Chapter 12 

 ECHOES AND TARGETS 



'2 1 FLUCTUATIONS OF RADAR ECHOES^ 



SINCE JUNE, 1943, the Propagation Group of the 

 Kadiatiou 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 

 eifects of atmospheric refraction can be neglected. 

 We have not paid much attention to moving targets 

 such as ships or jDlanes, 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. Li 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- 

 pnlse frequency changes are not greater than 0.1 or 

 0.3 mc, and frequency modulation inside the pulse is 

 less than 0.2 to 0.;5 mc. 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 signals 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 

 Eace 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. 



"By H. Goldstein, Radiation Laboratory, MIT. 



12.1.1 The Interference Concept 



When this research was started, it seemed to be a 

 connnon idea that changes in atmospheric refraction 

 in the j)ath 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 

 standijipes 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 GO 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 I/2 mc, which is larger 

 than the frequency instability of the system. Finally, 

 (he 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 tt the targets need only move 

 A/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 



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