Enhancements to NDBC's Digital Directional Wave Module

The present paper describes advancements that the National Data Buoy Center (NDBC) has achieved recently in improving its Digital Directional Wave Module (DDWM). Using the 3DM-GX1
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(MicroStrain
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, inc.) in the DDWM has expanded wave-measuring capability by addition of a triaxial acceleration package in place of a single, along-mast accelerometer used in the older NDBC wave system called the Angular Rate Sensor (ARS). NDBC has initiated two important changes in onboard processing. First, we transform along-mast accelerations to true vertical accelerations using measurements of pitch and roll angle. This change eliminates potential errors arising from a constant buoy heel angle. Second, NDBC has changed from a linear noise correction to an f
^-2
inverse power function. The NDBC Engineering Laboratory has carefully tested these changes using apparatus of the Dr. Ed Michelena Sensor Test Facility, NDBC's latest mechanical wave machine, the Desktop Wave Simulator (DTWS) and a detailed electronic computational wave model, the Directional Wave Simulation Software (DWSS). The DWSS was developed to examine extreme conditions that cannot be simulated in the laboratory. It also provides a method to obtain a known input for end-to-end system testing or evaluating future on-board processing modifications. In addition, triaxial accelerations are used to determine the average pitch and roll angles of the platform without need of a separate tilt sensor. The latest version of the DDWM can be used in either directional or nondirectional modes. A compact flash memory unit is now installed as a standard feature on all DDWM units, providing high-resolution records of raw measurements.