Definition of polarization angle. Left: the orientation of the main beam frame, (XYZ)_MB, with respect to the line-of-sight (LOS) frame, (XYZ)_LOS, is defined by the three angles θ_uv, φ_uv, and ψ_uv. The intermediate frame, (XYZ)_DX, is the detector frame, defined by the two angles θ_uv and φ_uv. Right: the angle ψ_pol is defined with respect to X_MB and represents the orientation of the polarization ellipse along the beam line of sight. It is very close to 0 or 90 deg for the S and M radiometers, respectively.