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



Our initial ideas were to drop SOFAR charges over accurately lo- 

 cated bottom hydrophones off Antigua, time the SOFAR arrivals 800 miles 

 away at Bermuda, and basically use this experimental sound-speed mea- 

 surement to calibrate sound velocimeters, in absolute terms, for the 

 ocean conditions at 3000- to 4000-ft depth and 3 C. This was the only 

 method we could conceive to obtain an absolute sound velocimeter cali- 

 bration for these temperature and pressure conditions. For example, 

 in 1962, when calibrating a sound velocimeter in the laboratory, there 

 was an elusive one-foot-per-second difference in various tabulated 

 values of sound speed for distilled water at surface temperatures 

 and pressures. 



In planning the experiment, we assumed that the axis sound speed 

 at any open-ocean location would be stable. We would use this 800-mile 

 travel-time measurement to calibrate the velocimeters in absolute terms 

 based on multiple lowerings along the transmission path. Since SOFAR 

 charges off Antigua could also be received on the MILS (Missile Impact 

 Location System) sound-channel axis hydrophones at Eleuthera, at 

 Fernando de Noronha off Brazil, at Barbados, and at Ascension, we 

 recorded on these as well. Looking ahead, since our ultimate problem 

 was to accurately locate a missile SOFAR charge in mid-Atlantic, a hy- 

 drophone was obviously needed in the northeast Atlantic to balance 

 any unknown bias from the existing MILS hydrophones to the south and 

 west. Such a hydrophone station was installed in the Canary Islands. 



Figure 2 shows a typical SOFAR signal received over a Sargasso 

 Sea transmission path. Typical, in that for a 900-mile transmission 

 path the signal has a 9-second duration and terminates with a sharp 

 cutoff. For this hydrophone buoyed up into the sound-channel axis. 



