Holographic Back-Projection Method for Calibration of Fully Digital Polarimetric Phased Array Radar

Phased Array Radar (PAR) technology is rapidly rising as a candidate for future weather radars because it can provide focused, high-quality meteorological observations. Due to the need for precise measurements of polarimetric weather variables, it is desired that the radar simultaneously transmits and receives linearly polarized horizontal (H) and vertical (V) fields through beams that are well matched in gain and shape at every pointing direction. These scanning characteristics are difficult to achieve, especially with current methods such as the "park and probe" calibration, because the radiation patterns of PAR antennas are inherently dependent on their pointing direction. Thus, polarimetric array calibration is critical to produce matched H/V copolar antenna patterns. In this article, we present a polarimetric antenna calibration procedure for the fully-digital PARs based on the holographic back-projection of electric fields. The fully digital S-band Horus radar is used to implement and evaluate the proposed calibration method. First, near-field measurements of a fully active Horus antenna panel are back-projected onto the plane of the array to derive the copolar magnitude and phase of the H/V fields radiated by each antenna element. Then, digital calibration parameters are produced from back-projected fields to compensate for excitation differences and produce uniform radiation at the plane of the array. The performance of the back-projection calibration method is evaluated for broadside and for electronically scanned angles and compared to the more conventional park and probe calibration method. A robustness analysis is conducted using simulations to evaluate the impact of excitation amplitude and phase errors that could result from practical near-field environments and amplifier performance degradation. Preliminary results show that through precise digital calibration based on back-projected fields, beam matching can be significantly improved to achieve the desired polarimetric calibration levels. For electronically steered beams, back-projection calibration can reduce H/V beam mismatch biases by approximately 25%, from 0.34 dB to 0.08 dB. This improves measurement accuracy of fully digital PARs by mitigating antenna-induced biases in meteorological estimates.