Calibration of WindSat polarimetric channels with a vicarious cold reference

Absolute calibration of WindSat's third and fourth Stokes brightness temperatures (T/sub 3/ and T/sub 4/) 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 T/sub 3/ and T/sub 4/ estimates. The method uses a vicarious cold reference brightness temperature applied to measurements made by WindSat at /spl plusmn/45/spl deg/ slant linear (T/sub P/ and T/sub M/) and left- and right-hand circular (T/sub L/ and T/sub R/) polarization. WindSat derives the third and fourth Stokes brightness temperatures by the differences T/sub P/-T/sub M/ and T/sub L/-T/sub 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.