Proposal for a New Style Beacon Experiment to Determine ∫Nds for the Ionosphere and Magnetosphere

Authors

  • W. Dieminger,

  • G. K. Hartmann,

  • G. Schmidt


Abstract

A new-style beacon experiment for measuring the difference-differential-Doppler (DDD) effect and the Faraday effect by using signals from geostationary satellites is proposed. By using the beacon frequencies of a geostationary satellite with frequency ranges ≥ 130 MHz, it can be shown that above 5000 km the upper atmosphere does not contribute measurably to the Faraday rotation angle. This is due to the weighting function of the earth's magnetic field and the decrease in electron content with increasing altitude. The socalled differential Doppler is not affected by the above-mentioned weighting function, and therefore ∫Nds is measurable along the total range up to 36,000 km, which is the distance of the geostationary satellite. In general, the Faraday rotation and the differential Doppler only display the variation of ∫Nds as a function of time; e.g., the total amount of ∫Nds is not known. By measuring the Faraday rotation on two closely spaced frequencies, however, one can calculate the total amount of ∫Nds up to about 5000 km. This experiment was designed so that the DDD effect and the Faraday effect could be measured on two closely spaced frequencies. To measure the total electron content (TEC) up to 36,000 km, the following new method (DDD) is proposed: Over a time interval of approximately 4 minutes, the beacon frequency of 138 MHz is slowly shifted to 137.5 MHz and then slowly back to 138 MHz; simultaneously, the frequency 414 MHz is shifted to 412.5 MHz and back to 414 MHz. By designing an appropriate phase-locked receiving system, it seems possible to measure the so-called difference-differential Doppler with an accuracy of better than 5%, e.g., to calculate the total amount of ∫Nds from 0 to 36,000 km. No other known method can achieve such an accuracy. The effective radiated power of the satellite antenna never exceeds 1 watt. Taking into account navigation systems, ∫Nds represents, apart from a constant well-known factor, the total range error that is due to the ionosphere and magnetosphere. Measurements of the Faraday effect and differential-Doppler effect (137.350 and 412.050 MHz) with signals from the ATS-3 satellite displayed very sensitively traveling ionospheric disturbances (TID) at our observing station in Lindau. By using several reasonably located observing stations, dynamic processes (TID) in the upper atmosphere might be easily investigated. Regardless of whether it will be possible to determine accurately the exospheric electron content and whether this proposal will be accepted, there is an absolute necessity for any measuring method that yields ∫Nds with a much higher accuracy than can be determined by the presently used methods. The determination of ∫Nds is required with at least the above-mentioned accuracy for any thorough investigation of so-called occultation experiments. Comparisons with other methods in use for calculating ∫Nds are presented, e.g., regular group-delay measurements. The proposed method seems to yield the most accurate measurements possible for these particular experiments. It is very likely that from a technical point of view the accuracy is greater than can be obtained with the group-delay methods now used. From a theoretical point of view, however, the degrees of accuracy obtained by the two methods are comparable.

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