Fully polarimetrie radiometer calibration: Determining retardation plate's characteristics

This paper presents a novel method to determine the phase shift of a phase retardation plate. Primarily, the method has been developed for fully polarimetric calibration standards, which consists of a linearly polarized (tri-polarimetric) calibration standard and a retardation plate. However, the method can also be applied for the phase retardation plates for other purposes. The presented method applies a fully polarimetric radiometer as the measuring instrument and it is based on the application of both linearly polarized (tri-polarimetric) and fully polarimetric calibration data, generated by the calibration standard. In the same process, the skew angle between the coordinates of the retardation plate and antenna polarization basis is determined. In addition, a method to determine the insertion loss of the phase retardation plate is described. It is based on the difference of the brightness temperatures when measuring the cold calibration scene with and without the retardation plate in the RF path. The feasibility of the presented methods has been demonstrated using the Fully Polarimetric Radiometer (FPoR), the Fully Polarimetric Calibration Standard (FPCS), and its retardation plate of Aalto University. Good match between measurement results and theoretical predictions as well as good repeatability have been achieved in determining the parameters of the retardation plate in 11 distinct measurements over a time period of almost three years.