Chapter 23 

 SUBMARINE TARGET STRENGTHS 



TARGET STRENGTHS of Submarines have been com- 

 puted mathematically from the size and shape of 

 a particular submarine, by the Fresnel zone method 

 outlined in Chapter 20. They have also been meas- 

 ured, both directly and indirectly, by use of the pro- 

 cedures and techniques described in Chapters 21 and 



Y 



ECHO-RANGING 

 VESSEL 



Figure 1. Definition of angles. 



22, and have been studied in general as a fimction of 

 orientation, submarine class, speed, range, pulse 

 length, and frequency of the echo-ranging sound. 

 This chapter presents the results of the different 

 methods of determining target strengths of subma- 

 rines and discusses their applicability to practical 

 echo ranging. 



23.] DEPENDENCE ON ORIENTATION 



Since a submarine is irregular in shape, the echoes 

 which it returns depend markedly on its orientation 

 with respect to the echo-ranging beam. The orienta- 



tion of such an irregular target is conveniently 

 described in terms of aspect and altitude angles, 

 defined in Figure 1. 



Consider a system of rectangular coordinates with 

 the origin at the center of the submarine. The aspect 

 angle is defined as the angle between the x axis and 

 the projection of the echo-ranging beam on the hori- 

 zontal {xz) plane. It is measured in degrees from the 

 bow of the submarine, in a clockwise direction as 

 viewed from above; bow aspect is degree, stern 

 aspect 180 degrees, while beam aspect will be 90 and 

 270 degrees for the starboard and port beams 

 respectively. 



The angle between the echo-ranging beam and its 

 projection on the horizontal {xz) plane is the altitude 

 angle. It is measured in degrees, positive when the 

 sound source is above the submarine, negative when 

 it is below the submarine. If the projector is at the 

 same depth as a level submarine, the altitude angle is 

 degree; .similarly, if it is directly above a level sub- 

 marine, the altitude is 90 degrees. The vertex of both 

 aspect and altitude angles is placed at the origin 

 of the coordinate system, which is taken at the 

 geometric center of the submarine. 



23.1.1 



Aspect Angle 



The strongest echo from a submarine is usually 

 found within 20 degrees of beam aspect — between 

 70 and 110 degrees, and between 250 and 290 de- 

 grees, from the bow of the submarine.^ These beam 

 and near-beam echoes average about as strong as the 

 echo from a sphere 35 yd in radius and correspond to 

 a target strength of 25 db. Actually, target strengths 

 as low as 7 db and as high as 40 db have been ob- 

 served at beam aspect, directly and indirectly; most 

 values, however, lie between 20 and 30 db. 



At other aspects, the target strength is much 

 smaller and averages between 5 and 15 db, depending 

 on the submarine and the altitude angle. At stern 

 aspect, for example, target strengths measured di- 

 rectly with standard gear vary from 4 to 19 db, de- 



388 



