396 



SUBMARINE TARGET STRENGTHS 



30 



o 



BOW 



60 90 



BEAM 

 ASPECT ANGLE IN DEGREES 



ISO 



ISO 

 STERN 



Figure 10. Target strength-aspect curves at different altitudes (optical). 



15, and 45 degrees. Figure 11 is a similar plot for the 

 acoustical measurements on the model of the U570 at 

 much smaller altitude angles of 0, 1.0, and 1.8 de- 

 grees. Occasionally a pronounced maximum of the 

 target strength has been observed at a very small and 

 critical altitude angle, where the conning tower gives 

 a prominent specular reflection or highlight. An ex- 

 ample may be seen in Figure 11, at an aspect angle 

 of about 105 degrees. Another example, at only one 

 aspect angle, is shown in Figure 12, where the target 

 strength of the U570 at bow aspect is plotted for five 

 different altitude angles used at Mountain Lakes; 

 here a strong reflection from the conning tower at an 

 altitude of 1.0 degree results in a target strength of 

 19 db, while the target strengths at altitude angles 

 only a fraction of a degree different are considerably 

 lower. Such maxima, however, are not common, and 

 are generally confined to such a small sector of alti- 

 tude angles that most of the time they are not likely 

 to be observed in actual echo ranging. Their possible 



effect on the direct measurements has not been veri- 

 fied because of the wide fluctuations in echo level 

 tending to obscure such fine detail. 



Negative altitude angles are not encountered for 

 surface vessels echo ranging on submarine targets. 

 The low target strengths at these negative angles, 

 measured in the optical studies, have already been 

 mentioned in the preceding section. 



At very large positive altitude angles, the differ- 

 ences between beam, bow, and stern target strengths 

 are less marked and the resulting target strength- 

 aspect curve is consideral^ly smoother, as illustrated 

 in Figure 9 for altitude angles between and 90 de- 

 grees, and in Figure 10 for altitude angles of 15 and 

 45 degrees. Direct measurements on a deep subma- 

 rine were also made at San Diego on the USS Tilefish 

 (SS307) at a depth of 400 ft,^ and gave results not 

 significantly different from measurements at shal- 

 lower depths except at quarter aspects. Values be- 

 tween 25 and 27 db were obtained at beam aspects in 



