Linear Depolarization Ratio (LDR)

Alternating polarization radars can transmit a single polarization and receive simultaneously in two channels, usually the co-polarized and cross-polarized components. For example, when transmitting horizontal, both horizontal (co-polarized) and vertical (cross-polarized) polarized energy are received by 2 separate channels.

In the case of vertical or horizontal, the ratio of the power Zcross or Zco is called the linear depolarization ratio (LDR). The amount of incident radiation that is depolarized by a particle, depends on the particle shape and orientation (for example, canting angle with respect to horizontal). Perfectly spherical particles do not depolarize either horizontal or vertical polarization, so the LDR is 0. Particles that are wet, tumbling and irregularly shaped give larger LDR values. Therefore, LDR values in rain tend to be small, for example, less than -25 dB. Larger values of LDR can occur in the bright band or in the presence of hail.

A radar and antenna system must be optimized to measure LDR by assuring that the antenna, feed and supporting struts, and radome are not themselves depolarizing the transmitted and received radiation. This is called "cross-pol isolation". The integrated cross-pol isolation of the antenna pattern must be more than 35 dB for LDR measurement. Stratiform rain typically causes the least amount of depolarization which had been thought to be around a value of -35 dB in LDR. However, as radar designs have improved, lower levels of depolarization in rainfall have been measured.

If the radar cannot achieve this level of cross-pol isolation it is limited to how accurate the rainfall estimates can be from the dual-polarization data.