Adjustment and Measurement of Polarization Angles in Microwave Radiometers

Abstract

A method for measuring and installing four switchable polarization planes in a microwave radiometer using a Faraday cell is considered. The calibrated radiometer is designed to record weak polarization effects, for example, azimuthal anisotropy that occurs during the formation of the intrinsic radiation of a rough sea surface. To solve such problems, it is necessary to absolutely link all polarization modes of the radiometer to the horizon level with an accuracy of no worse than 0.5°. In the proposed measurement scheme, radiation from a gas-discharge tube with an output antenna on horizontal polarization was used as a broadband microwave source, which was additionally reflected from the water surface at the Brewster angle. This provided additional suppression of the vertical component of the radiation by –12 dB and the formation of strictly horizontal polarization of the reflected wave since the free surface of the water in the bath is horizontal with an accuracy of greater than 0.05°. Traditional sources of polarized radiation do not provide horizontal orientation of the emitted signal with the specified accuracy. In the proposed method, the installation errors of the vertical, horizontal, and crossed at the angles of ±45° planes of polarization were no more than ±0.3°. The measurement of the installation angles of the polarization plane was carried out by with an accuracy of 0.1° rotating the radiometer around the axis of the receiving antenna and approximating the data by the Malus law. The adjustment of the angles was controlled by currents through the Faraday cell.