Novel Passive Calibration Method for Fully Polarimetric Near Field MIMO Imaging Radars

In this paper a novel method for calibration of multiple-input multiple-output (MIMO), fully polarimetric near field radar systems is presented. The developed approach allows for the first time a practical and effective calibration, i.e. the correction / equalization of the frequency transfer function of all polarimetric radar channels even in a near field scenario where the target distance is not large compared to the MIMO antenna array dimension. In a first step, the mutual coupling between all channels is corrected by an empty space measurement. The polarimetric calibration is based on measurements with two simple, passive calibration standards. An offset short calibration of the co-polarized channels is conducted by using a mirroring reflection of a plane metal plate. A dihedral reflector is employed for the calibration of the cross-polarized channels. Since the dihedral reflection is distinctly directive and due to the close range acquisition condition, only channels with adjacent antenna elements can contribute to the calibration process. However, it is shown in this paper, that the sparse calibration data obtained from these few antenna combinations are sufficient for a successive calibration of all cross-polarized channels. This way, the novel method allows for a simple but yet effective calibration of fully polarimetric high resolution near field imaging MIMO radars based on two cost-efficient passive standards and with very low adjustment efforts during the calibration.