160 TECHNICAL SURVEY 
some occasions very long ranges, up to 1,500 miles 
(Oman, Somaliland), have been observed on 200-mc 
radar on fixed echoes. 
When the southwest monsoon sets in early in 
June, superrefraction disappears on the Indian side 
of the Arabian Sea. However, along the western 
coasts conditions favoring superrefraction may still 
linger. This has been reported from the Gulf of Aden 
and the Strait of Hormuz, both of which lie on the 
outskirts of the main region dominated by the 
monsoon. The Strait of Hormuz is particularly inter- 
esting as the monsoon there has to contest against 
the shamal from the north. The Strait itself falls at 
the boundary between the two wind systems, forming 
a front, with the dry and warm shamal on top, and 
the colder, humid monsoon underneath. As a conse- 
quence, conditions are favorable for the formation of 
an extensive radio duct, which is of great importance 
for radar operation in the Strait. 
Tue Bay or BENGAL 
Such reports as are available from this region 
indicate that the seasonal trend is the same as in 
the Arabian Sea, with normal conditions occurring 
during the season of the:southwest monsoon, while 
superrefraction is found during the dry season. It 
appears, however, that superrefraction is much less 
pronounced than on the northwest side of the 
peninsula. 
Tue Pactric OcraN 
This region appears to be the one where, up to 
the present, least precise knowledge is available. 
There seems, however, to be definite evidence for the 
frequent occurrence of superrefraction at some loca- 
tions; e.g., Guadalcanal, the east coast of Australia, 
around New Guinea, and on Saipan. Along the 
Pacific coast of the United States observations indi- 
cate frequent occurrence of superrefraction, but no 
statement as to its seasonal trend seems to be 
available. The same holds good for the region near 
Australia. 
In the tropics there is found a very strong and 
persistent seasonal temperature inversion, the so- 
called trade wind inversion. It has no doubt a very 
profound influence on the operation of radar and 
short-wave communication equipment in the Pacific 
theater. 
Fluctuations in Signal Strength 
with Time 
A number of different causes tend to produce 
variations of signal strength with time. These are 
discussed briefly in the following paragraphs. 
Tarcet Mopuation 
Very rapid fluctuations having periods of only a 
smail fraction of a second frequently are encountered 
in radar observations, especially with centimeter 
waves. These fluctuations arise as a consequence of 
the internal motions of the target and are especially 
noticeable for aircraft. Similar effects have been 
observed with reflection of microwaves from wooded 
hills, the fluctuations in signal probably being caused 
by foliage moving in the wind. 
EFrrect oF WAVES ON THE SEA 
A similar phenomenon is observed when the trans- 
mitter and receiver are so situated that reflection 
from a water surface contributes to the received 
signal strength. Owing to irregularities of the water 
surface and their rapid change with time, variations 
in signal strength will appear. The fluctuations 
arising in this way have a time scale of the order of 
a second, in the case of a lightly ruffled sea (see 
Figure 34). Evidently rays reflected from different 
SECONDS 
Figure 34. Variation in signal strength with time in 
radiation reflected from the sea (direct radiation cut off). 
> = 9cm. 
parts of the water surface interfere, and with the 
changing form of the surface the interference pattern 
at the place of the receiver changes accordingly. The 
time scale of these changes must be connected with 
the speed, wavelength, and amplitude of the waves. 
‘but the exact relation is not known thus far. 
TipaL EFFEcTS 
The rise and fall of the tide produces a gradual 
variation in signal strength by changing the inter- 
ference between the direct and the reflected rays. 
The path difference between these rays is 2hihe/R, 
where Ju, hz are the heights of the transmitter and 
receiver relative to the instantaneous water level and 
R is the range. The corresponding difference in phase 
between the two rays is equal to 
Qhihe Qn 
sen 26 
RS ees (26) 
measured in radians. The variation in the signal 
strength depends upon the variation in ¢. It is small 
when the change in ¢ is small and increases to a 
maximum for a change in ¢ of z radians. It follows 
from equation (26) that the tidal effect increases 
with the variation in the water level of the tide and 
with the heights hf: and hz and decreases with the 
range and the wavelength. 
ScINTILLATIONS 
The really conspicuous fluctuations in propagation 
