ECHOES AND TARGETS 31] 
oe 5 4 db 
os 
for both polarizations. If the Rayleigh 1/A* law holds, 
the ratio should be +-18.5 db, which would seem to 
exclude spray drops as the scatterers. 
One of the difficulties of this type of measurement 
is to determine the average level of a signal that 
fluctuates as rapidly as does sea echo. To remove this 
source of trouble, a device has been developed that 
reads the average power directly. It might be de- 
scribed as a gated noise meter. With the aid of this 
instrument we have again begun making measure- 
ments of c on 8 and X bands, this time from Deer 
Island in Boston Harbor. The elevation is only 120 
ft, and the ranges are correspondingly small. 
The results obtained so far do not agree in all re- 
spects with the previous data obtained at Bar Harbor. 
When the sea is fairly calm (Beaufort 3 or less), the 
ratio of ox to og is reproducibly given by: 
= +12+2db _ horizontal 
os 
on horizontal polarization. The scatter is much greater 
on vertical polarization, and the ratio is much smaller: 
2, +4db vertical. 
os 
One set of worth-while measurements has been made 
with a sea that was considerably rougher (Beaufort 
4-5). The ratio was significantly smaller for both 
polarizations: 
of = 45+42db horizontal 
os 
Ox . 
— =-+2db vertical. 
os 
At the time these data were obtained the first 
Measurements were made with a calibrated experi- 
mental K-band set recently constructed. Only hori- 
zontal polarization was available. It was found that 
= 3toddb. 
ox 
Hence under these sea conditions the increase in o 
on going from S to X is about the same as when going 
from X to K. 
An interesting by-product of these measurements 
was the comparison of polarizations, keeping the wave- 
length the same. This ratio was quite variable, ne 
ing from 
*¥ =—9db Xband 
oH 
on X band under stormy conditions, to 
“Y > 425db Sband 
OH 
on S$ band with a calm sea. In general the ratio de- 
ereases as sea becomes rougher and is almost always 
less on X than on § band. 
It is too early in the investigation to attempt a de- 
tailed interpretation of the results. It does seem that 
scattering from small spray drops is not the sole 
mechanism, despite the popular observation that sea 
echo seems to increase rapidly with the appearance 
of whitecaps. Other evidence also seems to confirm 
this. Under favorable conditions sea echo appears as 
discrete signals, moving with the wind, that can be 
tracked for 15 to 20 sec. This seems longer than one 
would expect from a breaking wave. On the other 
hand, reflection from large wave surfaces cannot be 
the whole story either. This is indicated by the fairly 
rapid increase of o with frequency and by the com- 
plicated changes with polarization. It is probable 
that we are dealing with a combination of mech- 
anisms, and it will be a difficult task to unscramble 
the contribution of each to the total signal. 
It should be emphasized that these measurements 
were taken near the coast, though outside the break- 
ers. Conditions on the high seas might conceivably be 
quite different. 
Discussion 
It was stated that individual sea echoes which 
persist for many seconds cannot be caused either by 
specular reflection from an inclined water-air inter- 
face or by random (Rayleigh) scattering from in- 
dividual drops of spray. Instead, an aerated surface 
layér created by a breaking whitecap may persist for 
many seconds and may be responsible for persistent 
echoes. Such a layer constitutes an irregular network 
of air-water interfaces and may give rise to consider- 
able scatter of microwaves. The actual mechanism by 
which such a layer gives rise to a sea echo is likely to 
be different at different sea states. If a large area is 
covered with foam, then in the presence of strong 
swell the chief return should be expected from a wave 
crest, and the radar signal would appear to travel 
slowly on the radar screen as the wave crest pro- 
gresses. 
The author stated that so far no consideration had 
been given to such involved mechanisms as the one 
suggested, but added that data already collected might 
well lead to such an investigation. 
It was suggested that several mechanisms, includ- 
ing scattering from droplets, were probably respon- 
sible for sea echo in rough weather. Experiments in 
Britain reported by the British Army Operational 
Research Group showed that echoes from shell splashes 
viewed on an S-band gunnery radar could be resolved 
into two parts. One was from the “boil,” a solid wall 
of water with enclosed air bubbles, which could be 
readily distinguished from the superimposed response 
from the larger portion of the splash called the 
“plume,” which is a region of isolated water drop- 
lets. Echoes from the droplets in the “plume” region 
were of many seconds duration, and it seemed likely 
