Datawell Directional Waverider Buoy
General
The Datawell Datawell Directional Waverider is a 90cm diameter spherical buoy which measures wave height, period and direction. The directional measurement is based on a translational principle so that horizontal motions rather than wave slopes are measured. Consequently the measurement is independent of buoy roll motions and a relatively small spherical buoy may be used. A single point vertical mooring ensures adequate symmetrical horizontal buoy response to resolve small motions at low frequencies. Two 15m lengths of rubber cord are normally included in the mooring. Data are transmitted to a shore based station and/or an Argos satellite.
Sensors
- Heave-pitch-roll sensor Hippy40
- Three axis fluxgate compass
- Two fixed "x" and "y" accelerometers
- Temperature sensor
Principles of Measurement
The Directional Waverider measures vertical accelerations by means of an accelerometer placed on a gravity stabilised platform. This platform is formed by a disk which is suspended in the fluid within a plastic sphere placed in the bottom of the buoy. Two vertical coils are wound around the plastic sphere and one small horizontal coil is placed on the platform. The pitch and roll angles are defined by the amount of coupling between the fixed coils and the coil on the platform. This coupling is measured. The result of the measurement gives the sine of the angles between the coil axes (x and y axes) and the horizontal plane (=platform plane). The fluxgate compass measures the components of the Earth's magnetic field in the direction of the x, y and z axes. All of these measurements define the buoy's position with respect to fixed (north, south and vertical) coordinates.
A transformation matrix of the relation between vector components in buoy coordinates and fixed coordinates is formed. The acceleration of the buoy is measured in the x and y directions (two fixed accelerometers) and in the vertical direction (accelerometer mounted on a stabilised platform).
With the help of the transformation matrix and these measured accelerations, the accelerations in the north-south and east-west directions can be calculated. All the analogue outouts of the sensors are filtered using a low-pass filter with a cut-off frequency of 1.5Hz. At this frequency, the buoy's motions are already strongly attenuated due to the small wavelengths compared to the buoy's dimensions (geometric attenuation).
The filter outputs are sampled at a rate of 3.84Hz. As transformation calculations are performed at this rate by the on-board processor, the three accelerations are available at the same rate. By means of further digital processing, the acceleration data are filtered (low-pass 10th order filter with a cut-off at 0.6Hz) and double integrated to give three translation signals in the frequency range 0.033Hz - 0.6Hz (3dB) (0.05Hz - 0.55Hz 0.31dB). These translation signals are transmitted over the radio link in real time.
A complete spectrum is calculated every half hour. Parameters are calculated at each of 64 frequency points (0.025Hz - 0.100HZ at 0.005Hz intervals and 0.110Hz to 0.580Hz at 0.010Hz intervals). The parameters calculated at each frequecny point are: mean direction, angle of spread, second harmonics of angular energy distribution check factor and relative power density.
Reference
Datawell bv, Zomerlustraat 4, 2012 LM Haarlem, the Netherlands (1995). Operation and Service Manual for "Directional Waverider" from serial no. 30109 and including 30076, 30101, 30106, 30108.
