270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



minutes at a time. This effect occurs when a wave passes through a 

 region like the earth's ionosphere witli a magnetic field present. Un- 

 der certain conditions, the plane of polarization of the radar signal 

 was being sufficiently twisted, as it passed twice through the iono- 

 sphere, that it arrived back at the receiving antenna in a cross-polar- 

 ized orientation, producing zero output. 



By unraveling the sources of the fading, it became possible to elimi- 

 nate the ionospheric effects (through the use of circularly polarized 

 transmissions, for instance), in order to study more directly the reflec- 

 tive properties of the lunar surface. In addition, the Faraday rota- 

 tion could also be employed as a new tool to probe the properties of 

 the terrestrial ionosphere. 



An unexpected property of lunar reflections at radio wavelengths 

 came to light with the discovery, by J. H. Trexler at the Naval Re- 

 search Laboratory, that when a short pulse was sent out, most of the 

 returned signal power was confined to an interval of a few hundred 

 microseconds. So brief an echo could have been produced only by a 

 lunar terrain having relatively gentle slopes, as contrasted to the pre- 

 cipitous and craggy surface shown in popular illustrations. Further 

 verification was soon provided by accurate measurements of the travel 

 time, proving that the sharp echo originated in the nearest region of 

 the moon. 



It had long been known that at optical wavelengths the disk of the 

 full moon exhibits a striking uniformity in apparent brightness from 

 center to edge. The radar work made it clear that the nature of scat- 

 tering from the moon's surface was distinctly different when measured 

 with wavelengths of tens of centimeters instead of tenths of microns. 

 Unlike its visual appearance, the moon at radar wavelengths has a 

 strong highlight in the center. 



As radar transmitters became more powerful and antennas larger, 

 the echoes received back from the moon stood higher and higher above 

 the receiver noise level. Within the past few years, sufficient signal 

 has become available at several stations to show that there is in fact 

 observable echo power ail the way out to the lunar limb. Figure 2 

 shows the results of one such recent measurement. In addition to the 

 highlight or specular component, there is a diffuse contribution Avhich 

 very nearly obeys a Lambert scattering law. A reasonable fraction 

 of the moon's surface, therefore, must have irregularities that are 

 comparable in size to radar wavelengths — a conclusion of some impor- 

 tance to those who may wish to land there. 



How can we learn where these rough portions are? Photographs, 

 of course, tell quite a bit about the topography, and measurements of 

 the lengths of shadows cast by objects on the moon's surface have given 

 us much information about the height scale of its gi'oss features. The 



