372 



DIRECT MEASUREMENT TECHNIQUES 



30 



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K 

 lU 



Q. 



cn 20 



-I 

 ui 

 a 



u 

 111 



a 



z 



- 15 



I- 

 z 



bJ 

 U 



O 

 u 



z 



10 



10 



20 30 40 50 60 



FREQUENCY IN KILOCYCLES 



70 



80 



90 



100 



Figure 7. Attenuation coefficient as a function of frequency. 



transmission loss by the latter method is the presence 

 of four vessels in the operating area, that is, sub- 

 marine, escort vessel, echo-ranging vessel, and trans- 

 mission measuring vessel. The use of a hydrophone 

 mounted on a submarine is a definite improvement 

 but introduces new horizontal and vertical directivity 

 problems as well as installation complications. 



21.5.2 Inadequacy of Transmission- 

 Loss Measurements 



All three methods have been used to measure 

 transmission loss during direct target strength tests. 

 Where ample and consistent data have been taken 

 by any one of these methods, the transmission loss 

 calculated from these data has been used to evaluate 

 the target strength. 



Often, however, data have not been consistent. 

 During one run at San Diego, for example, the plot 

 of the echo level, corrected for inverse square law 

 spreading against range, indicated an attenuation 



coefficient of 19 db per kyd at a frequency of 60 kc 

 while measurements aboard the submarine when 

 analyzed showed a value of only 10 db per kyd. An- 

 other identical run the following day gave values for 

 the attenuation coefficient of 11.5 and 16 db per kyd, 

 respectively, as measured by the two methods. Ap- 

 parently the errors were not systematic. This lack of 

 consistency between two methods was not infre- 

 quent. Recent San Diego target strength measure- 

 ments, however, based on transmission loss measured 

 with a nondirectional hydrophone mounted on the 

 submarine, have been more consistent; this method 

 promises to eliminate much of the uncertainty in the 

 evaluation of the transmission loss. However, the 

 measurements reported '^~'* indicate that even this 

 method does not eliminate systematic error in the 

 determination of target strength, possibly because of 

 peculiarities of transmission at short ranges, possibly 

 because of calibration uncertainties. Certain 60-kc 

 measurements on the USS S-37 (SS142) at San 

 Diego gave a beam target strength of 28.7 db with a 



