Retardation Plate for the Calibration of a Fully Polarimetric Radiometer: Determination of 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 consist of a linearly polarized (tripolarimetric) calibration reference and a retardation plate. However, the method can also be applied to phase retardation plates for other purposes. The method presented here applies a fully polarimetric radiometer as the measuring instrument and it is based on the application of both tripolarimetric 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 in the brightness temperatures when the cold calibration scene is measured with and without the retardation plate in the RF path. The feasibility of the methods presented here has been demonstrated using Aalto University's Fully Polarimetric Radiometer (FPoR), the Fully Polarimetric Calibration Standard (FPCS), and its retardation plate. A good match between the measurement results and theoretical predictions, as well as good repeatability, was achieved in determining the parameters of the retardation plate in 11 distinct measurements over a time period of almost three years. The results suggest that the method is sufficiently accurate in terms of the calibration accuracy requirements for airborne and spaceborne remote sensing radiometers.