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^® (MicroStrain^®, 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.