Nonetheless, serious challenges must be overcome before this technology is ready for routine weather surveillance. The main issues are biases in polarimetric variable estimates caused by the beamsteering-dependent horizontal (H) and vertical (V) copolar patterns and significant cross-polar patterns. The scan-dependent biases (caused by copolar patterns) must be corrected using appropriate values at each boresight location. These correction values must be obtained via sufficiently accurate characterization of copolar pattern main beams at beamsteering angles of interest. The significant cross-polar patterns induce cross coupling between returns from the horizontally and vertically oriented fields resulting in the cross-coupling biases of polarimetric variable estimates. This effect can be mitigated using a pulse-to-pulse phase coding in either the H or V ports of the transmission elements. This method has been experimentally verified and does not require knowledge of cross-polar patterns.
To evaluate the suitability of PPAR technology for weather surveillance, the Advanced Technology Demonstrator (ATD) was installed at the National Severe Storms Laboratory (NSSL) in Norman, OK (https://nssl.noaa.gov/tools/radar/atd/). It is a full-size, S-band, planar, proof-of-concept PPAR. The ATD infrastructure includes a far-field calibration tower near the ATD site to facilitate calibration. Atop the tower is an S-band standard gain horn attached to a motorized platform that allows it to rotate about its axis and set the horn polarization in horizontal, vertical, or any other desired position. This infrastructure is used to accurately measure the array parameters needed for weather calibration. Herein, the latest efforts aimed at ATD weather calibration will be presented with particular attention to the broadside calibration of differential reflectivity and phase.

