations, the meteorological subsystem was not incorporated in our buoy, while fixed and gimballed accelerometers and a two-axis inclinometer were used instead of a Datawell Hippy sensor. Since the use of a vertically stabilized Datawell Hippy sensor still produced spurious energy at low frequencies, which was found by Steele et al. (1985) to grow at an increasing rate with significant wave