ACOUSTIC SYSTEMS 



Precise underwater navigation requires a means of measuring 

 velocity through the water and, if possible, relative to the bottom. The 

 ability to detect and identify underwater objects is also necessary, not 

 only to navigation but also to undersea engineering operations and 

 scientific studies. The most effective means of measuring velocity and 

 detecting and identifying objects are the use of acoustic sensors. The 

 two prime systems currently in use for velocity sensing are the 

 electromagnetic log and the sonic doppler log. 



The electromagnetic log has hitherto been able to measure only fore- 

 aft velocity relative to the surrounding water. Experimental studies with 

 a flush-mounted, bi-directional sensor on a submarine indicate that the 

 electromagnetic log can and should be adapted to measure athwartships 

 as well as fore-aft velocity. 



The sonic doppler log now in use senses velocity relative to the bot- 

 tom, but is effective only within 500 feet of the seafloor. An experimen- 

 tal pulse sonic doppler log system tested at the Atlantic Undersea Test 

 and Evaluation Center offers the potential for accurate navigation in 

 waters 1.500 feet or more above the bottom. However, the space re- 

 quired for the system and its weight (several hundred pounds) limit its 

 use to longer subsurface vessels. 



Many USN missions require the detection and positive identification 

 of underwater objects. The development of technology for these pur- 

 poses is the goal of the USN Acoustic Imaging Program. To accomplish 

 this objective, a system is being designed to achieve a recognizable im- 

 age, in turbid water, of a beer can at ranges to 25 feet, anywhere within a 

 22° by 11° field of view. Larger objects may be imaged at ranges to 150 

 feet. Pictures will initially be produced at the rate of one every 5 sec- 

 onds. With continued development of the system, it is expected that 30 

 pictures per second can be produced. 



The acoustic-imaging system will be installed aboard manned sub- 

 mersibles descending to depths of 12,000 feet. The system will be used in 

 conjunction with a search sonar to provide target identification, similar 

 to a television, but with greater range and less sensitivity to water tur- 

 bidity. As a visual system, it will enable the operators of underwater 

 work systems to see what they are doing, even through the mud turned 

 up by bottom operations. 



RESEARCH TO SUPPORT DIVER OPERATIONS 



Until recently, diving scientists, living saturated in oxygen-nitrogen 

 mixtures in habitats on the Continental Shelf, have not had tables to tell 

 them how long they can stay at lower depths without making 

 decompression stops on the way back to the habitat. Such tables have 

 now been developed in a USN/NOAA project. The tables are used in 



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