230 Lecture 12 
50 
FOCUSING RANGE >a = a | 
40 
30 
60-KC DIRECT SIGNAL 
© =SHALLOW DEPTH (50 ft) 
=DEEP DEPTH (100 to I50 ft) 
RELATIVE STANDARD DEVIATION (per cent) 
ie) 200 400 600 800 1000 1200 1400 1600 
RANGE (yd) 
(a) 
50 
FOCUSING RANGE ——*| 
25-KC DIRECT SIGNAL 
© = SHALLOW DEPTH (50 ft) 
x = DEEP DEPTH (I00 to 150 ft) 
40 
30 
RELATIVE STANDARD DEVIATION (per cent) 
(0) 200 400 600 800 1000 1200 1400 1600 
RANGE (yd) 
(b) 
Fig. 12.19, Direct-signal fluctuations measured in one day in July; (a) at 60 kc, (b) at 25 kc. 
As can be seen, the variability of the surface reflection is relatively high, 52% 
at short distances, but tends to decrease somewhat with increasing range. The 
variability of the direct transmission increases with distance, until at long 
ranges the same variability may be expected for the two paths. The slight de- 
crease in variability of the surface-reflected signal may be attributed to the 
fact that the sea surface becomes more like a perfect mirror and less like a 
