38. The short and intermediate arrays are nested within the long array 

 and are tuned to ranges of wavelengths to be resolved. The short arrays are 

 used to measure directional distributions of shorter wavelength wind waves; 

 the longer arrays are used to measure longer wavelengths. The frequency and 

 corresponding wavelength groupings are: 



Wavelengths , m 

 Subarray Name Frequencies. Hz (in 8 m of water) 



Short Array 0.19 to 0.33 38 to 14 



Short- Intermediate Array 0.14 to 0.19 57 to 38 



Long- Intermediate Array 0.08 to 0.14 107 to 57 



Long Array 0.05 to 0.08 175 to 107 



39. Detailed analysis of the behavior of high-resolution spectral 

 estimators has suggested the design of these nested subarrays . When waves are 

 directionally spread about a central angle, superposition of the different 

 longshore wavelengths makes the Co and Quad data appear as sinusoids which 

 decay exponentially with increasing lag. If the sensors are too far apart, 

 the exponential decay causes Co and Quad curves to approach zero. If too much 

 of the lag axis contains these low values, high-resolution estimators mis- 

 behave and directional spectra are less reliable. 



40. The FRF linear array has been designed to avoid this problem by 

 proportionally truncating the size of the array for different ranges of 

 wavelength. This limits the amount of decay in the Co and Quad curves and so 

 increases reliability of estimation algorithms. 



A15 



