Directional Wave Spectral Data from a Large Discus Buoy

Abstract

A 12-meter diameter discus buoy was i n s t r umented by t h e N O M Data Buoy Office (NDBO) f o r t h e measurement and near-real-time reporting of wave d i r e c t i o n a l s p e c t r a . Time series of buoy heave displacement, pitch, roll, and heading angles were processed into covariance functions on board the buoy. The covariances were then t e l e m e t e r e d t o s h o r e f o r f u r t h e r p r o c e s s i n g i n t o a Fourier series azimuthal approximation of the wave energy frequency/direction spectrum. The data processing procedures are described and examples of t h e d a t a are presented herein. Plans for continued development of the technique are a l s o o u t l i n e d . 1 . I n t r o d u c t i o n An automated data acquisition and processing system based on a 16-bit minicomputer w a s i n s t a l l e d and operated on board a 12-meter d i scus buoy (XERB) by NDBO, t o o b t a i n n e a r -r e a ltime estimates of t h e sea s u r f a c e d i r e c t i o n a l spectrum via the technique of Longuet-Higgins, Cartwright, and Smithl. Buoy motion was measured by a heave sensor, a p i t c h / r o l l s e n s o r , and a gyrocompass ( l a t e r r e p l a c e d by a t r i a x i a l magnetometer). Digitized time series of sensor outputs were processed into covariances on the buoy f o r t r a n s m i s s i o n t o s h o r e v i a a n HF r a d i o link. Further processing was performed on shore t o p r o d u c e t h e d i r e c t i o n a l s p e c t r a l e s t i m a t e . D i g i t i z e d s e n s o r d a t a w a s a l s o s t o r e d on board t h e buoy on magnetic tape. (A more d e t a i l e d desc r i p t i o n of the system hardware i s c o n t a i n e d i n a companion paper.2) This paper describes the techniques used for handling and processing the d a t a , t h e methods used t o e v a l u a t e t h e p r o c e s s e d data, and some preliminary results. Modifications made t o and proposed f o r t h e d a t a p r o c e s s i n g procedure, and NDBO p l a n s f o r f u r t h e r development of a d i r e c t i o n a l s p e c t r a l measurement c a p a b i l i t y , a r e a l s o p r e s e n t e d . 2. Description of the Technique The buoy motion sensors were r i g i d l y mounted i n a coordinate system, X, f i x e d i n t h e buoy, which may b e t a k e n t o b e as shown i n Figu r e 2 of S t e e l e , et a1.2 However, t h e s p e c t r a l estimate S(f,$) is computed from time s e r i e s of t h r e e q u a n t i t i e s which a r e e x p r e s s e d i n a n e a r t hfixed system, x, having the +x-axis in the east d i r e c t i o n and the +z-axis toward the zenith. The q u a n t i t i e s r e q u i r e d f o r t h e s p e c t r a l estimate, e x p r e s s e d i n c o n t i n u o u s n o t a t i o n , a r e d2 ' l o w = ;iE2rl (xo,y0,t) where q & , y , t ) = sea s u r f a c e e l e v a t i o n a t ( x , y ) and ll ( t ) = -r l (X3YYt) Ix = xo, = yo a Y aY * (3) It was assumed t h a t t h e buoy remained suff i c i e n t l y n e a r t o (xo, yo) during the data acq u i s i t i o n c y c l e and ...