Polarimetric radar calibration using distributed target

Accurate knowledge of the polarization scattering matrix is essential for understanding physical properties of the target. Conventional calibration uses a point target located in the boresight of the antenna. This calibration assumes that it is sufficient to measure amplitude and phase in one point of the antenna pattern. However most of the investigated targets are spatially distributed and observed by the complete antenna pattern. Thus a new calibration technique taking into account the complete antenna pattern is necessary. The simplest way for such a calibration is to use a distributed target with well known polarimetric properties. Since such an artificial target is not available, the choice of a natural target is unavoidable. Use of rain as the calibration target is discussed and results of the calibration are presented. This calibration was performed using the Delft Atmospheric Research Radar (DARR). DARR is a Doppler-polarimetric S-band radar located on the roof of a 92-m high building. The DARR system has a cross polarization isolation of 27 dB for a distributed target. As a result of the calibration, an effective polarization isolation of 34 dB is achieved.