398 



SUBMARINE TARGET STRENGTHS 



0.5 1.0 



ALTITUDE ANGLE IN DEGREES 



Figure 12. Altitude dependence (Mountain Lakes). 



12 db; and for a large fleet-type submarine about 19 

 db. At beam aspect, the target strengths are more 

 nearly the same. 



Unfortunately, no comparative measurements have 

 been made on two different vessels by a single 

 group during a single operation. In view of the sys- 

 tematic errors of 5 to 10 db that may be present in 

 target strength determinations, suggested in Sections 

 21.4, 21.5, and 21.6, the differences shown in Table 2 

 are not too conclusive. More accurate data are re- 

 quired to allow any conclusion to be drawn about the 

 variation of target strength between different sub- 

 marines. 



23.2.2 Indirect Measurements 



Table 3 lists target strengths for different sub- 

 marines measured indirectly at an altitude angle of 

 degrees. These values were obtained under con- 

 trolled conditions and are more self-consistent than 

 the values measured directly; as a result, fluctuations 

 are smaller and the differences between the values are 

 probably more significant than in the direct measure- 

 ments. Beam target strengths may vary between 25 

 and 30 db; at stern aspect, the limits are 1 and 9 db. 



Table 2. Dependence of target strength on submarine class (direct measurements). 



23.2.1 



Direct Measurements 



Representative target strengths of different sub- 

 marines measured directly are listed in Table 2. 

 S-boats, R-boats, and fleet-type submarines have all 

 been targets in direct measurements, but the experi- 

 mental error is so great that it generally obscures any 

 possible correlation of target strength with submarine 

 class. However, Table 2 shows an apparent depend- 

 ence of target strength on submarine class at stern 

 aspect. The target strength of R-boats at stern aspect 

 varies from 4 to 8 db; for an S-boat it averages about 



The possibility that a fleet-type submarine may re- 

 turn a strong echo at stern aspect, which is suggested 

 in Table 2, is confirmed by the optical measurements 

 on the USS Sand Lance (SS381) quoted in Table 3. 

 These measurements give a stern target strength of 

 9 db, which is less than most similar values measured 

 directly but still considerably larger than for any 

 other submarine tested optically at that aspect. 

 Since the indirect measurements at off-beam aspects 

 are much lower than the direct measurements at 

 those aspects, however, these optical values are 

 probably not too significant. 



