VARIABILITY OF ECHOES 



377 



21.6.2 



Fluctuation 



Many factors contribute to the ob.served fluctua- 

 tions of echoes. Much of thi.s rapid change in echo in- 

 tensity may be ascribed to the loll and pitch of the 

 echo-ranging vessel, which by changing the direction 

 of the sound beam causes the received eciioes to vary 

 in inten.sity. Altliougli gyroscopic stabihzation of the 

 transducer was employed at Fort Lauderdale to re- 

 duce fluctuations arising from the roll and pitch of the 

 ship, as described in Section 21.2.5, this .system has 

 not been used elsewhere for this purpose. Errors in 

 training the echo-ranging transducer toward the sub- 

 marine have also been responsible for some of the 

 fluctuations encountered ; training on the bearing of 

 maximum intensity, by means of cut-ons, is approxi- 

 mate and introduces variability in the received echo 

 intensities by changing the direction of the beam 

 relative to the submarine. 



In addition, surface reflection and interference 

 phenomena may be expected to account for part of 

 the fluctuations observed, as the sound beam fre- 

 quently follows multiple paths to reach the submarine 

 and return back again to the transducer. Chapter 7 

 of Part I of this volume discusses the evidence show- 

 ing that transmission fluctuations are very much re- 

 duced when surface-reflected sound is minimized. 

 Correlation has been observed between the depth of 

 the transducer below the ocean surface, and the mag- 

 nitude of the fluctuations observed in echoes from a 

 sphere two ft in diameter; ^ at a range of the order of 

 65 to 75 yd, elevating the transducer from a depth of 

 50 to 10 ft below the surface increased the standard 

 deviation of the echo intensity from 18 to 39 per cent. 

 In addition, the overall fluctuation appears to de- 

 crease as the signal length increa.ses. At other than 

 beam aspects, interference between echoes from dif- 

 ferent parts of the submarine is undoubtedly respon- 

 sible for part of the fluctuation observed, giving rise 

 to an irregular "hashed" echo structure described in 

 Sections 23.8.2 and 23.8.3. 



21.6.3 Effects on Echo Level and 

 Echo Structure 



Variation affects echo intensity; fluctuation affects 

 both echo intensity and echo structure. Echo en- 

 velopes never repeat exactly, and successive echoes 

 at the same range, aspect, frequency, and signal 

 length often appear totally different. This diversity 

 of echo structure not only complicates measurement 



of the intensity of the echo, but also makes it difficult 

 to resolve the length of the echo and the center of the 

 echo, in effect preventing precise measurement of the 

 range of an individual echo. 



Likewise successive echo intensities seldom repeat. 

 As a result, some sort of average must be taken over 

 successive echoes. If target strength is regarded as a 

 measure of the fraction of the incident sound energy 

 reflected by the target, the total reflected energy 

 should be compared to the total transmitted energy. 

 Such an analysis would require squaring the echo 

 amplitudes to give the echo intensities, then inte- 

 grating the intensities over the duration of the echo 

 to give the total echo energy; this same procedure 

 would be followed with the signal to yield the total 

 signal energy. Such an analysis has not been found 

 practical because of the complex instruments re- 

 quired. In addition, it may be that aural and non- 

 aural detection devices respond more to peak echo 

 intensity rather than to total echo energy, and that 

 therefore peak intensities are more significant. 



21.6.4 Peak versus Mean Echo 

 Intensity 



Since it has not been feasible to compare the re- 

 flected and transmitted energy directly, peak echo 

 amplitudes have been used to compute target 

 strengths. Observations show that these average peak 

 amplitudes do not differ significantly from the rms 

 peak amplitudes, which would correspond to peak 

 intensities. Thus the San Diego results may be re- 

 garded as giving average peak intensities. Not only is 

 this method simple and easy to apply, but also it pro- 

 vides values which may be compared directly with 

 recognition differential measurements where peak- 

 echo intensities alone are considered. Peaks, however, 

 fluctuate enormously, especially for off-beam echoes ; 

 a sample survey of 100 oscillograms of submarine 

 echoes at San Diego showed a maximum fluctuation 

 of 25 db between peaks, with fluctuations of 10 db 

 not uncommon. 



An approximate comparison of reflected and trans- 

 mitted energy might be made by measuiing the mean 

 echo intensity, averaged along the entire length of the 

 echo, and correcting this intensity for the pulse length 

 since the echo length generally is longer than the 

 pulse length. Then the ratio of the signal and echo 

 intensities, based on the same pulse length, would be 

 equal to the ratio of the signal and echo energies. 



This procedure was attempted for six echoes 



