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. Citation: Davydenko, M. A., N. V. Ilyin, and V. V. Khakhinov (2008), A method for restoration of radio channel transfer function by chirp sounding of the ionosphere, Int.
We model theoretically the received spectrum in the case of sounding of the ionospheric HF radio channel by a chirp signal. It is shown that the result of processing of an individual time sample of the received signal is equivalent to the sounding of the radio channel by a complex narrow-band pulsed signal such that the group delays of its propagation modes determine the maxima in the received spectrum. We analyze the quadrature components of realizations of the received signal at the intermediate frequency at the bandpass-filter output in the receiving channel of the chirp ionosonde. The results of our analysis show the possibility of reconstructing the transfer function of a HF radio channel in the sounding-frequency band for the delay range determined by the characteristics of the intermediate-frequency bandpass filter. We propose a method for reconstructing the transfer function of the ionospheric radio channel, which involves supplementing the circuit of primary processing of the signal by a corrective digital filter with specified amplitude-frequency and phase-frequency characteristics. The proposed method can be used for all operating regimes of the chirp ionosonde in the case of digital recording and processing of signals.
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