Compensation of Bending due to Gravity for MetOp-SG Scatterometer Antenna On-Ground Calibration at the DTU-ESA Antenna Test Facility

The on-ground calibration of the MetOp-SG Scatterometer Antenna Subsystem (SAS) requires spherical near-field measurements of large and heavy antennas mounted on a roll-over-azimuth positioner. Due to the significant mass of the antenna under test, the supporting structure experiences gravity-induced deformation; this results in the near-field signal of the antenna under test to be measured in points that are non-uniformly distributed on the measurement sphere. This paper proposes a compensation technique based on a non-uniform 2D Fast Fourier Transform to compute the spectral representation of the irregularly sampled near-field signal. The near-field values at the nominal sampling locations are subsequently reconstructed from this spectrum by interpolation, enabling recovery of the field corresponding to the ideal measurement geometry. The method was validated using measured and simulated data from an antenna with characteristics comparable to those of the MetOp-SG instruments, demonstrating negligible reconstruction error, and robustness with respect to uncertainty of the bending angle. Then, the technique was successfully applied to real data from the MetOp-SG SAS FM2 near-field measurement campaign conducted at the Technical University of Denmark in 2022, confirming its effectiveness in practical measurement scenarios. Additionally, it is proposed that the method can be adapted to compensate for a variety of systematic measurement setup errors; this is demonstrated by applying the method to real measured data of a standard gain horn affected by an axis-intersection error.