The first processing stage creates the attitude-drift GTI, the housekeeping GTI, the source list, the intermediate event lists, and the unfinished combined event list. Nothing in this stage depends upon the choice of primary source, and there will rarely be any reason for the user to restart rgsproc here. In preliminary steps two exposure-independent data files, the attitude time-series and (optionally) the pulse-height offsets, are created; these are used only within this stage.
