Chapter 22 

 INDIRECT MEASUREMENT TECHNIQUES 



REFLECTIONS FROM SUBMARINE models have been 

 studied in order to discover the principal reflect- 

 ing surfaces on a submarine and to measure sub- 

 marine target strengths under controlled conditions. 

 Both visible light and supersonic sound have been 

 used in these model tests. 



In the investigation of reflection from submarines, 

 models have many advantages over actual subma- 

 rines. Generally, experimental conditions can be con- 

 trolled much more easily under laboratory conditions 

 than in the field. Laboratory use of carefully con- 

 structed scale models makes possible a reasonably 

 reliable e^-aluation of target strength as a function of 

 aspect and altitude angles, as well as submarine class, 

 and provides both a theoretical guide and a con- 

 venient check on the direct measurements. 



22.1 PRINCIPLES OF INDIRECT 



MEASUREMENT 



Three groups have participated in the indirect 

 measurements of reflections from submarine models; 

 University of California Division of War Research at 

 the U. S. Navy Radio and Sound Laboratory, San 

 Diego, California [UCDWR]; Underwater Sound 

 Laboratory, Massachusetts Institute of Technology, 

 Cambridge, Massachusetts [MIT-USL]; and the 

 Underwater Sound Reference Laboratories, Columbia 

 University Division of War Research, Mountain 

 Lakes, New Jersey [USRL]. Only qualitative results 

 were obtained at San Diego while actual target 

 strength values were measured at MIT and at 

 USRL. 



22.1.1 



San Diego 



Early experiments were carried out at San Diego ' 

 on a 1:60 scale model of the U570 or HMS/M 

 Graph, a 517-ton German Type VIIC U-boat which 

 was captured in 1941 off Iceland and served in the 

 British fleet. The model, made of wood and finished 

 with glossy white enamel, was illuminated by a 



standard projection bulb and photographed in vari- 

 ous positions. 



The bulb was enclosed in a metal housing with a 

 hole l}4 in. in diameter on one side, and was placed 

 as close as possible to the camera lens so that the 

 angle between the incident and the reflected light at 

 the submarine was only about 3 degrees. Photographs 

 were made at different aspect angles, first with the 

 submarine finished with enamel, then with the sub- 

 marine covered in part by horizontal and vertical 

 corrugations, and finally with coarse emery cloth 

 covering certain areas on the model. The corruga- 

 tions and emery cloth were affixed to the submarine 

 in an effort both to reduce prominent reflections and 

 to suggest locations for possible absorption treat- 

 ment. 



These experiments were wholly qualitative, since 

 no measurements were made. The main purpose was 

 to discover the highlights on a submarine which 

 might be largely responsible for strong reflections. 

 Photographs for different aspect angles of the sub- 

 marine model, without the corrugation or emery 

 cloth, are reproduced in Figure 1. 



22.1.2 Massachusetts Institute of 

 Technology 



Quantitative experiments using visible light re- 

 flected from scale models were conducted at MIT- 

 USL to calculate target strengths of four different 

 submarines.^"* A series of measurements were made 

 on models of HMS/M Graph, an old S-boat, the 

 USS Perch (SS313) and the USS Sand Lance (SS381) ; 

 these models were from 60 to 120 times smaller than 

 the original submarines and were finished with a 

 glossy black enamel. 



In order to compute the target strength of one of 

 the submarines, light reflected from the submarine 

 model was compared with light reflected from a 

 sphere also enameled in glossy black. The target 

 strength of the submarine was calculated from the 



379 



