ANALYTICAL PROCEDURES 



367 



-130 



700 



600 



500 



300 



400 

 RANGE IN YARDS 



Figure 4. Echo maxima at Fort Lauderdale. 



200 



100 



21.3 



ANALYTICAL PROCEDURES 



Target strengths reported here were obtained for 

 the most part from measurements of amplitudes of 

 echoes recorded photographically. Sometimes, how- 

 ever, operating conditions were so poor that upon 

 examination of the photographs it was difficult to 

 identify individual echoes and to distinguish them 

 from noise signals. Therefore echo recognition is im- 

 portant in the study of target strengths and is par- 

 ticularly relevant to a discussion of analytical proce- 

 dures. 



Echo recognition depends not on the intensity of 

 the echo alone, but primarily on the difference be- 

 tween the intensity of the echo and the intensity of 

 the background. '2 At close ranges, echoes are usually 

 strong enough to be easily recognized and are clear 

 and well defined. Distant echoes, however, are often 

 so weak that they cannot be distinguished from the 

 background, and irregular spines and patches may 

 effectively obscure the echo; hence a study of the 



structure of echoes from submarines and other tar- 

 gets at short ranges may be useful in the recognition 

 and identification of distant echoes from these same 

 targets. Weak echoes may sometimes be attributable 

 to poor training of the transducer or roll and pitch 

 of the echo-ranging vessel, both of which may direct 

 the sound beam away from the target. A high back- 

 ground of reverberation and noise may make an echo 

 hard to recognize. Rough seas and a wide transducer- 

 beam pattern contribute to a high reverberation 

 level, while a surface vessel at moderate speeds or a 

 shallow submarine at high speeds may originate 

 enough self-noise in the transducer dome to mask the 

 echo. Reverberation is treated in detail in Chapters 

 11 to 17. 



Once the echo is recognized and definitely identified 

 as the desired echo, the problem becomes one of 

 measurement and analysis. Various analytical pro- 

 cedures have been employed by different groups in 

 processing the raw material from the oscillogram to 

 the computed target strength. 



