Beam switching is used in the context of continuum drifts. The various positions involved are described in different parts of the FITS and CLASS headers.
In the FITS header, the relevant information from e.g. iram30m-nbc-20220817s29-imb.fits looks like:
IMBF-scan:
Header:
OBJECT (C) = Mars / Source name
CTYPE1 (C) = RA-SFL / Basis system (longitude) --
CTYPE2 (C) = DEC-SFL / Basis system (latitude) -- XLAT-SFL
EQUINOX (R8) = 2022.62422998199 / [Julian yrs] Equinox
LONGOBJ (R8) = 56.1815486637719 / [deg] Source longitude in basis frame
LATOBJ (R8) = 18.2461304985798 / [deg] Source latitude in basis frame
SWTCHMOD (C) = beamSwitching / Switch mode
Table:
SYSOFF (C) = Nasmyth projection / WARNING! Added a dummy 'projection' value
XOFFSET (R4) = -1.9150140E-04 0.000000 / WARNING! Added a dummy 'projection' value
YOFFSET (R4) = 2.6664753E-05 0.000000 / WARNING! Added a dummy 'projection' value
IMBF-antenna (subscan #1):
Header:
SUBSTYPE (C) = onTheFly / Subscan type
SYSTEMOF (C) = horizontalTrue / System for offsets
Table:
LONGOFF (R8) = -2.908680211E-04 ... 2.908678739E-04 / [rad] long. offset from source in user native f
LATOFF (R8) = 0.00000000 ... 0.00000000 / [rad] lat. offset from source in user native fr
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The scan HDU is used to build the position section in the CLASS header. In this example, the reference position is taken from LONGOBJ and LATOBJ (in RA-Dec equinox 2022.6 coordinates in this example). According to the scan table, the position offset are (0,0) here (but they could be non-zero) with the radio projection (SYSOFF is projection).
The antenna HDU is used to build the drift offsets from the position described above. The table provides the whole drift offsets. It is stored in the CLASS RX array as it is not regularly spaced in the general case. Its associated system of coordinates is saved in the drift section (ctype, horizontal system in this example3.2). Its unit (angles here) is described in the general section (xunit parameter).