DEPENDENCE ON ORIENTATION 



389 



pending on the submarine-"^ (see Section 23.2.1). 

 Negative target .strengths have been observed in the 

 optical studies at certain aspects and altitudes; for 

 example, at l>o\v and stern aspects the target strength 

 of the German U570 (HMS/M Graph) varies from 

 — 4 to — 6 db when the echo-ranging beam is below 

 the submarine, at altitude angles between —5 and 

 — 15 degrees.* Since such negative altitude angles are 

 not encountered in practice when echo ranging from a 

 surface vessel on a submerged submarine — because 



furthermore, the uncertainty in the aspect angle in 

 some of the measurements was rather large. Con.se- 

 quently, the beam target strengths do not apply to an 

 aspect angle of exactly 90 or 270 degrees. The altitude 

 angle in all ca-ses was small. In the direct measure- 

 ments reported in this table, the submarine was sel- 

 dom submerged to a keel depth greater than 100 ft, 

 which at a range of 500 yd corresponds to an altitude 

 angle of 4 degrees, while for the indirect measure- 

 ments quoted the altitude angle was degree. 



Table 1. Submarine target strengths. 



* San Diego measurements at 60 kc are not included here. 

 t Beam focused on conning tower, 

 J Beam focused on screws. 



Average of all values in a 30-degree sector centered at beam aspect 

 Average of maximum values in the sector for each run. 



the projector is always above the target — these low 

 target strengths are not very significant. Moreover, 

 they may result from errors in the construction of the 

 model (see Section 23.6.2) or from possible systematic 

 errors inherent in the optical method (see Section 

 22.4). 



Table 1 summarizes submarine target strengths at 

 beam, bow, and stern aspects for the theoretical cal- 

 culations ^ and for the direct "~" and indirect meas- 

 urements.-' '^ Certain controversial values discussed 

 later in this chapter are omitted, as, for example, cer- 

 tain San Diego measurements at 60 kc. Most values 

 were averaged in sectors of roughly 10 or 20 degrees; 



Ranges varied from 200 to 1,000 yd, and the fre- 

 quency from 12 to 60 kc for all tests except the optical 

 studies at MIT, where the full-scale frequency was 

 much higher. Although the results of the mathemati- 

 cal studies are only approximate and the results of 

 the direct measurements highly variable, all the data 

 are generally consistent. 



The early San Diego values for a fleet-type sub- 

 marine of the Tambor class ' and an S-boat * are not 

 reliable. An early experimental frequency-modulated 

 gear was used to echo range on the Tambor-cla.ss sub- 

 marine; since these results are difficult to interpret in 

 terms of standard echo-ranging gear, they cannot be 



