TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 151 
the reflected radiation is appreciable only at angles 
near grazing (less than 1° under the conditions found 
at San Diego). Furthermore, other things being 
equal, the reflection coefficient increases with increas- 
ing wavelength. This feature distinguishes the reflec- 
tion by a layer from the duct effects produced by 
this layer, as the latter generally tend to become less 
pronounced for longer waves. The reflection gives 
rise to an additional field strength near the ground, 
often well beyond the optical horizon. 
Transmission experiments carried out at San Diego 
at frequencies between 50 and 500 me gave results 
that are explained satisfactorily on the basis of 
reflections of the type just described but not on the 
duct theory. Thus most of the ducts caused by 
reversals of the M curve of the type shown in Figure 
24D will be beyond cutoff for a frequency of 50 me, 
as indicated on page 150. No guided propagation 
should therefore be expected, whereas the observed 
field at the receiver, located well beyond the optical 
horizon, was consistently very high. 
At a frequency of 500 me the reflection is found 
to be highly critical with respect to the angle of 
incidence at the reflecting layer. When meteorological 
conditions are such that the layer is high (3,000 to 
4,000 ft), and therefore the angle of incidence large, 
the intensity of the reflected radiation is found to 
be very low; when the layer forms at a low level 
(a few hundred feet only) the reflected radiation 
becomes very strong. This behavior agrees with the 
predictions of electromagnetic theory. 
So far, the experiment at San Diego is the only 
instance where a clear-cut case of reflection by an 
elevated layer has been found, although indications 
of similar effects have been observed elsewhere. 
Whether or not this phenomenon will occur at other 
places in or near the subtropical belt is not conclu- 
sively known since our knowledge of meteorological 
conditions in these climates is far from complete. 
Tf it does occur, it will obviously be of great opera- 
tional significance. 
Operational Applications 
RaDAR 
Ground radars have experienced most of the effects 
of propagation in nonstandard atmospheres so far 
observed operationally. Phenomenal ranges on ship 
and low-flying airplane targets have been observed, 
especially in the Mediterranean area, the Arabia- 
India area, in Australia, and the Southwest Pacific 
theaters. In the United States and Europe ground- 
based ducts over land have occasionally produced 
fixed echo clutter seriously interfering with the 
plotting of aircraft targets over land. This ground 
clutter interference is especially troublesome with 
microwave early warning sets plotting targets over 
land. On ground radars with high pulse repetition 
rates, echoes from large distances frequently return 
on the second or later traces. Such echoes interfere 
with first sweep echoes and sometimes.are misinter- 
preted as having ranges appropriate to the first 
sweep, with serious tactical consequences. 
One of the most serious operational consequencés 
of superrefraction is a secondary effect, that of 
misleading operators as to the overall performance 
of the equipment. Long-range echoes caused by 
superrefraction have frequently been assumed to 
indicate good condition of the equipment, when 
precisely the opposite is actually the case. The 
phenomenon of superrefraction does not, however, 
in the same degree invalidate the measurement of 
signal-to-noise ratio of nearby echoes, as a criterion 
of relative overall set performance. Field strengths 
frem nearby objects well within the optical horizon 
are far less subject to propagation variations. Echo 
strengths (signal-to-noise ratio) from nearby objects 
are still considered a good relative index of overall 
performance, provided that easily recognized echoes 
can be measured which are not sensitive to very 
small changes in the radar frequency. There are 
other sources of echo fluctuations such as the motion 
of objects (trees, towers) caused by the wind (import- 
ant at wind speeds above 15 miles per hour). Great 
care is needed in the choice of fixed echo “standards” 
so that they are kept free of the effects enumerated. 
Sometimes artificial echoing objects are constructed 
of flat mesh screens perpendicular to the beam in 
order to secure suitable echoes which are not 
frequency sensitive. The extreme variability of long- 
range fixed echoes emphasizes the operational need 
for reliable test equipment for making quantitative 
tests on the components as well as on the overall 
performance of the equipment aspects of radars, as 
distinct from propagation effects. 
In addition to the direct electrical checks on set 
performance there are a number of ways of making 
sure indirectly whether any failures of detection by 
radar may be due to a deformation of the coverage 
pattern by superrefraction. In the first place, super- 
refraction rarely affects detection at angles of eleva- 
tion above about 1.5°. Any irregularity at higher 
angles must be attributed to other causes. Even 
between 0.5° and 1.5° failures of detection are excep- 
tional and oecur only where there are very strong 
ducts. A clue to the probability of occurrence of such 
conditions can be ascertained from a study of the 
primary meteorological effects which cause them; 
and even with only a moderate amount of meteoro- 
logical information it is usually possible to make an 
estimate of this probability. Such superrefractive 
conditions almost invariably show up in intensified 
and extended ground echoes (ground clutter on the 
scopes) and, in case of an overwater path, in extended 
ranges of ship detection. A record of meteorological 
data will be very helpful in deciding, after the fact, 
whether any specific failure of aircraft detection 
might have been ascribed to weather. Even if this 
