Polarization overview for ionospheric HF radars

Ionospheric HF radars detect targets at very long distances, beyond the horizon, using high frequency (HF) signals (between 3 and 30 MHz) that are refracted through the ionosphere or back-scattered from it. These signals may be polarized in different ways, such as linear polarization (vertical, horizontal or oblique), circular polarization (right or left hand side), or elliptical polarization (a mixture from both types). During the transmission from an antenna array with elements equipped to receive signals with a single linear polarization, either horizontal or vertical, if there is a rotation of the signal, part of the information may be lost. To overcome lost of information, it’s necessary to exploit the benefits of polarization, which also includes: interference rejection, clutter suppression, target classification, signal authentication, enhanced spacial resolution, and discrimination of decoy sources. In the ionosphere, the changes in the wave polarization, also called as Faraday Rotation, are basically due to the presence of the Earth’s magnetic field.