  We describe the preliminary results of a ground-based observing campaign
aimed at building a grid of approximately 200 spectro-photometric standard
stars (SPSS), with an internal $\simeq 1$\% accuracy (and sub-percent
precision), tied to CALSPEC Vega and Sirius systems within $\simeq 1$\%, for
the absolute flux calibration of data gathered by {\it Gaia}, the European
Space Agency (ESA) astrometric mission. The criteria for the selection and a
list of candidates are presented, together with a description of the survey's
strategy and the adopted data analysis methods. All candidates were also
monitored for constancy (within $\pm 5$ mmag, approximately). The present
version of the grid contains about half of the final sample, it has already
reached the target accuracy but the precision will substantially improve with
future releases. It will be used to calibrate the {\it Gaia} (E)DR3 release of
spectra and photometry.
