X-Band Phased-Array Weather-Radar Polarimetry Testbed: Bias Expectation and Preliminary Results

The University of Massachusetts Microwave Remote Sensing Laboratory is implementing a testbed to evaluate phased-array weather radar polarimetry at X-band. The project seeks to determine the impacts of electronic scan polarization errors on weather radar measurables in order to evaluate methods for polarimetric bias correction. The testbed consists of a planar dual-polarization X-band phased-array radar operated in tandem with a mechanically scanned polarimetric reference radar.

The phased-array, on long-term loan to UMass by Raytheon, consists of 2560 dual-polarized microstrip patch elements which may be excited in either vertical or horizontal polarization. Antenna elements are arranged in 128-element (16x8) tiles arranged in a 4x5 grid. The radar operates at 9.6 GHz and employs pulse compression with a duty cycle of up to 20%. The scan range is 90 degrees in azimuth and 30 degrees in elevation (+/-15 degrees off boresight assuming the array panel is tilted 15 degrees from the vertical).

The reference radar operates at 9.4 GHz employing a 1.2 m diameter mechanically-scanned parabolic reflector. It uses a magnetron transmitter to transmit and receive in both vertical and horizontal polarizations simultaneously (STSR), but also accommodates an LDR mode where the vertically polarized transmission is dumped to a load. This radar operates with a similar frequency, sensitivity and beamwidth to the phased-array in order to allow for easier comparison of collected data.

At least two methods of polarimetric calibration are being evaluated. The first uses near-field measurements of the co- and cross-polarized element patterns to construct a correction matrix that is based on the projection of the radiated fields at the array face to the world-relative horizontal and vertical polarization. The projection is considered in the case of a tilted array aperture. The second uses quasi-vertical scans of precipitation to determine system biases in Zdr, PhiDP, and LDR. Preliminary estimates of these biases are presented as are comparisons to the reference radar.