This is compiled "chm" file, the same type Microsoft uses for their help files. It is zipped and linked to at our general nav pubs site www.starpath.com/navpubs. They are the standard calculators for tables that appear in Bowditch. These are available free online at the NIMA site, but they are an Internet app, so these are more convenient as you do not have to be online to use them. We use them frequently in our online celestial navigation, marine weather, and coastal navigation courses. You may find other things of interest at our navpubs links as well.