Radio emission of the sea surface near the Brewster angle at 0.86-and 1.3-cm wavelength

Abstract

A great deal of attention is currently being devoted to the study of the background radio emission of the sea surface from airplanes, ships, and satellites. The objective of these investigations is to obtain information about the state of the sea surface along extended tracks. For observations from above, the range of visibility angles is 0-50 ~ from nadir, due to known limitations. It seems to us that low visibility angles have received little attention, however, and besides, this range of angles is of undisputed interest both for remote measurements of the state of the sea surface, and for antenna calibration in radio astronomical studies at sea. Carrying out these studies successfully requires advanced development of measurement techniques, particularly of operational control of radio telescope parameters at sea. Under land-based conditions, the methods which have been developed and widely used for antenna calibration include those in which natural radio sources of known signal strength are employed [i] (portions of forests, the earth's surface observed at the Brewster angle, mountain slopes, etc.). Under sea-going conditions, most of these regions of known emission temperature are inaccessible to observation. Under those circumstances, one may take as "reference"regions with known signal strength a segment of the surface of the water near the Brewster angle, which is quite "black," and an emitting region of the atmosphere near the horizon. With such a choice of "reference" regions, one assumes that the brightness temperature of the surface of the water [2], or, for atmospheric calibration, the emission temperature of a region near the horizon is taken to be equal to that of the ambient air. In both this and the other case, we ignore the relationship between the rough sea surface and the radiation temperature of the reference regions, which can lead to errors in the calibration of antenna parameters. The influence of these factors on the effective emission temperature requires that both experimental studies and modeling of the emission be carried out for the rough water surface. As is well known, the brightness temperature of the smooth water surface is determined by its intrinsic emission and by the reflection of atmospheric radiation. The contribution of intrinsic emission to the brightness temperature is proportional to the water temperature To, and the proportionality factor is the emissivity I--~(~,00). The Fresnel (power) reflection coefficient R(e, 00) for vertical or horizontal polarization depends on the complex dielectric constant of the water ~---~i~f~2 and the observing angle e0. The real and imaginary parts of the dielectric constant, which depend on the wavelength, temperature, and salinity, can be calculated using the well-known formulas of Debye [3]. The component of the brightness temperature of the surface of the water due to reflected atmospheric radiation is proportional to the brightness temperature of the atmosphere Tba(F , 0o) in the given