areas of brackish or "marble-ized" water. The complexity and expense of a 

 suitable device (XSTD) , at this time may be too high to be justified as an 

 expendable instrument in normal situations, but such a determination can- 

 not be made until the instrument, currently under development at NAVOCEANO, 

 is field tested. Interim alternatives in the form of hybrid systems are 

 possible, however, and while they would be compromises, they would add only 

 a fraction to the cost of an expendable BT cast, and need by made only as 

 frequently as necessary to determine the oceanographic conditions. The 

 first involves salinity recognition and the second direct sound speed 

 measurement . 



When the XBT probe is falling through water that is of lower sa- 

 linity than inferential sound speed calculations would permit within 

 accuracy requirements of the tactical situation, an indication can be ob- 

 tained. 



It is possible to modify the expendable BT so that the output of 

 the XBT is modulated, that is, interrupted when it is in brackish water. 

 This rate of interruption would vary with conductivity below any given value 

 of conductivity, with no interruption above that value. It is possible that 

 such a circuit could give sufficient information that the BT information 

 derived could be used within certain depth intervals if perhaps not in others, 

 It would not be necessary that every BT cast use the modified device, but 

 only one every nth cast when in waters where oceanographic conditions are 

 known to be variable. The record would show where brackish water existed, 

 and appropriate action could then be taken, such as use of an XSV. 



48 



Arthur ai.lLittlc.Knr. 



