Polarization mixing

In optics, polarization mixing refers to changes in the relative strengths of the Stokes parameters caused by reflection or scattering—see vector radiative transfer—or by changes in the radial orientation of the detector.

When the bases are rotated by 45 degrees around the viewing axis, the definition of the third Stokes component becomes equivalent[dubious – discuss][clarification needed] to that of the second, that is the difference in field intensity between the horizontal and vertical polarizations.

Ideally, in a polarimetric radiometer, especially a satellite mounted one, the polarisation axes are aligned with the Earth's surface, therefore we define the instrument viewing direction using the following vector: We define the slope of the surface in terms of the normal vector,

The Pol-Ice 2007 campaign included measurements over sea ice and open water from a fully polarimetric, aeroplane-mounted, L-band (1.4 GHz) radiometer.

Moreover, emissivity over calm water and to a lesser extent, sea ice, can be effectively modelled using the Fresnel equations.

In particular, the campaign included both circular and zig-zagging overflights which will produce strong mixing in the Stokes parameters.

To test the calibration of the EMIRAD II radiometer[3] used in the Pol-Ice campaign, measurements over open water were compared with model results based on the Fresnel equations.

[2] The first plot, which compares the measured data with the model, shows that vertically polarized channel is too high, but more importantly in this context, are the smeared points in between the otherwise relatively clean function for measured vertical and horizontal brightness temperature as a function of viewing angle.

These are the result of polarization mixing caused by changes in the attitude of the aircraft, particularly the roll angle.

Many of the radiance measurements over sea ice included large signals in the third Stoke component, U.

It turns out that these can be predicted to fairly high accuracy simply from the aircraft attitude.

The plot below shows the dependence on surface-slope and azimuth angle for a refractive index of 2 (a common value for sea ice[4]) and a nominal instrument pointing-angle of 45 degrees.