Hi Paul,

At present, the satellite magnitude data comes from the
TLEs themselves. If you look at the element file you're
using with Guide, you'll see that the "two-line elements"
actually are "three-line elements", with the first line
giving an object name and (usually) magnitude and size data,
as described at

http://users2.ev1.net/~mmccants/molcdesc.html

I assume that _somebody_ out there (or several somebodies)
keeps track of this magnitude data and updates it, but I dunno
who that would be.

Many TLEs really _are_ two-line, with no name/magnitude/size
line. Guide will use these, but it will usually know nothing of
their magnitudes (except for a few older satellites; Guide has
a little list with some of their magnitudes.)

-- Bill