Abstract
Ionospheric clutter is a major factor affecting the performance of high-frequency surface wave radar (HFSWR). Previous studies have been mainly focused on the development of ionospheric clutter suppression methods involving delicate signal processing techniques or additional antennas. However, ionospheric clutter originates from the interaction between HF waves and the ionosphere, thus it contains the characteristics of the latter. Therefore, ionospheric clutter can be analyzed to obtain the parameters of the reflecting ionosphere, expanding the value of HFSWR. This article presents the preliminary coordinated observation and analysis results of the characteristics of the ionosphere at mid-latitudes of China using both HFSWR and ionosonde. The results demonstrate the existence of an oblique skywave propagation path (0.5 jump, 1 jump, etc.) in addition to the vertical reflection path and ionosphere-ocean mixed path. HFSWR beams were also found split into an O-trace and an X-trace after entering the ionosphere. Furthermore, range-folded ionospheric echoes are related to the short-term thickening of the F2-layer, showing strong fluctuation in the range-Doppler spectrum of HFSWR. These observed characteristics of ionospheric echoes are useful for the development of an efficient ionospheric clutter suppression algorithm for HFSWR and further investigation of the ionospheric mechanisms at mid-latitudes.
Highlights
HIGH frequency surface wave radar (HFSWR) can detect targets beyond the horizon due to the mechanism of high-frequency wave diffraction along the ocean surface [1,2,3,4,5,6]
For a specific layer of ionosphere, the critical frequency of the ionosphere observed by HFSWR corresponds to the maximum of the O-trace and X-trace frequencies while ionosonde always picks up the O-trace component
The main conclusions include: 1) We confirmed the existence of skywave propagation paths for the ionospheric echoes of HFSWR in addition to the vertical reflection path and the ionosphere-ocean mixed path
Summary
HIGH frequency surface wave radar (HFSWR) can detect targets beyond the horizon due to the mechanism of high-frequency wave diffraction along the ocean surface [1,2,3,4,5,6]. Due to the limitations of radar antenna systems, namely, imperfection of radiation pattern, poor ground and wind-induced antenna motions, some transmitted waves travelling upward into the ionosphere are reflected along various paths back to the radar receiving system under certain conditions Such echo is referred to as ionospheric clutter [7,8]. Height-spread Es appears to extend over a range of heights, probably in the form of patches migrating with gases This type of layer may cause more interference for HFSWR because the signal disperses after being reflected and may not be present in the overhead null of the radar antenna pattern. We will investigate the ionosphere phenomenon that causes the range-folded ionospheric echoes in HF radar Doppler spectra based on both HF radar data and ionograms.
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