HTOF parses the intermediate data from Hipparcos and Gaia and fits astrometric solutions to those data. It computes likelihoods and parameter errors in line with the catalog and can reproduce five, seven, and nine (or higher) parameter fits to their astrometry.
https://doi.org/10.5281/zenodo.4104383 and https://ui.adsabs.harvard.edu/abs/2021arXiv210906761B