A compact passive microwave calibration target for full-polarized radiometer

Abstract

Passive microwave polarimeter is a microwave radiometer which can be used for measuring the polarized information of the target radiation. To achieve the quantitative apply of polarimeter, the calibration targets are key tools for converting the outputs of a radiometer into brightness temperature. In this paper, an advanced full-polarized passive calibration target was designed. A new idea is devised that a wave plate is applied to produce circular-polarization radiation after a linear-polarized source. Any combinations of the four Stokes brightness temperatures can be produced by automatically rotating the polarization grid around its normal line, which results in an imbalance waveform between the rotation angles and the four Stokes parameters The theoretical derivation of the four Stokes in brightness temperature after the wave plate are given in detail. The device of this compact passive microwave calibration target is developed and has been tested using polarimeters at 23.8 and 37.0 GHz. In the test, two traditional calibration targets at ambient temperature and boiling point of the liquid nitrogen are used outside for correcting the differences of the first two Stokes parameters due to antenna sidelobe contribution. Finally, the test results like specification of the compact passive microwave calibration target and groups of its full polarized brightness are given. Above all, this advanced full-polarized passive calibration target is easy to operate, and can evaluate and correct the uncertainties in antenna sidelobe of the polarimeter with the help of two traditional calibration target during the calibration when the antenna is not located so close as possible.