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The Polarization of the Thermal Radiation of the Moon at 14.5 GHz.

1965; Institute of Physics; Volume: 70; Linguagem: Inglês

10.1086/109511

1538-3881

J. W. M. Baars, P. G. Mezger, N.L. Savin, H. J. Wendker,

Spacecraft and Cryogenic Technologies

In the spring of 1964, the thermal radiation of the moon was measured in two perpendicular polarization directions. As pointed out originally by Troitskii and Zeitlin (Radiofisica 3, 1127, 1960), it is possible to derive the dielectric constant E of the surface layer of the moon from these measurements. From the measured drift and declination scans we derived the emissivity in both polarization angles (normalized to unity in the center of the moon) and the tangential polarization (i.e. the ratio of the emissivity of a point on the surface in the two polarization directions). In order to find the dielectric constant we computed the emissivity of a smooth sphere for values of E separated by steps of 0.1 in the range l.l-~E-~2.S We convolved these curves with the gaussian beam pattern of our 85-ft antenna (3'.35 arc beamwidth) and derived the tangential polarization. As a next step we assumed a large-scale surface roughness for the moon with the normals to the surface having a Gaussian distribution around the line of sight with a "half-width" of 5, 10, 15, 20, 25, and 30 deg. The theoretical curves computed with these models were also convolved with the antenna beam. The comparison of the computed model curves with the average of all our measurements shows that the role of the roughness is not serious in the central part of the lunar disk up to about 0.6 of the radius from the center. Within this radius e= 1.8 gives the best fit to the measurements. In the region 0.6 to 0.9 of the radius, the roughness is important and a model with roughness of 15 deg half-width and dielectric constant E= 1.5 yields the best fit. Close to the edge of the moon at >0.9 of the radius from the center, the polarization decreases rapidly. This decrease may be explained by the more normal incidence of the radiation coming from the mountains near the lunar limb. Our work, together with previously published values of the dielectric constant at different frequencies, suggests that the dielectric constant decreases with increasing frequency.