Compensation of Gravity Bending of MetOp-SG on-ground Calibration Measurements

The ongoing on-ground calibration of the MetOp-SG SAS instruments involves the mounting of very heavy antennas in a roll-over-azimuth AUT positioner. The weight of these instruments causes the support structure of the antennas to bend under gravity; this causes the near-field of the antenna under test (AUT) to be measured in points that are non-uniformly distributed on the measurement sphere. A method to compensate this bending has been developed, based on a non-uniform 2D Fast Fourier Transform (FFT) to compute the spectrum of the non-uniformly sampled signal of the AUT. The near-field in the nominal points can then be interpolated from this spectrum, thus compensating the effect of gravity bending. The proposed algorithm was first tested and validated using measured and simulated data of an antenna of similar characteristics as the MetOp-SG instruments, demonstrating its capacity to compensate the effects of gravity bending with negligible error. Subsequently, the method was used to compen-sate the near-field measurements of the MetOp-SG SAS FM2 campaign performed at the Technical University of Denmark (DTU) during 2022.