272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 



MAPPING THE MOON BY RADAR 



Suppose a pulse of radar energy reflected from the moon is ob- 

 served. The echo will have a longer duration than the original pulse, 

 because the reflection from the edge of the moon reaches us later than 

 that from the center of the disk. Hence, by selecting a part of the 

 returning signal within a limited time interval, we know that we are 

 observing a ring-shaped portion of the disk, centered on its midpoint. 

 But how do we isolate the energy reflected from a particular part of 

 this ring ? 



To achieve tliis, advantage is taken of the changing libration of 

 the moon, which causes a slow apparent turning of the moon as seen 

 by a terrestrial observer. At any moment, half of the moon's face is 

 approaching us and half receding, with respect to the center of the 

 disk. Thus the frequency of the energy returned from the ring differs 

 from point to point, in a predictable way, because of the Doppler 

 effect. Figure 3 shows the relation between range and frequency for 

 a turning spherical body such as the moon. 



If we could arrange to measure simultaneously both range and 

 frequency with sufficient precision, some semblance of a map could be 

 prepared. The separation of returns from different parts of the lunar 

 surface is possible because the energy received at a given range and 

 given frequency must have been reflected only from two definite points 

 on the moon. Tecluiiques which have been available for years give 

 adequate accuracy in range. The frequency measurement, on the 

 other hand, calls for a new level of stability, several parts in 10" over 

 the observing interval of 2.5 seconds, if useful resolution is to be 

 achieved. 



This stability has recently been obtained, and figure 4 shows some 

 experimental results gathered by this teclinique. It is hoped that 

 a number of these measurements will make it possible to match the 

 observed spectra with specific parts of the limar disk, and build up a 

 picture of the moon in terms of radar reflectivity. 



The span of frequencies covered by the echo is determined by the 

 product of the target's radius and turning rate. Also, the total time 

 duration of the echo is a direct measure of the radius. Hence, the 

 rotational rate of a planet may be found by radar. Such a study of 

 Venus would be very important, since the length of that planet's day is 

 still miknown. Perhaps the most interesting property of these metli- 

 ods is that angular resolution is not required. As radar capability 

 improves, Venus and Mars may be studied in detail with good surface 

 resolution, without recourse to impossibly small antenna beam widths. 



FUTURE TECHNIQUES AND EQUIPMENT 



The application of these methods to very distant targets will require 

 continuing efforts in four major areas: Transmitters, antennas, low- 



