Separation of O/X Polarization Modes on Oblique Ionospheric Soundings

Abstract

The oblique‐incidence sounder (OIS) is a well‐established instrument for determining the state of the ionosphere, with several advantages over vertical‐incidence sounders (VIS). However, the processing and interpretation of OIS ionograms is more complicated than that of VIS ionograms. Due to the Earth's magnetic field, the ionosphere is birefringent at radio frequencies and a VIS or OIS will typically see two distinct ionospheric returns, known as the O and X modes. The separation of these two modes on a VIS, using a polarimetric receive antenna, is a well‐established technique. However, this process is more complicated on an OIS due to a variable separation in the phase difference between the two modes, as measured between the two arms of a polarimetric antenna. Using a polarimetric antenna that can be rotated and tilted, we show that this variation in phase separation within an ionogram is caused by the variation in incidence angle, with some configurations leading to greater variation in phase separation. We then develop an algorithm for separating O and X modes in oblique ionograms that can account for the variation in phase separation, and we demonstrate successful separation even in relatively difficult cases. The variation in phase separation can also be exploited to estimate the incident elevation, a technique which may be useful for other applications of HF radio.

Response of a Polarimetric Antenna to Ionospherically Propagated Signals

“…a spherical mirror model (SMM), a relationship between a signal's group delay and its elevation of arrival can be derived. By applying the Breit-Tuve theorem and Martyn's equivalent path theorem [6], [12], [13] we can show that:…”

“…In previous work [6] a method for separating the polarization modes for OIS was developed, however this method assumes that both modes are circularly polarized. In this paper we will develop a model for the polarization of incoming OIS signals and demonstrate its validity using OIS soundings received by a polarimetric receive antenna, which specifically show some of the unique phenomena predicted by the model.…”

A Robust Feature Extraction and Parameterized Fitting Algorithm for Bottom‐Side Oblique and Vertical Incidence Ionograms

“…A customized configuration of the DST Group system for VI measurements with o / x separation has previously been reported (Harris et al, ). DST Group has also developed a solution for the reliable automatic separation of oblique o / x returns using twin‐polarized antennas (Harris et al, ), although it is not part of the standard DORS configuration.…”

A New Ionogram Automatic Scaling Method

“…Jiang et al (, ) presented a method to obtain the F2 parameters and F1 critical frequency by using the quasi‐parabolic segment model and empirical orthogonal functions. Harris et al () and Harris and Pederick () developed a new method to separate the O/X mode for vertical and oblique ionosonde based on direct digital receiver technology and algorithm. Even though considerable progress has been made in the recent years, the reliability and accuracy of the automatically scaled data still remained a challenge.…”