Cross-coupling, antenna errors and simultaneous horizontal and vertical polarization transmit data

Many radars around the world are currently adapting the simultaneous horizontal (H) and vertical (V) polarization transmit technique to achieve dual polarization measurements. The technique is based on the observation that the mean canting angle of large ensembles of raindrops is zero degrees. This implies that there is no cross-coupling of the H and V transmitted signals as they propagate. It is know that if the mean canting angle of the precipitation particles is non zero, then there will be cross-coupling and biases will become evident in the polarimetric signatures. This can occur in the ice phase regions of storms. Previous radar observations have supported and shown this. Similar bias effects can also be caused by antenna polarization errors. Polarization errors will occur because of non-ideal antenna and feed horn, support struts, etc. This talk examines the biases caused by cross-coupling of the H and V transmitted signals. A model is developed and the magnitudes of the errors are quantified with particular attention to the measurement of differential reflectivity, Zdr. The model demonstrates how the errors are a strong function of the differential propagation phase, phi_dp. The effect of the phase difference between the H and V transmit components is also examined. Estimation of antenna polarization errors is discussed and TiMREX (Terrain-influenced Monsoon Rainfall Experiment) data from NCAR's S-band polarimetric radar, S-Pol, are used to illustrate some of the concepts.