RADIO-ECHO OBSERVATIONS OF THE MOON AT 3.6-CM WAVELENGTH

Radio-echo studies of the lunar surface in the wavelength range 3 meters to 10 cm indicate that the surface is smooth and undulating for the most part, with an average surface gradient of the order of one in ten. Photometric studies of the brightness distribution over the moons disk, on the other hand, demonstrate the existence of microstructure which causes the surface to appear very rough at these much shorter wavelengths. This report describes radio-echo measurements of the reflection properties of the moon at a wavelength of 3.6 cm. The results show that the surface appears a good deal rougher at 3.6 cm than at meter wavelengths. Some 30 per cent of the reflected power is returned from scatterers that are uniformly distributed over the surface. The remainder is reflected from a region at the center of the visible disk which has a radius of about half the lunar radius. In this region, the surface appears to be describable by means of a Gaussian spatial autocorrelation function with a mean surface gradient of one in three.