Doppler shifts of HF radio waves along oblique propagation paths in the ionosphere
Abstract
Urgency. The ionosphere is widely used as a channel for radar, radioastronomy, radio navigation, communication links, and space-based sounding the Earth. To reveal changes in the radio channel parameters and variations in dynamic ionospheric processes, measurements of the Doppler shift and signal amplitude of HF radio waves should be taken along oblique propagation paths of various orientation. The Doppler radio sounding exhibits a great sensitivity to the dynamic processes in the ionosphere, therefore, the need for further development of the theoretical basis for oblique HF radio sounding the ionosphere, as a main simple and cheap technique for monitoring the ionospheric radio channel, is urgent.
The aim of this work is to develop the theoretical basis for the Doppler sounding the ionosphere along oblique propagation paths and to derive simple analytical relations for the Doppler shift.
Methods and Methodology. In developing the theoretical basis for oblique HF sounding, the undisturbed ionosphere
is assumed to be plane stratified, and the ionospheric disturbances to follow periodic and aperiodic models. The Snell law, the general formula for the Doppler shift, and the index of refraction taken to be that for the isotropic ionosphere are involved in the analytical derivations. The main technique is calculations using an analytical approach.
Results. The corrected secant law describing the condition for HF radio waves to reflect from a loss-free smooth spherically concentric isotropic ionosphere has been obtained. A simple analytical expression for estimating the maximum usable frequency along oblique HF propagation paths, making use of the known maximum plasma frequency, has been determined. For the conditions of oblique HF sounding the ionosphere, simple analytical relations are obtained for estimating the relative amplitude of quasi-sinusoidal disturbances and the magnitude of aperiodic disturbances of the electron density in the ionosphere. The applicability of the obtained relations has been demonstrated in practice.
Conclusions. The scientific base has been developed for making use of it in oblique HF sounding the ionosphere.
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References
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