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Depolarization in Radar

Figure 1: Explanation of the mechanism of polarization rotation.

Figure 1: Explanation of the mechanism of polarization rotation.

Depolarization in Radar

In radar, depolarization is the change in the orientation of the electric field during a reflection or (more rarely) diffraction. This definition concerns the fully polarized waves which occur exclusively in radar. This is because the polarization of an electromagnetic wave is largely dependent on the geometry of the transmitting antenna. This geometry cannot take chaotic states. Therefore, the state of polarization is constant for the time periods considered here in the electromagnetic wave. (For only partially polarizable waves, for example, the light, the term depolarization has a completely different meaning for historical reasons).

The process of depolarization depends on the geometrical and dielectric properties of the reflecting object. To understand how such depolarization can occur, let's assume that the reflector is a resonant dipole at a slight angle to the direction of polarization of the incident wave. It will nevertheless absorb some energy, albeit with losses, i.e. not with a maximum possible magnitude. It will now re-radiate this energy in exactly the polarization direction corresponding to its geometric orientation. The electric field of the reflected energy is now slightly rotated with respect to the transmitting antenna of the radar. If the transmitting radar antenna is horizontally polarized, then a somewhat weaker signal is received in the horizontally polarized receiving antenna. On the other hand, with a dual polarized radar, the vertically polarized receiving antenna also receives a weak signal. From the magnitude ratio of both signals, it can now be calculated in which position this reflecting resonant dipole is located. In practice, for example, this mechanism of rotation of polarization described here can occur due to the spatial orientation of ice crystals.

Since depolarization is a relative process, all quantitative polarimetric information is preserved in the amplitudes and phases between the different polarizations. Therefore, even in the case of an accumulation of rather chaotic reflections within a volume target (rain area), the echo signals of individual raindrops from different geometric positions and distances are coherent with each other and superimpose in the far-field to form a common echo signal with a uniform polarization which may then also be twisted with respect to the emitted signal.

Depolarization is of particular importance in precipitation radar for characterizing the type of precipitation. In air surveillance and air defense radars, this effect is currently negligible.