## IRAM context

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.