Modeling the recorded spectrum and reconstructing the transfer function of the wideband ionospheric HF radio channel in the case of chirp sounding

Abstract

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.