Methods for Estimation of Ionospheric Layer Height Characteristics from Doppler Frequency and Time of Flight Measurements on HF Skywave Signals
Abstract. We demonstrate a methodology to estimate ionospheric virtual layer height characteristics using Doppler measurements from frequency locked time standard stations in conjunction with ionosonde measurements and ray-tracing models. We consider data from three events: the solar eclipse of 21 August 2017, a observations of the dawn terminator on 1 October 2019, and a time-of-flight study conducted 29 January 2020. Observations are consistent with a model in which mode splitting originates from different path length velocities associated with single and multiple hop modes as the virtual layer height changes. Support for this hypothesis comes from the complementary processes of 1) calculating Doppler shifts from virtual layer height changes and virtual layer height changes from Doppler shifts, and 2) the analysis of intermittent low-Doppler shift modes including correlation with ionosonde observations to help identify multihop propagation modes. We find that observations are in good agreement with measured data and simulations. We also find that the use of a precision frequency standard, such as a GPS-disciplined oscillator, at the receiving station is vital for ionospheric height measurements, since small errors in frequency estimation can lead to uncertainties on the order of tens of kilometers in resulting estimations of ionospheric height. The methods discussed herein provide a means to calculate path length estimates from distributed stations when integrated with other ionospheric measurements, helping to address the problem of under-sampling of the bottomside ionosphere.
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