Another interesting observation is that Equation (23) also approaches Equation 

 (12) with the values of 2s = l6. 



Based on some field data, Forristal and his co-workers proposed a Fouric 

 series of the spreading function as 



Hg(f,e) =![!+ I c n . cos(ne) • cos(ne) + I c n • sin(ne) • sin(ne)] (2*+) 

 17 2 n=l n=l 



where 



c n = r 2 (s + l)/[r(s + n + 1) r(s - n + 1)] (25) 



The function Hg(f,e) is intended to be fitted to Equations (12) and (13). In 

 practice, the values of c^ and Cp can be computed from: 



c x = s/(s + 1) = nUi 2 + Bj 2 ) 3 ^ (26) 



c 2 = s(s - l)/(s + l)(s + 2) = tr(A 2 2 + B^) 1 ^ (27) 



This model has been used to increase the statistical reliability of the results 

 by combining several instruments measured in the same location. 



MEASUREMENT OF DIRECTIONAL WAVES BY AN ORBITAL FOLLOWING BUOY 

 The development of directional wave spectra entails four efforts: deploying 

 the buoys; collecting the data; editing the data; and running the data through the 

 analysis routines, resulting in spectral densities, mean directions and spreading. 



MEASURING PROCEDURE 



Upon deployment of a buoy, it may be allowed to free drift, it may be 

 moored, or it may be tethered to the ship. In the free drift mode, a navigational 

 fix is made at the position of deployment. The buoy then free floats completely 

 isolated from the ship with only wind, wave, and current forces acting on it. In 

 this mode, the ship is free to perform other activities, such as maneuvering and 

 seakeeping trials. When these activities are completed, the buoy is retrieved and 

 another fix is determined. These two fixes allow one to calculate the average 

 velocity of the buoy due to the wind and current. 



