Buoy pitch and roll computed using three angular rate sensors

Abstract

The National Data Buoy Center (NDBC), a part of the US National Weather Service (NWS), operates networks of data buoys. Each buoy measures, and reports to shore each hour, air temperature, water temperature, barometric pressure, wind speed, wind direction, and nondirectional wave data. Some 15 discus-type buoys report directional wave data each hour. The systems aboard these buoys apply measurement and analysis techniques that require time-series records of hull east and north slopes, which are, in turn, based on measurements of hull bow azimuth, pitch, and roll. Pitch and roll are normally measured on NDBC hulls using the Datawell (Netherlands) Hippy 40 model sensor. Time-series records of pitch and roll thus measured are stored in the onboard computer memory of a wave processing module. These pitch and roll records are used in conjunction with hull azimuth and heave acceleration time series to compute directional wave data. The Hippy 40 heave, pitch, roll sensor consumes little power, is reliable when handled correctly, and provides sufficiently accurate measurements of pitch and roll. On the other hand, it is heavy, large, requires special handling, and is expensive. Thus, NDBC needs an alternative basis for the measurement of hull pitch and roll. In recent years, low-cost, solid-state angular rate sensors have become available. Three of these small, inexpensive sensors, when mounted orthogonally in a buoy, can produce estimates of the body-fixed components of the angular velocity vector. From the time-series outputs of these sensors aboard a discus hull, there are mathematical means to produce time-series records of wave-frequency pitch and roll relative to mean pitch and roll. Other sensors provide mean values of pitch and roll, so that by summing, instantaneous total hull pitch and roll records can be produced. The NDBC existing onboard-buoy wave processing module (WPM) system can process these total pitch and roll records just as if they had come from a Hippy 40, so that little change to NDBC's existing WPM hardware/software, or to the shore-based supporting software, is necessary to produce a new directional wave measurement system based on angular rate sensors. This paper describes the techniques NDBC is employing to produce wave-frequency pitch and roll relative to mean pitch and roll, and the necessary additional sensors needed to determine mean pitch and mean roll. The validity of these techniques is demonstrated by comparing pitch and roll records resulting from their application to pitch and roll records taken directly from the Hippy 40. NDBC's plans for the employment of this new pitch-roll measuring technique in a modified WPM system are summarized.