This helped me out , not sure if it's what your looking for ...

http://www.x-plane.org/home/robinp/Apt850.htm


On Thu, May 28, 2009 at 6:45 PM, Andrew Gillanders <andrew.gillanders@uqconnect.edu.au> wrote:
Is the content of 'apt.dat' documented somewhere, or do I need to
look in the source to see how to interpret it?

Thanks
Andrew