The problem of converting geographical coordinates to geomagnetic coordinates in atmloc.f

Dear Fluka experts
As I said in the topic, I would like to ask where the GEOGMG (shown in the screenshot) function in atmloc.f can be looked up, and I would like to know what model the Fluka authors used to convert geographical coordinates to geomagnetic coordinates.
If you know, hope you can tell me, thank you for your generous answer.
Thank you,
Yongce


atmloc.f (17.9 KB)

The indicated function is a FLUKA core routine included in the source package.
The geomagnetic coordinates are defined in a system with centre at the planet centre, dipole axis inclined towards the magnetic north pole, and longitude measured from the plane containing the geographical and geomagnetic axes.