B. Problems and Recommendations 



1. Depth Accuracy 



Accurate depth location is required for two reasons: to ensure 

 correct profile determination and, in the case of indirect methods, for the 

 application of a correct pressure correction to the sound speed measured. 



Expendable instruments are necessarily located by a precalculated 

 sink rate and elapsed time. Possible errors in depth location by this 

 method have been considered and compared with experimental data. The 

 analyses have neglected some possible hydrodynamical problems (e.g. Ref- 

 erence 38) . Other instruments are located in depth by means of pressure 

 transducers or by inverted echo sounder. When using pressure transducers, 

 such as the "Vibrotron," depth is calculated from hydrostatic pressure. 

 Problems may arise when measuring great depths in regions of gravity anom- 

 alies or when the exact density of the water is in question. The inverted 

 echo sounder, on the other hand, is particularly suited for use in this 

 system. This is a method where depth is calculated from the round trip time 

 of a pulse from the echo sounder which is attached to the velocimeter to 

 the sea surface and back to the echo sounder at the velocimeter. Because 

 the velocimeter measures sound velocity during descent, the output of the 

 echo sounder can be reduced using these true velocity data directly, thus 

 avoiding all errors due to uncertainties in water density and local gravity. 



2. Velocity Accuracy 



The accuracy of the available velocimeters as listed in Table III-l 

 is excellent for any naval requirement. Temperature-coefficients of the 

 acoustical path and of the gain and delay of transducers and amplifier, as 

 well as pressure coefficients due to buckling of bulkheads, etc. and other 



43 



Arthur 2l.1Littlc.Knr. 



