Beam size monitoring using OTF raster scans. Geometrical FWHM at 1 mm (top panel) and 2 mm (bottom panel) as a function of the observation time in UT hours are shown using scans of giant planets (filled circles) and bright point-like sources with a flux density higher than 1 Jy (filled stars) for the three Reference observation campaigns (N2R9, N2R12, and N2R14). The cross-hatched areas correspond to the observing time periods that are discarded using the Baseline scan selection, as described in Sect. 4.6.