NDBC's digital directional wave module

The National Data Buoy Center (NDBC) has developed a compact, low power, directional wave measurement system called the Digital Directional Wave Module (DDWM). It represents the latest advance in NDBC's 30-year history of producing wave measurement systems. The DDWM consists of embedded electronics; a commercial off-the-shelf (COTS) motion sensor; custom, embedded software and hardware; and a mounting plate. The COTS sensor measures the earth's magnetic flux density and the buoy's, acceleration and angular rate about three orthogonal axes, aligned with its bow, starboard beam and mast. The module uses the Angular Rate System (ARS) method of determining pitch and roll angles, from which wave directions are derived. Using the same wave-data processing techniques, algorithms, and software as those from earlier NDBC wave systems has reduced development and operating costs and has avoided problems associated with changing data message formats. The DDWM's small size and weight, compared to previous systems, has reduced shipping costs and eased handling, thereby facilitating safe and efficient at-sea servicing. The module's high tolerance for extreme environmental conditions has allowed NDBC to loosen its special handling procedures associated with less robust sensors. New quality assurance tests and test equipment were developed for the digital sensor, the integrated processing package, and digital sensor output. This paper presents DDWM's development strategy, system description, quality assurance, laboratory tests and field tests. Test results show that primary wave parameters from DDWM are within NDBC accuracy standards.