Unfolded level-2 spectra for the horizontal polarization mixer (blue line), vertical polarization mixer (red line), and averaged spectrum (black line) obtained with the WBS on UT 1.5 September 2010. Baselines are fitted with a linear combination of sine functions obtained by masking (−1, 1) km s^-1 windows around the expected position of negative (left panel) and positive (right panel) phases of the frequency-switched line rest frequency of the H₂O J_KaKc (1₁₀–1₀₁) transition. Note that the scale in the vertical axes is the main brightness temperature corrected by the forward and beam efficiencies (the vertical axis scale is inverted in the left panel). The upper horizontal axis is the lower sideband frequency, and the lower horizontal axis shows the Doppler velocity with respect to the nucleus rest frame.