53 - 



With this convention, the program as written will generate the 

 directional distribution with the greatest energy concentrated about 

 the compass heading to which the waves are moving towards with DPG 

 arm 1 =0 degrees. (The heading is later adjusted to represent the 

 compass direction from which the waves propagate with respect to true 

 north.) 



The absolute signal is then reduced to pounds per square inch 

 absolute (PSIA) . 



4. Instrument Tilt Approximation 



The mean of each differential pressure record is compared to 

 the zero-wave condition tare as determined in the laboratory. If the 

 mean of a differential record is assumed to represent a zero-wave 

 condition in the field, then a discrepancy between the mean and the 

 laboratory tare could indicate instrument tilt above the seabed. An 

 approximation of the tilt of each arm is calculated using this 

 difference between the mean and tare for each transducer record. If 

 the arm tilts about the horizontal an angle a, as shown in Figure 

 IV-3, then a pressure difference between the sensors is detected. 

 This pressure differential is converted to a pressure head, Q, and 

 the tilt a approximated from the arctangent of Q/ax. Specifically, 



AP 



(Y 



\f> 



1 

 Ax 



(4.34) 



