Calibration of WindSat polarimetric channels with a vicarious cold reference

Absolute calibration of WindSat\´s third and fourth Stokes brightness temperatures (T₃ and T₄) is needed at the tenth of Kelvin level in order to adequately resolve their dependence on wind direction. Previous aircraft based fully polarimetric microwave radiometers have generally relied on "circle flights", during which a single area of the ocean is observed at all azimuth angles, to estimate residual biases in the calibration of its polarimetric channels. WindSat, the first spaceborne fully polarimetric microwave radiometer, operates in low Earth orbit and thus cannot execute this traditional calibration technique. A new method is presented to estimate the residual biases that are present in WindSat\´s T₃ and T₄ estimates. The method uses a vicarious cold reference brightness temperature applied to measurements made by WindSat at ±45° slant linear (T_P and T_M) and left- and right-hand circular (T_L and T_R) polarization. WindSat derives the third and fourth Stokes brightness temperatures by the differences T_P-T_M and T_L-T_R, respectively. The method is demonstrated by applying it to the 10.7-GHz WindSat observations. Calibration biases of 0.2-0.6 K are determined with a precision of 0.04 K.