A Robust Approach to Polarimetric Calibration for NSSL's Advanced Technology Demonstrator

Weather radars are a critical tool for monitoring and predicting weather patterns and are particularly useful for detecting and tracking severe weather, such as thunderstorms and tornadoes. It is commonly used by meteorologists and weather forecasters to provide accurate and timely weather information to the public. An antenna is the key component of a radar system and most weather radars, in use today, rely on parabolic antennae. A significant limitation of weather radars with parabolic antennas is that they have to scan via mechanical antenna movement angles to build a complete picture of the weather. The time it takes to complete a full scan can be several minutes, which may delay the availability of radar data and make it harder to provide real-time weather information. Phased array weather radar, on the other hand, can scan the sky much faster than traditional parabolic antenna radar due to its ability to electronically steer the radar beam, without physically moving the antenna. For this reason, polarimetric phased array radar (PPAR) is one of the candidate platforms for the next generation of weather radars.

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.