This paper proposes a novel synchronous wideband frequency domain method for measuring time domain response of long-distance channel. Its core consists of: (1) baseband signal generators at the transmission terminal and the reception terminal respectively are used to generate the wideband signal of the same frequency; (2) the two GPS clock frequency reference sources locked on the same satellite are used to yield the high-stability 10MHz signal as the external reference source of the baseband signal generator so that the initial phases of the wideband signals are basically the same; (3) the pulse per second (PPS) signal generated by the GPS clock frequency reference source is used as trigger signal to ensure that the baseband signal generator and the vector network analyzer (VNA) can transmit and receive signals synchronously; (4) the time domain response of the channel is indirectly obtained through the inverse Fourier transform of amplitude and phase of the frequency domain response. To verify the measurement method, experiments were performed, in which the sea surface evaporation waveguide which is tens of kilometers apart from each other was selected as the channel. The experimental results, given in Figs. 4 and 5, and their analysis show that the measurement method can obtain amplitude and phase of the signal whose band is hundreds of MHz and whose equivalent pulse width reaches 5ns. The measurement method is used to obtain the time domain response of the long-distance channel, verifying that the measurement method is effective.
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