So it is widely used for propagation predictions of high frequency (HF) radio communications, over-the-horizon radar localization and for basic research purposes (Rao 1973 Yau et al. It offers several important advantages over vertical sounding, such as the opportunity of monitoring the ionosphere across large, otherwise inaccessible areas such as ocean surfaces, and the ability of a receiver to detect signals from many different directions (Ippolito et al.

Oblique incidence ionospheric sounding (Davies 1990), which consists of a transmitter and a receiver located several hundreds or thousands of kilometers apart, is a powerful tool for monitoring the prevailing propagation conditions in real time and is also important for tracking ionospheric effects caused by space weather events. In the case of east–west propagation, the separation decreases to a minimum near ground range of 2000 km and then increases very slowly with increasing ground range, while it monotonically increases for the north–south propagation path. The main conclusions are as follows: (a) the separation on east–west path is susceptible to ionospheric variability, while the separation on north–south path does not show a significant correlation with local time and season (b) a general diurnal tendency and a summer anomaly on east–west propagation are first proposed and discussed, which may be related to the diurnal variation of hmF2 above the reflection point and the strong lower layers below the reflection point (c) the separation varies approximately as a cosine function with the propagation direction owning two maxima in the north–south direction and two minima in the east–west direction (d) the variation patterns of the separation with the propagation length are obviously not the same in different directions. The dependence on local time, season, geomagnetic activity, O-wave JF and group path, solar activity, direction, and length of propagation is investigated. This paper presents a statistical and simulation study of the separation in JFs between O- and X-waves based on observed oblique ionograms over three mid-latitude paths within China and a 3D ray-tracing program. This requires the correct identification of O- and X-mode traces, so it is very helpful and worthy to grasp the relative position between the two modes. The most important aim in interpreting an oblique ionogram is to obtain the accurate Junction Frequencies (JFs) of the ordinary (O) and extraordinary (X) mode.

Xwave radio