Chapter 21 

 DIRECT MEASUREMENT TECHNIQUES 



SUBMARINE TARGET STRENGTHS have been calcu- 

 lated theoreticallj' and measured experimentally. 

 The theoretical calculations described in Section 20.5 

 are based on assumptions simplifying the geometry 

 of the hull and conning tower, and the way in which 

 the submarine reflects sound. Actual measurements 

 in the field are necessary to verifj^ and amplify these 

 theoretical predictions and to assess their accuracy. 



Measurements have been both direct and indirect.' 

 Direct measurements consist of echo ranging, with 

 short pulses of supersonic sound, on a submerged sub- 

 marine at various ranges, depths, and speeds. The in- 

 tensities of the received echoes are then measured and 

 converted to target strengths. This chapter describes 

 in detail the various experimental procedures and 

 techniques employed by different laboratories in the 

 direct measurements of submarine target strengths. 



Indirect measurements, on the other hand, use con- 

 tinuous sound or light reflected from a scale model of 

 a submarine, and interpret these results in terms of 

 supersonic sound reflected from an actual submarine 

 of the same shape; Chapter 22 describes how target 

 strengths are measured indirectly. The results of both 

 the direct and indirect submarine target strength 

 measurements are presented and discussed in Chapter 

 23 while both the techniques and results of target 

 strength measurements on surface vessels are treated 

 in Chapter 24. 



21.1 PRINCIPLES OF DIRECT 



MEASUREMENT 



In order to calculate target strengths, echoes from 

 a submarine may be compared with echoes received 

 at the same time and under the same conditions from 

 a sphere. From the relative intensities of the echoes 

 from the submarine and from the sphere, and from 

 the expression for the target strength of a sphere 

 [equation (10) in Chapter 19], the target strength 

 of the submarine could be readily computed. Since 



only the relative intensities of two echoes would need 

 to be determined, no absolute measurements or cali- 

 brations would be required. But at sea, a sphere large 

 enough to return a strong echo at ranges normally 

 used in echo ranging is too awkward to handle easily 

 and therefore cannot be used in practice to obtain 

 target strengths. 



Instead, target strengths are always found by using 

 the fundamental definition [equation (6) in Chapter 

 19], which defines the target strength of any object 

 in terms of the echo level, the source level, and the 

 two-way transmission loss from the projector to the 

 target and back to the projector again, all expressed 

 in decibels. This expression is simple and easy to use 

 and has the advantage that all the quantities appear- 

 ing in it may, in principle, be measured directly. Only 

 the difference between the echo level and the source 

 level, and the transmission loss which the signal un- 

 dergoes as it travels from the projector to the tar- 

 get need to be known in order to find the target 

 strength. 



Unfortunately, the difficulties of calibration and 

 other practical problems not yet resolved make the 

 fundamental definition less useful than may be sup- 

 posed. In particular, the calibration of the transducer, 

 described in Section 21.4 as the measurement of its 

 output as a projector and its sensitivity as a receiver, 

 and the determination of the transmission loss, de- 

 scribed in Section 21.5, as well as the large fluctua- 

 tions and variations normally encountered in under- 

 water sound experiments, introduce numerical im- 

 certainties which cannot be accurately evaluated. 

 Nevertheless, the fundamental definition of target 

 strength introduced in Section 19.1.3 has been used 

 in all direct measurements and has led to reasonably 

 consistent results. 



21.2 EXPERIMENTAL PROCEDURES 



Four groups have measured submarine target 

 strengths directly. They are : University of California 



303 



