at least in the vertical direction is necessary for determining the 

 path of the transmitted sound signal. The data should be sufficiently 



accurate to define — . For sophisticated ray tracing techniques, knowledge 

 dz 



of higher derivatives may be useful. More sophistication might require 



knowledge of horizontal variations in v due to ocean patchiness as well 



as measurements of the sound speed structure in the ocean bottom. 



The needs for sound speed data may be divided into "tactical" 



and "survey." Their requirements overlap in certain cases, e.g., the 



propagation of sound in deep sound channels, and where bottom-bounce 



is utilized. The required absolute accuracy in the measurement of v(z) 



varies. For those active sonars where the upper portion of the water 



column is used, knowledge of the shape of the profile v(z) is sufficient, 



and only relative accuracy is required. For range estimation based on 



propagation in the deep sound channel absolute accuracy in the knowledge 



of v(z) is desirable. 



There are two methods of determining v(z) and -7— . The indirect 



dz 



method consists of measuring the variation of temperature in the vertical 

 direction and computing the sonic velocity from equations such as Equation 

 (1). Generally, the salinity term is taken from reference tables but 

 alternatively, the salinity is measured simultaneously either in situ 

 or from collected water samples. The temperature profile is measured 

 with bathythermographs, thermistor chains or reversing thermometers. 



The direct method uses sound velocimeters which measure the 

 local sound speed at the instrument by transmitting an acoustic signal 

 across a path determined by the instrument size which is generally of 

 the order of a few centimeters. Because of this short path length the 



arthur 21. kittle, Ifnr. 



