Once the ASTRA program has been started it reads in the initial data on the model to be run from the MODELDATA.DAT and SHELL.DAT files. This gives it all the information it needs on the shape of the cloud in order to be able to calculate the geometry of the lines of sight throughout the cloud.
There are two geometry routines (labeled geometry and geometry2), the first calculates the entire geometry of the cloud with all its lines of sight and is described in detail in this section. The second (ie. geometry2) accepts parameters describing just one line of sight and calculates the cylinder/disk intersections for just that one line - it is described in section 4.8.2.