Polarimetric Radiometry – Coherent vs. Incoherent Detection

There are two popular approaches to the design of a polarimetric radiometer: using either coherent or incoherent detection. Coherent detection implies that a direct cross-correlation is performed of the vertical and horizontal components of the incident electrical field. Incoherent detection derives the third and fourth Stokes parameters indirectly, by differencing the brightness temperatures detected at +/45 degree linear and left and right hand circular polarizations. In this paper, we consider the calibration accuracy of both types of radiometers. A relationship is developed between the true Stokes parameters and the measurements, as functions of non-ideal antenna characteristics. The sensitivity of calibration equations, which correct for these characteristics, to errors in knowledge of the polarization impurity is examined. It is found that, in most cases, the incoherent approach is considerably more tolerant of polarization impurity than is the coherent approach.