ZDR Measurement

The differential reflectivity (ZDR) is a relative power measurement of the signals acquired in the horizontally and vertically polarized channels in the polarimetric weather radar.

ZDR measurement accuracy must be at the level of 0.1 dB for applications of quantitative rainfall and echo identification. Such accuracies are challenging if pursued through absolute power calibrations of each radar channel independently. Accounting for this, RDA calculates the differential reflectivity independent of the reflectivity dBZ, and the related calibration coefficients, while a tunable offset parameter is reserved to maintain the balance of the ZDR calibration, explicitly.

Depending on the radar system stability, regular and semi-continuous monitoring of the ZDR calibration and occasional update of the offset parameter are necessary to maintain an accurate ZDR calibration.

It is operationally feasible to maintain the relative power balance of two radar channels at sub-decibel accuracy, by using the external 0 dB reference defined by the atmospheric echo inputs that are averaged over rapid full rotations of the vertically pointing antenna ("bird bath" scans).¹