424 



SURFACE VESSEL TARGET STRENGTHS 



speed of the Elcobel had to be held down to a few 

 knots when the transducer was in operation, while 

 the target ships were traveling at least several times 

 faster. Consequently a special procedure was de- 

 veloped to fit these conditions. 



As the ship approached, the Elcobel maneuvered 

 so that it neared the ship at an aspect angle just off 

 the bow of the target ship. When the range was 

 closed to about 600 yd, the test began, and the 

 Elcobel followed a course that kept the transducer 

 constantly aimed at the stern of the ship; the sound 

 beam was wide enough to cover the entire ship even 

 at close ranges. It is possible that echoes were also 

 obtained from the wakes of the ships, although such 

 echoes were probably distinguishable from ship 

 echoes for pulse lengths of 3 msec. Observations were 

 made as frequently as possible, and the range and 

 aspect angle were estimated at the time of the ob- 

 servations. Actually both ships deviated from their 

 nominal courses because of the effects of the wind 

 and sea state as well as inaccuracies in steering, so 

 that the ranges and aspect angles changed rather 

 irregularly. 



expected to depend on aspect angle in much the same 

 way as the target strength of a submarine depends 

 on its aspect. In fact, since most surface vessels are 

 more nearly flat at beam aspects than submarines, a 

 sharper dependence might be predicted as long as re- 

 flections come exclusively from the hull, in other 

 words, as long as the ship is anchored, or moving 

 through the water very slowly, and gives rise only to 

 specular reflection. If a moving ship reflects sound 

 diffusely, some change of target strength with aspect 

 angle might be expected, but not so marked a change 

 close to beam aspect as results from specula re- 

 flection. 



Table 1 lists beam and off-beam target strengths 

 for still and moving ships, together with the ranges 

 at which they were measured and the number of in- 

 dividual observations — each comprising at least 

 five echoes — which were averaged to obtain the 

 tabulated results. Here the values given for beam 

 target strength include all measurements at esti- 

 mated aspect angles between 70 and 110, and be- 

 tween 250 and 290 degrees from the bow, whereas 

 off-beam target strengths include measurements at 



Table 1 -Aspect dependence. 



24.3 



ASPECT DEPENDENCE 



Surface vessel target strengths have been measured 

 at San Diego and New York for different aspect 

 angles, ranges, speeds, types of ships, pulse lengths, 

 and frequencies. A dependence on aspect angle and 

 on range is suggested by the reported data; in addi- 

 tion, the target strength of ships under way appears 

 to be considerably different from the target strengths 

 of still vessels. Sufficient infonnation, however, is 

 not available to permit evaluation of the effects of 

 the class of ship, pulse length, or frequency on the 

 target strengths measured. 



The target strength of a surface vessel might be 



all other aspect angles. Ranges, speeds, ship types, 

 pulse lengths, and frequencies are not separated. 

 Table 1 is illustrated graphically in Figure 1. 



24.3.1 



Still Vessels 



Only for the anchored ships is the difference be- 

 tween beam and off-beam target strengths roughly 

 the same as the scatter of the observations, as repre- 

 sented by the standard deviation. The dependence 

 of target strength on aspect angle for the individual 

 measurements on these still vessels is shown in Fig- 

 ure 2. Two target strengths at an aspect angle of ap- 

 proximately 100 degrees are conspicuously higher 



