Facets of Planar Polarimetric Phased Array Radar Use for Weather Observations

While the functionality of Phased Array Radar (PAR) technology for point targets is well-established, accurate polarimetric measurements of distributed targets using this technology have not yet been demonstrated. Currently, the planar and cylindrical antenna architectures are being considered for weather observations. To achieve sufficiently accurate polarimetric weather measurements, the planar presents a greater calibration challenge (at least in theory) but provides superior flexibility to perform point target and weather functions. Also, planar PARs are ubiquitous for point target detection and tracking, which makes them preferable for weather use from the manufacturing aspect. The complexity of using polarimetric PARs for weather functions arises from the inherent antenna gain dependency on the electronic beam steering angle. Furthermore, these dependencies are not the same for the two polarimetric channels (i.e., horizontal-H and vertical-V). Consequently, this induces a variable systematic bias in the measurements of differential reflectivity which must be accounted for. This motivates the need to characterize the copolar antenna patterns for all steering locations of interest to provide for adequate corrections. Another serious issue is the polarization purity which is reflected in the existence of cross-polar antenna patterns on transmit and receive. These patterns describe the departure of the true polarizations from the intended ones (e.g., the excitation of the H/V port generates a field which is not H/V polarized), where the differences between the true and the intended polarizations increase as the beams are steered farther away from the principal planes. To address the polarization purity issue, a pulse-to-pulse phase coding technique has been developed at the National Severe Storms Laboratory (NSSL). It is capable of sufficiently mitigating the cross-polar pattern effects if these are ~26 dB or more below the copolar patterns (which characterize the intended polarization). Hence, the effectiveness of this method declines as H and V beams are steered away from principal planes. As a result, the cross-polar patterns must be characterized at locations where the suppression of the cross-polar pattern effects is insufficient (to perform the error corrections using the knowledge of both the copolar and cross-polar patterns).

To evaluate the performance of a planar polarimetric PAR (PPPAR) for weather surveillance, Advanced Technology Demonstrator (ATD) is installed in Norman, OK. To perform calibration functions, a tower with the horn antenna will be installed near the ATD site. It will be used to measure the peaks of the ATD antenna patterns on transmit and receive. To account for the system induced errors, these measurements will be used to correct the power measurements from the H and V channels, as well as the cross-correlation estimates. However, the limited accuracy of the beam peak measurements will affect the correction quality.

Herein, an overview of the problems related to the PPPAR use for weather observations is presented as well as the prospective solution methods. Also, an assessment of the beam peak measurement errors and their effects on the accuracy of polarimetric variable estimates is shown.