Measurements of Faraday polarization rotation of VHF radio waves transmitted from geostationary satellites have been made by many workers at various locations throughout the world to study the total electron content (TEC) of the earth's ionosphere. In order to determine the absolute amount of total Faraday rotation a knowledge of the initial plane of polarization as transmitted from the satellite with respect to a known reference is necessary. A knowledge of this initial polarization angle is particularly important when the total Faraday rotation is small, such as at near-equatorial regions, and at all locations during nighttime, particularly under solar minimum conditions. By using a VHF lundar radar at a time when the moon passed nearly behind each satellite this initial polarization has been determined with respect to a known reference for 10 different geostationary satellites. The initial polarizations of the signals emitted from the ionospheric beacon on the ATS 6 satellite were measured before the satellite launch. One of these initial polarizations, the one at the VHF frequency most often used for Faraday measurements, was confirmed by the lunar technique. |