D. G. Tucker 47 
SPHERE BOTTOM 
ECHOES ECHOES 
Fig. 2.17. Echoes from sphere and sea bottom on different trans- 
missions. Duration of first and second pulses in each transmission 
=0.25 msec; separation=2.5 msec. 
to evaluate. It is doubtful if they have received anything like their proper share 
of research effort. 
An illustration of the sort of thing that may make higher resolution an em- 
barrassment, and automatic recognition of targets impracticable, is an effect 
which has been studied at Birmingham recently [24]. It is the rather startling 
influence of small transducer movements on the echo amplitude (and waveform) 
from complex targets. One particular result may be quoted to demonstrate this. 
A 50-kcps beam of about 12° by 14° was pointed vertically downwards from the 
ship Discovery II anchored in about 12 fathoms ofwater in a tideway. The trans - 
ducer was stabilized against roll, although the water was dead calm. An 18-in. - 
diameter sphere was suspended a few feet above the bottom. The sonar equip- 
ment emitted, on every cycle of repetition, two 0.25-msec pulses spaced a few 
milliseconds apart. Figure 2.17 shows the oscilloscope record of the returned 
signals for two different emissions. The echoes from the sphere (which is a 
simple single target) remained substantially constant throughout the test, but 
the echo from the bottom (which represents a complex target) was completely 
randomized in amplitude and waveform from one emission to another. Indeed, 
as can be seen from the records in Fig. 2.17 the lower amplitude return is 
partially randomized even between the two pulses whichare only 2.5 msec apart. 
The only possible explanation for this effect is horizontal movement of the ship 
due to yaw, but the magnitude of this movement cannot even approach the wave- 
length in 2.5 msec, although from one pair of pulses to another it could be several 
wavelengths. Laboratory tests confirm this explanation, as can be seen from the 
results of a tank test at 500 kcps with different kinds of bottom configuration, 
shown in Fig. 2.18. 
