Implementation of a Far-Field Tower for Calibrating a Dual-Polarization Planar Phased-Array Radar

The NOAA National Severe Storms Laboratory is currently in the calibration and test phase of the Advanced Technology Demonstrator (ATD), a full-scale, S-band, dual-polarization, 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, as uncalibrated H- and V-channel beams do not necessarily illuminate the exact same volume and electronic steering introduces cross-polar components that bias dual-pol variable estimates. To produce reliable polarimetric variable estimates, it is critical that the H- and V-channel beam peaks are spatially aligned for all scanning angles and that steering-dependent cross-polar patterns are accurately measured and accounted for.

To support these measurements, a 45 m tower was erected in the far field of the ATD, 425 m to the north. Atop the tower is an S-band standard gain horn on a motorized platform that allows it to rotate about its axis, enabling measurements in horizontal or vertical orientation. Although the tower location is fixed, measurement at any electronic steering angle is possible by mechanically positioning the ATD antenna such that the steering angle of interest points toward the calibration tower. An equipment shelter local to calibration tower contains a network of switches and other RF equipment to route signals and adjust power levels in support of multiple modes of measurement (e.g. transmit only, receive only, two-way with delay, single element measurement). This shelter is connected to the ATD by underground network and RF-over-fiber links. Using a dedicated processor and a custom interface board to control the tower equipment, intricate multi-part measurement strategies can be automated in the ATD software (for example, collecting H- and V- channel beam peak measurements for hundreds of steering angles). This poster focuses on the implementation of the calibration tower infrastructure to support all desired measurement types, including challenges and mitigations resulting from preliminary measurements.