Constraints on the polarization purity of a Stokes microwave radiometer

Measurement of the third and fourth Stokes parameters of microwave thermal emission can be degraded by nonideal radiometer characteristics. Of particu lar importance is the level of polarization purity and the knowledge of pol arization impurity on the part of the instrument. An explicit relationship is developed between the true Stokes parameters and the measurements, as fu nctions of nonideal polarization characteristics. In principle, knowledge o f the characteristics allows the degradation to be removed by a postprocess ing calibration procedure. However, errors in the knowledge are possible, r esulting from inexact initial testing and/or subsequent drifting of the ins trument. The effects of these errors on overall calibration are considered. It is found that there is considerably greater tolerance to the errors whe n incoherent detection is used (differencing +/-45 degrees linear and left- or right-hand circular polarizations), as opposed to coherent detection (d irect cross correlation of vertical and horizontal linear polarizations). T his is particularly true when cross-polar leakage is imbalanced. The level of nominal polarization purity that is required is also more relaxed in the incoherent case. On the other hand, coherent detection has the capability for superior performance with regard to precision.