II. REVIEW OF INDIRECT METHODS FOR THE MEASUREMENT OF SOUND SPEED 



A. Introduction 



As indicated in section I, the indirect method of measuring 

 the sound speed profile consists of measuring the vertical temperature 

 profile and computing the sound speed from this. At present, there are 

 five types of instruments in use to determine the vertical temperature 

 profile. These are the thermistor chain, the reversing thermometer, the 

 mechanical bathythermograph, the expendable bathythermograph, or XBT, and 

 the electronic S.T.D. 



B. Thermistor Chain 



This instrument is intended entirely for survey purposes and 

 would never be used in a tactical situation. The thermistor chain used 

 by USNEL is a string of 34 temperature sensors that operates in essen- 

 tially a subsurface vertical line as the ship moves on the surface 

 (Reference 20) . 



The signals from the sensors are scanned electronically and 

 interpolated for all whole degree centigrade isotherms which are printed 

 on a continuous chart. In addition to this analogue output of two dimen- 

 sional temperature distribution, the water temperatures at the 34 levels 

 are planned to be recorded on punched tapes. The depth of the array 

 is monitored by a pressure transducer at the submerged end, and ducted 

 current meters monitor shear currents. An accelerometer is being applied 

 for stability monitoring. 



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