Calibration for Bias Correction of a Polarimetric, X-band, Phased-Array, Flat Panel Radar

The NSF Engineering Research Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) has been developing new radar technologies for better low-altitude weather observations based on dense networks of small X-band, polarimetric radars. CASA has developed a low-cost polarimetric phased array radar, the CASA X-Band Phase-Tilt Radar. The phase-tilt antenna performs electronic beam steering in the azimuth direction while mechanically steering (tilting) in the elevation direction. Phased arrays offer greater beam agility than traditional reflector antennas which allows for faster updates of rapidly developing events. However, a characteristic of polarimetric phased arrays is the variation of antenna gain with electronic scan angle. This variation is not consistent between polarization which leads to biases in differential reflectivity and other polarimetric measurements. Methods for correcting these biases have been proposed based on the antenna patterns of an individual array element in isolation. A shortcoming of these methods is that the patterns of an isolated antenna element can vary greatly from one located in an array. This effect, known as mutual coupling, will alter the pattern of each element in a way that depends on the location of the element within the array. Measuring these variations is necessary for proper bias correction of the polarimetric variables so that they may be used in detection algorithms and assimilated into models. This paper looks at residual errors based on different levels of element characterization (individual element vs. array of coupled elements) to understand the level of characterization vs accuracy which could point to the most cost effective measurement/calibration approach for a given level of performance.