National Data Buoy Center 1.8-meter Discus Buoy, Directional Wave System

Since year 2005, the national data buoy center (NDBC) has been developing, testing, evaluating and refining a pitch-roll-heave directional wave measuring system. It consists of a 1.8-meter-diameter isomer foam flotation ring in the center of which is placed a cylindrical battery and instrument compartment. A MicroStrainreg 3DM-GX1 motion sensor, located at the center of flotation, provides a stream of nine measurements subsampled at a rate of 2,048 samples in 1,200 seconds. The buoy transmits standard NDBC directional wave spectral information each hour via an iridiumreg satellite modem after computing directional wave spectra from triaxial components of earth magnetic flux density and angular rate, and along-mast acceleration. Additionally, high-resolution data are stored onboard the station for post-deployment analysis. Testing has been done in seven steps. First, static testing of the 3DM-GX1 revealed that the measurement mode of the sensor yields varying amounts of electronic and processor noise. The best mode and time constants for the sensor were determined in the second step of development, using a desktop wave simulator designed exclusively for that purpose. It provided precisely known simulated directional wave information. In the third step, the complete buoy payload, including an air-depolarized, alkaline battery pack manufactured by cegasa International was placed on the one-meter radius NDBC directional wave simulator, with which a full end-to-end test was conducted. Fourth, after sufficient laboratory testing, the complete system was deployed near NDBC station 42007 in the northern Gulf of Mexico for several days, and there was found close agreement between measurements from the two platforms. Additional, as a fifth step, a second 1.8-m buoy with a different battery pack, consisting of 700 alkaline D-cells, was evaluated and found to be adequate, yet the batteries interfered with magnetometer measurements more than the smaller, lighter cegasareg batter- y pack. In the sixth step, the buoy with cegasa batteries was deployed in shallow water at the U.S. Army Corps of Engineers Field Research Facility, Duck, North Carolina, from December 2006 until April 2007. This deployment provided two sources of accurate directional wave data as a basis for comparison, the FRF 8-meter array of bottom-mounted pressure sensors and a datawell waveriderreg buoy. Comparisons indicated that the 1.8-m buoy gives excellent non-directional wave spectra and accurate wave directions. A small drawback, an area of on-going research, is that the buoy gives lower than desired spreading function values. It produces characteristically lower values than does the lighter, particle-following waverider buoy. Finally, NDBC deployed an identical, second 1.8-meter buoy off Mission Bay, California, next to another datawell waverider, from February to June 2007. Directional wave accuracy, using swell waves from a distant storm as ground truth, has proven to be excellent, although, as with the first deployment at Duck, directional spreading is less than desired. We speculate this is due to size and shape of hull and limitations arising from pitch-roll signal -to-noise ratio.