Brightness temperature spectrum from the North Ecliptic Pole (NEP) data cube at , illustrating aspects of the baseline removal. The negative feature at 370 km s^-1 results from the 2.5 MHz frequency-switching: it is an offset half-amplitude inversion of the −170 km s^-1 feature. The upper panel shows a typical third-order polynomial baseline fit to the central 4 MHz of the frequency-switched spectrum; the horizontal line segments mark the “emission-free” channels determined iteratively to be part of the baseline (see text). The lower panel displays the spectrum following baseline removal.