Evaluation of several shaped dihedrals useful for polarimetric calibration

Calculation, measurement and improvement of two-bounce passive reflectors designed for the external cross-calibration of linearly polarized polarimetric radars are addressed. Derived from the standard right angled dihedral, the reflectors under consideration are designed so that they have broad Co- and X-pol backscattering diagrams. These broad beamwidths are highly required for in situ SAR calibration, where uncertainties on the direction of flight and incidence angle may exist. Several examples are discussed. The first part is concerned with the standard dihedral which is made of two orthogonal conducting plates. Due to the narrow X-pol backscattering beamwidth of this type of dihedral, X-pol channels calibration is harder to achieve, especially outside laboratory. Therefore, dihedrals with elliptical vertex, including the circular case, are studied. The results are compared with the standard dihedral. Subsequently, simulations conducted on this type of target have determined the vertex ellipticity influence on the Co- and X-pol backscattering diagrams. Finally the authors set up a synthesis process to design an optimal shaped dihedral as long as the X-pol backscattering diagram is required to be as flat and as wide as possible. The optimization leads to an helicoidal vertex. Advantages and drawbacks for each kind of structures are also discussed