We have implemented a method for constructing the transfer function (TF) of an ionospheric radio channel from results of ionospheric sounding by a frequency-modulated continuous wave. This method is based on insertion of a digital correction filter with given amplitude-frequency and phase-frequency characteristics into a sounding signal processing circuit. The problem of synthesis of the correction filter in a time domain was solved by restricting the filter's infinite-impulse response in a finite time interval. We analyzed amplitude and phase errors associated with such restriction and discussed results of processing of radiostation interference by the correction filter.
The correction filter is demonstrated to be a matched filter for the radiostation interference passing through the reception path. This allows us to detect the interference more effectively through the threshold processing and to remove them with only a slight distortion of a useful signal.
We show that the correction filter allows solving the well-known problem of the difference between frequencies of a heterodyne and a sounding signal passing through an ionospheric channel. This problem is pressing for high rate of FMCW ionosonde frequency sweep. After a signal has been processed by the correction filter, the sounding signal carrier frequency matches the heterodyne frequency.
This method was implemented in the processing circuit of sounding signal of the ISTP SB RAS-developed multifunctional FMCW ionosonde. We give an example of processing of ionospheric vertical sounding data obtained in a frequency band of 1.3–10 MHz at a rate of the frequency sweep of 2 MHz/s. We present results of constructing the ionospheric HF radio channel TF in the form of an ionogram and a signal amplitude segment representing an ionospheric radio channel TF in the vicinity of the frequency corresponding to equal time delays of ordinary and extraordinary waves.
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