SOLAR SYSTEM AND RADAR — GREEN AND PETTENGILL 269 



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: RADAR DETECTABILITY RELATIVE TO THE MOON 



Figure 1. — For the planets, a few satellites, and some asteroids, the radar detectabilities 

 are compared with that of the moon. These results have been deduced only from sizes 

 and relative distances, without regard to possible differences in surface reflectivity. For 

 the asteroids, approximations are given, since their sizes are uncertain. All illustrations 

 with this article are from MIT's Lincoln Laboratory, unless otherwise credited. 



surfaces or the absorptivities of their surrounding atmospheres. As 

 a matter of fact, the observed strength of the return signal can indi- 

 cate the reflectivity of a planet, since the diameter and distance are 

 already known. 



RADAR TO THE MOON 



The first echoes whose characteristic delay time and Doppler fre- 

 quency shift positively identified them as reflections from the moon 

 were obtained by the U.S. Army Signal Corps in 1946. But it was 

 puzzling that lunar echoes were not always observed even though con- 

 ditions appeared to be favorable. Some hitherto unsuspected effect 

 must have been taking place. 



Australian and British scientists showed that the observed fading 

 had two causes. A relatively rapid component stemmed from chang- 

 ing interference among simultaneous reflections from different regions 

 of the lunar surface, as changing librations caused it to turn under the 

 radar beam. And a propagation effect, Faraday rotation, was re- 

 sponsible for the slow fading which caused the signal to disappear for 



