Abstract
[1] Analysis of both in-phase and quadrature components of received frequency-modulated continuous wave (FMCW) signal at the output of intermediate frequency (IF) band-pass filter of the chirp ionosonde demonstrates a new possibility of recovering the transfer function of the HF radio channel within the band of operating frequencies and the interval of time delays that are defined by the IF filter characteristics. The transfer function is determined by the parameters of all parts comprising the radio channel: transmitter, receiving-transmitting antenna-feeder devices, Earth surface, ionosphere, as well as the receiver circuitry participating in the signal processing. However, characteristics of all parts except the ionosphere can be considered stationary and well-known or controlled. It is the ionosphere that displays significant temporal variability. A new technique is proposed for recovering the transfer function of the ionospheric radio channel, together with its implementation as a signal preprocessing circuit containing a correcting digital filter that adapts to the current amplitude frequency and phase frequency characteristics of the channel. INDEX TERMS: 2441 Ionosphere: Ionospheric storms; 2407 Ionosphere: Auroral ionosphere; 2447 Ionosphere: Modeling and forecasting; KEYWORDS: FMCW sounder, chirp sounding technique, ionospheric radio channel, transfer function, digital signal processing.
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