Chapter 6 — PRINCIPLES OF SONAR 



detected sound. A supersonic converter is used 

 to increase tlie spectrum of detectable frequen- 

 cies. 



Accurate bearing is determined by training 

 the hydrophone back and forth across the direc- 

 tion of the sound source. Outputs of the right 

 and left halves of the hydrophone are fed to a 

 right-left indicator (RLI). Any phase difference 

 between the right and left signals causes the 

 meter to deflect. 



Operation of the RLI is diagramed in figure 

 6-8. In part A of the illustration, the hydro- 

 phone is on target. Signals in both halves (right 

 and left) produce a phase difference equal to 

 zero. No deflection is indicated by the meter 

 needle. In part B, the hydrophone is trained 

 off the tcirget to the right, and the meter needle 

 is deflected in the "train left" direction. The 

 meter indication informs the operator that he 

 must train left to obtain a phase difference of 

 zero. The hydrophone in part C is trained to 

 a position left of the target. A right train is 

 necessary to obtain the desired bearing. 



The majority of passive sonar systems are 

 equipped with the automatic target follower (ATF) 

 feature. With the ATF feature, right and left 

 signals are fed back into the training system, 

 causing the hydrophone to follow the target 

 automatically. 



Array Sonar 



Modern sonars utilize a hydrophone array, 

 which is installed in one of two configurations. 

 The conformal array is curved around the sub- 

 marine's hull, with an open end aft. The circular 

 array consists of a num'aer of hydrophones 

 arranged vertically in a circle and mounted 

 in or under the submarine's bow. 



MODERN PASSIVE SONAR THEORY 



The theory of modern passive sonar consists 

 of two basic steps: reception and presentation. 

 Figure 6-9 is a simplified block diagram of an 

 array type of passive sonar. The array cannot 

 be trained physically. Instead, a compensator 

 switch is added that, in effect, trains the system 

 electronically. 



Reception 



Signals received in an array type of passive 

 sonar system are converted to electrical energy 



PRE 

 AMP 



COMP 

 SW. 



LAG 



LINES 



AUDIO 



AMP 





A 



71.55 

 Figure 6-9. — Block diagram of an array-type 

 passive sonar. 



amd then are fed to a preamplifier to be ampli- 

 fied to a usable level. There is one preamplifier 

 for each hydrophone in the array. The output 

 from each preamplifier is connected to a slip- 

 ring in the compensator switch. The slipring 

 couples the preamplifier output to a rotor plate. 



One side of the rotor plate consists of an 

 equal niunber of brushes and sliprlngs, each 

 brush riding on a slipring. On the other side 

 of the rotor plate, a set of brushes is arranged 

 in a scale model of the hydrophone array. 

 These latter brushes couple the rotor plate 

 output to the stator plate. The stator plate has 

 two sets of bars inlaid on the plate, one set 

 on each half of the plate. Brushes on the rotor 

 facing the stator plate make contact with the 

 bars as the rotor is trained, and the signals 

 present on the brushes are picked off by the 

 bars. Half of the brushes on the rotor plate 

 are always in contact with the stator plate, 

 thereby utilizing half of the array at any given 

 time. The center of the stator plate, being the 

 reference point, makes it possible to determine 

 the exact bearing of the target. 



The arrangement of the hydrophones causes 

 the signals to be out of phase with each other 

 at the output of the preamplifiers. For the 

 signals to be usable, they must be placed in 

 phase with each other, hence, it is necessary 

 to delay the signal. Each bar in the stator plate 

 is connected to lag lines. The purpose of lag 

 lines is to delay the first received signals a 

 proportional amount until the last received sig- 

 nals can catch up. 



Once the signals are in phase with each 

 other, they are additive. As a result we have 

 a strong signal to feed to the audio amplifier. 

 There the signal is amplified and then is fed 

 to an indicator. 



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