HAMILTON: TIME VARIATIONS OF SOUND SPEED OVER LONG PATHS IN THE OCEAN 



in the South Atlantic, and Barbados just 300 miles to the south of 

 Antigua. The obvious conclusions we reached from this were that we 

 couldn't predict the axis sound speed with the required accuracy for 

 missile-splash location. 



Note the long-term correlation between Eleuthera and Bermuda. 

 For the first half of the program, there is a six-month cycle, rather 

 well displayed, that occurs four months later at Eleuthera than Bermuda. 



Shown in Figure 7 are the results of an eight-day SCAVE with sig- 

 nals at hourly intervals. The dots are the actual sound speeds measured. 

 The dark line is a seven-point moving average. Although the sound- 

 speeds change over this eight-day period, they change slowly. Hydrophones 

 with closely adjacent transmission paths have similar changes. It was 

 from these data that we developed a system for using SOFAR signals to pro- 

 vide accurate missile-impact location estimates. 



Essentially we calibrated or measured the axis sound speed for each 

 missile test for each receiving hydrophone. This was done by firing 

 SOFAR charges in the missile-impact area over bottom transponders which 

 had already been located. Ten SOFAR charges were fired before the 

 test and ten after the test. An average measured SOFAR speed for each 

 hydrophone was used for calculating the missile splash position. 



In Figure 7, results from the Bermuda suspended phone for the 

 first 3 days suggest a sound-speed variation with the period of a semi- 

 diurnal tide. This Bermuda phone was at the 4000' sound-channel axis 

 depth, but buoyed 5000' off the bottom. It was not unreasonable to 

 suspect that this hydrophone moves back and forth with tidal currents 

 at Bermuda. Without data on this hydrophone's movements, it is there- 

 fore impossible to state whether this is an actual sound-speed varia- 

 tion or an artifact of hydrophone motion. The hour-to-hour variations 



17 



