Fig. 2. Stabilized transducer housing. 



The ultrasonic transducers are mounted on the 

 bottom of the housing. 



The vertical gyro, which has a low erection 

 rate after spinup, acts as a long-period pendu- 

 lum and provides a vertical reference accurate 

 to l/8 degree in spite of normal motions of the 

 ship's bow and independent of the accuracy of 

 the mounting surface. The housing is roll stabi- 

 lized and the inertial quality, temperature regu- 

 lated accelerometer is additionally stabilized 

 in pitch. It is mounted on a small servo driven 

 pitch cradle that is erected to synchro signals 

 received from the vertical gyro. 



The output of the accelerometer is a direct 

 current proportional to the true vertical accel- 

 eration including gravity. The current is con- 

 verted to voltage by means of a precision pick- 

 off resistor located in the interior electronic 

 package shown in Fig. 3- The gravity component 

 is removed and the signal doubly integrated to 

 produce a voltage proportional to the vertical 

 displacement of the instrument housing. Distance 

 from the housing to the local sea surface is 

 obtained by means of a pulsed, ultrasonic, echo- 

 ranging height sensor previously developed by 

 RCA for hydrofoil craft control applications. 

 The two signals are subtracted and combined with 

 an adjustable bias to give the instantaneous dis- 

 placement of the local sea surface from mean sea 

 level. Correction for the variation of the 

 velocity of sound with temperature is accom- 

 plished by means of a single dial setting. This 

 could be made completely automatic, if desired, 

 by addition of a thermistor temperature sensing 

 circuit. 



TRANSDUCER BEAMWIDTH 



Most of the problems encountered in developing 

 the wave height sensor arose, naturally enough, 

 from such properties of the ocean surface as its 

 local slope, reflectivity, displacement and 

 velocity. The first two properties primarily 

 affect the design of the echo-ranging part of 



Fig. 3- Interior electronic package. 



the wave height sensor. As shown in Fig. k, the 

 apparent profile of a wave is distorted by an 

 excessively wide beamwidth. Fig. 5 shows quan- 

 titatively the error due to beamwidth in the 

 presence of sloping wave surfaces. The lower 

 group of curves corresponds to the shortest range 

 within the beamwidth and the upper group to the 

 longest range. For pulsed operation the lower 

 group represents the first return. The extreme 

 curve of the lower group shows the return for 

 specular reflection, which is necessarily from 

 perpendicular incidence. It implies operation 



102 



