Far-Field Antenna Pattern Measurement Using a UAS-Based Dynamic Gimbal Scan System

As weather radar systems evolve in complexity, there's an increased need for accurate in-situ measurements of antenna patterns, especially in the realm of 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 degrees, respectively, highlighting the system's precision. The nearly negligible error contributions from the RFSonde positioning system, 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.