There is adequate documentation on the TRMM VIRS fire WWW page to get a sense of how FITS works for that data set.

There is a useful WWW page of FITS documentation. The basic idea is that a file is made up of "header and data units" (HDUs), and the HDUs may include both metadata and data values (values you could do math with, for example, an map of fire locations). A file may include HDUs where the data is in a mix of data types and precision. For example, the file may include maps in short integers and other maps in floating point.

There is a very useful set of scripts, provided by Peter Rydesäter of the Institute for Information Technology which will read FITS-formatted data written for the Auroral Large Imaging System. These files are posted on a MATLAB ftp site.

I modified one of Peter's scripts, fitsload.m, to read the TRMM VIRS fire count FITS-formatted files. The script, fitsload2.m, is linked here.

You should certainly look at the FITS documentation URL that is listed above, but the basic ideas that I needed are as follows: