Chapter 7 
ECHOES AND TARGETS 
FLUCTUATIONS OF RADAR ECHOES 
INCE JUNE, 1948, the Propagation Group of the 
S Radiation Laboratory has had a project under way 
to investigate the nature and origin of fluctuations 
from close targets. This work has been done in the 
microwave region using the mobile S- and X-band 
sets belonging to the group. Most of the work has been 
on S band. We have restricted ourselves to targets 
sufficiently close to the radar that the more usual 
effects of atmospheric refraction can be neglected. 
We have not paid much attention to moving targets 
such as ships or planes, as their echoes are easily ac- 
counted for by the changing aspect of the target, 
propeller modulation, etc. 
One of the obvious sources of signal fluctuation is 
instability in the system. In our case system instabil- 
ity was chiefly due to ripple in the receiver, and sen- 
sitivity to changes in line voltage affecting the modu- 
‘lator, receiver, and indicator units. After considerable 
effort these forms of instability have been reduced but 
not completely eliminated. The transmitted pulse 
shows an average fluctuation about the mean of +0.1 
db with a maximum deviation about 0.5 db. Pulse-to- 
pulse frequency changes are not greater than 0.1 or 
0.2 me, and frequency modulation inside the pulse is 
less than 0.2 to 0.3 me. These figures are for the S- 
band set, and instability is somewhat greater on X. 
The r-f signal intensity is measured by comparison 
with a pulse from a calibrated signal generator. This 
pulse shows a fluctuation as large as the transmitted 
pulse, i.e., about +£0.12 db. It is believed that this 
apparent change is not in the signal generator but 
rather in the receiver and indicator units. 
Some radar signais show almost as little fluctuation. 
These are large man-made targets in isolated posi- 
tions viewed over land. Some examples that we have 
found are the Provincetown standpipe as viewed from 
Race Point in Provincetown and the Winthrop stand- 
pipe in Boston viewed from Deer Island. In these 
cases the average pulse to pulse deviation from the 
mean is +0.14 db. Such steady signals are the rare 
exception. Most echoes show changes that are much 
larger than can be accounted for by instability in the 
system tests. 
The Interference Concept 
When this research was started, it seemed to be a 
common idea that changes in atmospheric refraction . 
“By H. Goldstein, Radiation Laboratory, MIT. 
306 
in the path between the target and set could account 
for the observed variations. We have found little evi- 
dence for this belief. If the targets are closer than 10 
miles, the effects due to the atmosphere, if there are 
any, must be small compared to the more important 
phenomena shown by the echoes. The behavior of the 
radar echo is determined by the fact that a radar signal 
is usually not the return from a single target but rather 
the sum of returns from all targets within the area 
illuminated by the set. Since the radar beam is co- 
herent, the individual signals must be added in am- 
plitude taking into account the relative phase of the 
echoes. The total signal is the result of the interference 
between these component echoes. In the case of the 
standpipes mentioned above there were intervening 
hills so that only the top portions of the targets were 
seen by the radar, but in most other cases there is 
more than one target present, and the interference 
between these targets will determine the nature of 
the total echo. 
In the Boston region, we have found one very simple 
dual target consisting of two radio towers 500 ft high 
and 60 yd apart in range. Both constructive and de- 
structive interference has been observed in this case. 
The changes in the phase between the component 
signals might be due to several causes. If the index of 
refraction in the path between the two towers changes, 
then the optical path length would change. However, 
the deviation of the index from 1 would have to double 
in order to produce sufficient phase change. A change 
in the frequency of the transmitter could also account 
for the phase change. To produce the observed effect 
it would have to be greater than 42 me, which is larger 
than the frequency instability of the system. Finally, 
the towers themselves could physically move relative to 
each other and produce the phase change in a manner 
similar to that in the Michelson interferometer. To 
produce a phase change of x the targets need only move 
d/4 relative to each other. At S band this amounts to 
1 in. It does not seem unnatural that such tall struc- 
tures might sway in the wind by even a greater amount. 
To test this conclusion the signal from these towers 
was measured over a period of 4 days. The amount of 
fluctuation was estimated visually every half hour. 
These results showed a definite correlation with the 
speed of the wind. Large fluctuations occurred only 
with high winds. It was calculated that if the fluctua- 
tion had indeed been independent of wind speed the 
odds against getting the set of readings obtained by 
these measurements would be 10,000,000 to 1. 
