[time-nuts] OT- RMC sentence and magnetic variation/compensation in GPS module

[Giuseppe Marullo]

Hi all,
I am unable to understand how to get this field from a RMC sentence, on 
the GPS I am using (LS20031) the field is empy so I am wondering if this 
is just a problem of implementation or what.
AFAIK, the data is highly  variable (location and time) so how is the 
GPS module  able to provide it? Is it something provided by satellites? 
Will the module compute it from the path comparing the bearing with an 
onboard compass?

Could some good soul shed some light on the matter?

Thanks in advance.

Giuseppe Marullo