Wave Spectra Corrections for Measurements of Hull-Fixed Accelerometers

The NOAA Data Buoy Center (NDBC) routinely operates approximately forty nondirectional wave measurement systems aboard a variety of hull types. In almost all cases, the wave sensor is a linear accelerometer with its axis fixed parallel to the hull vertical and thus not vertically stabilized relative to the earth. From earlier NDBC work, it is known that fixing the accelerometer introduces spurious energy into acceleration spectra at frequencies, particularly low frequencies, where there is not actual wave energy. Since 1977, a frequency dependent, but sea-state independent, correction function has been used to remove this spurious energy and to produce satisfactory wave data for most wave situations. This paper describes development of a new improved correction method that depends on sea-state as well as frequency. Sea-state dependence is included in a computationally simple manner through the use of mean-square hull slope, as estimated from acceleration spectra measured by the fixed accelerometer. This sea-state dependent noise correction function substantially improves the accuracies of low frequency swell measurements.