Stokes Q_azi,elv (left column) and U_azi,elv (right column) data sets recorded at 4.85 GHz for the unpolarized, point-like source NGC 7027 in the two scanning directions; the azimuth in the top row and the elevation in the bottom row. Instead of the expected flat, noise-like pattern, the presence of spurious signals is clearly visible in both scanning directions. The instrumental polarization, calculated by the instrument model M, is shown in each panel with a smooth black line. The dashed blue lines mark the source position along the scanning direction.