0.1 second and the highest non-ambiguous envelope 

 frequency is about 10 cps. A carrier frequency 

 in the neighborhood of 35 to ^0 Kcps is suitable 

 for either type of modulation since it is high 

 enough to avoid machinery noise interference and 

 low enough to avoid the effect of the rapidly 

 increasing attenuation in the region of 50 Kcps 

 and above . 



For a relative horizontal velocity of 2 feet 

 per second, the 38 Kcps transmitted frequency is 

 shifted by ±19 cps for a half beamwidth of 

 8 degrees. A continuous spectrum is generated 

 by echoes from various points within the beam. 

 The envelope of such a signal approaches 100$ 

 modulation at noise frequencies up to 19 cps and 

 interferes with the necessary signal modulation 

 of 10 cps. Pulse modulation was chosen for the 

 RCA height sensor to avoid this source of noise 

 and to obviate the necessity for heavy filtering 

 with attendant low time response. Operation on 

 the leading edge of the return pulse eliminates 

 the effects of doppler shift. 



MINIMIZATION OF THE EFFECTS OF SPRAY 



The effect of spray on the distance measure- 

 ment is minimized by the action of a slow auto- 

 matic gain control loop. Spray droplets are 

 generally small compared to the transmitted wave- 

 length and, therefore, tend to scatter the energy 

 incident upon them. If a sheet of spray is suf- 

 ficiently dense the true echo may be so reduced 

 in intensity that it will not exceed the auto- 

 matic gain control threshold and hence will be 



lost. To provide for this case the previous cor- 

 rect range must be held. Since the height sensor 

 measures range by counting clock pulses, the pre- 

 vious count can be held exactly for any desired 

 period of time. If the spray persists, the auto- 

 matic gain control loop gain is automatically 

 adjusted, within limits to pass the true signal. 

 It is unlikely that a persistent wind-borne spray 

 would produce a sharply defined echo at suffi- 

 cient intensity to exceed that of the true echo 

 because the particles of such a spray are neces- 

 sarily finer and more uniformly distributed than 

 those which occur momentarily in dense sheets 

 near the surface of the water. 



SHIP MOTION COMPENSATION 



Ocean waves further affect the design and per- 

 formance of the wave height sensor by causing 

 motions of the ship's bow upon which the wave 

 height sensor is mounted. The principal motions 

 are pitch, roll and the attendant translations 

 of the housing. It is required that the ultra- 

 sonic transducers and the input axis of the accel- 

 erometer be maintained approximately vertical but 

 the necessary accuracy for the accelerometer is 

 much greater than for the transducers . It is for 

 this reason that the accelerometer is pitch sta- 

 bilized while the transducers are not. Surge and 

 sway accelerations are eliminated by keeping the 

 accelerometer vertical and heave acceleration is 

 used to derive the heave displacement height com- 

 ponent which is subtracted from the echo ranging 

 height . 



Fig. 6 shows the Neumann spectrum for a fully 

 developed state 6 sea and also the rough relative 



60 



50 



40- 



SPECTRAL 



DENSITY 



[AM] 2 30 



IN 



FT. 2 -SEC 



20- 



-5 



-4 



RELATIVE 

 3 SHIP 

 RESPONSES 



-2 



6 8 10 12 



FREQUENCY IN RAD. /SEC. 



Fig. 6. Neumann spectrum of state 6 sea. 



1.6 



1014- 



