LCP (red circles) and RCP (black triangles) system temperature (T_sys) measurements versus the airmass (AM) for one observing session at 4.85 GHz. The AM range is split into a given number of bins and the data points with the lowest T_sys within each bin (filled markers) are used to fit the two lower envelopes (solid lines). Their slopes are practically identical, indicating that the atmospheric absorption does not influence the polarization of the transmitted radiation.