44 Lecture 2 
SCANNING 
SWITC 
R. RECEIVER switcH 
TO DISPLAY 
(RANGE ) 
SCANNING 
SWITCH. 
TO DISPLAY 
(BEARING) 
Fig. 2.14. Double FM sonar system. 
may eventually improve the use of visual displays. Of course, great progress 
has been made in what may be called the mechanics of displays, e.g., the 
writing of strobes, markers, and numbers in between signal scans, but we have 
not been concerned with this. 
2.4.1. Integration and Correlation 
Sonar systems are only too often concerned with threshold detection, and it 
is therefore important to make the maximum use of information which may be 
repeated in successive range or bearing scans. The improvement of detection 
obtained with a paper (chemical) recorder when a target appears on several 
successive range scans, and thus appears as a line on the record, is well known 
[20]. The improvement of threshold signal—noise level has been shown to be 
about 2.4 db per doubling ofthe number of scans. The same method can be applied 
in cathode-ray displays of similar type, but it is, of course, not suitable for 
P.P.I. or sector-scan displays. For these latter, some improvement of threshold 
arises from either integration of successive signals on the phosphor itself, or 
from a human memory effect when successive pictures are mentally superposed. 
A more satisfactory basis of improvement, however, is properly designed in- 
strumental integration. 
Integrators using a delay line (e.g., mercury) as the storage element are 
well known, and used occasionally. Cooper and Griffiths [21] have not only estab- 
lished a basis for optimum design, but have also developed an improved inte- 
grator using two loopcircuits,as shownin Fig. 2.15. Working on the assumption 
of a P.P.I. display obtained from a single-beam echo-ranging system, with the 
beam rotated at a relatively slow rate (giving several echoes from any particular 
target on each sweep), and assuming a directional pattern of Gaussian shape, 
they have determined the proper loop gain for single- and double-loop integrators. 
These factors lead to the following optimum performance figures; all are ex- 
pressed as decibels-improvement in video signal-noise ratio relative to that of 
