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At the IRAM 30m telescope, the observations are traditionally
performed so that the spectra coordinates, and the resulting gridded
cubes created from on-the-fly projects, are expressed as an absolute
position on the celestial sphere plus relative offsets projected on a
2D plane using what we call the radio projection. This
projection is correctly supported within the GILDAS frame (including
the GILDAS Data Format), but also when exporting cubes to FITS using
the equivalent GLobal-Sinusoidal (hereafter GLS) projection
inherited from AIPS legacy.
However, recent versions of third-party software (CASA, CARTA) have broken support of the GLS projection in FITS files. This memo analyzes the current FITS standard regarding spatial projections, makes a complete review of the various projections supported in GILDAS, and describes the actions performed to ensure GILDAS users have no problem when exporting or importing their data to/from FITS.