38 Lecture 2 
Bearing 
a6" o 8 6+6 
1 1 
— 
~~ 
Qu. 
u 
a 
Fig. 2.7. Scanning echo-sounder 
display with fish passing through 
sector. (Southern North Sea, depth 
about 20 fathoms; maximum range 
on scale, 25 fathoms.) (Courtesy, 
——— Institute of Navigation, London.) 
25 
fathoms 
This method of scanning has been very successful, and the experimental 
equipment has been useful in examining fish shoals in connection with a fisheries 
research program of the Fisheries Laboratory at Lowestoft (see Fig. 2.7). The 
equipment has also been effectively demonstrated [15] as an aid to hydrographic 
and oceanographic surveying, both with horizontal beam (Fig. 2.8) and with ver- 
tical beam (Fig. 2.9); in the latter application it shows a complete bottom profile 
on a single pulse transmission, and this proves useful in bad weather when 
"quenching," by suppressing nearly all the transmissions, makes ordinary echo 
sounders useless. In these experiments, the beam was about 1.5° wide between 
3-db points in the plane of scanning, and 12° at right angles to this plane. The 
frequency was 37 kcps, and of 1-msec pulse duration. 
The analysis and experimental investigation of the factors which limit the 
performance of this type of scanning system have been taken a long way [16], 
partly with the help of an electromagnetic analog [17] (i.e., a radar version of 
it), and currently a high-resolution system is nearing completion which exploits 
the system fully. This has 35 sections to the array, and with an operating fre- 
quency of 400 kcps will give a resolution of ie in bearing, 6 in. in range, over 
a 17° sector, with a maximum range approaching 100 yards; it is intended, among 
other things, to allow observation of the movements of individual small fish in 
a reservoir. 
The most striking achievement in this development at Birmingham is the 
successful application of the multiplicative principle to the scanning system [3]. 
Figure 2.10 shows in block form how it is done. This has resulted in a genuine 
doubling of the angular resolution for a given length of array, as illustrated in 
Fig. 2.11. In the experimental trial, of which this is one of the records obtained, 
