An error in travel time or knowledge of V causes the points of inter- 

 section of the hyperbolae to become areas. The size of the area depends 

 on the bearing angles <j> 1 , <|>„ of the listening stations. Let us consider 

 a simple example. The travel time in the sound channel for an acoustic 



pulse over a distance of 3000 km to each of two stations at 90° to each 



3 

 other would be approximately 2 x 10 seconds. If there is an error in 



the value of V of + 2 meters per second, then the source of sound may 

 o — 



lie anywhere in an area of 64 square km. If the bearing between the 



stations is much less than 90°, this area can become a long, thin band 



so that rapid location of a moving source is difficult. The persistence of 



the sound channel indicates that accurate measurements in the channel can 



possibly define the channel over considerable distances for periods of 



time as long as a day. Probably a single profile is not sufficient to 



fully define the channel over a large area, however. For optimum ranging 



an accurate value of V is required. It is worth noting in our example 

 o 



that + 2 meters per second corresponds to a temperature error no 

 greater than about + 1°F, or in considering the pressure correction to a 

 depth error of 110 meters at a depth of 1500 meters coupled with a possible 

 temperature error of + 0.2°F. From the point of view of current tech- 

 nology we should note that velocimeters are quoted by the manufacturers 

 to be accurate to 0.15 meters per second. For field use we might 

 safely double this. The temperature error in XBT measurements is + 0.2°F 

 and the error in depth location (from relatively shallow tests) is 



currently quoted as 2 - 3% of depth implying an error in V of + 0.7 meters 



(1) 

 per second at 1500 meters. 



Obvious questions with regard to tactical situations are: 



1. Would present systems benefit if either deeper data or data 



with higher absolute accuracy were available. 



(1) 



See Conclusions - Section V. 



11 3tthur ZD.Itittlcilnr. 



