On the processing of FMCW signals for ionospheric sounding applications

FMCW (chirp) signals are often employed to measure the propagation characteristics of an HF channel. The duration and frequency span of the chirps may vary from short duration in real time channel evaluation (RTCE) systems to long, continuous sweeps covering the entire HF band. Interest has developed in using such sounding techniques to provide high resolution measurements by the use of modern signal processing techniques. Spectral analysis procedures, including the prior weighting of the signal samples by a window function, are usually employed to extract the signal parameters from the received chirp signal. These procedures are not ideal and their limitations need to be appreciated in order to avoid misleading results. Furthermore, ionospheric effects, in particular, changes in the group path delay over the frequency range of the analysis intervals, also significantly affect the results of the spectral analysis procedures. Several of these effects are outlined in the paper and the associated limitations of the analysis procedures discussed