Separation of O/X Polarization Modes on Oblique Ionospheric Soundings

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. Key Points Separation of O and X modes more difficult for oblique ionograms than for vertical ionograms Experimental results show that difficulty is due to variation in incidence angle, leading to greater variation in phase separation Have developed an algorithm for separating modes in oblique ionograms that can account for the variation in phase separation