PPAR Advanced Technology Demonstrator Polarimetric Calibration for Spoiled Beams

Polarimetric phased array radars (PPAR) are poised to be strong contenders for the next generation of radar systems. Their unique ability to steer beams electronically makes them invaluable for advancing weather surveillance strategies and refining weather radar products. However, significant challenges must be addressed before these technologies can be routinely deployed for weather surveillance.

Foremost among these challenges are the biases in polarimetric measurements resulting from beam-steering-dependent horizontal (H) and vertical (V) copolar patterns and significant cross-polar patterns. The biases associated with copolar patterns, which depend on the scan direction, require correction with precise values at each boresight position. These correction values must be determined by accurately characterizing the copolar pattern main beams at relevant beam-steering angles.

Moreover, significant cross-polar patterns cause cross-coupling between returns from horizontally and vertically oriented fields, introducing biases in polarimetric measurements. Using pulse-to-pulse phase coding in either the H or V ports of the transmission elements has been experimentally validated as a way to mitigate this effect. This method offers the advantage of not requiring explicit knowledge of cross-polar patterns. Additionally, a slight antenna tilt has been proposed to help reduce cross-coupling. However, the effectiveness of these mitigation strategies decreases as the level of cross-polar patterns increases relative to the copolar patterns.

In scenarios where cross-polar patterns are particularly strong, the previously described mitigation strategies may be inadequate at certain beam-steering angles. In such cases, a correction strategy based on a comprehensive understanding of both copolar and cross-polar antenna patterns has been proposed. This approach is more complex, requiring precise characterization of both cross-polar and copolar beams, and its experimental validation is still pending.

To assess the viability of PPAR technology for weather surveillance, the Advanced Technology Demonstrator (ATD) was installed at the National Severe Storms Laboratory (NSSL) in Norman, OK. The ATD, a full-size, S-band, planar proof-of-concept PPAR, is the centerpiece of PPAR polarimetric calibration research. To support this, the ATD setup includes a nearby far-field calibration tower. Atop this tower is an S-band standard gain horn mounted on a motorized platform, allowing rotation about its axis to adjust polarization to horizontal, vertical, or any desired orientation. This setup enables the measurement of array parameters essential for weather calibration purposes. This presentation will explore the latest efforts in weather calibration for “spoiled beams” using the ATD and its associated calibration infrastructure. A "spoiled beam" in radar terminology refers to a deliberately widened radar transmit beam to cover a larger area but with a reduced focus and intensity. Multiple receive beams are formed simultaneously via amplitude and phase weighting of data received from 24 subarrays in the case of ATD. This technique is used to achieve broader surveillance coverage at the expense of finer resolution and sensitivity. It results in faster scan update times which is important for radar meteorologists interpreting weather radar products.