Khuda Burdi

This article presents advancements in the development and validation of the Radio Frequency Sonde (RFSonde) [1], an unmanned aerial system (UAS) specifically designed for in-situ antenna pattern measurements of fielded weather radar systems. The RFSonde employs a 3-axis gimbal stabilization system, ensuring accurate mechanical polarization alignment of the probe during flights for scanning antenna patterns. The methodology was tested using two operational phased array radar (PAR) systems—Horus [2] and the Advanced Technology Demonstrator (ATD) [3]. A simulation framework was developed to assess critical measurement uncertainties arising from factors such as GNSS positional errors, environmental wind effects, and polarization tracking accuracy. Measured and simulated antenna pattern comparisons allowed us to establish UAS requirements for accurate weather radar calibration. This study also quantified transmit co-polar beam matching biases in differential reflectivity (ΔZ_DR,tx) and co-polar correlation coefficient (ρ_hv,tx). Combined with prior work measuring transmitted differential phase (ϕ_DP,tx) [4], these results demonstrate that the RFSonde can reliably achieve stringent calibration requirements under realistic field conditions. Meeting such calibration standards is essential for accurate weather radar products, such as quantitative precipitation estimation (QPE). Thus, this work validates the RFSonde as a practical, precise, and scalable solution for routine weather radar calibration and validation, potentially enhancing the reliability of radar-based meteorological observations.

The growing complexity of weather radar systems necessitates precise in-situ antenna-pattern measurements, particularly for emerging phased array weather radars. Our study introduces an innovative method using the Radio Frequency Sonde (RFSonde), a custom-modified uncrewed aerial system (UAS). It conducts in-flight power measurements in the far-field region of operational radar antennas, employing a unique 3-axis gimbal control strategy to maintain continuous polarization alignment with the antenna under test (AUT). This strategy, consistent with the Ludwig-3 definition of cross-polarization, avoids distortions due to probe misalignment and effectively utilizes the full dynamic range of the onboard power sensor. In multiple field experiments, the RFSonde demonstrated the viability and repeatability of this method, capturing co-polar and cross-polar radiation patterns of an X-band weather radar. The maximum absolute differences in azimuth and elevation beamwidth estimates were 0.011° and 0.006°, respectively, highlighting the system’s precision. The nearly negligible error contributions, amounting to just 0.014% of the power measured, further showcase the system’s accuracy. Additionally, we verified that our dynamic gimbal scan algorithm accurately measures the AUT without needing posterior probe correction. This groundbreaking work contributes to the emerging field of aerial RF measurement technology and has potential applications for the polarimetric calibration of phased array radars.