is especially so in a military instrumentation system where doctrine may 

 establish the manner of utilization of a system or of its components. The 

 user comes to rely on the schemata of the system rather than upon an inter- 

 pretation of the meanings of each of the measured variables in the system. 



A particular problem in system performance arises when there is no 

 alternative method either for measuring an important variable or for 

 assessing the context in which the measurement is being made. A case in 

 point which particularly applies here is the use of a bathythermograph 

 aboard a ship in an ASW exercise. In most operating areas, it would be 

 quite reasonable to assume a fairly uniform salinity structure and therefore 

 be able to infer local sound speed structure from the temperature trace 

 recorded. However, if the area is a so-called shallow water area, that is, 

 one where internal waves or strong salinity gradients are present, the tem- 

 perature profile measurement at only one point may not provide adequate 

 information useful to the sonar operator. In fact, it may lead him to 

 erroneous conclusions. Historical data will allow him to hazard a guess as 

 to the utility of the measurements he is receiving, but in coastal areas or 

 off the mouths of sizable rivers, the exact meander of the fresh water 

 plume and its consequent eddies may be unknown. Some indication of the 

 existence of such a problem can be gained by monitoring the intake water 

 salinity, but it is unlikely that under stress the sonar crew will be able 

 to quickly recognize such indications, even if they are capable of inter- 

 preting them. 



A study has been made (Reference 39) which shows that a rather 

 careful measurement of conductivity must be made in order to yield sound 

 velocity structure to the accuracy a sonar officer would like to have in 



47 



Srthur 3l.3Uttlc.Ilnr. 



