INTRODUCTION TO SONAR 



be able to provide sonar data on contacts many 

 miles distant. 



Sound has a natural tendency to seek paths 

 toward the cooler layers of the sea. Because the 

 temperature of the sea normally decreases with 

 depth, the path of a transmitted pulse of sound 

 usually is in a downward direction. 



If a cross section or a profile of the sea's 

 temperatures were taken, a normal condition 

 might show a layer of water of uniform tempera- 

 ture (less than 1/2° temperature change) from 

 the surface to varying depths. This condition is 

 called isothermal. Next, there would be a region 

 of water in which the temperature decreased 

 rapidly with depth. Such an area is known as a 

 thermocline. Finally, for the remainder of the 

 measured depth, the temperature would decrease 

 only slightly with depth. 



THERMOCLINES 



The thermocline can play havoc with a pulse 

 of acoustical energy. As the transmitted sound 

 pulse reaches the thermocline, one of two effects 

 is apparent. 



First, the thermocline can prevent passage of 

 the pulse, reflecting it back to the surface. Targets 

 beneath the thermocline may possibly be un- 

 detected. This possibility is one of the reasons 

 submarine commanding officers seek the cover 

 of such thermoclines, thus hoping to evade 

 detection by surface ships or aircraft. 



Second, the thermocline can allow passage 

 of the sound pulse but alter its direction con- 

 siderably in so doing. This effect is called 

 refraction. If a sound pulse enters a thermo- 

 cline at, for instance, a 30° angle from the 

 sea's surface, it is possible for the angle to be 

 altered to 70° or more while traveling through 

 the thermocline, and change again as it emerges. 

 The result of this refraction can be a distorted 

 path of sound travel that affects the accuracy 

 of target presentation at the sonar console. 



SUBMARINE BATHYTHERMOGRAPH 



More information is required from a sub- 

 marine BT than from its shipboard counterpart. 

 Because salinity, temperature, and pressure 

 affect the ability of a submarine to maintain 

 desired operating depth, knowledge of these 

 conditions and their effect on buoyancy is neces- 

 sary so that the diving officer can maintain 

 trim of the submarine. The AN/BSH-2D and the 

 AN/BQH-IA (which is replacing the AN/BSH-2D) 



are the types of BTs that provide the necessary 

 data. 



AN/BSH-2D 



The AN/BSH-2D is designed for use in all 

 types of submarines. It indicates changes in 

 sound velocity with depth, and shows direction 

 of buoyancy change. It can measure and indicate 

 temperatures between 40°F and 90°F, salinity 

 content from 20 to 40 parts per thousand parts 

 of water, depths between and 800 feet, and 

 buoyancy from to 100,000 pounds. Whenever 

 a change occurs in salinity, temperature, or 

 depth, their new values are indicated within 

 a few seconds. 



The AN/BSH-2D consists of five units: two 

 salinity-temperature elements (one on the peri- 

 scope shears and one on the hull below the 

 waterline), an amplifier computer, a recorder, 

 and a switch for selecting a salinity-temperature 

 element. 



The selector switch chooses one of the salinity- 

 temperature elements and connects it to the 

 amplifier computer. Changes in salinity and 

 temperature cause the element to change its 

 electrical resistance characteristics. The ampli- 

 fier computer converts the change in electrical 

 resistance into corresponding voltage changes, 

 amplifies the signal, computes the sound velocity 

 and buoyancy changes, and transmits the informa- 

 tion to the recorder unit. This unit then displays 

 the data on a card. Accuracy of the AN/BSH-2D 

 is ±8 feet in depth, ± 6 feet per second in sound 

 velocity, and 2-1/2 percent of the change in 

 buoyancy. 



DEPTH-SOUND SPEED MEASURING 

 SET AN/BQH-IA 



The AN/BQH-IA is a completely transis- 

 torized device used to provide accurate informa- 

 tion concerning soi.md ranging conditions in the 

 surrounding water and buoyancy during diving 

 operations. 



The equipment is capable of measuring sound 

 velocity in sea water over a range of 4600 to 

 5100 fps. Accurate depth measurement is provided 

 by a depth element and its associated circuitry. 

 Velocity of sound and depth of the sensing element, 

 which measures this velocity, are displayed on a 

 two-channel (pen and drum) servomechanism 

 recorder (fig. 5-1). 



Maintaining the trim of a moving submarine 

 is aided greatly by indicating on the recorder 

 chEirt a series of isoballast lines. These lines 



58 



