Very high-accuracy calibration of radiation pattern and gain of a near-field probe

In this paper, very high-accuracy calibration of the radiation pattern and gain of a near-field probe is described. An open-ended waveguide near-field probe has been used in a recent measurement of the C-band Synthetic Aperture Radar (SAR) Antenna Subsystem for the Sentinel 1 mission of the European Space Agency. The measurement has been performed with the planar near-field technique in which the probe uncertainty is directly contributing to the final uncertainty of the SAR instrument. Thus, given the uncertainty requirements for the Sentinel 1 mission, very stringent requirements were set for the probe calibration uncertainty, 0.03 dB (3 sigma) for the directivity within the 3 dB beamwidth and 0.10 dB (3 sigma) for the gain. In order to achieve the requested uncertainty, an extensive investigation was carried out involving a detailed study of all uncertainty sources in the spherical near-field technique used for the probe calibration. The results of the investigation illustrating the significant uncertainty sources and the ways of their compensation to achieve the required probe calibration uncertainty are presented and discussed.