Calibration of a Dual-Polarization Planar Phased-Array Radar Using a Far-Field Tower

The NOAA National Severe Storms Laboratory is completing the installation of the Advanced Technology Demonstrator (ATD), a full-scale, dual-polarization, S-band, active planar phased array weather/air traffic radar. Calibration to achieve satisfactory polarimetric performance remains among the greatest risks for dual-polarization phased array weather radars. This is due to the inherent steering angle dependent gains, as well as the existence of antenna cross-polar patterns in both the horizontal and vertical channels. Such behavior induces biases in the estimates of polarimetric variables which must be corrected for. To perform the bias corrections, a precise knowledge of both the copolar and cross-polar pattern beam peaks on transmit as well as receive is required.

To support these measurements, the calibration infrastructure for the ATD includes a 45 m far-field calibration tower, located 435 m north of the ATD. Atop the tower is an S-band standard gain horn on a motorized platform that allows it to rotate about its axis. RF-over-fiber links connect the ATD and calibration tower, allowing coherent calibration measurements. A matrix of switches, attenuators, and amplifiers supports the following multiple modes of measurement: (1) remote horn connected to a continuous-wave source for non-coherent receive measurements of the ATD antenna, (2) remote horn connected to the ATD exciter for coherent receive measurements of the ATD antenna, (3) remote horn connected to the ATD receiver for coherent transmit measurements of the ATD antenna, and (4) remote horn connected to a delay line for two-way measurements. Mechanical positioning of the ATD antenna allows beam peak measurement at any steering angle, enabling calibration data to be collected for all electronic scan positions of interest. Herein, an overview of the ATD calibration tower concept is presented, including details on equipment, measurement strategies, and challenges and mitigations.