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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 (FF) region of operational radar antennas, employing a unique three-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 copolar 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 (DGS) algorithm accurately measures the AUT without needing posterior probe correction (PC). 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 (PARs).