Range Aliasing Effect of Frequency Modulation Interrupted Continuous Wave (FMICW) High-Frequency Surface Wave Radar – Ionospheric Clutter Examination

In this study, a theoretical expression of the range spectrum for frequency modulation interrupted continuous wave (FMICW) radar echoes is derived. It shows that the resultant range spectrum is the result of convolving the original range spectrum with the Fourier transform of the periodic sequence of the gating function and multiplying by a weighting function that is the Fourier transform of the gating function. The overlapping between adjacent cycles of the repeatedly occurring range spectra accounts for the range aliasing of the resultant range spectrum. In addition, the amplitude modulation of the resultant range spectrum is also investigated. In order to validate the theoretical results, the ionospheric clutters detected by Pengjiayu FMICW ground wave sea radar (PGWSR) at Taiwan were employed to compare with the ionogram traces recorded by Chung-Li ionosonde. After correcting the range aliasing effect, the ranges of PGWSR-observed ionospheric specular echoes are consistent with those observed by the ionosonde; however, no one-to-one trace counterparts can be found in the ionograms for the diffuse type ionospheric clutters. With the help of the International Geomagnetic Reference Field (IGRF) model, we find that perpendicular distance of field-aligned irregularities (FAIs) are comparable to those of the diffuse type ionospheric echoes, suggesting the targets responsible for the diffuse type ionospheric echoes are very likely the FAIs at decameter.