Polarization is an extremely important weather radar parameter for assessing the composition of the atmosphere. The relationship between simultaneous horizontally and vertically polarized returns has shown to provide quantifiable estimates of rainfall as well as improve discrimination of other weather phenomena. This capability is now being integrated into the WSR- 88D parabolic antenna radar system. The next generation of weather radars will begin to incorporate phased arrays with inertia free scanned beams that will shorten radar surveillance times. With flexible scanning strategies, phased arrays can provide multiple independent radar beams that can be used for multiple functions. While one beam is used to provide a continuous scan of the entire 360 degree coverage, special beams can be used to focus on special severe weather phenomena to provide high temporal rate data.

The addition of polarimetric capability to the phased array for weather radar brings new technical challenges. The quality of the polarimetric radar products is directly related to the isolation between the two nominally orthogonal polarizations. It is desired to measure Zdr, the ratio of the reflected horizontal to vertically polarized field to within 0.1 dB. Narrow beam phased arrays use thousands of identical radiating elements which are electronically delayed with respect to each other to form the antenna beam. The relative isolation between the two polarizations is determined by the electromagnetic characteristics of the individual radiating element imbedded in a large field of identical radiating elements. Ideal radiating elements would provide a radiation characteristic such that the two polarization states provide orthogonal fields in the antenna far-field. Due to practical limitations, isolation greater than 25 dB presents a significant technical and cost challenge. In this paper, an approach to calibrating the phased array through the characterization of the antenna in its near field is presented. Calibration data obtained in this manner is integrated along with the measured radar data to compensate for the finite isolation of the radiating element itself. Isolation is also affected by the relative gain and phase of the transmit and receive channels connected to the radiating elements. Approaches to addressing these challenges will also be discussed