ATS-6 radio beacon measurements of modulation phase and Faraday rotation made at Ootacamund, India, in 1975–1976 are discussed with emphasis on the measurement and analysis errors. The modulation-phase measurements are insensitive to the geomagnetic field and provide an accurate determination of the total columnar electron content. Comparison of modulation-phase measurements at different frequencies shows a minor sensitivity to the ATS-6 pitch angle. For the low geomagnetic latitude and nearly transverse propagation conditions of Ootacamund, the use of a fixed conversion coefficient gives an unreliable Faraday content. However, the Faraday rotation measurements may be used to determine the shape factor F, which provides information about the electron density altitude profile. Corrections to the measured Faraday rotation for the ATS-6 pitch and yaw are important, particularly at night when the rotation on 140 MHz can be of the order of 10° to 20°. Thus the reduction of data to plasmaspheric electron content is not recommended. Sample data are presented showing that, in the daytime, total content is about twice that in middle latitudes. The shape factor shows a low predawn minimum indicating the nearly complete erosion of the F2 layer peak.
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